A glacier is a large, long-lasting mass of ice and snow that moves very slowly over the land like a slow-moving frozen river. A glacier is formed as layers upon layers of snow are compacted. As new snow falls, older layers are compressed into dense ice.

Glacial ice is made of frozen fresh water (not salt water); glaciers contain the largest reservoir of fresh water on Earth. Glaciers contain about 75% of the fresh water on earth and cover about 10% of the land. During the last Ice Age (which ended about 10,000 years ago), glaciers covered almost a third of the land.

The word glacier comes from the Latin word for ice, "glacies." The study of glaciers is called glaciology; a scientist who studies glaciers is called a glaciologist.

Where Glaciers Are Located
Glaciers are located in cold areas that get snow in the winter and have relatively cool summers. Most glaciers are located near the North and South Poles or are high in the mountains. Glaciers can be found on every continent on Earth except Australia (but there are glaciers on other islands of Oceania). Glaciers are also found on some other planets, including dry-ice glaciers on Mars.

How Glaciers Form and Shrink
Glaciers form when snow falls and accumulates in cold areas in which the snow does not all melt in the summer. A glacier will grow in mass when winter snowfall is greater than summer snowmelt. A glacier will shrink in mass when winter snowfall is less than summer snowmelt.

As snow accumulates in a glacier it becomes heavier, and the snow is compressed under the pressure, turning into ice. The top of a glacier is snow, but deeper layers are made of snow that has been compressed into ice. The bottom of a glacier is a thin layer of water (from glacial melting or from water that seeps through cracks in the glacier).

The mass of glacial ice combined with the slope of the Earth under it cause the ice to inch its way downwards or outwards, sliding on the thin layer of water. Mountain glaciers flow downhill and continental glaciers flow outwards. As a moving mass of ice and snow, the ice mass is classified as a glacier.

When one parts of a glacier moves more rapidly than other parts, that area becomes relatively unstable, and deep cracks or chasms (called crevasses) form in the ice.

Compressed glacial ice looks bluish in color (glacial ice that looks white is full of bubbles).

Types of Glaciers
Two of the main types of glaciers are ice sheets (also called continental glaciers) and Alpine glaciers (also called mountain glaciers).

Ice sheets are huge masses of glacial ice and snow that cover large areas of land; they are dome-shaped. Ice sheets cover much of Antarctica and Greenland.

Alpine glaciers are glaciers that form on mountains; these high-altitude glaciers exist all over the world.

Glaciers and Erosion
Erosion is the wearing down of an object by friction or abrasion. Glaciers cause erosion by crushing and scraping along the soil and rock they sit on. The mass of snow and ice often pluck up rocks and boulders and deposit them downstream. Glacial till is assorted sediment, including soil, rock, and boulders that are moved around by glaciers. Also, grooves and striations are often made in underlying rock by the massive moving ice.

Many geological formations are created by glacial movement, including fjords (long, deep inlets with steep sides), U-shaped glacial valleys (V-shaped valleys are rounded by a glacier), cirques (bowl-shaped depressions formed at the beginning or end of a glacier -- a cirque has high walls except for the downhill portion where the glacier flowed), and many others.

Icebergs
Calving is the process in which large chunks of ice break off a glacier. Icebergs are huge chunks of floating fresh-water ice that break off a glacier or ice shelf and float out to sea. Almost 90% of an iceberg is hidden beneath the water.

Glaciers and the Future
As the Earth's temperature warms, most of the Earth's mountain glaciers have been decreasing in size; ice sheets in Greenland are also decreasing in size. The ice sheets in Antarctica are changing less dramatically.

Glaciers on Mars
Glaciers cover much of the polar regions (the ice caps) of Mars. The glaciers on Mars are made of frozen carbon dioxide, CO₂ -- dry ice (and not water, H₂O).

