View source for Earth:Snowball Earth

{{Short description|Worldwide glaciation episodes during the Proterozoic eon}}
[[Image:Snowball_Huronian.jpg|thumb|Artist's rendition of a fully-frozen Snowball Earth with no remaining liquid surface water.]]
{{Graphical timeline
| title=Proterozoic snowball periods
| align=right | height=550 | width=16 | height-unit=px | plot-colour=transparent
| from=-1000 | to=-542 | scale-increment=50
| period1 = Tonian
| period1-border-width = 1
| period2 = Cryogenian
| period3 = Ediacaran
| period3-border-width = 1
| period3-nudge-down = 1.5
| period4 = Neoproterozoic
| period4-right = 0.15 | period4-text = <nowiki />
| period4-border-width = 1
| period4-colour = #FFFFB3
| bar1-from=-717 | bar1-to=-660
| bar1-left=0.1 | bar1-right=0.9
| bar1-text = [[Earth:Sturtian glaciation|Sturtian]]<ref name=Pu /> | bar1-colour = #77bb77
| bar2-from=-650 | bar2-to=-635
| bar2-left=0.1 | bar2-right=0.9
| bar2-text = Marinoan<ref name=Pu /> | bar2-colour = #77bb77
| bar3-text = ← [[Earth:Gaskiers glaciation|Gaskiers]] | bar3-colour = #77bb77
| bar3-from = -580 | bar3-to = -579.3
| bar3-left = 0.1 | bar3-right = 0.9 
| bar4-from = -540 | bar4-to = -530
| bar4-text = <small>(millions of years ago)</small>    | bar4-colour = transparent
| bar5-text=Baykonurian | bar5-colour = #77bb77
| bar5-from=-547 | bar5-to = -540
| bar5-left = 0.1 | bar5-right = 0.9
| legend1= [[Earth:Neoproterozoic|Neoproterozoic]] era
| legend1-colour=#FFFFB3
| legend2= Snowball Earth
| legend2-colour=#77bb77
| caption=Estimate of [[Earth:Proterozoic|Proterozoic]] glacial periods.<ref name=Smith2009>{{cite journal
| last = Smith
| first = A. G.
| title = Neoproterozoic timescales and stratigraphy
| journal =Geological Society, London, Special Publications
| year= 2009
| volume = 326
| issue = 1
| pages= 27–54
| doi = 10.1144/SP326.2|bibcode = 2009GSLSP.326...27S | s2cid = 129706604
}}</ref><ref name=Pu/>
 Dating of pre-[[Earth:Gaskiers glaciation|Gaskiers glaciation]]s is uncertain. As for the Kaigas, its very existence is doubted by some. The [[Earth:Huronian glaciation|Huronian glaciation]], is not shown, there is a lack of any significant evidence for a Snowball Earth during the time period.
}}
The '''Snowball Earth''' is a [[Earth:Historical geology|geohistorical]] hypothesis that proposes during one or more of Earth's [[Earth:Greenhouse and icehouse Earth|icehouse]] climates, the planet's surface became entirely or nearly entirely frozen with no liquid oceanic or surface water exposed to the atmosphere. The most academically referred period of such [[Earth:Glacial period|global glaciation]] is believed to have occurred sometime before 650 [[Astronomy:Myr#Usage|mya]] during the [[Earth:Cryogenian|Cryogenian]] period.

Proponents of the hypothesis argue that it best explains sedimentary [[Earth:Deposition (geology)|deposits]] that are generally believed to be of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean,<ref name=Kirschvink1992>{{cite book
 | author = Kirschvink, J. L.| year = 1992
 | chapter = Late Proterozoic low-latitude global glaciation: The snowball Earth
 | title = The Proterozoic Biosphere: A Multidisciplinary Study
 | pages = 51–2| publisher = Cambridge University Press | editor = Schopf, J. W. | editor2 = Klein, C.
 | chapter-url=http://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf}}</ref><ref name=nature_geo>{{cite journal| doi = 10.1038/ngeo355
 | title = Sedimentary challenge to Snowball Earth| year = 2008
 | author = Allen, Philip A.| journal = Nature Geoscience| volume = 1| pages = 817–825
 | last2 = Etienne| first2 = James L.
 | issue=12|bibcode = 2008NatGe...1..817A }}</ref> and they emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether Earth was a full snowball or a "slushball" with a thin equatorial band of open (or seasonally open) water. The snowball-Earth episodes are proposed to have occurred before the sudden radiation of [[Biology:Multicellular organism|multicellular]] bioforms known as the [[Earth:Cambrian explosion|Cambrian explosion]]. The most recent snowball episode may have triggered the evolution of multicellularity.

==History==
{{Life timeline}}
===First evidence for ancient glaciation===
Long before the idea of a global glaciation was first proposed, a series of discoveries occurred that accumulated evidence for ancient [[Earth:Precambrian|Precambrian]] glaciations. The first of these discoveries was published in 1871 by J. Thomson, who found ancient glacier-reworked material (tillite) in Islay, Scotland. Similar findings followed in {{wipe|Australia}} (1884) and {{wipe|India}} (1887). A fourth and very illustrative finding, which came to be known as "[[Earth:Reusch's Moraine|Reusch's Moraine]]," was reported by Hans Reusch in northern {{wipe|Norway}} in 1891. Many other findings followed, but their understanding was hampered by the rejection (at the time) of [[Earth:Continental drift|continental drift]].<ref name=Hoffman2011>{{cite book |last1=Hoffman |first1=Paul F.|editor-last=Arnaud|editor-first=E. |editor-last2=Halverson|editor-first2=G.P.|editor-last3=Shields-Zhou|editor-first3=G. |year=2011 |title=The Geological Record of Neoproterozoic Glaciations |chapter=A history of Neoproterozoic glacial geology, 1871–1997 |issue=36 |pages=17–37 |series=Geological Society, London, Memoirs|publisher=Geological Society of London}}</ref>

===Global glaciation proposed===
[[Biography:Douglas Mawson|Douglas Mawson]] (1882–1958), an Australian [[Geologist|geologist]] and Antarctic explorer, spent much of his career studying the [[Earth:Stratigraphy|stratigraphy]] of the [[Earth:Neoproterozoic|Neoproterozoic]] in South Australia, where he identified thick and extensive glacial sediments. As a result, late in his career, he speculated about the possibility of global glaciation.<ref name="Mawson">{{Cite journal | last1 = Alderman | first1 = A. R. | last2 = Tilley | first2 = C. E. | doi = 10.1098/rsbm.1960.0011 | title = Douglas Mawson 1882-1958 | journal = Biographical Memoirs of Fellows of the Royal Society | volume = 5 | pages = 119–127 | year = 1960 | title-link = Douglas Mawson | doi-access = free }}</ref>

Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and those of other continents where low-[[Earth:Latitude|latitude]] glacial deposits are found, have remained constant through time.{{fact|date=July 2023}} With the advancement of the [[Earth:Continental drift|continental drift]] hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes.

In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial [[Earth:Till|till]]ites in [[Place:Svalbard|Svalbard]] and [[Place:Greenland|Greenland]] were deposited at tropical latitudes.<ref name="Harland">{{cite journal
| author=W. B. Harland
| title=Critical evidence for a great infra-Cambrian glaciation
| journal=International Journal of Earth Sciences
| year=1964
| volume=54
| issue=1
| pages=45–61
| bibcode=1964GeoRu..54...45H
| doi=10.1007/BF01821169| s2cid=128676272
}}</ref> From this [[Earth:Paleomagnetism|palaeomagnetic]] data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued that an [[Earth:Ice age|ice age]] occurred that was so extreme that it resulted in marine glacial rocks being deposited in the tropics.

In the 1960s, Mikhail Budyko, a Soviet climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a [[Feedback|feedback loop]] ensued where the increased reflectiveness ([[Earth:Albedo|albedo]]) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium.<ref name="Budyko">{{cite journal
| author=M.I. Budyko
| title=The effect of solar radiation variations on the climate of the Earth
| journal=[[Earth:Tellus A|Tellus A]]
| year=1969
| volume=21
| issue=5
| pages=611–619
| doi=10.3402/tellusa.v21i5.10109
| bibcode=1969Tell...21..611B
| doi-access=free
}}</ref>

While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had, in fact, never happened, as his model offered no way to escape from such a feedback loop. 

In 1971, Aron Faegre, an American physicist, showed that a similar energy-balance model predicted three stable global climates, one of which was snowball Earth.<ref name="Faegre">{{cite journal
| author=A. Faegre
| title=An Intransitive Model of the Earth-Atmosphere-Ocean System
| journal=Journal of Applied Meteorology
| year=1972
| volume=11
| issue=1
| pages=4–6
| doi=10.1175/1520-0450(1972)011<0004:AIMOTE>2.0.CO;2
| bibcode=1972JApMe..11....4F
| doi-access=free
}}</ref> This model introduced [[Biography:Edward Norton Lorenz|Edward Norton Lorenz]]'s concept of [[Intransitivity|intransitivity]], indicating that there could be a major jump from one climate to another, including to snowball Earth.

The term "snowball Earth" was coined by Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning the biology of the [[Earth:Proterozoic|Proterozoic]] eon.<ref name="Kirschvink" /> The major contributions from this work were: (1) the recognition that the presence of [[Earth:Banded iron formation|banded iron formation]]s is consistent with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a completely ice-covered Earth—specifically, the accumulation of [[Chemistry:Carbon dioxide|CO<sub>2</sub>]] from [[Earth:Volcano|volcanic outgassing]] leading to an ultra-[[Earth:Greenhouse effect|greenhouse effect]].

Franklyn Van Houten's discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle". His studies of [[Chemistry:Phosphorus|phosphorus]] deposits and banded iron formations in sedimentary rocks made him an early adherent of the "snowball Earth" hypothesis postulating that the planet's surface froze more than 650 million years ago.<ref>{{cite web| url = http://www.princeton.edu/main/news/archive/S28/44/69O99/index.xml?section=topstories| title = Princeton University - Franklyn Van Houten, expert on sedimentary rocks, dies at 96}}</ref>

Interest in the notion of a snowball Earth increased dramatically after Paul F. Hoffman and his co-workers applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in Namibia and elaborated upon the hypothesis in the journal ''Science'' in 1998 by incorporating such observations as the occurrence of [[Earth:Cap carbonate|cap carbonate]]s.<ref name="Hoffman">{{Cite journal
| last3=Halverson
| first1=P. F.
| first2=A. J.
| last2=Kaufman
| first3=G. P.
| first4=D. P.
| last1=Hoffman 
| title = A Neoproterozoic Snowball Earth
| last4=Schrag
| journal = Science
| volume = 281 
| issue=5381
| pages = 1342–1346
| year = 1998
| doi = 10.1126/science.281.5381.1342
| pmid=9721097
| bibcode = 1998Sci...281.1342H
| s2cid=13046760
| url=https://semanticscholar.org/paper/37b1ffb6aadc8b436c029fb8a3311c0b26e30d4e
}}</ref>

In 2010, Francis A. Macdonald, assistant professor at [[Organization:Harvard University|Harvard]] in the Department of Earth and Planetary Sciences, and others, reported evidence that [[Earth:Rodinia|Rodinia]] was at equatorial latitude during the [[Earth:Cryogenian|Cryogenian]] period with glacial ice at or below sea level, and that the associated [[Earth:Sturtian glaciation|Sturtian glaciation]] was global.<ref>{{cite journal |last1=Macdonald |first1=F. A. |last2=Schmitz |first2=M. D. |last3=Crowley |first3=J. L. |last4=Roots |first4=C. F. |last5=Jones |first5=D. S. |last6=Maloof |first6=A. C. |last7=Strauss |first7=J. V. |last8=Cohen |first8=P. A. |last9=Johnston |first9=D. T. |last10=Schrag |first10=D. P. |title=Calibrating the Cryogenian |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1241–1243 |doi=10.1126/science.1183325 |pmid=20203045 |bibcode=2010Sci...327.1241M |s2cid=40959063}}
*{{cite press release |date=5 March 2010 |title=Snowball Earth: New evidence hints at global glaciation 716.5 million years ago |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2010/03/100304142228.htm}}</ref>

==Evidence==
The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes.<ref name=Harland1964>{{cite journal
| author = Harland, W.B.
| year = 1964
| title = Critical evidence for a great infra-Cambrian glaciation
| journal = International Journal of Earth Sciences
| volume = 54
| issue = 1
| pages = 45–61
| bibcode = 1964GeoRu..54...45H
| doi = 10.1007/BF01821169
| s2cid = 128676272
}}</ref> According to modelling, an [[Earth:Ice–albedo feedback|ice–albedo feedback]] would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25°<ref name="Meert1994pm" /> to 30°<ref name=Budyko1969>{{cite journal
| author = Budyko, M.I.
| year = 1969
| title = The effect of solar radiation variations on the climate of the earth
| journal = Tellus
| volume = 21
| pages = 611–9
| doi = 10.1111/j.2153-3490.1969.tb00466.x
| issue = 5
| bibcode = 1969Tell...21..611B
| citeseerx = 10.1.1.696.824
}}</ref> of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover.

Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove three things:
# that a bed contains sedimentary structures that could have been created only by glacial activity;
# that the bed lay within the tropics when it was deposited.
#that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.

This last point is very difficult to prove. Before the [[Earth:Ediacaran|Ediacaran]], the [[Earth:Biostratigraphy|biostratigraphic]] markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at precisely the same time. The best that can be done is to estimate the age of the rocks using [[Astronomy:Radiometry|radiometric]] methods, which are rarely accurate to better than a million years or so.<ref name=Eyles2004/>

The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as {{Ma|200}} can be riddled with difficulties.<ref name=Briden1971>{{cite journal
| author = Briden, J.C. |author2=Smith, A.G. |author3=Sallomy, J.T.
| year = 1971
| title = The geomagnetic field in Permo-Triassic time
| journal = Geophys. J. R. Astron. Soc.
| volume = 23
| pages = 101–117
| doi = 10.1111/j.1365-246X.1971.tb01805.x
|bibcode=1971GeoJ...23..101B | doi-access = free
}}</ref>

===Palaeomagnetism===
The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.

When [[Earth:Sedimentary rock|sedimentary rock]]s form, magnetic minerals within them tend to align themselves with [[Earth:Earth's magnetic field|Earth's magnetic field]]. Through the precise measurement of this palaeomagnetism, it is possible to estimate the [[Earth:Latitude|latitude]] (but not the [[Earth:Longitude|longitude]]) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator,<ref name="Evans">{{cite journal
| author=D.A.D. Evans
| title=Stratigraphic, geochronological, and palaeomagnetic constraints upon the Neoproterozoic climatic paradox
| journal=American Journal of Science
| year=2000
| volume=300
| issue=5
| pages=347–433
| doi = 10.2475/ajs.300.5.347| bibcode=2000AmJS..300..347E
}}</ref> although the accuracy of this reconstruction is in question.<ref name=Eyles2004 /> This palaeomagnetic location of apparently glacial sediments (such as [[Earth:Dropstone|dropstone]]s) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation, or the existence of localized, possibly land-locked, glacial regimes.<ref name=Young1995>{{cite journal
| author = Young, G.M.
| date = 1 February 1995
| title = Are Neoproterozoic glacial deposits preserved on the margins of Laurentia related to the fragmentation of two supercontinents?
| journal = Geology
| volume = 23
| issue = 2
| pages = 153–6
| doi = 10.1130/0091-7613(1995)023<0153:ANGDPO>2.3.CO;2
|bibcode = 1995Geo....23..153Y }}</ref> Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.<ref name=Meert1994nmse>{{Cite journal| doi = 10.1016/0012-821X(94)90253-4| title = The Neoproterozoic (1000–540 Ma) glacial intervals: No more snowball earth?| year = 1994| last1 = Meert | first1 = J. G.| last2 = Van Der Voo | first2 = R.| journal = Earth and Planetary Science Letters| volume = 123| issue = 1–3| pages = 1–13| bibcode=1994E&PSL.123....1M | hdl = 2027.42/31585| hdl-access = free}}</ref>

Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of [[Earth:Structure of the Earth|Earth's core]], it is possible that during the Proterozoic, the [[Physics:Magnetic field|magnetic field]] did not approximate a simple [[Physics:Dipole|dipolar]] distribution, with north and south [[Physics:Magnet|magnetic poles]] roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a 'West Pole' rather than the North Pole. Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north pole would occur around the same time as the [[Earth:Gaskiers glaciation|Gaskiers glaciation]].<ref>{{Cite journal| doi = 10.1016/j.epsl.2010.02.038| title = Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis| year = 2010| last1 = Abrajevitch | first1 = A.| last2 = Van Der Voo | first2 = R.| journal = Earth and Planetary Science Letters| volume = 293| issue = 1–2| pages = 164–170| bibcode=2010E&PSL.293..164A}}</ref>

Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building [[Physics:Orogeny|orogeny]] releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations.<ref name=Meert1994pm>{{cite journal
| author = Meert, J.G.
| author2 = Van Der Voo, R.
| author3 = Payne, T.W.
| year = 1994
| title = Paleomagnetism of the Catoctin volcanic province: A new Vendian-Cambrian apparent polar wander path for North America
| journal = Journal of Geophysical Research
| volume = 99
| issue = B3
| pages = 4625–41
| doi = 10.1029/93JB01723
| bibcode=1994JGR....99.4625M
}}</ref> Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles.<ref name=Font2010pm>{{cite journal
| author = Font, E
| author2 = C.F. Ponte Neto
| author3 = M. Ernesto
| year = 2011
| title = Paleomagnetism and rock magnetism of the Neoproterozoic Itajaí Basin of the Rio de la Plata craton (Brazil): Cambrian to Cretaceous widespread remagnetizations of South America
| journal = Gondwana Research
| volume = 20
| issue = 4
| pages = 782–797
| doi = 10.1016/j.gr.2011.04.005
| bibcode = 2011GondR..20..782F
}}</ref><ref name=Rowan2010pm>{{cite journal
| author = Rowan, C. J.
| author2 = Tait, J.
| year = 2010
| title = Oman's low latitude "Snowball Earth" pole revisited: Late Cretaceous remagnetisation of Late Neoproterozoic carbonates in Northern Oman
| journal = American Geophysical Union, Fall Meeting
| volume = 2010
| pages = GP33C–0959
| bibcode = 2010AGUFMGP33C0959R
}}</ref>

There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date is well-constrained, and the signal is demonstrably original.<ref name=Sohl1999>{{cite journal
| author = Sohl, L.E. |author2=Christie-blick, N. |author3=Kent, D.V.
| year = 1999
| title = Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia; implications for the duration of low-latitude glaciation in Neoproterozoic time
| journal = Bulletin of the Geological Society of America
| volume = 111
| issue = 8
| pages = 1120–39
| doi = 10.1130/0016-7606(1999)111<1120:PPRIMC>2.3.CO;2
|bibcode = 1999GSAB..111.1120S | url = https://academiccommons.columbia.edu/doi/10.7916/D80P18JS/download
}}</ref>

===Low-latitude glacial deposits===
[[Image:PocatelloFm.JPG|thumb|Diamictite of the Neoproterozoic Pocatello Formation, a "snowball Earth"-type deposit]]
[[File:Elatina Fm diamictite.JPG|thumb|Elatina Fm [[Earth:Diamictite|diamictite]] below Ediacaran GSSP site in the Flinders Ranges NP, South Australia. A$1 coin for scale.]]
Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the ''snowball Earth'' hypothesis many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, it is worth remembering that many sedimentary features traditionally associated with glaciers can also be formed by other means.<ref name=Arnaud2002>{{cite journal
| author = Arnaud, E.
| author2 = Eyles, C. H.
| year = 2002
| title = Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway
| journal = Sedimentology
| volume = 49
| issue = 4
| pages = 765–88
| doi = 10.1046/j.1365-3091.2002.00466.x
| bibcode = 2002Sedim..49..765A
| s2cid = 128719279
}}</ref> Thus the glacial origin of many of the key occurrences for snowball Earth has been contested.<ref name="Eyles2004" />
As of 2007, there was only one "very reliable"—still challenged<ref name="Eyles2004" />—datum point identifying tropical tillites,<ref name="Evans" /> which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the Sturtian is accumulating.<ref name='Macdonald2010'>{{cite journal |last1=Macdonald |first1=F. A. |last2=Schmitz |first2=M. D. |last3=Crowley |first3=J. L. |last4=Roots |first4=C. F. |last5=Jones |first5=D. S. |last6=Maloof |first6=A. C. |last7=Strauss |first7=J. V. |last8=Cohen |first8=P. A. |last9=Johnston |first9=D. T. |last10=Schrag |first10=D. P. |title=Calibrating the Cryogenian |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1241–1243 |doi=10.1126/science.1183325 |bibcode=2010Sci...327.1241M |pmid=20203045|s2cid=40959063 }}</ref><ref>{{cite journal |last1=Kerr |first1=R. A. |title=Snowball Earth Has Melted Back To a Profound Wintry Mix |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1186 |doi=10.1126/science.327.5970.1186 |pmid=20203019 |bibcode=2010Sci...327.1186K |doi-access=free }}</ref>
Evidence of possible glacial origin of sediment includes:
* Dropstones (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena.<ref name=Donovan1997>{{cite journal
| author = Donovan, S. K.
| author2 = Pickerill, R. K.
|year = 1997
| title = Dropstones: their origin and significance: a comment
| journal = Palaeogeography, Palaeoclimatology, Palaeoecology
| volume = 131
| issue = 1
| pages = 175–8
| doi = 10.1016/S0031-0182(96)00150-2
| bibcode = 1997PPP...131..175D
}}</ref>
* Varves (annual sediment layers in periglacial lakes), which can form at higher temperatures.<ref name=Thunell1995>{{cite journal
| author2 = Tappa, E.
| author3 = Anderson, D. M.
| date = 1 December 1995
| title = Sediment fluxes and varve formation in Santa Barbara Basin, offshore California
| journal = Geology
| volume = 23
| issue = 12
| pages = 1083–6
| doi = 10.1130/0091-7613(1995)023<1083:SFAVFI>2.3.CO;2
|bibcode = 1995Geo....23.1083T }}</ref>
* [[Earth:Glacial striation|Glacial striation]]s (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by [[Earth:Mudflow|mudflow]]s or tectonic movements.<ref name=Jensen1996>{{cite journal
| author = Jensen, P. A.
| author2 = Wulff-pedersen, E.
| date = 1 March 1996
| title = Glacial or non-glacial origin for the Bigganjargga tillite, Finnmark, Northern Norway
| journal = Geological Magazine
| volume = 133
| issue = 2
| pages = 137–45
| doi = 10.1017/S0016756800008657
| bibcode = 1996GeoM..133..137J
| s2cid = 129260708
}}</ref>
* [[Earth:Diamictite|Diamictite]]s (poorly sorted conglomerates). Originally described as glacial [[Earth:Till|till]], most were in fact formed by [[Earth:Debris flow|debris flow]]s.<ref name=Eyles2004>{{cite journal
| author = Eyles, N.
| author2 = Januszczak, N.
| year = 2004
| title = 'Zipper-rift': A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma
| journal = Earth-Science Reviews
| volume = 65
| issue = 1–2
| pages = 1–73
| doi = 10.1016/S0012-8252(03)00080-1
| bibcode=2004ESRv...65....1E
}}</ref>

