Antarctica: Ice Sheet, Ice Shelves and Sub-Glacial Lakes

🗻 Antarctica: Ice Sheet, Ice Shelves and Sub-Glacial Lakes

Antarctica is the most extreme continent on Earth, and its physical geography is unlike anywhere else on the planet. Understanding its key features — the ice sheet, ice shelves, katabatic winds, and sub-glacial lakes — is essential for both the physical geography section and later topics on polar change.

The Antarctic Ice Sheet

The Antarctic Ice Sheet is the largest single mass of ice on Earth. It contains approximately 26.5 million km³ of ice — roughly 90% of all the freshwater ice on the planet and approximately 70% of Earth's total fresh water. The average thickness of the ice sheet is about 2.1 kilometres; at its thickest point — above Lake Vostok — it reaches 4.8 kilometres.

The ice sheet has been accumulating for approximately 34 million years, built up from snowfall that compresses into ice under its own weight. At the bottom of the ice sheet, the ice is under enormous pressure and has been there for hundreds of thousands of years — ice cores drilled from Antarctica provide a climate record stretching back 800,000 years, showing evidence of past ice ages, warm periods, and atmospheric composition.

If the entire Antarctic Ice Sheet melted, global sea levels would rise by approximately 58–61 metres. This would submerge most of the world's coastal cities — London, New York, Shanghai, Mumbai, Sydney — under tens of metres of water. Even partial melting of the West Antarctic Ice Sheet (which is less stable than the East Antarctic Ice Sheet) could raise sea levels by 3–4 metres, threatening hundreds of millions of people living in low-lying coastal areas.

Ice Shelves

Where the Antarctic Ice Sheet extends beyond the coastline and floats on the ocean, it forms ice shelves — thick floating platforms of glacial ice attached to the continent. The largest is the Ross Ice Shelf, which covers an area roughly the size of France and is up to 750 metres thick. Ice shelves are formed by glaciers and ice streams flowing off the continent and flowing out over the ocean.

Ice shelves are critical for the stability of the ice sheet. They act as a buttress — holding back the glaciers and ice streams behind them. When ice shelves collapse (as the Larsen B Ice Shelf did spectacularly in 2002, when 3,250 km² disappeared in 35 days), the glaciers behind them can accelerate dramatically, pouring more ice into the ocean and raising sea levels. The collapse of an ice shelf does not itself raise sea levels (the shelf was already floating), but the acceleration of land-based glaciers it causes does.

Katabatic Winds

Antarctica experiences the most extreme winds on Earth: katabatic winds. These form when the elevated ice plateau cools the air immediately above it. Cold air is denser than warm air, so this chilled air sinks and flows rapidly downslope from the high interior plateau towards the coast. By the time it reaches the coast, it has accelerated to speeds of up to 200 km/h — more than a Category 5 hurricane. The word "katabatic" comes from the Greek for "going downhill".

Katabatic winds are a direct result of Antarctica's unique topography: a high, cold, domed interior plateau that chills air dramatically, creating a persistent downslope air flow that intensifies near the coast. They drive blizzards that reduce visibility to zero and make outdoor work impossible for days at a time.

Sub-Glacial Lakes

One of Antarctica's most remarkable physical features is what lies beneath the ice: sub-glacial lakes — bodies of liquid water trapped beneath kilometres of ice. Lake Vostok, beneath 4 kilometres of ice in East Antarctica, is approximately 250 km long, 50 km wide and up to 1,000 metres deep — making it one of the largest lakes in the world by volume, despite never having been seen by human eyes until radar surveys revealed it.

How can water remain liquid beneath 4 km of ice at −60°C surface temperatures? Two mechanisms: the enormous pressure of the overlying ice lowers the melting point of water slightly; and geothermal heat from the Earth's interior warms the base of the ice sheet. Lake Vostok has been isolated from the atmosphere for approximately 15–25 million years. Scientists believe it may harbour unique microorganisms that have evolved in complete isolation — making it one of the most scientifically significant places on Earth.

The Antarctic Circumpolar Current

The Antarctic Circumpolar Current is the largest ocean current on Earth. It flows continuously around Antarctica in a westward direction, driven by powerful westerly winds and unimpeded by any landmass. The current acts as an oceanic barrier, isolating Antarctica's surrounding waters from warmer water to the north. This thermal isolation amplifies Antarctica's cold: without the circumpolar current, warmer water from the Atlantic, Pacific and Indian Oceans would reach Antarctica's coasts and melt coastal ice. The Drake Passage — the 800 km wide gap between South America and Antarctica — is the stormiest sea on Earth, where the full force of the circumpolar current is compressed between two continents.