Imprisoned in

Continental glaciations leave evidence of their former presence: a characteristic topography on the landscape, grooves and scratches caused as the passing glaciers ground over hard rock, and (perhaps most important) telltale sedimentary deposits called tillites. The latter are deposits of angular rock fragments, which were carried and then left by moving glaciers. The recently concluded ice ages of 2.5 million to 12,000 years ago left many such deposits in both the Northern and the Southern hemispheres. Such tillite deposits are also found in much older rocks. Thick tillite deposits have been recovered from two different intervals in Precambrian Earth history: around 2.4 billion years ago and during the interval from about 800 to 650 million years ago. The unusual aspect of these features is that they are recovered from virtually all latitudinal regions of the globe, which shows that the glaciations extended to near equatorial latitudes (in contrast to the more recent glaciations, which extended from the poles to mid-latitudes). It may be that no region then on Earth escaped the glaciation. So much of the planet was covered by ice in these two Precambrian ice ages that in 1992, Dr. Joseph Kirschvink of Cal Tech dubbed them "Snowball Earth" events. Far different from the later ice ages, they were times when Earth teetered dangerously close to becoming too cold for any life. The Snowball Earth theory received a boost in August 1998 with Harvard geologist Paul Hoffman's publication, in Science, of new evidence that ice extended to near equatorial latitudes in the late Precambrian, about 700 million years ago.

The more recent glaciations, those that occurred since skeletons evolved about 550 million years ago, affected only land regions; except for an increase in icebergs, or at most ice cover near the continents, the oceans remained open. Such may not have been the case in the Precambrian glaciations. During these two "Snowball Earth" episodes, all of the oceans may have been covered with ice to considerable depths. And although the deeper regions of the seas remained liquid, thick icebergs, or pack ice to depths of 500 to 1500 meters, may have covered the ocean. Earth would have been cold indeed. Average surface temperatures on the planet would have varied between -20°C and -50°C.

These extremely cold temperatures would have had an enormous influence on the surface of our planet. For example, continental weathering would have slowed or even stopped. In the interior of continents, the covering of ice would eventually ablate (evaporate) away, just as it does in the dry valleys of Antarctica today, leaving behind a sterile rock surface. Dust from these regions would be blown out to sea, making the pack-ice cover of the oceans brown from terrigenous material. From space, Earth would have looked white and brown—the white being the ice covers on the oceans, the brown the denuded land areas.

The presence of the pack ice covering the sea would act as a lid on a pot. Normally, much free exchange occurs across the vast interface of ocean and atmosphere. Water evaporates from the sea into the air and then rains back into the sea. If the sea were covered with ice, however, the ocean and the atmosphere would become "decoupled." Chemical changes in the ocean would be separated from the atmosphere by the kilometer-thick lid of ice on the ocean surface. Very drastic chemical change could—and according to Kirschvink and others, did—occur within the sea itself.

Even with the icy cover, volcanism would have continued both on the land's surface and along the mid-ocean volcanic ridges at the bottoms of the world's oceans. At such sites today (see Chapter 1), great volumes of metal-rich fluids gush forth from these submarine volcanoes. In a covered ocean, this material would have become toxic, producing what are known as reducing conditions. The oceans would have begun to accumulate with metal ions, mainly iron and manganese. For as long as 30 million years, the glaciers and ice never relaxed their frigid grip on the planet's surface.

All of this global cold would surely have adversely affected life in the shallow-water regions of all the world's oceans. The biosphere became restricted to a narrow belt around the equator and to deep-sea hot springs and hydrothermal vent settings. Perhaps some life also survived in occasional Yellowstone-like hydrothermal systems.

Astronomers once thought that a previously warm world's descent into such an "icehouse" or "snowball" would be irreversible. Their reasoning was that as a planet gets more and more thickly coated by ice, the fraction of sunlight reflected back into space increases and solar heating of the surface declines. On Earth today, sunlight is adsorbed by the darker land and seas but is reflected into space by cloud cover. A planet completely covered with ice would reflect most sunlight into space, causing the planet to become ever cooler. Yet it is clear that Earth was able to escape from the deep freeze—not once but several times. The means of that escape was through the volcanic emissions of greenhouse gases such as carbon dioxide into the atmosphere, producing a "greenhouse effect."

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