Snowball Earth is a theory that much or all of Earth’s surface was covered by ice at various times in geologic history. The name refers to Earth’s being white and reflective, like a snowball. Scientists have divided the geological history of Earth into long stretches of time called eons. Earth scientists think that several Snowball Earth periods occurred during the Proterozoic Eon that began 2.5 billion years ago and ended 539 million years ago. Scientists believe that the first Snowball Earth occurred about 2.2 billion years ago. Another is thought to have occurred about 700 million years ago. Evidence suggests that the most recent Snowball Earth event occurred about 620 million to 600 million years ago.
Scientists think that during Snowball Earth periods, ice covered the polar areas and extended to the equatorial regions. The equator is usually warm and ice-free because it receives large amounts of solar radiation compared with higher latitudes. However, so much bright white ice and snow covered Earth during these periods that much of the incoming sunlight was reflected back into space, cooling Earth. The percentage of solar radiation reflected by Earth is known as its albedo. Albedo was much higher during Snowball Earth periods than today. The Snowball Earth events created what scientists call a positive feedback loop. The more solar energy that was reflected, the cooler Earth became and the more bright, reflective, snow and ice covered the surface.
Evidence.
Earth scientists have found several kinds of evidence for the Snowball Earth periods in the geologic record. Most significant is evidence for where the continents were covered with glaciers at latitudes near the equator in the past. For example, rocks that show evidence of past glaciation are found today in what is now southern Africa and southern Australia.
Evidence of widespread glaciers is seen in deposits of unsorted silt, sand, and rocks called tillites. The deposits formed as glaciers scrape the ground as they move over it. Large rocks called dropstones found in the deep ocean are evidence that icebergs once moved across ocean regions near the equator. The rocks were transported from land by icebergs that calved (broke off) glaciers when the glaciers reached the ocean. The rocks fell to the ocean floor as the icebergs melted.
Scientists know that Earth’s continents have moved over time (see Plate tectonics). Earth scientists measure the past latitude of a region using the orientation of magnetic grains in rock. The magnetic orientation of the rocks indicates the glacial deposits were produced near the equator hundreds of millions of years ago.
End of Snowball Earth.
Each Snowball Earth period appears in the geological record to end abruptly. Geologists observe in deposits a thick layer of limestone (calcium carbonate) that marks the end of the event. They believe these “cap carbonates” may reflect changes in the rate of weathering of rocks around the world or in the cycling of carbon and calcium during and after each Snowball Earth period.
Snowball Earth periods all occurred before the rise of complex multicellular organisms, oxygenation of the atmosphere, and the rise of land plants. None has occurred since. Earth scientists believe that levels of carbon dioxide in the atmosphere are more stable since the rise of plants. Carbon dioxide is a powerful greenhouse gas that keeps Earth warm. On Earth, plants and other forms of life stabilize the carbon cycle and help regulate carbon dioxide in the atmosphere. After the last Snowball Earth period, the first simple, soft-bodied animals appear in the fossil record. The appearance of this diverse group of life forms marks what Earth scientists call the Ediacaran Period in Earth’s history.
