Page 572 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

536 part III The earth–atmosphere interface
F cus Study 17.1 Natural Hazards Snow Avalanches
a skier moves swiftly down a steep, snowy slope. Suddenly the snowpack shatters like a pane of glass and breaks apart in a snow avalanche, capturing the skier and sending thousands of metric tonnes of snow cascading down the mountain side (Figure 17.1.1 and the opening photo of Chapter 1). Snow avalanches move with forces equal to or exceeding those in tornadoes and
hurricanes. an avalanche at rogers Pass, British Columbia, in 1910 killed 58 men clearing the Canadian Pacific railway line. With the high frequency and danger of avalanches in rogers Pass, this helped lead to the decision to excavate the 8-km long Connaught Tunnel through Mount Macdonald. What conditions cause ava- lanches and how do people trigger them?
Snow avalanches are defined as the sudden release and movement of mas- sive amounts of snow down a mountain slope. a snow avalanche is a type of mass-wasting event in that the snow moves under the influence of gravity and may carry other materials picked up as it moves downslope. However, these natural hazards are associated with snow rather than soil or rock, and they are understood by examining snow characteristics.
The Role of Terrain, Snowpack, and Weather
avalanches happen under specific conditions related to terrain, snowpack conditions, and weather. avalanche
terrain consists of mountain slopes that
are steeper than 30 degrees and mostly unforested. On these slopes the mountain snowpack accumulates in layers that reflect differences in the atmospheric conditions, temperatures, and wind conditions associ- ated with each storm. Once deposited, these layers change constantly as snow metamorphism processes differentiate
them further. The snowpack normally consists of both stronger and weaker layers; when a stronger layer, called a slab, overlies a weaker layer, avalanches are possible.
Weather, especially new snowfall, also has an important role in avalanches. The weight of new snow adds considerable stress to the snowpack, increasing the probability of avalanches. Wind, too, is important in avalanche formation, be- cause it is capable of transporting huge volumes of snow from the windward sides of ridges and gullies onto the lee slopes, where the added weight increases the avalanche danger.
in certain areas—where terrain, snowpack, and weather conditions are right—snow avalanches occur repeat- edly. in these mountain regions, ava- lanche paths are visible features of the landscape (Figure 17.1.2). On forested slopes, trees are sometimes completely cleared from these paths; continued ava- lanche activity prevents new trees from establishing.
Avalanche Triggers
Once conditions are favourable for ava- lanche release, the only missing ingredient is a trigger. Triggers can be natural, such as the additional load from new snow or the weight of a falling cornice (an over- hanging ledge of snow formed by wind, usually at the top of a mountain ridge), or human, such as a skier. a human trigger often becomes an avalanche victim.
▲Figure 17.1.1 Skier triggering an avalanche in the Wasatch Range, Utah. [Lee Cohen/Corbis.]
occurred for extended periods at lower latitudes than they do today, allowing snow to accumulate and persist year after year.
Alpine Glaciers
With few exceptions, a glacier in a mountain range is an alpine glacier, or mountain glacier. The name comes from the Alps of central Europe, where such glaciers abound. Alpine glaciers have several subtypes. Val- ley glaciers are masses of ice confined within a valley originally formed by stream action. These glaciers range in length from as little as 100 m to more than 100 km. How many valley glaciers do you see joining the main glacier in Figure 17.1a? Figure 17.3 shows a valley gla- cier in the Tien Shan in central Asia, one of the largest
continuous mountain ranges in the world. The two high- est peaks in the central part of this range, both shown in the photo, are Xuelian Feng at 6527 m and Peak 6231, aptly named at 6231 m above sea level.
A glacier that forms within the snow filling a cirque, or bowl-shaped recess at the head of a valley, is a cirque glacier. Several cirque glaciers may jointly feed a valley glacier (Figure 17.3). A piedmont glacier is formed wher- ever several valley glaciers pour out of their confining valleys and coalesce at the base of a mountain range. A piedmont glacier spreads freely over the lowlands, as demonstrated by the remnants of the Malaspina Glacier, which flows into Yakutat Bay, Alaska.
As a valley glacier flows slowly downhill, it erodes the mountains, canyons, and river valleys beneath its mass, transporting material within or along its base.