===Open-water deposits===
It appears that some deposits formed during the snowball period could only have formed in the presence of an active [[Physics:Water cycle|hydrological cycle]]. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition.<ref name=Condon2002>{{cite journal
| author = Condon, D.J.
|author2=Prave, A.R. |author3=Benn, D.I. 
| date = 1 January 2002
| title = Neoproterozoic glacial-rainout intervals: Observations and implications
| journal = Geology
| volume = 30
| issue = 1
| pages = 35–38
| doi = 10.1130/0091-7613(2002)030<0035:NGRIOA>2.0.CO;2
|bibcode = 2002Geo....30...35C
}}</ref> It is considered{{by whom|date=June 2015}} possible that [[Earth:Ice stream|ice stream]]s such as seen in [[Earth:Antarctica|Antarctica]] today could have caused these sequences.
Further, sedimentary features that could only form in open water (for example: wave-formed ripples, far-traveled ice-rafted debris and indicators of [[Earth:Photosynthesis|photosynthetic]] activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent "[[Earth:Oasis|oases]]" of [[Earth:Meltwater|meltwater]] on a completely frozen Earth,<ref name=Halverson2004>{{cite journal
| author = Halverson, G.P.
|author2=Maloof, A.C. |author3=Hoffman, P.F. 
| year = 2004
| title = The Marinoan glaciation (Neoproterozoic) in northeast Svalbard
| journal = Basin Research
| volume = 16
| issue = 3
| pages = 297–324
| doi = 10.1111/j.1365-2117.2004.00234.x
| bibcode = 2004BasR...16..297H
|citeseerx=10.1.1.368.2815 |s2cid=53588955 }}</ref> computer modelling suggests that large areas of the ocean must have remained ice-free; arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models.<ref name="Peltier">
{{cite book
| last= Peltier
| first= W.R.
| editor= Jenkins, G.S.
| editor2= McMenamin, M.A.S.
| editor3= McKey, C.P.
| editor4= Sohl, L.
| title= The Extreme Proterozoic: Geology, Geochemistry, and Climate
| year= 2004 |publisher=American Geophysical union |pages=107–124
| chapter= Climate dynamics in deep time: modeling the "snowball bifurcation" and assessing the plausibility of its occurrence
}}
</ref>

===Carbon isotope ratios===
There are two stable [[Physics:Isotope|isotope]]s of [[Chemistry:Carbon|carbon]] in sea water: [[Physics:Carbon-12|carbon-12]] (<sup>12</sup>C) and the rare [[Physics:Carbon-13|carbon-13]] (<sup>13</sup>C), which makes up about 1.109 percent of carbon atoms.

[[Biology:Biochemistry|Biochemical]] processes, of which [[Earth:Photosynthesis|photosynthesis]] is one, tend to preferentially incorporate the lighter <sup>12</sup>C isotope. Thus ocean-dwelling photosynthesizers, both [[Biology:Protist|protist]]s and [[Biology:Algae|algae]], tend to be very slightly depleted in <sup>13</sup>C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower <sup>13</sup>C/<sup>12</sup>C ratio within organic remains, and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in <sup>13</sup>C. 

Silicate weathering, an inorganic process by which carbon dioxide is drawn out of the atmosphere and deposited in rock, also fractionates carbon. The emplacement of several [[Earth:Large igneous province|large igneous province]]s shortly before the Cryogenian and the subsequent chemical weathering of the enormous continental [[Earth:Flood basalt|flood basalt]]s created by them, aided by the breakup of Rodinia that exposed many of these flood basalts to warmer, moister conditions closer to the coast and accelerated chemical weathering, is also believed to have caused a major positive shift in carbon isotopic ratios and contributed to the beginning of the Sturtian glaciation.<ref name="FloodBasaltWeathering">{{cite journal |last1=Cox |first1=Grant M. |last2=Halverson |first2=Galen P. |last3=Stevenson |first3=Ross K. |last4=Vokaty |first4=Michelle |last5=Poirier |first5=André |last6=Kunzmann |first6=Marcus |last7=Li |first7=Zheng-Xiang |last8=Denyszyn |first8=Steven W. |last9=Strauss |first9=Justin V. |last10=Macdonald |first10=Francis A. |date=15 July 2016 |title=Continental flood basalt weathering as a trigger for Neoproterozoic Snowball Earth |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=446 |issue= |pages=89–99 |doi=10.1016/j.epsl.2016.04.016 |bibcode=2016E&PSL.446...89C |doi-access=free }}</ref>

During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of <sup>13</sup>C to <sup>12</sup>C.<ref name=Rothman2003>{{cite journal |author1=D.H. Rothman |author2=J.M. Hayes |author3=R.E. Summons | title=Dynamics of the Neoproterozoic carbon cycle | journal=Proc. Natl. Acad. Sci. U.S.A. | year=2003 | volume=100 | issue=14 | pages=124–9 | doi = 10.1073/pnas.0832439100 | pmid=12824461 | pmc=166193|bibcode = 2003PNAS..100.8124R |doi-access=free }}</ref> Close analysis of the timing of <sup>13</sup>C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic.<ref name=Kaufman1997>{{cite journal
| author = Kaufman, Alan J.
|author2=Knoll, Andrew H. |author3=Narbonne, Guy M. 
| date = 24 June 1997
| title =Isotopes, ice ages, and terminal Proterozoic earth history
| journal = Proc. Natl. Acad. Sci. U.S.A.
| volume = 94
| issue = 13
| pages = 6600–5
| doi = 10.1073/pnas.94.13.6600
| pmid =11038552
| pmc = 21204
|bibcode = 1997PNAS...94.6600K |doi-access=free }}</ref>

===Banded iron formations===
[[Image:Black-band ironstone (aka).jpg|thumb|2.1 billion-year-old rock with black-band ironstone]]
Banded iron formations (BIF) are sedimentary rocks of layered [[Chemistry:Iron oxide|iron oxide]] and iron-poor [[Earth:Chert|chert]]. In the presence of oxygen, [[Chemistry:Iron|iron]] naturally [[Chemistry:Rust|rusts]] and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of Earth's atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and [[Chemistry:Precipitation|precipitated]] out as iron oxide.

The bands were produced at the [[Earth:Tipping point (climatology)|tipping point]] between an [[Earth:Hypoxia (environmental)|anoxic]] and an oxygenated ocean. Since today's atmosphere is [[Chemistry:Oxygen|oxygen]]-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with [[Earth:Cryogenian|Cryogenian]] glacial deposits.

For such iron-rich rocks to be deposited there would have to be [[Earth:Anoxic event|anoxia]] in the ocean, so that much dissolved iron (as ferrous oxide) could accumulate before it met an oxidant that would precipitate it as [[Chemistry:Ferric|ferric]] oxide. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice.<ref name="Kirschvink">{{cite book
| last=Kirschvink
| first=Joseph
| editor=J. W. Schopf
| editor2=C. Klein
| title=The Proterozoic Biosphere: A Multidisciplinary Study
| year=1992
| publisher=Cambridge University Press
| chapter=Late Proterozoic low-latitude global glaciation: the Snowball Earth}}</ref> At the {{clarify span|terminus of a glaciation|reason=Is this a location i.e. a [[Earth:Glacier terminus|Glacier terminus]]? Or is it a time i.e. the ending of a glacial period?|date=May 2023}}, a reestablishment of gas exchange between the ocean and atmosphere oxidised seawater rich in ferrous iron.<ref>{{cite journal |last1=Wu |first1=Chang-Zhi |last2=Zhao |first2=Fei-Fan |last3=Yang |first3=Tao |last4=Lei |first4=Ru-Xiong |last5=Ye |first5=Hui |last6=Gao |first6=Bing-Fei |last7=Li |first7=Feiqiang |date=15 July 2022 |title=Genesis of the Fulu Cryogenian iron formation in South China: Synglacial or interglacial? |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926822001334 |journal=Precambrian Research |volume=376 |page=106689 |doi=10.1016/j.precamres.2022.106689 |bibcode=2022PreR..37606689W |s2cid=248622156 |access-date=20 May 2023}}</ref> A positive shift in δ<sup>56</sup>Fe<sub>IRMM-014</sub> from the lower to upper layers of Cryogenian BIFs may reflect an increase in ocean acidification, as the upper layers were deposited as more and more oceanic ice cover melted away and more carbon dioxide was dissolved by the ocean.<ref>{{cite journal |last1=Zhu |first1=Xiang-Kun |last2=Sun |first2=Jian |last3=Li |first3=Zhi-Hong |date=1 September 2019 |title=Iron isotopic variations of the Cryogenian banded iron formations: A new model |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926818303243 |journal=Precambrian Research |volume=331 |page=105359 |doi=10.1016/j.precamres.2019.105359 |bibcode=2019PreR..33105359Z |s2cid=189975438 |access-date=21 May 2023}}</ref> Opponents of the hypothesis suggest that the rarity of the BIF deposits may indicate that they formed in inland seas.

Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's Black Sea; a sufficient input of iron could provide the necessary conditions for BIF formation.<ref name=Eyles2004 /> A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments;<ref name=Young1995/> such interbedding has been suggested to be an artefact of Milankovitch cycles, which would have periodically warmed the seas enough to allow gas exchange between the atmosphere and ocean and precipitate BIFs.<ref>{{cite journal |last1=Mitchell |first1=Ross N. |last2=Gernon |first2=Thomas M. |last3=Cox |first3=Grant M. |last4=Nordsvan |first4=Adam R. |last5=Kirscher |first5=Uwe |last6=Xuan |first6=Chuang |last7=Liu |first7=Yebo |last8=Liu |first8=Xu |last9=He |first9=Xiaofang |date=7 July 2021 |title=Orbital forcing of ice sheets during snowball Earth |journal=Nature Communications |volume=12 |issue=1 |page=4187 |doi=10.1038/s41467-021-24439-4 |pmid=34234152 |pmc=8263735 |bibcode=2021NatCo..12.4187M |hdl=20.500.11937/90462 |hdl-access=free }}</ref> BIFs are also strikingly absent during the [[Earth:Marinoan glaciation|Marinoan glaciation]].{{Citation needed|date=March 2008}}

===Cap carbonate rocks===
[[Image:Grosser Aletschgletscher 3178.JPG|thumb|left|A present-day glacier]]

Around the top of [[Earth:Neoproterozoic|Neoproterozoic]] glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary [[Earth:Limestone|limestone]] or [[Earth:Dolomite (rock)|dolomite]] metres to tens of metres thick.<ref name="Kennedy">{{cite journal
| author=M.J. Kennedy
| title=Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolomite: deglaciation, d13C excursions and carbonate precipitation
| journal=Journal of Sedimentary Research
| year=1996
| volume=66
| issue=6
| pages=1050–64
| doi=10.2110/jsr.66.1050|bibcode = 1996JSedR..66.1050K }}</ref> These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in [[Chemistry:Ocean chemistry|ocean chemistry]].<ref name="Spencer">{{cite journal
| author=Spencer, A.M.
| title=Late Pre-Cambrian glaciation in Scotland
| journal=Mem. Geol. Soc. Lond.
| year=1971
| volume=6}}</ref>
[[File:Redoubt pre-eruption 2009.jpg|thumb|right|Volcanoes may have had a role in replenishing {{co2}}, possibly ending the global ice age of the Cryogenian Period.]]

