Glaciers and Ice Ages

About this topic

Glaciers are large masses of ice that flow slowly under their own weight, formed where snow accumulates faster than it melts. They carve U-shaped valleys, cirques, fjords and moraines. Ice sheets cover Antarctica and Greenland, holding enough ice to raise sea levels by many metres.

Ice ages are periods of cooling with advancing ice; the last glacial period ended about 11,700 years ago. Milankovitch cycles in Earth's orbit trigger these changes. Ice cores preserve ancient air bubbles, recording past climates. Today glaciers are retreating.

During the last glacial maximum about 20,000 years ago, ice sheets up to 3 km thick covered much of Canada, northern Europe and the northern US, and sea level was about 120 m lower, so you could have walked from Britain to France. The ice's weight pushed the land down, and Scandinavia and Canada are still slowly rising back up, called isostatic rebound. Ice flows by internal deformation and by sliding on meltwater at the base; Alpine glaciers move metres per year, while some outlet glaciers in Greenland move kilometres a year.

Glacier size depends on the balance between accumulation in the upper zone and ablation (melting) in the lower. Moving rock fragments act as sandpaper, creating striations and carving valleys, and erratics are boulders carried far from home. Glaciers also store fresh water that feeds rivers in dry seasons for hundreds of millions of people in Asia and South America. Ice cores from Antarctica reach back about 800,000 years and show that CO2 and temperature go together. A common misconception is that melting sea ice raises sea level; floating ice displaces its own weight, so it is melting land ice that raises it. Another is that ice ages are single events; there have been many cycles.