Glacier

I

Introduction

Glacier, an enduring accumulation of ice, snow, water, rock, and sediment that moves under the influence of gravity. Glaciers form where the temperature is low enough to allow falling snow to accumulate and slowly transform into ice. This accumulation is most common in the polar regions, but can also occur at high altitudes on mountains even near the equator. Glaciers are complex systems that grow and shrink in response to climate. At the present, glacier ice covers about 15 million sq km (5.8 million sq mi), or 10 percent, of Earth’s land area.

Glaciers occur on all continents except Australia. Antarctica and the North American island of Greenland have the largest continuous ice masses. Ice covers their land areas almost completely. About 80 percent of the fresh water on Earth is frozen in ice sheets and glaciers. If all of this ice melted, sea level would rise by about 60 m (200 ft). Much of the world’s population resides in coastal areas that would be underwater. Glaciers are an intriguing part of Earth’s natural environment and their majestic beauty in wild and inaccessible mountain settings is unparalleled.

II

Types of Glaciers

Glaciers occur in many different forms and locations, from the big ice sheet that covers the entire continent of Antarctica to the small valley glaciers that are present in many parts of the world. They are generally divided into several categories depending on their size and location. Glaciers categorized by size include ice fields, ice caps, and ice sheets. Glaciers categorized by location include alpine, valley, and piedmont glaciers.

A

Ice Sheets, Ice Caps, and Ice Fields

Ice sheets are the largest ice masses found on Earth, covering huge land areas. The ice sheet in Antarctica covers 13 million sq km (5 million sq mi). It is over 4 km (14,000 ft) thick and its weight has depressed the continent below sea level in many places. If this weight were removed, the continent would slowly rise and readjust itself, as Europe still does after the melting of the ice sheet that covered that continent during the last ice age. Antarctica’s ice sheet and the similar but smaller ice sheet that covers Greenland both flow slowly downslope. Embedded in these flows are fast outlet glaciers that break up when they reach the ocean, forming large icebergs. The largest outlet glacier, the Lambert Glacier in Antarctica, is 40 km (25 mi) wide and 400 km (250 mi) long, draining one million sq km (about 400,000 sq mi) of eastern Antarctica. The iceberg that sank the Titanic originated with an outlet glacier in Greenland.

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Ice caps are smaller than ice sheets. They form when snow and ice fill a basin or cover a plateau to a considerable depth. There are many ice caps in the Canadian Arctic, Alaska, and other heavily glaciated areas. When these ice caps become thick enough, tongues of ice overflow the basins and discharge ice through valley glaciers.

Ice fields develop where large interconnecting valley glaciers are separated by mountain peaks and ridges that project through the ice. These exposed rock features in the surrounding ice are called nunataks, an Inuit word. Ice fields are common in Alaska, where they occupy up to 4,000 sq km (1,500 sq mi) each.

B

Valley, Alpine, and Piedmont Glaciers

Valley glaciers flow from ice caps or originate in high mountain basins where snow accumulates. They can erode the landscape very effectively, producing U-shaped valleys in their descent. These U-shaped valleys are a common feature in many landscapes that were once glaciated and that remain after the glacier is gone. Some of these valley glaciers can be up to 1,000 m (3,000 ft) thick and 160 km (100 mi) long, but most extend only a few miles.

Alpine glaciers are also sometimes called mountain or cirque glaciers. They are located high up in the mountains and tend to be smaller than valley glaciers. A cirque is a rounded bowl-shaped depression in which snow accumulates easily. Ice flows out over a lip in the cirque and often cascades down into valley glaciers through icefalls or terminates halfway in steep hanging glaciers.

Piedmont glaciers are broad lobe-shaped ice masses that form when one or more valley glaciers flow from a confined valley and spread over low-lying slopes below the mountains. There are two good examples of piedmont glaciers in Alaska, the Malaspina and Bering glaciers. Each covers more than 2,000 sq km (800 sq mi). The Malaspina glacier is larger than the state of Rhode Island.

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