Glacial Systems

Glaciation

A glacier is a natural accumulation of land ice showing movement at some time. Many times during Earth's history, great ice sheets waxed and waned over the surface. What caused these periods of glaciation is still not fully understood and no single reason will probably be found.

Causes of glaciation

The onset of a period or stage of glaciation is due to a change in Earth temperature and circulation. It is generally accepted that a global decrease of  4^o to 5^o C, especially during the summer, and a substantial increase in the amount of snowfall in subarctic and arctic regions is necessary for the onset of a glacial episode. Several theories have been proposed for such a change in climate -- reductions in solar radiation due to meteorite collisions with the Earth, increased volcanism, the shifting location of continents, and the uplift of vast mountain regions. Milutin Milankovitch proposed one of the most significant theories to account for climate change by variations in Earth orbit. Changes in the eccentricity of earth orbit, the degree of deviation of the orbit from a perfect circular path, is thought to cause the necessary change in insolation to decrease global temperatures. Recall that the Earth's orbit is elliptical, but over periods of 100,000 years the shape varies. The changes in orbit have been correlated with ocean sediments that record the history of glacial stages. The cyclical nature of warming and cooling correspond well with the estimated dates of glacial and interglacial periods. In addition to the change in orbit, the Earth "wobbles" on its axis which alters the amount of insolation reaching the surface of the Earth. [For more about the causes and stages of glaciation in earth history see: "Why were there four long, generally cool periods during which continent-sized glaciers advanced and retreated?" from the Illinois State Museum.]

Figure GS.1 Snowball Earth
Courtesy National Geographic

Previous | Continue