These [[Earth:Cap carbonate|cap carbonate]]s have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples.<ref name="HoffmanSchrag">{{cite journal
|author1=P. F. Hoffman |author2=D. P. Schrag | title=The snowball Earth hypothesis: testing the limits of global change
| journal=Terra Nova
| year=2002
| volume=14
| pages=129–55
| doi = 10.1046/j.1365-3121.2002.00408.x
| issue=3| bibcode=2002TeNov..14..129H| doi-access=free
}}</ref>
The formation of such sedimentary rocks could be caused by a large influx of positively charged ions, as would be produced by rapid [[Earth:Weathering|weathering]] during the extreme greenhouse following a snowball Earth event. The {{delta|13|C|link}} isotopic signature of the cap carbonates is near −5&nbsp;‰, consistent with the value of the [[Earth:Mantle (geology)|mantle]]—such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis.

The precise mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant {{co2}} from the atmosphere to form [[Chemistry:Carbonic acid|carbonic acid]], which would fall as [[Earth:Acid rain|acid rain]]. This would weather exposed [[Chemistry:Silicate|silicate]] and [[Chemistry:Carbonate|carbonate]] rock (including readily attacked glacial debris), releasing large amounts of calcium, which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an abiotic "[[Earth:Cap carbonate|cap carbonate]]" sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis.

However, there are some problems with the designation of a glacial origin to cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within—starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin.<ref name=Eyles2004 /> An alternative mechanism, which may have produced the Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48&nbsp;‰—{{delta|13|C|}} values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments.<ref>{{cite journal
| title=Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China
| doi = 10.1130/G24513A.1
| year=2008
| author=Wang, Jiasheng
| journal=Geology
| volume=36
| pages=347–350
| last2=Jiang
| first2=Ganqing
| last3=Xiao
| first3=Shuhai
| last4=Li
| first4=Qing
| last5=Wei
| first5=Qing
| issue=5
| bibcode=2008Geo....36..347W
}}</ref>

===Changing acidity===
Isotopes of the element [[Chemistry:Boron|boron]] suggest that the [[Chemistry:PH|pH]] of the oceans dropped dramatically before and after the Marinoan glaciation.<ref name=Kasemann2005>δ<sup>11</sup>B, in {{cite journal
| author = Kasemann, S.A.
|author2=Hawkesworth, C.J. |author3=Prave, A.R. |author4=Fallick, A.E. |author5=Pearson, P.N. 
| year = 2005
| title = Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change
| journal = [[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]]
| volume = 231
| issue = 1–2
| pages = 73–86
| doi = 10.1016/j.epsl.2004.12.006
| bibcode=2005E&PSL.231...73K
}}</ref>
This may indicate a buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form [[Chemistry:Carbonic acid|carbonic acid]]. Although the [[Chemistry:Boron|boron]] variations may be evidence of extreme [[Earth:Climate variability and change|climate change]], they need not imply a global glaciation.

===Space dust===
Earth's surface is very depleted in the element [[Chemistry:Iridium|iridium]], which primarily resides in Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An [[Chemistry:Iridium anomaly|iridium anomaly]] has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years,<ref name=Bodiselitsch>{{cite journal
| author = Bodiselitsch, Bernd.
|author2=Koeberl, C. |author3=Master, S. |author4=Reimold, W.U. 
| date = 8 April 2005
| title =Estimating Duration and Intensity of Neoproterozoic Snowball Glaciations from Ir Anomalies
| journal = Science
| volume = 308
| issue = 5719
| pages = 239–42
| doi = 10.1126/science.1104657
| pmid =15821088
|bibcode = 2005Sci...308..239B |s2cid=12231751 }}</ref> but this does not necessarily imply a ''global'' extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large [[Astronomy:Meteorite|meteorite]].<ref name=Grey2003>{{cite journal
| author = Grey, K. |author2=Walter, M.R. |author3=Calver, C.R.
| date = 1 May 2003
| title = Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?
| journal = Geology
| volume = 31
| issue = 5
| pages = 459–62
| doi = 10.1130/0091-7613(2003)031<0459:NBDSEO>2.0.CO;2
| bibcode=2003Geo....31..459G
}}</ref>

===Cyclic climate fluctuations===
Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods.<ref name="Rieu">{{cite journal
|author1=R. Rieu |author2=P.A. Allen |author3=M. Plötze |author4=T. Pettke | title=Climatic cycles during a Neoproterozoic "snowball" glacial epoch
| journal=Geology
| year=2007
| volume=35
| issue=4
| pages=299–302
| doi = 10.1130/G23400A.1|bibcode = 2007Geo....35..299R }}</ref> This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to [[Earth:Timeline of glaciation#Nomenclature of Quaternary glacial cycles|Pleistocene]] [[Earth:Ice age|ice age]] cycles than to a completely frozen Earth.

In addition, glacial sediments of the Port Askaig Tillite Formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments.<ref name=Young1999>{{cite journal
| author = Young, G.M.
| year = 1999
| title = Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland
| journal = Journal of the Geological Society
| volume = 156
| issue = 6
| pages = 1097–1111
| doi = 10.1144/gsjgs.156.6.1097
| bibcode = 1999JGSoc.156.1097Y
| s2cid = 128600222
}}</ref> The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Portaskaig group is 8&nbsp;km stratigraphically above the beds of interest. Its dating to 600&nbsp;Ma means the beds can be tentatively correlated to the [[Earth:Sturtian glaciation|Sturtian glaciation]], but they may represent the advance or retreat of a snowball Earth.

==Mechanisms==
[[Image:SnowballSimulations.jpg|350px|thumb|right|One computer simulation of conditions during a snowball Earth period<ref name=Hyde2000 />]]

The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's [[Earth:Albedo|albedo]], which would result in [[Positive feedback|positive feedback]] for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where [[Physics:Solar radiation|solar radiation]] is most direct.

Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a [[Earth:Supervolcano|supervolcano]], a reduction in the atmospheric concentration of [[Chemistry:Greenhouse gas|greenhouse gas]]es such as [[Chemistry:Methane|methane]] and/or [[Chemistry:Carbon dioxide|carbon dioxide]], changes in Solar energy output, or perturbations of [[Astronomy:Earth's orbit|Earth's orbit]]. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen [[Astronomy:Equator|equator]] as cold as modern [[Earth:Antarctica|Antarctica]].

[[Earth:Global warming|Global warming]] associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium.{{Citation needed|date=April 2019}}

===Initiation of glaciation===
A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth.<ref name=Hoffman2005/>
Firstly, tropical continents are more reflective than open ocean, and so absorb less of the Sun's heat: most absorption of Solar energy on Earth today occurs in tropical oceans.<ref name=Jacobsen2001>{{cite journal
| author = Jacobsen, S.B.
| year = 2001
| title = Earth science. Gas hydrates and deglaciations
| journal = Nature
| volume = 412
| issue = 6848
| pages = 691–3
| doi = 10.1038/35089168
| pmid = 11507621
| s2cid = 4339151
}}</ref>

Further, tropical continents are subject to more rainfall, which leads to increased river discharge—and erosion.
When exposed to air, [[Chemistry:Silicate|silicate]] rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form
: Rock-forming mineral + CO<sub>2</sub> + H<sub>2</sub>O → cations + bicarbonate + SiO<sub>2</sub>
An example of such a reaction is the weathering of [[Chemistry:Wollastonite|wollastonite]]:
: CaSiO<sub>3</sub> + 2 CO<sub>2</sub> + H<sub>2</sub>O → Ca<sup>2+</sup> + SiO<sub>2</sub> + 2 {{chem|HCO|3|−}}

The released [[Chemistry:Calcium|calcium]] cations react with the dissolved [[Chemistry:Bicarbonate|bicarbonate]] in the ocean to form [[Chemistry:Calcium carbonate|calcium carbonate]] as a chemically precipitated [[Earth:Sedimentary rock|sedimentary rock]]. This transfers carbon dioxide, a greenhouse gas, from the air into the [[Earth:Geosphere|geosphere]], and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.

As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis.<ref name=Meert2004>{{cite book
| author = Meert, J.G.
|author2=Torsvik, T.H.
 | year = 2004
| title = Paleomagnetic Constraints on Neoproterozoic 'Snowball Earth' Continental Reconstructions
|journal=Washington DC American Geophysical Union Geophysical Monograph Series
 | editor = GS Jenkins
| editor2 = MAS McMenamin
| editor3 = CP McKey
| editor4 = CP Sohl
| editor5 = L Sohl
| publisher=American Geophysical Union
| volume = 146
| pages = 5–11
| doi = 10.1029/146GM02
| series = Geophysical Monograph Series
| isbn = 978-0-87590-411-5
|bibcode = 2004GMS...146....5M |citeseerx=10.1.1.368.2259
 }}</ref> Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth.<ref name=Smith2003>{{cite journal
| author = Smith, A.G.
|author2=Pickering, K.T.
 | year = 2003
| title = Oceanic gateways as a critical factor to initiate icehouse Earth
| journal = Journal of the Geological Society
| volume = 160
| issue= 3
| pages= 337–40
| doi = 10.1144/0016-764902-115
|bibcode=2003JGSoc.160..337S
 |s2cid=127653725
 }}</ref>

Additional factors that may have contributed to the onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with [[Earth:Atmospheric methane|methane in the atmosphere]], oxidizing it to carbon dioxide, a much weaker greenhouse gas,<ref name=Kerr1999>{{cite journal
| author = Kerr, R.A.
| year = 1999
| title = Early life thrived despite earthly travails
| journal = Science
| volume = 284
| issue = 5423
| pages = 2111–3
| doi = 10.1126/science.284.5423.2111
| pmid = 10409069
| s2cid = 32695874
}}</ref> and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic.<ref name=Eyles2004 />

Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this '[[Negative feedback|negative feedback]]' process limits the magnitude of cooling. During the [[Earth:Cryogenian|Cryogenian]] period, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This let ice advance beyond the polar regions. Once ice advanced to within 30° of the equator,<ref name=Kirschvink2002>{{cite journal
| author = Kirschvink, J.L.
| year = 2002
| title = When All of the Oceans Were Frozen
| journal = La Recherche
| volume = 355
| pages = 26–30
| url = http://web.gps.caltech.edu/~jkirschvink/pdfs/laRechercheEnglish.pdf
}}</ref> a positive feedback could ensue such that the increased reflectiveness ([[Earth:Albedo|albedo]]) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered.

