Glaciers

A glacier is a mass of ice derived from snow that has accumulated and been compacted over a long period of time. About 70 percent of the Earth’s freshwater resource is stored in glacial ice.

How are glaciers formed?

Snowflake

Glaciers are formed from layers of snow converted to ice. When snowflakes fall, they are feathery and light. Due to partial melting and compaction, the accumulated snow becomes rounded ice crystals with interconnecting air spaces. See the snow to ice animation. This is called firn or névé. As firn is compacted further by the weight of the overlying snow, air spaces are reduced and dense glacial ice is produced. This can take many years. The density of the ice formed is about 0.9 mg m-³. See snow and ice density dataset.

Movement of glaciers

Gravity and the weight of accumulating snow and ice moves the glacier downhill. Rates at which glaciers move varies from 20 m per year to 10 km per year. At the terminal end of the glacier, either the ice melts or, if the glacier flows out over the sea or a lake, chunks of ice break off producing icebergs.

Erosion and deposition

Ice is very effective at eroding rock. Melting and refreezing occurs on the underside of the glacier, causing rocks to be frozen into the lower surface of the ice, producing an abrasive surface rather like sandpaper. Striations (parallel scratches) are left in the rocks as the glacier moves over them. Sediment and rocks accumulated by the glacier are deposited a distance away from where they were collected. These deposits are called till. Ancient striations and till have been used as evidence of continental drift.

The past climate is recorded in the ice

Terminal of Miers Glacier

As ice sheets can persist for many thousands of years, the air bubbles trapped in them can provide a record of past atmospheric composition. Dating of ice cores taken from glaciers is done in various ways, including measuring changes in electrical conductivity, recording seasonal changes in the ice, comparison of volcanic ash with known eruptions, comparison with other ice cores, a mathematical technique called isolation and modelling of accumulation and flow rate changes of the ice.

Glaciers on Antarctica

About 30 million km³ of freshwater is locked up in Antarctica’s ice. This is 90 percent of the Earth’s ice. Total Antarctic ice is estimated to cover 13.72 million km² – 98 percent of the total land area. The ice sheet covers mostly land mass but on the western side of the continent, it extends into the sea. It is so cold in central Antarctica (-30 to -80°C) that to form glacial ice from snow takes several thousand years. The oldest ice to have been found was from the EPICA Dome C core and is estimated to be about 950,000 years old. There are many valley glaciers as well as the ice sheet on Antarctica.