Glaciers

While the Greenland and Antarctic ice sheets are enormous, they are only a fraction the size of the kilometers-deep ice sheets that have covered large portions of the Earth during extensive periods of glaciation, such as during an Ice Age. Geologists assume that glaciers have expanded to mammoth proportions at least six times over the past 960 million years, sweeping slowly down from the polar regions every 250 million years or so and persisting, usually, for 5–10 million years.

Hubbard Glacier calving. © Mark Newman/Phototake NYC. Reproduced by permission.

Most glaciers that exist today are remnants of the last glacial period, which lasted from 1.8 million to 11,000 years ago and which occurred in four periods of advance and retreat. At their maximum, the glaciers of this period covered 30–of the Earth's land surface, particularly in the Northern Hemisphere. As the glaciers advanced, they lowered sea levels by hundreds of feet, creating land bridges between continents. This is the most likely explanation for how humans reached North America from Asia—that is, glaciers probably crossed over via land that was exposed between Asia and Alaska.

As a glacier advances it grinds up the land beneath it, scooping up rocks and soil. These add to the glacier's weight and abrasive power; V-shaped valleys can be altered to U-shaped valleys, and mountains can go from peaky to rounded. As they melt, this burden of rock, gravel, and dirt is dropped in place. This material is termed glacial till. Glacial till, which accumulates preferentially along the leading edges of the advancing glacier, is deposited in huge mounds along glacier's edge when it ceases to advance and begins to melt, creating new hills, or moraines. Formerly placated areas are covered by 200 –1,200 ft (61–366 m) of till that was carried and dropped by glaciers. Chunks of ice buried in this till create large depressions that later become lakes called "kettle lakes." Glaciers also scour the land to great depths, creating larger lakes such as the Great Lakes of North America.

During the last ice age, much of the Earth's surface was depressed due to the weight of the glaciers. As the glaciers retreated, the crust began to rise. This crustal rebounding, as it is called, is still occurring at in parts of North America and Europe.

Glaciers advance relatively slowly, moving anywhere from a few centimeters per year to a few meters per day. When ice melts under the glacier as a result of pressure from above and friction with the ground, accumulated meltwater may act like a lubricant to increase the glacier's rate of flow; this sudden increase in speed is termed a surge