Polar continents, due to low rates of [[Physics:Evaporation|evaporation]], are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from the [[Earth:Carbon cycle|carbon cycle]]. A gradual rise of the proportion of the [[Physics:Isotope|isotope]] carbon-13 relative to carbon-12 in sediments pre-dating "global" glaciation indicates that {{co2}} draw-down before snowball Earths was a slow and continuous process.<ref name=Schrag2002>{{cite journal |last1=Schrag |first1=Daniel P. |last2=Berner |first2=Robert A. |last3=Hoffman |first3=Paul F. |last4=Halverson |first4=Galen P. |title=On the initiation of a snowball Earth |journal=[[Chemistry:Geochemistry, Geophysics, Geosystems|Geochemistry, Geophysics, Geosystems]] |date=June 2002 |volume=3 |issue=6 |page=1036 |doi=10.1029/2001GC000219 |doi-access=free |bibcode=2002GGG.....3.1036S }}</ref>

The start of snowball Earths are always marked by a sharp downturn in the δ<sup>13</sup>C value of sediments,<ref name=Hoffman1998>{{cite journal
| author = Hoffman, P.F.
|author2=Kaufman, A.J. |author3=Halverson, G.P. |author4=Schrag, D.P. 
| date = 28 August 1998
| title = A Neoproterozoic Snowball Earth
| journal = Science
| volume = 281
| issue = 5381
| pages = 1342–6
| doi = 10.1126/science.281.5381.1342
| pmid = 9721097
| bibcode=1998Sci...281.1342H
|s2cid=13046760 |url=https://semanticscholar.org/paper/37b1ffb6aadc8b436c029fb8a3311c0b26e30d4e }}</ref> a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans.

In January 2016, Gernon et al. proposed a "shallow-ridge hypothesis" involving the breakup of the [[Earth:Supercontinent|supercontinent]] [[Earth:Rodinia|Rodinia]], linking the eruption and rapid alteration of [[Earth:Hyaloclastite|hyaloclastite]]s along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events.<ref>{{cite journal |last1=Gernon |first1=T. M. |last2=Hincks |first2=T. K. |last3=Tyrrell |first3=T. |last4=Rohling |first4=E. J. |last5=Palmer |first5=M. R. |title=Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |date=18 January 2016 |volume=9 |issue=3 |pages=242–8 |doi=10.1038/ngeo2632 |bibcode=2016NatGe...9..242G |s2cid=1642013 |url=https://research-information.bristol.ac.uk/files/60401094/snowball_NG_gernon_final.pdf}}
*{{cite web |author=Thomas Gernon |date=18 January 2016 |title=How 'Snowball Earth' volcanoes altered oceans to help kickstart animal life |website=The Conversation |url=https://theconversation.com/how-snowball-earth-volcanoes-altered-oceans-to-help-kickstart-animal-life-53280}}</ref>

Dating of the Sturtian glaciation's onset has found it to be coeval with the emplacement of a large igneous province in the tropics. Weathering of this equatorial large igneous province is believed to have sucked enough carbon dioxide out of the air to enable the development of major glaciation.<ref>{{cite journal |last1=Hoffman |first1=Paul F. |last2=Abbot |first2=Dorian S. |last3=Ashkenazy |first3=Yosef |last4=Benn |first4=Douglas I. |last5=Brocks |first5=Jochen J. |last6=Cohen |first6=Phoebe A. |last7=Cox |first7=Grant M. |last8=Creveling |first8=Jessica R. |last9=Donnadieu |first9=Yannick |last10=Erwin |first10=Douglas H. |last11=Fairchild |first11=Ian J. |last12=Ferreira |first12=David |last13=Goodman |first13=Jason C. |last14=Halverson |first14=Galen P. |last15=Jansen |first15=Malte F. |last16=Le Hir |first16=Guillaume |last17=Love |first17=Gordon D. |last18=MacDonald |first18=Francis A. |last19=Maloof |first19=Adam G. |last20=Partin |first20=Camille A. |last21=Ramstein |first21=Gilles |last22=Rose |first22=Brian E. J. |last23=Rose |first23=Catherine V. |last24=Sadler |first24=Peter M. |last25=Tziperman |first25=Eli |last26=Voigt |first26=Aiko |last27=Warren |first27=Stephen G. |date=8 November 2017 |title=Snowball Earth climate dynamics and Cryogenian geology-geobiology |journal=Science Advances |volume=3 |issue=11 |pages=e1600983 |doi=10.1126/sciadv.1600983 |pmid=29134193 |pmc=5677351 |bibcode=2017SciA....3E0983H }}</ref>

===During the frozen period===
[[Image:AntarcticaDomeCSnow.jpg|thumb|250px|Global ice sheets may have created the bottleneck required for the evolution of multicellular life.<ref name=Kirschvink1992 />]]
Global temperature fell so low that the equator was as cold as modern-day [[Earth:Antarctica|Antarctica]].<ref name=Hyde2000>{{cite journal |last1=Hyde |first1=William T. |last2=Crowley |first2=Thomas J. |last3=Baum |first3=Steven K. |last4=Peltier |first4=W. Richard |title=Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model |journal=Nature |date=May 2000 |volume=405 |issue=6785 |pages=425–429 |doi=10.1038/35013005 |pmid=10839531 |bibcode=2000Natur.405..425H |s2cid=1672712 }}</ref> This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect. Degassing of carbon dioxide has been speculated to have been unusually low during the Cryogenian, enabling the persistence of global glaciation.<ref>{{cite journal |last1=Mills |first1=Benjamin J. W. |last2=Scotese |first2=Christopher R. |last3=Walding |first3=Nicholas G. |last4=Shields |first4=Graham A. |last5=Lenton |first5=Timothy M. |date=24 October 2017 |title=Elevated CO2 degassing rates prevented the return of Snowball Earth during the Phanerozoic |journal=Nature Communications |volume=8 |issue=1 |page=1110 |doi=10.1038/s41467-017-01456-w |pmid=29062095 |pmc=5736558 }}</ref>

===Breaking out of global glaciation===
The carbon dioxide levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere.<ref name=Crowley2001>{{cite journal
| author = Crowley, T.J.
|author2=Hyde, W.T. |author3=Peltier, W.R. 
| year = 2001
| title = CO 2 levels required for deglaciation of a 'near-snowball' Earth
| journal = [[Earth:Geophysical Research Letters|Geophysical Research Letters]]
| volume = 28
| pages = 283–6
| doi = 10.1029/2000GL011836
| bibcode=2001GeoRL..28..283C
| issue = 2
| doi-access = 
|s2cid=129246869 }}</ref> Since Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of [[Earth:Siliceous rock|siliceous rock]]s. Over 4 to 30 million years, enough {{co2}} and [[Chemistry:Methane|methane]], mainly emitted by volcanoes but also produced by [[Biology:Microorganism|microbes]] converting organic carbon trapped under the ice into the gas,<ref>{{cite web| url = http://www.antarcticglaciers.org/modern-glaciers/glacier-ecosystems/| title = Glacier ecosystems| date = 17 July 2020}}</ref> would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed; this would be darker than the ice, and thus absorb more energy from the Sun—initiating a "[[Positive feedback|positive feedback]]".<ref name=Pierrehumbert2004>{{cite journal
| author = Pierrehumbert, R.T.
| year = 2004
| title = High levels of atmospheric carbon dioxide necessary for the termination of global glaciation
| journal = Nature
| volume = 429
| pages = 646–9
| doi = 10.1038/nature02640
| pmid = 15190348
| issue = 6992
|bibcode = 2004Natur.429..646P | s2cid = 2205883
}}</ref>

The first areas to become free of permanent ice cover may have been in the mid-latitudes rather than in the tropics, because a rapid hydrological cycle would have inhibited the melting of ice at low latitudes. As these mid-latitude regions became ice free, dust from them blew over onto ice sheets elsewhere, decreasing their albedo and accelerating the process of deglaciation.<ref>{{cite journal |last1=De Vrese |first1=Philipp |last2=Stacke |first2=Tobias |last3=Rugenstein |first3=Jeremy Caves |last4=Goodman |first4=Jason |last5=Brovkin |first5=Victor |date=14 May 2021 |title=Snowfall-albedo feedbacks could have led to deglaciation of snowball Earth starting from mid-latitudes |journal=Communications Earth & Environment |volume=2 |issue=1 |page=91 |doi=10.1038/s43247-021-00160-4 |doi-access=free |bibcode=2021ComEE...2...91D }}</ref>

Destabilization of substantial deposits of methane hydrates locked up in low-latitude [[Earth:Permafrost|permafrost]] may also have acted as a trigger and/or strong positive feedback for deglaciation and warming.<ref>{{cite journal |last1=Kennedy |first1=Martin |last2=Mrofka |first2=David |last3=von der Borch |first3=Chris |title=Snowball Earth termination by destabilization of equatorial permafrost methane clathrate |journal=Nature |date=29 May 2008 |volume=453 |issue=7195 |pages=642–645 |doi=10.1038/nature06961 |pmid=18509441 |bibcode=2008Natur.453..642K |s2cid=4416812 }}</ref>

Methanogens were an important contributor to the deglaciation of the Marinoan Snowball Earth. The return of high primary productivity in surficial waters fueled extensive microbial sulphur reduction, causing deeper waters to become highly euxinic. Euxinia caused the formation of large amounts of methyl sulphides, which in turn was converted into methane by methanogens. A major negative nickel isotope excursion confirms high methanogenic activity during this period of deglaciation and global warming.<ref>{{cite journal |last1=Zhao |first1=Zhouqiao |last2=Shen |first2=Bing |last3=Zhu |first3=Jian-Ming |last4=Lang |first4=Xianguo |last5=Wu |first5=Guangliang |last6=Tan |first6=Decan |last7=Pei |first7=Haoxiang |last8=Huang |first8=Tianzheng |last9=Ning |first9=Meng |last10=Ma |first10=Haoran |date=11 February 2021 |title=Active methanogenesis during the melting of Marinoan snowball Earth |journal=Nature Communications |volume=12 |issue=1 |page=955 |doi=10.1038/s41467-021-21114-6 |pmid=33574253 |pmc=7878791 |bibcode=2021NatCo..12..955Z }}</ref>

On the continents, the melting of [[Earth:Glacier|glacier]]s would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as [[Chemistry:Phosphorus|phosphorus]], which combined with the abundance of {{co2}} would trigger a [[Biology:Cyanobacteria|cyanobacteria]] population explosion, which would cause a relatively rapid reoxygenation of the atmosphere, which may have contributed to the rise of the [[Earth:Ediacaran biota|Ediacaran biota]] and the subsequent [[Earth:Cambrian explosion|Cambrian explosion]]—a higher oxygen concentration allowing large [[Biology:Multicellular organism|multicellular]] lifeforms to develop. Although the [[Positive feedback|positive feedback]] loop would melt the ice in geological short order, perhaps less than 1,000 years, replenishment of atmospheric oxygen and depletion of the {{co2}} levels would take further [[Millennium|millennia]].

