An icefall is a portion of some glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

s characterized by rapid flow and a chaotic crevassed surface. Perhaps the most conspicuous consequence of glacier flow, icefalls occur where the glacier bed steepens and/or narrows. The term icefall is formed by analogy with the word waterfall, a similar, but much higher speed, flow phenomenon.

Most glacier ice flows at speeds of a few hundred metres per year or less. However, the flow of ice in an icefall may be measured in kilometres per year. Such rapid flow cannot be accommodated by plastic deformation of the ice. Instead, the ice fractures forming crevasses. Intersecting fractures form ice columns or serac

Serac

A serac is a block or column of ice formed by intersecting crevasses on a glacier. Often house-sized or larger, they are dangerous to mountaineers since they may topple with little warning...

s. These processes are imperceptible for the most part, however, a serac may collapse or topple abruptly and without warning. This behavior often poses the biggest risk to mountaineers

Mountaineering

Mountaineering or mountain climbing is the sport, hobby or profession of hiking, skiing, and climbing mountains. While mountaineering began as attempts to reach the highest point of unclimbed mountains it has branched into specialisations that address different aspects of the mountain and consists...

 climbing in an icefall.

Below the icefall, the glacier bed flattens and/or widens and the ice flow slows. Crevasses close and the glacier surface becomes much smoother and easier to traverse.

Icefalls vary greatly in height. The Roosevelt Glacier icefall, on the north face of Mount Baker

Mount Baker

Mount Baker , also known as Koma Kulshan or simply Kulshan, is an active glaciated andesitic stratovolcano in the Cascade Volcanic Arc and the North Cascades of Washington State in the United States. It is the second-most active volcano in the range after Mount Saint Helens...

 (Cascade Range

Cascade Range

The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...

, U.S.), is about 730 metres (2,400 ft) high (photograph at right). The ice cliff of the left side of the ice fall and above the debris covering the glacier is 20 to 40 metres (70 to 140 ft) high. Typical of mountain glaciers, this icefall forms as the ice flows from a high elevation plateau or basin accumulation zone to a lower valley ablation zone. Much larger icefalls may be found in the outlet glaciers of continental ice sheet

Ice sheet

An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...

s. The icefall feeding the Lambert Glacier in Antarctica (photograph at left) is 7 kilometres (4.4 mi) wide and 14 kilometres (9 mi) long, even though the elevation difference is only 400 metres (1,300 ft), a little more than half that of the Roosevelt Glacier icefall.

Icefalls are climbed

Ice climbing

Ice climbing, as the term indicates, is the activity of ascending inclined ice formations. Usually, ice climbing refers to roped and protected climbing of features such as icefalls, frozen waterfalls, and cliffs and rock slabs covered with ice refrozen from flows of water. For the purposes of...

 because of their beauty and the challenge they pose. In some cases, an icefall may provide the only feasible or the easiest route up one face of a mountain. An example is the Khumbu Icefall

Khumbu Icefall

The Khumbu Icefall is an icefall at the head of the Khumbu Glacier.The icefall is found at on the Nepali slopes of Mount Everest not far above Base Camp and southwest of the summit. The icefall is regarded as one of the most dangerous stages of the South Col route to Everest's summit...

 on the Nepal

Nepal

Nepal , officially the Federal Democratic Republic of Nepal, is a landlocked sovereign state located in South Asia. It is located in the Himalayas and bordered to the north by the People's Republic of China, and to the south, east, and west by the Republic of India...

ese side of Mount Everest

Mount Everest

Mount Everest is the world's highest mountain, with a peak at above sea level. It is located in the Mahalangur section of the Himalayas. The international boundary runs across the precise summit point...

, variously described as "treacherous" and "dangerous." It is about 5,500 metres (18,000 ft) above sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

.