Page 478 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

442 part III The earth–Atmosphere Interface
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Notebook
Madison River Landslide
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Landslide scar
Landslide debris
▲Figure 14.23 Madison River landslide. Cross section showing geologic structure of the Madison River Canyon in Montana, where an earth- quake triggered a landslide in 1959. The landslide debris blocked the canyon and dammed the Madison River, visible in the inset photo. [USGS Professional Paper 435-K, August 1959, p. 115. Inset photo by Bobbé Christopherson.]
These classes range in volume of material (small to mas- sive), moisture content (dry to wet), and rate of move- ment (rapid free-falling rock to slow-moving creep). Figure 14.24 displays the specific types of mass move- ment discussed ahead according to the moisture and speed categories.
Rockfalls and Debris Avalanches Rockfalls and debris avalanches are types of mass movement that occur at faster rates and in materials that have little to intermedi- ate water content. A rockfall is simply a volume of rock that falls through the air and hits a surface (Figure 14.8). During a rockfall, individual pieces fall independently and characteristically form cone-shaped piles of irregu- lar broken rocks known as talus cones that coalesce in a talus slope at the base of a steep incline (Figure 14.25 on page 444).
A debris avalanche is a mass of falling and tum- bling rock, debris, and soil travelling at high velocity owing to the presence of ice and water that fluidize the debris. The extreme danger of a debris avalanche results from its tremendous speed. In 1962 and again in 1970, debris avalanches roared down the west face of Nevado Huascarán, the highest peak in the Peruvian Andes. An earthquake initiated the 1970 event in which upward of 100 million m3 of debris travelling at 300 km · h−1 buried the city of Yungay, killing 18000 people (Figure 14.26 on page 444).
Landslides A sudden rapid movement of a cohesive mass of regolith or bedrock that is not saturated with moisture is a landslide—a large amount of material failing simultaneously. Surprise creates the danger, for the downward pull of gravity wins the struggle for equilibrium in an instant. Focus Study 14.1 describes one such surprise event that struck near Frank, Alberta, in 1903.
To eliminate the surprise element, scientists are using the Global Positioning System (GPS) to monitor landslide movement. With GPS, scientists measure slight land shifts in vulnerable areas for clues to an impending danger of mass wasting. At two sites in Japan, GPS effec- tively identified pre-landslide movements of 2–5 cm per year, providing information to expand the area of hazard concern and warning.
Slides occur in one of two basic forms: transla- tional or rotational (see Figure 14.24 for an idealized view of each). Translational slides involve movement along a planar (flat) surface roughly parallel to the angle of the slope, with no rotation. The Madison Can- yon landslide described earlier was a translational slide. Flow and creep patterns also are considered translational in nature.
Another slide event with deadly consequences oc- curred 6 km east of Oso, Washington State, on March 22, 2014. A mudslide with a volume estimated by the USGS to be 8 million m3 caused 43 deaths and destroyed
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