It is possible that carbon dioxide levels fell enough for Earth to freeze again; this cycle may have repeated until the [[Earth:Continental drift|continents had drifted]] to more polar latitudes.<ref name=Hoffman1999>{{cite journal
| author = Hoffman, P.F.
| year = 1999
| title = The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth
| journal = Journal of African Earth Sciences
| volume = 28
| issue = 1
| pages = 17–33
| doi = 10.1016/S0899-5362(99)00018-4
| bibcode=1999JAfES..28...17H
}}</ref>

More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state.<ref>{{cite journal | last1 = Peltier | last2 = Richard | first2 = W. | last3 = Liu | first3 = Yonggang | last4 = Crowley | first4 = John W. | year = 2007 | title = Snowball Earth prevention by dissolved organic carbon remineralization | journal = Nature | volume = 450 | issue = 7171| pages = 813–818 | doi = 10.1038/nature06354 | pmid = 18064001| bibcode = 2007Natur.450..813P | s2cid = 4406636 }}</ref>

During millions of years, [[Earth:Cryoconite|cryoconite]] would have accumulated on and inside the ice. [[Biology:Psychrophile|Psychrophilic]] microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers. Once the ice started to melt, these layers would become visible and color the icy surfaces dark, helping to accelerate the process.<ref>{{cite journal| pmid=27422766 | doi=10.1111/gbi.12191 | volume=14 | issue=6 | title=Cryoconite pans on Snowball Earth: supraglacial oases for Cryogenian eukaryotes? | year=2016 | journal=Geobiology| pages=531–542 | author=Hoffman PF| bibcode=2016Gbio...14..531H | s2cid=21261198 }}</ref>

Also, ultraviolet light from the Sun [[Chemistry:Hydrogen peroxide#Other sources|produced hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>) when it hit water molecules. Normally H<sub>2</sub>O<sub>2</sub> breaks down in sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, increasing atmospheric oxygen.<ref>{{cite web| url = https://www.newscientist.com/article/dn10662-did-snowball-earths-melting-let-oxygen-fuel-life/| title = Did snowball Earth's melting let oxygen fuel life?}}</ref>

===Slushball Earth hypothesis===
While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the [[Astronomy:Equator|equator]], allowing for a continued hydrologic cycle.

This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water, or rapidly moving ice (which would require somewhere ice-free to move to). Recent research observed geochemical cyclicity in clastic rocks, showing that the "snowball" periods were punctuated by warm spells, similar to [[Earth:Ice age|ice age]] cycles in recent Earth history. Attempts to construct computer models of a snowball Earth have also struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet.

A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation.<ref name=Harland2007>{{cite journal
| last1=Harland
| first1=W. B.
| year= 2007
| title=Origin and assessment of Snowball Earth hypotheses
| journal=Geological Magazine
| volume=144
| issue=4
| pages=633–42
| doi=10.1017/S0016756807003391| bibcode=2007GeoM..144..633H
| s2cid=10947285
}}</ref> Synthesised evidence has produced models indicating a "slushball Earth",<ref name=Fairchild2007>{{cite journal
| last1=Fairchild
| first1=I. J.
| last2=Kennedy
| first2=M. J.
| year=2007
| title=Neoproterozoic glaciations in the Earth System
| journal=Journal of the Geological Society
| volume=164
| pages=895–921
| doi=10.1144/0016-76492006-191
| issue=5
| citeseerx=10.1.1.211.2233
| bibcode=2007JGSoc.164..895F
| s2cid=16713707
}}</ref> where the stratigraphic record does not permit postulating complete global glaciations.<ref name=Harland2007/> Kirschvink's original hypothesis<ref name=Kirschvink/> had recognised that warm tropical puddles would be expected to exist in a snowball Earth.

The snowball Earth hypothesis does not explain the alternation of glacial and interglacial events, nor the oscillation of glacial sheet margins.<ref name=Chumakov2008>{{cite journal
| last1=Chumakov
| first1= N. M.
| year=2008
| title=A problem of Total Glaciations on the Earth in the Late Precambrian
| journal=Stratigraphy and Geological Correlation
| volume=16
| issue=2
| pages=107–119
| doi=10.1134/S0869593808020019 |bibcode = 2008SGC....16..107C | s2cid= 129280178
}}</ref>

==Scientific dispute==
The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "snowball Earth" deposits. Evidence for such melting comes from evidence of glacial dropstones,<ref name=Condon2002/> geochemical evidence of climate cyclicity,<ref name="Rieu"/> and interbedded glacial and shallow marine sediments.<ref name=Young1999/> A longer record from Oman, constrained to 13°N, covers the period from 712 to 545 million years ago—a time span containing the [[Earth:Cryogenian|Sturtian and Marinoan]] glaciations—and shows both glacial and ice-free deposition.<ref name=Kilner2005>{{cite journal
| author = Kilner, B. |author2=Niocaill, C.M. |author3=Brasier, M.
| year = 2005
| title = Low-latitude glaciation in the Neoproterozoic of Oman
| journal = Geology
| volume = 33
| issue = 5
| pages = 413–6
| doi = 10.1130/G21227.1
|bibcode = 2005Geo....33..413K }}</ref>

There have been difficulties in recreating a snowball Earth with global climate models. Simple GCMs with mixed-layer oceans can be made to freeze to the equator; a more sophisticated model with a full dynamic ocean (though only a primitive sea ice model) failed to form sea ice to the equator.<ref name=Poulsen2001>{{cite journal
| author = Poulsen, C.J. |author2=Pierrehumbert, R.T. |author3=Jacob, R.L.
| year = 2001
| title = Impact of ocean dynamics on the simulation of the Neoproterozoic ''snowball Earth''
| journal = [[Earth:Geophysical Research Letters|Geophysical Research Letters]]
| volume = 28
| issue = 8
| pages = 1575–8
| doi = 10.1029/2000GL012058
| bibcode=2001GeoRL..28.1575P
|s2cid=2190435 | doi-access = free
}}</ref> In addition, the levels of {{co2}} necessary to melt a global ice cover have been calculated to be 130,000 ppm,<ref name=Crowley2001/> which is considered by some to be unreasonably large.<ref>{{cite journal |last1=Bao |first1=Huiming |last2=Lyons |first2=J. R. |last3=Zhou |first3=Chuanming |title=Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation |journal=Nature |date=22 May 2008 |volume=453 |issue=7194 |pages=504–506 |doi=10.1038/nature06959 |pmid=18497821 |bibcode=2008Natur.453..504B |s2cid=205213330 }}</ref>

Strontium isotopic data have been found to be at odds with proposed snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation. Therefore, methane release from permafrost during [[Earth:Marine transgression|marine transgression]] was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation.<ref name=Kennedy2001>{{cite journal |last1=Kennedy |first1=Martin J. |last2=Christie-Blick |first2=Nicholas |last3=Sohl |first3=Linda E. |title=Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals? |journal=Geology |year=2001 |volume=29 |issue=5 |pages=443 |doi=10.1130/0091-7613(2001)029<0443:APCCAI>2.0.CO;2 |bibcode=2001Geo....29..443K }}</ref>

==="Zipper rift" hypothesis===
<!-- I don't feel I've done this paper justice here; it may be because I've not grasped it fully, but may also be that their model's not quite as awe-inspiring as it's made out to be. Please do feel free to have a look at the paper—message Verisimilus if you can't find it online—and improve this section if you feel you can! -->
Nick Eyles suggests that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure.<ref name=Eyles2004 /> The "Zipper rift" hypothesis proposes two pulses of continental "unzipping"—first, the breakup of the supercontinent Rodinia, forming the proto-Pacific Ocean; then the splitting of the continent [[Earth:Baltica|Baltica]] from [[Earth:Laurentia|Laurentia]], forming the proto-Atlantic—coincided with the glaciated periods.
The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography; this high ground could then host glaciers.

Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and [[Earth:Anoxic waters|anoxic waters]] to form; however, the limited extent of the Neoproterozoic banded iron deposits means that they may not have formed in frozen oceans, but instead in inland seas. Such seas can experience a wide range of chemistries; high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form.

Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels.

===High-obliquity hypothesis===
A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's [[Astronomy:Axial tilt|axial tilt]] was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce.<ref>{{cite web
| url= http://www.livescience.com/7105-day-earth-fell.html
| title= LiveScience.com: The Day The Earth Fell Over| website= Live Science| date= 25 August 2006}}</ref> A less extreme possibility would be that it was merely Earth's [[Astronomy:Poles of astronomical bodies|magnetic pole]] that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depend on the magnetic and rotational poles being relatively similar. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today; severe changes to Earth's climate are not necessary.

===Inertial interchange true polar wander===
The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange [[Earth:True polar wander|true polar wander]] (IITPW).<ref name=Kirschvink1997>{{cite journal
| author = Kirschvink, J.L.
|author2=Ripperdan, R.L. |author3=Evans, D.A. 
| date = 25 July 1997
| title = Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander
| journal = Science
| volume = 277
| issue = 5325
| pages = 541–545
| doi = 10.1126/science.277.5325.541
|s2cid=177135895 }}</ref><ref name=Meert1999>{{cite journal
| author = Meert, J.G.
| year = 1999
| title = A palaeomagnetic analysis of Cambrian true polar wander
| journal = [[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]]
| volume = 168
| issue = 1–2
| pages = 131–144
| doi = 10.1016/S0012-821X(99)00042-4
| bibcode=1999E&PSL.168..131M
}}</ref>
This hypothesis, created to explain palaeomagnetic data, suggests that Earth's orientation relative to its axis of rotation shifted one or more times during the general time-frame attributed to snowball Earth. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude.<ref>{{cite web |url=http://www.pppl.gov/colloquia_pres/WC01OCT08_Maloof.pdf |title= Rock magnetic evidence for rapid motion of the solid Earth with respect to its spin axis |year= 2008 |access-date=2010-05-13 |url-status=dead |archive-url=https://web.archive.org/web/20110607213103/http://www.pppl.gov/colloquia_pres/WC01OCT08_Maloof.pdf |archive-date=7 June 2011}}</ref> While the physics behind the proposition is sound, the removal of one flawed data point from the original study rendered the application of the concept in these circumstances unwarranted.<ref name=Torsvik1998>{{cite journal
| author = Torsvik, T.H.
| date = 2 January 1998
| title = Polar Wander and the Cambrian
| journal = Science
| volume = 279
| issue = 5347
| page = 9
| doi = 10.1126/science.279.5347.9a
|bibcode = 1998Sci...279....9T | doi-access = free
}}</ref>

Several alternative explanations{{clarify|which?  where can one read about these?|date=June 2021}} for the evidence have been proposed.{{cn|date=June 2021}}

==Survival of life through frozen periods==
[[Image:Blacksmoker in Atlantic Ocean.jpg|thumb|right|A black smoker, a type of hydrothermal vent]]
A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing non-oxidized iron-rich rocks to form.

Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as [[Earth:Stromatolite|stromatolite]]s and [[Earth:Oncolite|oncolite]]s prove that, in shallow marine environments at least, life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed.<ref name=Corsetti2003>{{cite journal |last1=Corsetti |first1=F. A. |last2=Awramik |first2=S. M. |last3=Pierce |first3=D. |title=A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA |journal=Proceedings of the National Academy of Sciences |date=7 April 2003 |volume=100 |issue=8 |pages=4399–4404 |doi=10.1073/pnas.0730560100 |pmc=153566 |pmid=12682298 |bibcode=2003PNAS..100.4399C |doi-access=free }}</ref> Proponents counter that it may have been possible for life to survive in these ways:
* In reservoirs of [[Biology:Anaerobic organism|anaerobic]] and low-oxygen life powered by chemicals in deep oceanic [[Earth:Hydrothermal vent|hydrothermal vent]]s surviving in Earth's deep oceans and [[Earth:Crust (geology)|crust]]; but [[Earth:Photosynthesis|photosynthesis]] would not have been possible there.
* Under the ice layer, in chemolithotrophic (mineral-metabolizing) [[Earth:Ecosystem|ecosystem]]s theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of volcanism or [[Earth:Geothermal activity|geothermal]] activity.<ref name=Vincent2000>{{cite journal
| author = Vincent, W.F.
| year = 2000
| title = Life on Snowball Earth
| journal = Science
| volume = 287
| issue = 5462
| pages = 2421–2
| doi = 10.1126/science.287.5462.2421b
| pmid = 10766616
| s2cid = 129157915
}}</ref>
* In pockets of liquid water within and under the ice caps, similar to [[Earth:Lake Vostok|Lake Vostok]] in Antarctica. In theory, this system may resemble microbial communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 20 m thick, and at the temperatures predicted by models, equatorial sublimation would prevent equatorial ice thickness from exceeding 10 m.<ref name=McKay2000>{{cite journal
| author = McKay, C.P.
| year = 2000
| title = Thickness of tropical ice and photosynthesis on a snowball Earth
| journal = [[Earth:Geophysical Research Letters|Geophysical Research Letters]]
| volume = 27
| issue = 14
| pages = 2153–6
| doi = 10.1029/2000GL008525
| pmid = 11543492
| bibcode=2000GeoRL..27.2153M
| s2cid = 559714
| doi-access = free
}}</ref>
* As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period.
* In small regions of open sea water: [[Earth:Polynya|polynya]]. These natural ice holes can occur due to the action of winds, currents or a local heat source (e.g. geothermal), even if the surrounding sea is completely frozen over. They could preserve enclaves of photosynthesizers (not multicellular plants, which did not yet exist) with access to light and {{co2}} to generate trace amounts of oxygen, enough to sustain some oxygen-dependent organisms. It is not necessary that  a hole form in the ice, merely that some parts of the ice become thin enough to admit light. These small regions may have occurred in deep ocean, far from the [[Earth:Supercontinent|supercontinent]] [[Earth:Rodinia|Rodinia]] or its remnants as it broke apart and drifted on the tectonic plates.{{cn|date=June 2021}} 
* In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice.<ref name=Barras2018>{{cite journal |last1=Barras |first1=Colin |title=Scott's dirty ice may solve mystery |journal=New Scientist |date=March 2018 |volume=237 |issue=3171 |pages=16 |doi=10.1016/S0262-4079(18)30558-X |bibcode=2018NewSc.237...16B }}</ref> Such environments may have covered approximately 12 per cent of the global surface area.<ref name=Hawes2018>{{cite journal |last1=Hawes |first1=I. |last2=Jungblut |first2=A. D. |last3=Matys |first3=E. D. |last4=Summons |first4=R. E. |title=The 'Dirty Ice' of the McMurdo Ice Shelf: Analogues for biological oases during the Cryogenian |journal=Geobiology |date=July 2018 |volume=16 |issue=4 |pages=369–77 |doi=10.1111/gbi.12280 |pmid=29527802 |hdl=1721.1/140848.2 |s2cid=3885072 |hdl-access=free }}</ref>
* In small oases of liquid water, as would be found near [[Earth:Geothermal activity|geothermal]] [[Earth:Hotspot (geology)|hotspots]] resembling Iceland today.<ref name=Hoffman2000>{{cite journal |last1=Hoffman |first1=Paul F. |last2=Schrag |first2=Daniel P. |title=Snowball Earth |journal=Scientific American |date=January 2000 |volume=282 |issue=1 |pages=68–75 |doi=10.1038/scientificamerican0100-68 |bibcode=2000SciAm.282a..68H }}</ref>
* In [[Earth:Nunatak|nunatak]] areas in the tropics, where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt pools, which would freeze at sunset.{{cn|date=June 2021}}
* Oxygenated subglacial meltwater, along with iron-rich sediments dissolved in the glacial water, created a meltwater oxygen pump when it entered the ocean, where it provided eukaryotes with some oxygen, and both photosynthetic and chemosynthetic organisms with sufficient nutrients to support an ecosystem. The freshwater would also mix with the hypersaline seawater, which created areas less hostile to eukaryotic life than elsewhere in the ocean.<ref>{{cite journal| title = Subglacial meltwater supported aerobic marine habitats during Snowball Earth - PNAS| journal = Proceedings of the National Academy of Sciences of the United States of America| date = 17 December 2019| volume = 116| issue = 51| pages = 25478–25483| doi = 10.1073/pnas.1909165116| last1 = Lechte| first1 = Maxwell A.| last2 = Wallace| first2 = Malcolm W.| last3 = Hood| first3 = Ashleigh van Smeerdijk| last4 = Li| first4 = Weiqiang| last5 = Jiang| first5 = Ganqing| last6 = Halverson| first6 = Galen P.| last7 = Asael| first7 = Dan| last8 = McColl| first8 = Stephanie L.| last9 = Planavsky| first9 = Noah J.| pmid = 31792178| pmc = 6926012| doi-access = free}}</ref>

However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a [[Biology:Extinction event|mass extinction]]. With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16&nbsp;million years.<ref name='Corsetti2009'>{{Cite journal| first1 = F. A.| title = Palaeontology: Extinction before the snowball| journal = [[Earth:Nature Geoscience|Nature Geoscience]]| last1 = Corsetti| volume = 2| pages = 386–387| year = 2009| doi = 10.1038/ngeo533    |bibcode = 2009NatGe...2..386C| issue=6}}</ref> Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed<ref name=Corsetti2006>
{{cite journal
|author=Corsetti, F.A.
|author2=Olcott, A.N. |author3=Bakermans, C.
 |year=2006
|issue=2–4
|pages=114–130
|title=The biotic response to Neoproterozoic Snowball Earth
|journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]]
| volume = 232
| doi = 10.1016/j.palaeo.2005.10.030
|bibcode=2006PPP...232..114C }}</ref>—in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth.<ref name=Grey2003 /> One rebuttal to this is the fact that in many of these places where an argument is made against a mass extinction caused by snowball Earth, the Cryogenian fossil record is extraordinarily impoverished.<ref>{{cite web |title=What is the evidence against the snowball earths? |url=http://www.snowballearth.org/against.html |website=www.snowballearth.org/}}</ref>

==Implications==
A snowball Earth has profound implications in the history of life on Earth. While many [[Biology:Refugium (population biology)|refugia]] have been postulated, global ice cover would certainly have ravaged [[Earth:Ecosystem|ecosystem]]s dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials.

Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted under a hot atmosphere rich in carbon dioxide, it would cover the oceans with a layer of warm (50°C) freshwater up to 2 kilometres thick. Only after the warm surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state.<ref>Owens, Brian, [https://www.newscientist.com/article/2130525-snowball-earth-melting-led-to-freshwater-ocean-2-kilometres-deep/ Snowball Earth melting led to freshwater ocean 2 kilometres deep], ''New Scientist'', 10 May 2017.</ref>

The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the [[Earth:Cryogenian|Cryogenian]] period.

===Effect on early evolution===
[[Image:DickinsoniaCostata.jpg|thumb|right|222px|''[[Biology:Dickinsonia|Dickinsonia]] costata'', an Ediacaran organism of unknown affinity, with a quilted appearance]]
The [[Earth:Neoproterozoic|Neoproterozoic]] was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This rapid development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles; in this sense, snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating [[Chemistry:Copper|copper]] levels and rising oxygen may have played a part. Many Sturtian diamictites unconformably overlie copper-mineralised strata in Greenland, North America, Australia, and Africa; the glacial breakup and erosion of rocks heavily enriched in copper during the Sturtian glaciation, combined with the chemical weathering of the Franklin Large Igneous Province, greatly elevated copper concentrations in the ocean. Because copper is an essential component of many [[Biology:Proteins|proteins]] involved in mitigating [[Medicine:Oxygen toxicity|oxygen toxicity]], synthesising [[Chemistry:Adenosine triphosphate|adenosine triphosphate]], and producing [[Biology:Elastin|elastin]] and [[Biology:Collagen|collagen]], among other biological functions, this spike in copper concentrations was essential to the explosive evolution of multicellular life throughout the latter portion of the Neoproterozoic. Elevated copper concentrations persisted into the [[Earth:Cambrian explosion|Cambrian explosion]] at the beginning of the [[Earth:Phanerozoic|Phanerozoic]] and likely influenced its course too.<ref>{{cite journal |last1=Parnell |first1=J. |last2=Boyce |first2=A. J. |date=6 March 2019 |title=Neoproterozoic copper cycling, and the rise of metazoans |journal=Scientific Reports |volume=9 |issue=1 |page=3638 |doi=10.1038/s41598-019-40484-y |pmid=30842538 |pmc=6403403 |bibcode=2019NatSR...9.3638P }}</ref>

One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much ''affect'' the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The cool period of the [[Earth:Huronian glaciation|Huronian glaciation]] is speculated to be linked to the decline in the atmospheric content of greenhouse gases during the [[Earth:Great Oxidation Event|Great Oxygenation Event]]. Similarly, the possible snowball Earth of the Precambrian's [[Earth:Cryogenian|Cryogenian]] between 580 and 850 million years ago (and which itself had a number of distinct episodes) could be related to the rise of more advanced multicellular animal life and life's colonisation of the land.<ref>Cowie, J., (2007) ''Climate Change: Biological and Human Aspects''. Cambridge University Press. (Pages 73 - 77.) {{ISBN|978-0-521-69619-7}}.</ref><ref>Lenton, T., & Watson, A., (2011) ''Revolutions That Made The Earth''. Oxford University Press. (Pages 30 -36, 274 - 282.) {{ISBN|978-0-19-958704-9}}.</ref> However, a very recent study, based on findings of previous studies, suggested [[Biology:Evolutionary history of plants#Colonization of land|land plant evolution]] was driven by the Cryogenian glaciations, which they also theorized to be the reason why the [[Biology:Zygnematophyceae|Zygnematophyceae]] (sister group of land plants) became unicellular and cryophilic, lost their flagella and evolved sexual conjugation.<ref>Zarsky, J. D., Zarsky, V., Hanacek, M., & Zarsky, V. (2021, July 21). Cryogenian glacial habitats as a plant terrestrialization cradle – the origin of the anydrophytes and Zygnematophyceae split. https://doi.org/10.3389/fpls.2021.735020</ref>

=== Effects on ocean circulation ===
Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation.<ref name="Ashkenazy2013">{{Cite journal | last1 = Ashkenazy | first1 = Y. | last2 = Gildor | first2 = H. | last3 = Losch | first3 = M. | last4 = MacDonald | first4 = F. A. | last5 = Schrag | first5 = D. P. | last6 = Tziperman | first6 = E. | doi = 10.1038/nature11894 | title = Dynamics of a Snowball Earth ocean | journal = Nature | volume = 495 | issue = 7439 | pages = 90–93 | year = 2013 | pmid =  23467167| bibcode = 2013Natur.495...90A | s2cid = 4430046 | url = https://dash.harvard.edu/bitstream/1/13454620/1/final-with-figs.pdf }}</ref>

