Glaciers are large masses of moving ice. Glaciers form by the accumulation of snow over tens, hundreds, and even thousands of years. Glaciers grow in cold places where more ice forms than melts each year, namely, close to the north and south poles and at high elevations (near the summits of tall mountains.)
Today, ice covers about one tenth of Earth’s surface. Huge dome-shaped masses of glacial ice, called continental glaciers or ice sheets, cover the arctic island of Greenland and the most of the continent of Antarctica at the South Pole. Mountain (alpine) glaciers flow down valleys in the Himalayas, Andes, Alps and other major mountain ranges.
Glacial ice affects Earth’s climate, drives ocean currents (a moving mass of water that may also differ from surrounding water in properties such as temperature or amount of salt (salinity), and determines global sea level. (When more water is bound up in ice on land, sea level falls.
When glaciers melt, sea level rises.) Glaciers sculpt the land beneath them as they advance and leave behind distinctive sedimentary deposits (fine soil or mineral particles) and and forms as they retreat.
How glaciers form
Glaciers form in places where more snow accumulates than melts each year, typically in climates with cold, snowy winters and cool summers. Permanent ice is common at high latitudes (imaginary lines on Earth that tell how far a place is from the equator) because the Sun’s rays are less direct near the poles, and at high elevations, because air cools as it rises.
Mountain glaciers grow above the snow line where it is cold enough for some snow to remain on the ground all year. The elevation of the snow line is very high near the equator, and at sea level near the poles.
Glaciers cover the peaks of only few very tall mountains near the equator, including Cotopaxi in South America and Kilimanjaro in Africa, while ice flows directly into the ocean in Alaska, Siberia, Norway, and Antarctica.
Cold regions are not always snowy. Many dry, wind-blown polar areas and high mountain peaks do not have glaciers because they do not receive enough snow to outpace the melting during warm spells and wind erosion.
Alpine glaciers typically form on the windward side of a mountain (the side of the mountain exposed to the wind) where where moisture-bearing winds drop heavy snows on mountain ranges.
Antarctica is a frozen desert (an area with the equivalent of less than 10 inches of rain a year or about 120 inches of snow if only snow falls) that only receives a few inches of snow each year. It has remained ice-covered only because temperatures are so frigid that snow almost never melts.
Once snow accumulates, it takes several years for thick, fluffy layers of loosely-packed snowflakes to compact into thin bands of dense glacial ice. The weight of the snow on the surface packs the snow laying underneath.
How glaciers move
Glacial ice moves downhill when it has grown too thick to resist the pull of gravity (force of attraction between two objects). The steeper the slope or the thicker the ice, the faster the glacier flows. A dome of very thick ice on flat ground, like the Greenland or Antarctic ice sheet, will spread out under its own weight like pancake batter spreading on a griddle.
Glacial movement is very slow compared to other forms of moving water. (When someone says, “You are moving like a glacier!” they are usually asking you to move faster.) Some glaciers, however, are slower than others.
Some glaciers cans move several hundred feet a year, others may advance or retreat on a few inches. Most glaciers flow by a combination of faster sliding and slower internal deformation (a process whereby ice shifts within the glacier itself to make the glacier appear to grow (advance) or retreat along the ground).
Like rivers, glaciers have flow patterns within them. Ice that forms from snowfall at the upper reaches of a mountain glacier moves down the valley to the ice front (end of the glacier) where it melts into streams or breaks off (calves) into the ocean.
The rate at which ice flows within the glacier does not affect the position of the ice front. Glaciers grow, or advance, when more snow accumulates than melts. They shrink, or retreat, when rising temperatures or drought (long period of dry weather) cause melting to outpace accumulation.
Friction slows the ice at the edges and bottom of a glacier. The fastest moving ice is along the centerline of the glaciers (an imaginary line through the center of the glacier).
Ice buried deep within the glacier flows around obstacles by stretching and bending, while brittle ice at the surface breaks forming huge cracks called crevasses. Ice can stretch or bend (a process also called internal deformation, due to the weight of ice and snow above or due to melting and refreezing of ice.
Glaciers are massive earth-moving machines. Advancing glacial ice tears up rock and bulldozes it toward the ice front. Mountain glaciers carve through the solid rock that lies under the soil (bedrock) as the glaciers advance and create distinctive features. Called erosional features, these include: U-shaped valleys, hanging valleys, fjords, glacial polish and striations:
- U-shaped valley: Mountain glaciers carve narrow V-shaped stream valleys into wide, flat-bottomed U-shaped valleys. Yosemite Valley in California is a U-shaped glacial valley.
- Hanging valley: Tributary glaciers (smaller glaciers that join the main glacier from side valleys) often enter a main glacier high above the main valley floor. After the glacier melts, streams flowing from the hanging valleys form tall waterfalls.
- Fjord: Unlike rivers, glaciers can erode (wear away) their valleys far below sea level. When a coastal glacier retreats, the sea fills its U-shaped valley forming a narrow, long, steep-sided bay called a fjord. Alaska, Norway, and Argentina have fjords along their rugged glacial coastlines.
- Glacial polish and striations: Sand stuck in the ice polishes the bedrock beneath the glacier, and rocks gouge out long grooves called striations.
Glacial ice is full of sediment, from huge boulders to fine specks of silt. When glaciers melt, they drop their immense load of sediment and meltwater flows from the retreating ice front. Glacial deposits and landforms cover areas that were once glaciated: glacial till, glacial flour, and outwash, moraines, erratics, and kettle ponds.
- Till, flour, and outwash: Melting glaciers deposit thick layers and mounds of unsorted sediment called till. Glacial flour is powder-fine sediment created by crushing and grinding beneath a glacier. Outwash is glacial sediment that has been carried, sorted, and deposited by meltwater streams.
- Moraine: Moraines are long, narrow mounds of glacial till. Moraines form at the ice front (end and terminal moraines), along valley edges (lateral moraines), and down the center of glacial valleys (medial moraines).
- Glacial erratic: Glaciers pluck large boulders from the ground and carry them far from their original locations before dropping them. Glacial erratics from northern Canada are strewn across the American Midwest and New England.
- Kettle pond: Glacial landscapes are dotted with small round ponds called kettle ponds that form where a retreating glacier left blocks of ice buried in its till deposits. When the blocks melt, pond water fills the round holes.