Glaciers

High in the mountains, it is very cold.

Snow falls and does not melt away. Year after year it piles up and turns into ice. The ice slowly slides down the mountain. That is a glacier!

A glacier is a huge mass of ice and snow that slowly flows over land. The National Snow and Ice Data Center (NSIDC) describes mountain glaciers, called alpine glaciers, as frozen rivers of ice, flowing very slowly down mountainsides and into valleys under their own weight.

The biggest glaciers of all are ice sheets. There are only two on Earth today, covering most of Greenland and Antarctica. Together, NSIDC says, they hold more than 70 percent of the frozen fresh water on Earth.

NSIDC defines a glacier as an accumulation of ice and snow that slowly flows over land. Alpine (valley) glaciers flow down mountainsides under their own weight; ice caps, covering less than 50,000 km², spread out in domes in high-latitude mountain regions; ice sheets, masses of glacial land ice larger than 50,000 km², exist today only on Greenland and Antarctica and spread in broad domes in all directions.

The two ice sheets together contain more than 70 percent of Earth's freshwater ice. Beyond them, NSIDC counts more than 200,000 separate glaciers, which would raise sea level by just under half a meter if they all melted. The largest single glacier by area, NSIDC says, is the Seller Glacier on the Antarctic Peninsula, at over 7,000 km².

Fluffy snow falls on the mountain.

More snow falls on top. The snow gets squished. Little by little, it turns into hard, blue ice.

A glacier starts with snow that does not melt in summer. Each year, new snow falls on top and squeezes the old layers below. NSIDC explains that the snowflakes slowly change into round grains about the size and shape of sugar grains.

After about a year, the snow turns into firn, something in between snow and ice. Over many more years, the grains grow bigger and the air between them is squeezed out, until the firn becomes solid glacier ice. For most glaciers this takes more than a hundred years. Very old, dense glacier ice can look blue, because it has so few air bubbles.

Glaciers begin where snow persists from year to year. Burial compresses each layer, forcing the snow to recrystallize into grains like sugar; the grains grow, air spaces shrink and density rises. After about a year the snow becomes firn, roughly two-thirds as dense as water. Continued compression produces glacier ice whose crystals can grow to several centimeters, and in very old ice to the size of a fist (NSIDC). For most glaciers, the transformation takes more than a hundred years.

Deep in the ice, at depths of hundreds of meters, the remaining air is squeezed into the ice's crystal structure, so dense glacier ice can lack visible bubbles yet still contain trapped air. Such ice looks blue: ice absorbs a little red light, and in bubble-free ice light travels far enough before scattering back out for the absorption to show (NSIDC).

Ice seems hard. But a glacier is so heavy that it slowly flows.

It creeps downhill, too slowly to see. Sometimes it slides on a little water underneath.

How can solid ice flow? A glacier is so thick and heavy that its own weight squeezes the ice and makes it slowly bend and creep downhill. NSIDC explains that ice is soft compared with rock, so it can be pushed into new shapes.

Glaciers also slide. A thin layer of water underneath, from ice melted by the weight above or from water trickling down through cracks, lets the glacier slip over the ground. The top of a glacier moves faster than the bottom, which rubs against the rock. Where the ice bends or speeds up, it cracks open into deep cracks called crevasses.

Glaciers move by internal deformation and by sliding at the base (NSIDC). The weight of overlying snow, firn and ice, with pressure from ice upstream and downstream, deforms the ice in a process called creep. Basal sliding occurs over a thin water film, from pressure melting or from meltwater reaching the bed through cracks, or over soft, wet sediment; it may account for most of the motion of thin, cold glaciers on steep slopes but only 10 to 20 percent for warm, thick glaciers on gentle slopes, though Alaska's Columbia Glacier moves mostly by sliding.

Friction makes the base slower than the surface. Where flow accelerates over obstacles, or drags along the margins, stresses exceed the ice's strength and it fractures into crevasses. Some glaciers surge: in 1986 the Hubbard Glacier in Alaska advanced 10 m a day across the mouth of Russell Fjord and dammed it into a lake within two months.

A glacier is like a giant, slow bulldozer.

