Introduction
Between 720 and 635 million years ago, during the Cryogenian Period, Earth experienced some of the most extreme climate conditions in its geologic history. Massive global glaciations, most notably the Sturtian and Marinoan events, are thought to have covered much of the planet in ice, possibly from pole to equator. This era is central to the Snowball Earth hypothesis, which proposes that continental landmasses and oceans were largely frozen over, triggered by decreased greenhouse gases and continental configurations that favored increased reflectivity from widespread ice and snow.
Tectonically, the supercontinent Rodinia was in the process of breaking apart, creating rift basins and expanding passive continental margins. These environments collected thick sedimentary sequences, some of which preserved glacial diamictites, dropstones, and distinctive cap carbonates. These carbonate layers signal rapid climate warming and a return to open oceans following global glaciation. Erosion by vast ice sheets may have contributed to the formation of the Great Unconformity by stripping away earlier rock layers and exposing basement rock.
Despite harsh surface conditions, life persisted, possibly near hydrothermal vents or in areas of thin equatorial ice. The Cryogenian set the stage for a major biological rebound, leading eventually to the rise of complex multicellular organisms in the Ediacaran Period.
While the Snowball Earth hypothesis paints a dramatic picture of a planet entirely encased in ice, several lines of evidence challenge the idea of a completely frozen globe. Sedimentary deposits from the Cryogenian period, such as wave-generated structures and non-glacial carbonates found at low latitudes, suggest that some open water may have persisted. Additionally, the survival of photosynthetic and eukaryotic life implies the existence of ecological refuges, possibly in equatorial regions or near hydrothermal vents. These observations raise the question—was Earth truly sealed in an unbroken shell of ice, or was it more of a "Slushball," with patches of open ocean and dynamic climate zones? (Hoffman & Schrag, 2002)
The Snowball Earth Theory
The Snowball Earth theory proposes that during the Cryogenian Period, between roughly 720 and 635 million years ago, Earth experienced several global-scale glaciations that may have encased much of the planet in ice. According to this theory, massive ice sheets extended to equatorial regions, drastically altering the planet’s climate and surface processes. These glacial periods, especially the Sturtian and Marinoan events, are recorded in sedimentary rocks across multiple continents. During this time, the supercontinent Rodinia was breaking apart, forming rift basins and passive margins that preserved glacial deposits such as diamictites and dropstones. Overlying these layers are cap carbonates, which signal a rapid climatic shift from glaciation to warming.
Many scientists who support the Snowball Earth theory believe that most life entered a state of dormancy during the Cryogenian glaciations. With thick ice covering oceans and reducing sunlight and nutrient circulation, surface ecosystems would have collapsed or paused. Microbial life likely survived in a low-energy, dormant state, similar to spores or cysts. (Hoffman & Schrag, 2002)
The Slushball Theory
The Slushball Earth theory offers a less extreme alternative to the Snowball Earth hypothesis. It suggests that while large portions of the planet were glaciated during the Cryogenian Period, equatorial oceans remained at least partially ice-free or covered by thin, translucent ice. This allowed some sunlight to penetrate, supporting limited photosynthesis and keeping ocean circulation partially active. As a result, life may have continued in a more active state than the dormancy proposed by the Snowball model. The theory helps explain geologic evidence that is inconsistent with a completely frozen Earth, such as ripple marks, stromatolites, and non-glacial carbonates in low-latitude regions. It also accounts for the survival and even modest evolution of early eukaryotic organisms. By proposing a more moderate global climate scenario, the Slushball Earth theory bridges the gap between total glaciation and ongoing biological activity, offering a nuanced view of Earth's complex cryogenic past. (Corsetti, Awramik, & Pierce, 2003; Kennedy, Christie-Blick, & Sohl, 2001)
The Evidence
In Namibia's Otavi Group, particularly within the Ghaub Formation, researchers have identified sedimentary structures consistent with wave-formed ripples. These features suggest the presence of open water or thin ice cover during deposition, challenging the notion of a completely frozen Earth. (Hoffman & Schrag, 2002)