==Occurrence and timing==
===Palaeoproterozoic===
{{main|Earth:Huronian glaciation}}
The Snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of {{wipe|Canada}}, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested,<ref name="Williams">{{cite journal
|author1=Williams G.E. |author2=Schmidt P.W. | title=Paleomagnetism of the Paleoproterozoic Gowganda and Lorrain formations, Ontario: low palaeolatitude for Huronian glaciation
| journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]]
| year=1997
| volume=153
| issue=3
| pages=157–169
| doi = 10.1016/S0012-821X(97)00181-7
| bibcode=1997E&PSL.153..157W}}</ref><ref name="Kopp" /> and stratigraphic evidence clearly shows only three distinct depositions of glacial material (the Ramsay, Bruce and Gowganda Formations) separated by significant periods without. The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were possibly deposited at tropical latitudes.<ref>{{cite journal |last1=Evans |first1=D. A. |last2=Beukes |first2=N. J. |last3=Kirschvink |first3=J. L. |title=Low-latitude glaciation in the Palaeoproterozoic era |journal=Nature |date=March 1997 |volume=386 |issue=6622 |pages=262–266 |doi=10.1038/386262a0 |bibcode=1997Natur.386..262E |s2cid=4364730 }}</ref> It has been proposed that rise of free oxygen that occurred during the [[Earth:Great Oxygenation Event|Great Oxygenation Event]] removed [[Earth:Atmospheric methane|atmospheric methane]] through oxidation. As the [[Astronomy:Solar irradiance|solar irradiance]] was notably weaker at the time, Earth's climate may have relied on methane, a powerful [[Chemistry:Greenhouse gas|greenhouse gas]], to maintain surface temperatures above freezing. In the absence of this methane greenhousing, temperatures plunged and a global glaciation could have occurred between 2.5 and 2.2 Gya, during the [[Earth:Siderian|Siderian]] and [[Earth:Rhyacian|Rhyacian]] periods of the [[Earth:Paleoproterozoic|Paleoproterozoic]] era.<ref name="Kopp">{{cite journal
|author1=Robert E. Kopp |author2=Joseph L. Kirschvink |author3=Isaac A. Hilburn |author4=Cody Z. Nash  |name-list-style=amp | title=The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis
| journal=Proceedings of the National Academy of Sciences of the United States of America
| year=2005
| volume=102
| issue=32
| pages=11131–6
| doi=10.1073/pnas.0504878102
| pmid=16061801
| pmc=1183582
| bibcode=2005PNAS..10211131K|doi-access=free }}</ref>

===Neoproterozoic===
{{main|Earth:Cryogenian|Earth:Sturtian glaciation|Earth:Marinoan glaciation}}
There were three or four significant ice ages during the late [[Earth:Neoproterozoic|Neoproterozoic]]. Of these, the [[Earth:Marinoan glaciation|Marinoan]] was the most significant, and the [[Earth:Sturtian glaciation|Sturtian]] glaciations were also truly widespread.<ref name=Stern2006>{{cite journal
| author = Stern, R.J. |author2=Avigad, D. |author3=Miller, N.R. |author4=Beyth, M.
| year = 2006
| title = Geological Society of Africa Presidential Review: Evidence for the Snowball Earth Hypothesis in the Arabian-Nubian Shield and the East African Orogen
| journal = Journal of African Earth Sciences
| volume = 44
|issue=1 | pages = 1–20
| doi = 10.1016/j.jafrearsci.2005.10.003|bibcode = 2006JAfES..44....1S }}</ref> Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long<ref name=Pu>{{cite journal
| last = Pu
| first = J.P.
| title = Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota
| journal = Geology
| year= 2016
| volume = 44
| issue = 11
| pages= 955–958
| doi = 10.1130/G38284.1| bibcode = 2016Geo....44..955P
| s2cid = 31142776
}}
</ref> Gaskiers glaciation did not lead to global glaciation,<ref name=Hoffman2005>{{cite journal
| author = Hoffman, P.F.
| year = 2005
| title = On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record
| journal = South African Journal of Geology
| volume = 108
| pages = 557–77
| doi = 10.2113/108.4.557
| issue = 4
}}</ref> although it was probably as intense as the [[Earth:Late Ordovician glaciation|late Ordovician glaciation]]. The status of the [[Earth:Kaigas|Kaigas]] "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age.<ref name=Smith2008>{{Cite journal | last1 = Smith | first1 = A. G. | doi = 10.1144/SP326.2 | title = Neoproterozoic timescales and stratigraphy | journal =  Geological Society, London, Special Publications| volume = 326 | pages = 27–54 | year = 2009 | issue = 1 | bibcode=2009GSLSP.326...27S| s2cid = 129706604 }}</ref> It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis.<ref name=nature_geo/>

===Karoo Ice Age===
{{main|Earth:Late Paleozoic icehouse}}
Before the theory of continental drift, glacial deposits in [[Earth:Carboniferous|Carboniferous]] strata in tropical continental areas such as India and South America led to speculation that the [[Earth:Karoo Ice Age|Karoo Ice Age]] glaciation reached into the tropics. However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the [[Earth:Supercontinent|supercontinent]] [[Earth:Gondwana|Gondwana]].{{Cn|date=August 2022}}

==See also==
<!-- Please keep entries in alphabetical order & add a short description WP:SEEALSO -->
{{Div col}}
* [[Astronomy:Europa (moon)|Europa]]
* [[Astronomy:Gaia hypothesis|Gaia hypothesis]]
* Global catastrophic risks
* [[Earth:Great Unconformity|Great Unconformity]]
* [[Earth:Interglacial|Interglacial]] and [[Earth:Stadial|stadial]] periods
* [[Earth:Lake Nyos|Lake Nyos]]
* [[Earth:Milankovitch cycles|Milankovitch cycles]]
{{div col end}}
<!-- please keep entries in alphabetical order -->

==References==
{{reflist|30em}}

==Further reading==
* {{Cite journal | last1 = Tziperman | first1 = E. | last2 = Halevy | first2 = I. | last3 = Johnston | first3 = D. T. | last4 = Knoll | first4 = A. H. | last5 = Schrag | first5 = D. P. | title = Biologically induced initiation of Neoproterozoic snowball-Earth events | doi = 10.1073/pnas.1016361108 | journal = Proceedings of the National Academy of Sciences | volume = 108 | issue = 37 | pages = 15091–15096 | year = 2011 | pmid =  21825156| pmc = 3174660| bibcode = 2011PNAS..10815091T | doi-access = free }}
* {{cite book
| isbn = 978-1-4051-8300-0
| chapter = Neoproterozoic glaciated basins: A critical review of the Snowball Earth hypothesis by comparison with Phanerozoic glaciations
| year = 2007
| author = Etienne, J.L.
| author2 = Allen, P.A.
| author3 = Rieu, R.
| author4 = Le Guerroué, E.
| name-list-style = amp
| title= Glacial Sedimentary Processes and Products
|editor1= Michael Hambrey |editor2=Poul Christoffersen |editor3=Neil Glasser |editor4=Bryn Hubbard
|series = IAS Special Publication
| volume = 39
| pages = 343–399
| publisher = IAS/Blackwell
| location = Malden, MA
| doi=10.1002/9781444304435.ch19
}}
* {{cite book|author=Walker, Gabrielle|year=2003|title=Snowball Earth|publisher=Bloomsbury Publishing|isbn=978-0-7475-6433-1}}
* {{cite journal
| author=Micheels, A.
| author2=Montenari, M.
| title=A snowball Earth versus a slushball Earth: Results from Neoproterozoic climate modeling sensitivity experiments
| journal=Geosphere
| volume=4
| issue=2
| pages=401–10
|year=2008
|doi=10.1130/GES00098.1| bibcode=2008Geosp...4..401M
| doi-access=free
}} (Geol. Soc. America).
* {{cite journal |author=Roberts, J.D. |title=Late Precambrian glaciation: an anti-greenhouse effect? |journal=Nature |volume=234 |pages=216–7 |year=1971 |doi=10.1038/234216a0 |issue=5326|bibcode = 1971Natur.234..216R |s2cid=34163139 }}
* {{cite journal |author=Roberts, J.D. |title=Late Precambrian dolomites, Vendian glaciation, and the synchroneity of Vendian glaciation |journal=Journal of Geology |volume=84 |issue=1 |pages=47–63 |year=1976 |doi=10.1086/628173 |bibcode=1976JG.....84...47R|s2cid=140664783 }}
* {{cite journal |last1=Sankaran |first1=A. V. |title=Neoproterozoic 'snowball earth' and the 'cap' carbonate controversy |journal=Current Science |year=2003 |volume=84 |issue=7 |pages=871–873 |jstor=24108043 }}
* {{cite journal |author=Torsvik, T.H. |author2=Rehnström, E.F. |title=Cambrian palaeomagnetic data from Baltica: Implications for true polar wander and Cambrian palaeogeography |journal= Journal of the Geological Society|volume=158 |pages=321–9 |year=2001 |doi=10.1144/jgs.158.2.321 |issue=2|bibcode=2001JGSoc.158..321T |s2cid=54656066 }}

Review of late Ediacaran glacial deposits: {{Cite journal | doi = 10.1016/j.earscirev.2023.104610| title = A late Ediacaran ice age: The key node in the Earth system evolution| date = 2023| last1 = Wang| first1 = Ruimin| last2 = Yin| first2 = Zongjun| last3 = Shen| first3 = Bing| journal = Earth-Science Reviews| volume = 247| bibcode = 2023ESRv..24704610W| s2cid = 265071916}}

==External links==
* [https://web.archive.org/web/20100613140722/http://studentresearch.wcp.muohio.edu/snowballearth/articles/Snowballearthpast99.pdf The Snowball Earth] 1999 overview by Paul F. Hoffman and Daniel P. Schrag, 8 August 1999
* [http://www.snowballearth.org/index.html Snowball Earth web site] Exhaustive on-line resource for snowball Earth by pro-snowball scientists Hoffman and Schrag.
* [https://www.sciencedaily.com/releases/2007/03/070323104746.htm New Evidence Puts 'Snowball Earth' Theory Out In The Cold] sciencedaily.com. 2007. Analyses in Oman produce evidence of hot-cold cycles in the Cryogenian period, roughly 850–544 million years ago. The UK-Swiss team claims that this evidence undermines hypotheses of an ice age so severe that Earth's oceans completely froze over.
* [http://www.channel4.com/programmes/catastrophe/4od#2917637 Channel 4 (UK) documentary, Catastrophe: Snowball Earth] {{Webarchive|url=https://web.archive.org/web/20160529143900/http://www.channel4.com/programmes/catastrophe/4od#2917637 |date=29 May 2016 }} episode 2 of 5, first screened Dec 2008, documentary narrated by Tony Robinson, advocates snowball Earth and contains interviews with proponents.
* [https://www.newscientist.com/article/mg20527461.100-first-breath-earths-billionyear-struggle-for-oxygen.html First breath: Earth's billion-year struggle for oxygen] New Scientist, #2746, 5 February 2010 by Nick Lane. Posits an earlier much longer snowball period, c. 2.4 – c. 2.0 Gya, triggered by the [[Earth:Great Oxygenation Event|Great Oxygenation Event]].
* [http://news.bbc.co.uk/2/hi/uk_news/scotland/1857545.stm 'Snowball Earth' theory melted] BBC News online (2002-03-06) report on findings by geoscientists at the [[Organization:University of St Andrews|University of St Andrews]], Scotland that casts doubt on the snowball Earth hypothesis due to evidence of sedimentary material, which could only have been derived from floating ice on open oceanic waters.
* [https://www.bbc.co.uk/news/science-environment-11992299 Life may have survived 'snowball Earth' in ocean pockets] BBC News online (2010-12-14) report on research presented in the journal Geology by Dr Dan Le Heron (''et al.'') of [[Organization:Royal Holloway, University of London|Royal Holloway, University of London]] who studied rock formations in Flinders Ranges in South Australia, formed from sediments dating to the Sturtian glaciation, which bear the unmistakable mark of turbulent oceans.
* [https://www.giss.nasa.gov/research/briefs/2002_sohl_01/ Snowball Earth animated simulation] by [[Organization:NASA|NASA]] and LDEO 

{{Ice ages}}

[[Category:Cryogenian]]
[[Category:Extinction events]]
[[Category:Paleoclimatology]]
[[Category:Hypotheses]]
[[Category:Meteorological hypotheses]]

{{Sourceattribution|Snowball Earth}}