Earth has twice been covered with ice according to the Snowball Earth theory. Ice may have covered the entire planet, perhaps comparable to the way ice covers Europa (a moon of Jupiter) today.
How could the earth totally freeze into a snowball during the Neoproterozoic Sturtian global glaciation 717 million years ago, and again during the Marinoan glaciation 650 million years ago?
The sun's output could have dimmed, or an increase of aerosols in the upper atmosphere could have reflected incoming radiation. A 2% reduction of incoming sunlight, if it occurred quickly over just 10,000 years, may have been sufficient to trigger a Snowball Earth event.
In addition, there may have been massive lava flows followed by rapid weathering of the newly-exposed rock. The weathered products incorporated much of the atmosphere's carbon dioxide into minerals, and those minerals were then consolidated into sedimentary formations. That process may have removed enough CO2 from the atmosphere to transform the climate.
Light from the sun continued to enter the earth's atmosphere in short wavelengths that passed through the atmosphere with minimal absorption of the energy. As carbon dioxide levels dropped, less of the energy in those longer wavelengths was captured. The planet cooled, and snow covered more of the surface. White snow reflected light directly back into space as albedo increased. Less solar energy was absorbed by the ground and then re-emitted in longer wavelengths, and the planet cooled further in a positive feedback loop.
With a short time period, perhaps just 10,000 years, ice sheets crept from the poles to the Equator.1
However, volcanoes continued to emit carbon dioxide. In an ice-covered Earth, CO2 was not incorporated into weathering rocks and sequestered in sediments. As levels of carbon dioxide increased, more energy was absorbed in the atmosphere. Temperatures rose and the ice melted. Ground was exposed with lower albedo, and with the high level of CO2 the average temperatures on earth skyrocketed in a climate reversal.
As the ice cover was removed from the oceans, the level of dissolved CO2 in the water increased. When covered with ice, the oceans had been enriched with dissolved minerals. The dissolved CO2 combined with calcium to create calcium and magnesium, which precipitated to the seafloor and became limestone and dolomite formations.
The sequestering of carbon dioxide in rock formations reduced the amount in the atmosphere. That reduced the greenhouse gas effect, cooled the planet, and left as evidence thick layers of carbonate formations just above evidence of glacial formations. The ice may have melted within just one million years.2
A second cycle of lava flows and ice sheet expansion occurred, but a third is less likely. The expansion of life on earth has created a new pattern of storing and recycling carbon, and that may occur too quickly for another Snowball Earth. However, the causes are still not well understood, so the possibility exists. Paul Hoffman, a pioneer Snowball Earth researcher, has noted:3
Sediments at Sharp Top Mountain appear to be the debris from the base of pre-Cambrian glaciers, 750 million years ago. These Virginia sediments are simlar to a contemporary formation in Namibia, indicating what is now Virginia was covered in ice during the Snowball Earth periods.
The Sharp Top sediments are diamictites, a conglomerate rock with stones of different sizes. They were formed by glaciers scraping together a debris pile that later consolidated into rock. The diamictites are exposed today in the Mechums River formation, southwest of Charlottesville near Batesville.
For roughly 10 million years, life in the oceans was suppressed. The earliest forms of complex life, such as sponges, somehow survived even when the ocean surface was covered with ice. Transfer of oxygen into the ocean may have occured where meltwater came from the ice sheet during the Snowball Earth period.
Eventually, the greenhouse effect was triggered by the extra carbon dioxide, the earth warmed, and the global ice sheets melted - forming the distinctive sedimentary deposits (diamictites) in Albemarle County. The Konnarock Formation at Mount Rogers provides evidence of glacially-influenced sedimentation at tropical latitudes 751 million years ago.
There may have been a series of dramatic extinctions, then re-blossomings of life after several freezing and warming cycles. The Cambrian "explosion" of life 580 million years ago may have brought an end to the cycles, in part due to the evolution of... bottom-dwelling worms. These animals have stirred/recycled the carbon in ocean sediments for the last 580 million years, perhaps maintaining the essential percentage of carbon dioxide and preventing another reduction in the ability of the atmosphere to retain heat from the sun.4
Whether or not the earth was a completely frozen popsicle, it is clear that colder climate changed the composition of the vegetation in Virginia in the most recent Ice Age . As shown by analysis of the pollen in the bottom of bogs in the Appalachians, the species we currently consider to be "northern" such as spruce and fir trees were common in Virginia 20,000 years ago.
At Great Falls on the Potomac, the forest was composed of spruce and jack pine trees 16,000 years ago - comparable to forests along the Canadian border today. Pollen from oaks and hemlocks appears in quantity 12,000 years ago, and by 8,000 years ago the forests were similar to the oak-hickory forest of today.4