Rocks stuck in the ice scrape the ground. The glacier carves a wide valley. When the ice melts, it leaves piles of rocks behind.

As a glacier flows, it picks up rocks and drags them along its bottom. The British Geological Survey (BGS) explains that the huge weight of the ice and the rock stuck in it scrape and wear away the ground underneath. Mountain glaciers can carve out whole valleys.

Glaciers can change the shape of a valley. The U.S. National Park Service (NPS) says glaciers in the Teton Range widened narrow, V-shaped river canyons into wide, U-shaped ones and sharpened the peaks. When a glacier melts back, it dumps its rocks in ridges called moraines, and can leave giant boulders far from where they started.

Glaciers are powerful agents of erosion: continental ice sheets erode large parts of continents and alpine glaciers entire valleys, as the weight of ice and embedded debris abrades the bedrock (BGS). NSIDC notes that glaciers plow up or push aside rock and debris, and that the ground beneath a large retreating glacier is typically scoured, leaving scars and debris on the bedrock.

In the Tetons, the NPS describes Ice Age glaciers widening V-shaped river canyons into U-shaped glacial canyons, gouging out basins now filled by lakes and sharpening the high peaks; the most recent advance lasted from about 50,000 to 12,000 years ago. The debris is deposited as till, an unsorted mix of clay to boulders, forming moraines, and boulders carried far from their source are left as glacial erratics (BGS).

Today, ice covers about one out of every ten parts of Earth's land.

Long ago, in the Ice Age, there was much more. Ice covered big parts of North America and Scandinavia.

NSIDC says glacial ice covers about 10 percent of Earth's land today, more than 15 million square kilometers. That includes mountain glaciers, ice caps and the giant ice sheets of Greenland and Antarctica.

It was not always like this. At the height of the last ice age, which ended about 12,000 years ago, glaciers covered about 32 percent of the land, and ice sheets spread across much of North America and Scandinavia. In the United States today, most glacier ice is in Alaska.

Glacial ice covers about 10 percent of Earth's land, more than 15 million km² (NSIDC). At the maximum of the last ice age, which ended about 12,000 years ago, glaciers covered about 32 percent of the land, with ice sheets over much of North America and Scandinavia. Antarctica has been at least partly covered by an ice sheet for the past 40 million years, and NSIDC describes a Little Ice Age, from about the early fourteenth to the mid-nineteenth century, when glaciers advanced in many regions.

NSIDC gives the Greenland Ice Sheet about 1.7 million km² and the Antarctic Ice Sheet almost 14 million km² holding 30 million km³ of ice; elsewhere the same page gives 12.3 million km² for Antarctica, a reminder that published figures differ. In the United States, glaciers cover over 90,000 km², 87,000 km² of it in Alaska.

When more snow falls than melts, a glacier grows.

When more ice melts than snow falls, it shrinks. Many glaciers are getting smaller today.

A glacier is like a bank account of ice. Snow is added high up, in the accumulation zone. Ice melts away lower down, in the ablation zone. If more snow is added than melts, the glacier grows forward. If more melts, its front pulls back. NSIDC points out that even a shrinking glacier still flows downhill, like a conveyor belt: it just melts faster than it moves.

Over the past 60 to 100 years, NSIDC says, glaciers around the world have mostly been shrinking. In Alaska, Muir Glacier pulled back about 12 kilometers between 1941 and 2004 and became more than 800 meters thinner.

Mass balance governs a glacier: snow accumulates in the higher accumulation area, melt and evaporation dominate the lower ablation area, and where they balance the glacier is in equilibrium. When the balance shifts, the terminus advances or retreats, but the ice keeps flowing downslope; a retreating glacier simply melts faster than it flows (NSIDC).

NSIDC reports that glaciers worldwide have tended to retreat over the past 60 to 100 years, with tidewater and small alpine glaciers especially sensitive. NSIDC cites the American Meteorological Society's State of the Climate in 2021, which reported alpine glaciers losing mass for a 34th consecutive year, with average loss per decade rising from 214 mm water equivalent in the 1980s to 896 mm in the 2010s. Muir Glacier in Alaska retreated 12 km and thinned by more than 800 m between 1941 and 2004.