Page 477 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 441
in 1845. On November 13, 1985, at 11 p.m., after a year of earthquakes and harmonic tremors (seismic energy re- leases associated with volcanoes), a growing bulge on its northeast flank, and months of small summit eruptions, Nevado del Ruiz violently erupted in a lateral explosion and triggered a mudflow down its slopes toward the sleeping city and villages below.
The mudflow was a mixture of liquefied mud and volcanic ash that developed as the hot eruption melted ice on the mountain’s snowy peak. This lahar, an Indo- nesian word referring to mudflows of volcanic origin, moved rapidly down the Lagunilla River toward the vil- lages below. The wall of mud was at least 40 m high as it approached Armero, a regional centre with a population of 25 000. The lahar buried the sleeping city: 23 000 peo- ple were killed; thousands were injured; 60000 were left homeless across the region. The debris flow generated by the 1980 eruption of Mount St. Helens was also a lahar.
Landslides are another type of mass movement that poses a major hazard, causing thousands of deaths on average each year on Earth. For more on land- slides, see www.nrcan.gc.ca/hazards/landslides or landslides .usgs.gov. Also, the American Geophysical Union land- slide blog (blogs.agu.org/landslideblog/) has information about recent events worldwide.
mass-movement mechanics
Mass movement, also called mass wasting, is the downslope movement of a body of material made up of soil, sediment, or rock propelled by the force of gravity. Mass movements can occur on land, or they can occur beneath the ocean as submarine landslides.
slope Angle and Forces All mass movements occur on slopes under the influence of gravitational stress. If we pile dry sand on a beach, the grains will flow downslope until equilibrium is achieved. The steep- ness of the resulting slope, called the angle of repose, depends on the size and texture of the grains. This angle represents a balance of the driving force (grav- ity) and resisting force (friction and shear). The angle of repose for various materials ranges between 33° and 37° (from horizontal) and between 30° and 50° for snow avalanche slopes.
As noted, the driving force in mass movement is grav- ity. It works in conjunction with the weight, size, and shape of the surface material; the degree to which the slope is oversteepened (how far it exceeds the angle of repose); and the amount and form of moisture available (frozen or fluid). The greater the slope angle, the more sus- ceptible the surface material is to mass-wasting processes.
The resisting force is the shear strength of the slope material—that is, its cohesiveness and internal friction, which work against gravity and mass wasting. To reduce shear strength is to increase shear stress, which eventu- ally reaches the point at which gravity overcomes fric- tion, initiating slope failure.
Conditions for slope Failure Several conditions can lead to the slope failure that causes mass movement. Failure can occur when a slope becomes saturated by a heavy rainfall; when a slope becomes oversteepened (40° to 60° slope angle), such as when river or ocean waves erode the base; when a volcanic eruption melts snow and ice, as happened on Nevado del Ruiz and Mount St. Helens; or when an earthquake shakes de- bris loose or fractures the rock that stabilizes an over- steepened slope.
Water content is an important factor for slope stabil- ity; an increase in water content may cause rock or rego- lith to begin to flow. Clay surfaces are highly susceptible to hydration (physical swelling in response to the pres- ence of water). When clay surfaces are wet, they deform slowly in the direction of movement; when saturated, they form a viscous fluid that fails easily with overlying weight. The 1995 La Conchita mudslide in California (see Figure 14.24a) occurred during an unusually wet year. The same slope failed again in 2005, after a two-week pe- riod of near-record rainfall.
The shocks and vibrations associated with earth- quakes often cause mass movement, as happened in the Madison River Canyon near West Yellowstone, Montana. Around midnight on August 17, 1959, an M 7.5 earth- quake broke a dolomite (a type of limestone) block along the foot of a deeply weathered and oversteepened slope (white area in Figure 14.23), releasing 32 million m3 of mountainside. The material moved downslope at 95 km · h−1, causing gale-force winds through the canyon. Momentum carried the material more than 120 m up the opposite side of the canyon, trapping several hundred campers with about 80 m of rock and killing 28 people.
The mass of material that dammed the Madi- son River as a result of this event created a new lake, dubbed Quake Lake. To prevent overflow and associ- ated erosion and flooding downstream, the U.S. Army Corps of Engineers excavated a channel through which the lake could drain.
The M 8.0 earthquake in the Sichuan Province of China in 2008 caused thousands of landslides through- out the region, many of which created dams on rivers and earthquake lakes. At the largest of these land- slide dams on the Qianjiang River, channel dredging successfully prevented overtopping and downstream flooding. In 2010 in northern Pakistan, a massive land- slide dammed the Hunza River. In this case, dredging was impossible due to muddy conditions at the site. The dam survived repeated overtoppings; then, in 2012, a spillway was blasted to reduce the water level of the lake.
Classes of mass movements
In any mass movement, gravity pulls on a mass of mate- rial until the critical shear-failure point is reached—a geomorphic threshold. The material then can fall, slide, flow, or creep—the four classes of mass movement.