Six Glacial Periods on Earth
The geological record of the planet contains evidence of six significant periods of glaciation or “ice ages,” when masses of ice covered extensive regions of the earth—or in some cases the whole earth.
The geological record of major glaciations goes back almost 3 billion years. The figure above shows the time and duration of these ice ages (blue boxes). The majority of these have occurred in the last billion years. The colored boxes shows the major time periods that scientists use to divide up the history of the earth.
The earliest discovered so far is the Pongola glaciation, occurring about 2.9 billion years ago (Ga). Glacial deposits from this time have been found in the Pongola supergroup of Wittsverd, South Africa from this period of time. There is no consensus on what may have caused this glaciation. It may have ended with increased tectonic activity and volcanism that accompanied the breakup of one of the very first supercontinents, Vaalbara, composed of cratons from South Africa and Western Australia—two of the few cratons that likely existed during that time—around 2.8 Ga.
The second ice age, called the Huronian glaciation, occurred between 2.4 and 2.1 Ga at the onset of the breakup of the second supercontinent, Kenorland. Its onset has been linked to the significant accumulation of oxygen in the atmosphere generated by photosynthetic bacteria over tens of millions of years, what scientists refer to as the Paleoproterozoic Great Oxidation Event. The increase in oxygen led to a decrease in atmospheric methane that was enough to shift the planet from what may have been a “greenhouse period” to an extensive ice age. During this period three regional glaciations were followed by a period when the entire earth appears to have been locked in ice—the so-called “snowball earth,” also called the Makganyene glaciation around 2.3 Ga. During this time temperatures around the globe may have fallen to as much as -60° F. This global ice age precipitated what was probably the first mass extinction in earth’s history among the cyanobacteria that had built up the atmosphere’s oxygen. Snowball earth came to an end as continued tectonic activity and related volcanism from Kenorland’s breakup caused a buildup of greenhouse gases and new oceanic areas enabled circulation of warmer waters into the polar regions.
Image: a rendering of snowball earth, with large masses of ice covering the continents and oceans locked up in ice. Image from https://www.google.com/url?sa=i&url=https%3A%2F%2Fdinopedia.fandom.com%2Fwiki%2FHuronian_glaciation&psig=AOvVaw16rlKAgpqhiPjTk-UbHKnC&ust=1613945802971000&source=images&cd=vfe&ved=0CA0QjhxqFwoTCIicxeO--e4CFQAAAAAdAAAAABAD. Used under a Creatives Common license.
The third major glaciation was the Cryogenian (715-635 Ma). This occurred during the breakup of Rodinia, and was contemporaneous with an important stage in the formation of Avalonia. There were two significant glaciations during this time, the Sturtian from about 715-650 Ma, and the Marinoan from about 650-635 Ma. During this time, the bulk of the continental masses were assembled near the south pole, and disruption of oceanic and atmospheric circulation may have triggered these ice ages. Both of these were “snowball earth” episodes; the ending of the Marinoan may have been precipitated by the renewed tectonic activity and volcanism that led to the breakup of Rodinia. Significant volcanic activity in Avalonia occurred during the end of the Marinoan glaciation.
The fourth was the Andean-Saharan glaciation (460-420 Ma). This was a regional event that appeared to affect mostly continental areas that were near the south pole at the time.
The fifth major ice age was the Karoo (360-320 Ma). This glaciation again occurred on continental masses located at the south polar region of the earth. The closing of the Rheic and Iapetus Oceans—events related to the separation of Avalonia from West Gondwana and its drift to ancient North America (Laurentia)—along with the presence of a significant continental mass near the south pole, may have induced this ice age.
Image right: a reconstruction of the continents at the time of the Karoo glaciation. Figure shows that a significant continental mass was located at the south pole. The purplish-blue area maps the hypothesized extent of the ice coverage during the Karoo. Image from: https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse#/media/File:Karoo_Glaciation.png.
The final major ice age was the Cenezoic. This began around 34 million and continues through today. This has also been a regional glaciation, with the accumulation of ice at the polar regions, and periodic advances of ice sheets from continental ice masses. Initial ice sheet formation occurred in the Antarctic. Over the last three million years, the accumulation and expansion of the northern ice sheet has occurred--a time demarcated by scientists as the Pleistocene. A series of glacial advances have occurred during this time, affecting large portions of North America and Eurasia. The last major glaciation, the Wisconsin glaciation, started about 75,000 years ago and ended around 10,000 years ago. The inter-glacial pattern of this ice age has been disrupted by human-generated carbon dioxide and methane, which is pushing the earth's climate into a warming cycle when the climate should be cooling off.
Image right: Map showing the maximum extent of the ice sheets during the Wisconsin glaciation. Image from: Hannes Grobe/AWI - own work - redrawn, supplemented and modified grafic from John S. Schlee (2000) Our changing continent, United States Geological Survey.
