The Greenland ice sheet is the world’s second largest ice sheet (and second largest glacier) after the Antarctic Ice sheet (see pp. 306–307), covering 660,000 square miles (1.71 million square km) of Greenland. It has an estimated volume of 680,000 cubic miles (2.85 million cubic km), with an average thickness of about 5,480 ft (1,670m) and a maximum thickness of 10,515 ft (3,205m). Some of its ice is thought to be up to 110,000 years old. Shape and movement The ice sheet’s surface is slightly domed, reaching a height of about 10,800ft (3,290m) above sea level at the top of the dome. From here, ice moves slowly down toward the ice sheet’s edges, where it is mostly constrained by coastal mountains; it only reaches the sea along a broad front in a few places. As a result, Greenland (unlike Antarctica) has no ice-shelves.

In many places, the ice flows through gaps in the mountains in the form of outlet glaciers. When these reach the coast, they discharge enormous numbers of icebergs into the sea. One outlet glacier, the Jakobshavn Glacier in western Greenland, is the world’s fastest-flowing glacier. At its sea end, the ice flows at about 3 ft (1 m) per hour. Threats to sea level and freshwater The Greenland ice sheet is melting due to global warming. If it melted completely, the world’s oceans would rise by about 24ft (7.2m), drowning many of the world’s major cities and leading to loss of about 6.5 percent of the world’s freshwater reserves.

The Columbia Glacier

The Columbia Glacier is one of Alaska’s largest glaciers, with an area of about 360 square miles (920 square km) and a length of 25 miles (40 km). It descends from the Chugach Mountains in southern Alaska to Prince William Sound (an inlet of the Gulf of Alaska), where it calves large numbers of icebergs into the sea. Shrinking record The Columbia Glacier is one of the fastest moving in North America, but its fast forward movement is offset by extremely rapid calving of icebergs at its terminus, at the rate of some 14 million tons (13 million tonnes) every day. As a result, since the 1960s, the glacier’s terminus has moved backward, and since 2011 it has become detached from some of its largest tributary glaciers. The retreat cannot be attributed solely to global warming, as other nearby glaciers are not shrinking at the same pace. Instead, experts believe it is connected to the shape of the bedrock channel under the glacier.

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The Kaskawulsh Glacier

Around 400 square miles (1,000 square km) in area, the Kaskawulsh Glacier snakes for some 45 miles (70 km) through the St. Elias Mountains of Canada’s Yukon Territory. At its terminus, or snout, it produces meltwater that maintains the level of one of the Yukon’s largest lakes, Kluane Lake. Bold stripes All glaciers constrained by valleys have dark bands of material running along their sides, called lateral moraines (see panel, p.150). These contain rock fragments that have been plucked off the walls of the valley and are being carried along by the glacier. When two glaciers merge to form a larger glacier, the lateral moraines of the smaller glaciers typically combine to form a bold dark stripe running down the merged glacier. This stripe is called a medial moraine. The lower half of the Kaskawulsh Glacier has several bold medial moraines, formed through a series of minimergers of its different arms, or tributaries, upstream. The glacier has three main arms, each with its own tributaries.

Enormous Malaspina Glacier

The enormous Malaspina Glacier is up to 40 miles (65 km) wide and extends up to 28 miles (45 km) forward from where its ice “root” emerges from between a gap in some mountains near the coast of Alaska. Studies of earthquake waves passing through it have shown that its ice is up to 2,000 ft (600m) thick. Patterned lobe The glacier has complex patterns of light and dark bands on its surface—the dark areas are moraines, containing rock debris. The bands are arranged in zigzags and swirls, and are thought to have been caused by alternating, perhaps seasonal, patterns of fast and slow ice flow. Also on the surface are colonies of tiny creatures called ice worms.

The Black Rapids Glacier

The Black Rapids Glacier flows down a valley in south central Alaska and is one of the world’s more accessible glaciers, because its snout lies only a few miles from a road. It is about 25 miles (40 km) long and has several tributaries. In 2002, an earthquake caused a huge part of it to be buried in rock debris. The Black Rapids is a surge-type glacier, in which periodic rapid transfers of ice occur from the upper to the lower parts, which increase in thickness and then The Black Rapids Glacier An unusual glacier, the largest in the Alaska Range, with a history of phases of exceptionally rapid advance NW. North America surge forward. The Black Rapids Glacier made national news in the U.S. for three months in 1937, when its ice suddenly began moving at 100 ft (30m) a day— more than 20 times its normal rate—and its snout began to rapidly advance down its valley. During this period, it became known as the Galloping Glacier. Since this surge, however, the Black Rapids has steadily retreated to its current length, which is at a historic minimum.

The Kennicott Glacier

The Kennicott Glacier flows down from the southern Wrangell Mountains, its principal source of ice being the flanks of Mount Blackburn—a snow-clad, eroded, extinct volcano and the fifth highest peak in the U.S. The glacier is about 26 miles (42 km) long. Arms and tributaries The upper part of the Kennicott Glacier has two arms, west and east, that flow around a ridge of rock and a nunatak (a small rocky peak poking through the ice) known as Packsaddle Island. After these two arms have joined, the impressively striped main trunk of the glacier flows south, where it merges with two large tributaries—the Gates and Root Glaciers—and later passes an abandoned mining community called Kennecott. It ends in a broad, rock-covered snout, surrounded by shallow meltwater pools and lakes. Several streams emerge from underneath the glacier and merge to form the Kennicott River. The Kennicott Glacier and Root Glacier are accessible from a road built to supply the old mining camp. Guided hikes onto the glaciers provide a chance to view the ice formations, as well as vivid blue pools and streams on the glaciers’ surfaces, or to explore the ice caves underneath them. Crossing the Root Glacier gives access to Donoho Peak. A strenuous scramble up this can give brilliant views of the whole area.

The Mendenhall

The Mendenhall is one of 38 large glaciers that flow down off the Juneau Ice-field. The glacier is about 13 miles (21km) long and ends in an iceberg-filled lake. It has been retreating and shrinking since at least the 1700s, with some phases of rapid retreat in recent years, and it is expected that it will recede out of the lake by about 2020. Exploring the glacier By paddling a canoe across the meltwater lake, it is possible to reach the side of the glacier’s terminus, clamber across its surface, and then explore its ice caves, with their deep blue ceilings and streams of water running over the rocks inside. Wildlife in the area includes beavers, bears, spawning salmon in season, and the occasional wolverine (a carnivore resembling a small bear).

The Yukon

The Yukon River is 1,980 miles (3,185 km) long, making it the third longest river in North America after the Missouri and Mississippi. Eight major rivers flow into the Yukon, and its tributaries drain an area of about 328,000 square miles (850,000 square km)—the fourth largest drainage basin in North America. Epic journey From its mountain source in British Columbia, the Yukon flows slowly northwest through the lowland forests of the Yukon Territory before entering Alaska, where it widens into a vast wetland area called the Yukon Flats. From here, its lower course turns southwest to wind across the entire breadth of the relatively flat central Alaska landscape to its mouth in the Bering Sea. The vast scale of the Yukon and the remoteness of the landscapes that it crosses have led to the romantic mythology of frontier life associated with its name. Once the principal means of transportation for the pioneers of the Klondike Gold Rush at the turn of the 19th century, the river remains an epic watercourse for those with an adventurous spirit.