Glacier Glossary

ablation - the loss of snow or ice from a glacier from melting, evaporation (sublimation), or calving. ablation zone (also called zone of wastage) - the part of a glacier in which there is net snow loss (through melting, calving, etc.) Alpine glacier (also called mountain glacier) - a glacier that forms on mountains calving - the process in which large chunks of ice break off a glacier cirque - a bowl-shaped depression formed at the beginning or end of a glacier -- a cirque has high walls except for the downhill portion where the glacier once flowed downwards continental glacier (also called continental ice sheet) - a dome-shaped glacier that covers relatively flat land, for example, the Antarctic and Greenland glaciers. These are the largest glaciers. crevasse - a deep crack or chasm in a glacier. Crevasses usually form in areas where ice flow is rapid (the fracture zone). erosion - the wearing down of an object by friction or abrasion. A glacier causes erosion by crushing and scraping along the soil and rock it sits on, often plucking up boulders and depositing them downstream. Grooves and striations are often made in underlying rock. firn - older snow on the upper portion of a glacier that is partially compacted fjord - a long, deep inlet with steep sides, formed by glacial movement glacial till - assorted sediment, including soil, rock, and boulders that are moved around by glaciers. glaciation - the process or condition in which glaciers form and cover the land glacier - a large, long-lasting mass of ice and snow that moves very slowly over the land like a slow-moving frozen river glacieret - a small glacier glaciologist - a scientist who studies glaciers glaciology - the study of glaciers iceberg - a floating mass of freshwater ice formed from ice that broke off a glacier (or ice shelf). Up to 90% of an iceberg is hidden beneath the water. icefall - the fast-moving part of glaciers that have many crevasses. The ice in an average icefall moves at a speed of up to a few hundred meters each year (some move more quickly). ice sheet - a huge mass of glacial ice and snow that covers a large area of land; it is dome-shaped (for example, the Antarctic and Greenland ice sheets) ice shelf - the part of an ice sheet that extends over the sea and floats in the water moraine - linear accumulations of rocks and sediment that are deposited at or near the edges of a glacier. Lateral moraines form at the edges of glaciers; medial (middle) moraines form when two glaciers (going in roughly the same direction) merge producing moraines near the middle of the glacier. mountain glacier (also called Alpine glacier) - a glacier that forms on a mountain - generally at high altitudes névé - partially compacted first-year snow on the upper portion of a glacier ogive - bands of light and dark material on a glacier snout (also called the terminus) - the end of a glacier terminus (also called the snout) - the end of a glacier tidewater glacier - a mountain glacier that ends at the ocean till - assorted sediment, including soil, rock, and boulders that are moved around by glaciers. U-shaped glacial valley - A type of valley with a cross-section that is shaped like the letter U. It is formed when a V-shaped valley's base is rounded by a glacier. valley glacier - a glacier that forms in a mountain valley (a type of Alpine glacier) zone of accumulation - the part of a glacier in which snow accumulates zone of wastage (also called ablation zone) - the part of a glacier in which there is net snow loss (through melting, calving, etc.)

Activities and Worksheets to Print

Glacier Cloze Worksheet Fill in the Blanks A printable fill-in-the-blanks worksheet about glaciers. Use words from a word bank to complete the passage about glaciers. Or go to the answers. Or go to the pdf version of the worksheet and the answers (subscribers only).	Glacier Matching Worksheet Match Each Word to Its Definition Match each glacier-related word to its definition (printable worksheet). Words: glacier, glaciology, glaciologist, Alpine glacier, ice sheet, crevasse, terminus, calving, iceberg, erosion. Or go to the answers. Or go to a pdf of these pages (subscribers only)	Write Eight Words Related to Glaciers Think of and write eight words related to glaciers. Then, for each word, write a sentence containing the word. Sample answers: snow, ice, crevasse, till, glaciologist, glaciology, calving, iceberg.
Write Glacier-Related Definitions In this worksheet, write the definition of a word, what part of speech it is, and use it in a sentence. Words: glacier, glaciologist, glaciology, crevasse, Alpine glacier, ice sheet, erosion, fjord, calving, iceberg. Or go to the answers. Or go to a pdf of the questions and answers (subscribers only).	Glacier Quiz A Worksheet to Print A short, printable worksheet quiz about glaciers. The short-answer worksheet asks 15 general questions about glaciers, for example, "The study of glaciers is ______." Select the answers from a word bank. Or go to the answers. Or go to a pdf file of both.	Write Ten Things About Glaciers A one-page printable worksheet. Write ten things about glaciers (plus one thing you would like to change about them).

References
All About Glaciers, from the National Snow and Ice Data Center.

Repeat photographs of Alaska Glaciers, from the National Snow and Ice Data Center.

Impact of Climate Warming on Polar Ice Sheets Confirmed, from Goddard Space Flight Center, NASA.

Ice - Glaciers, from National Oceanographic and Atmospheric Administration (NOAA).