Also within the Otavi and Nama Groups of Namibia, researchers have discovered Otavia antiqua, phosphatized microfossils interpreted as ancient sponge-like organisms. These fossils date from approximately 760 to 550 million years ago, spanning the Cryogenian glaciations. Their presence suggests that early multicellular life persisted through these glacial periods, challenging the notion of a completely frozen Earth. (Germs & Prave, 2001)
Stromatolites, layered structures formed by microbial mats, have been discovered in the Noonday Dolomite of Death Valley, USA. Their presence indicates that photosynthetic microorganisms were active during the Cryogenian, implying that some regions had conditions amenable to life, contrary to a fully glaciated planet. (Corsetti, Awramik, & Pierce, 2003)
In the Flinders Ranges of South Australia, cap carbonates overlying glacial deposits exhibit features such as giant wave ripples. These structures point to high-energy, open-water conditions immediately following glaciation, supporting the idea of episodic open water even during glacial periods. (Kennedy et al., 2001)
Equatorial Connection
It is very likely that many of the locations showing signs of biological activity and open water during the Cryogenian glaciations—such as Namibia, South Australia, and Death Valley—were positioned near the equator around 720 million years ago. During this time, the supercontinent Rodinia was breaking apart, and many continental fragments drifted into low-latitude regions. Paleomagnetic data supports this, showing that formations like the Otavi Group and Elatina Formation were deposited in equatorial zones. These areas would have been relatively warmer and less affected by thick, continent-crushing ice sheets, making them ideal refuges for life and allowing for the preservation of stromatolites, microbial mats, and sedimentary structures like ripple marks.
You might be wondering, how could Death Valley and the Grand Canyon, sitting relatively close today, tell such different geologic stories? Around 720 million years ago, Death Valley and the Grand Canyon were not as close together as they are today, but they were part of the same ancient landmass, Laurentia, the core of modern North America. At the time, Laurentia was positioned near the equator and oriented differently due to plate tectonics. As a result, it likely experienced a much milder climate and showed little to no evidence of glacial ice—especially when compared to areas like the Grand Canyon. Both regions lay along its western margin and were shaped by the rifting of the supercontinent Rodinia. (Li & Powell, 2001)
Conclusion
In the end, the Snowball Earth theory remains one of the most dramatic proposals in Earth science, but the geologic and fossil record increasingly supports a more nuanced view. The Slushball Earth model offers a compelling alternative—one that accounts for both the severity of global glaciations and the persistence of life in equatorial refuges. Evidence from sedimentary structures, microbial fossils, and paleogeographic reconstructions reveals that Earth during the Cryogenian was far from uniformly frozen. Instead, it was a planet of extremes, with icy continents, dynamic oceans, and life clinging to the margins. As we study these ancient climates, we not only unravel Earth's deep past, but we also gain insight into the resilience of life and the long-term drivers of planetary climate—lessons that feel more relevant than ever today.
References
Hoffman, P. F., & Schrag, D. P. (2002). The Snowball Earth hypothesis: Testing the limits of global change. Terra Nova, 14(3), 129–155. https://doi.org/10.1046/j.1365-3121.2002.00408.x
Germs, G. J. B., & Prave, A. R. (2001). Otavia antiqua: Phosphatized sponge-like fossils from the Neoproterozoic of Namibia. South African Journal of Science, 97(3–4), 107–109.
Corsetti, F. A., Awramik, S. M., & Pierce, D. (2003). Stromatolites and stable isotopes in the Noonday Dolomite, Death Valley, California: Evidence for Proterozoic glacial events. Precambrian Research, 120(3–4), 295–314. https://doi.org/10.1016/S0301-9268(02)00171-2
Kennedy, M., Christie-Blick, N., & Sohl, L. E. (2001). Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals? Geology, 29(5), 443–446. https://doi.org/10.1130/0091-7613(2001)029<0443:APCCAI>2.0.CO;2
Li, Z. X., & Powell, C. M. (2001). An outline of Neoproterozoic to early Paleozoic plate configurations and their implications for the origins of supercontinents. Earth-Science Reviews, 53(3–4), 237–277. https://doi.org/10.1016/S0012-8252(00)00051-0