Review Questions and Answers; Mass Wasting

Mass wasting is the downslope movement of soil and weathered rock debris. Streams can deepen valleys by downcutting (erosion), but widening or enlarging of the valley via erosion of the sides or slopes is accomplished largely through mass wasting. Mass wasting delivers the weathered rock material and soil to the streams, which carry it away to some site of deposition.

2. How did the building of a dam contribute to the Vaiont Canyon disaster? Was the disaster avoidable? (Box 9.1.)

The devastating flood (Box 9.1) associated with the Vaiont Canyon slide could have been avoided by not building the dam. The section of the canyon above the dam was recognized as having a high potential for large-scale, mass movements. Weak sedimentary strata, some highly porous and others rich in clays, dip almost parallel to the north-facing, canyon slope. The site was a "textbook" example of where not to build a dam. As the reservoir was filling, strata low on the slopes became saturated and substantially weakened; unfortunately, these strata provided lateral support to inclined strata higher up the slope. For weeks prior to the catastrophic slide (Oct. 9, 1963), downhill creep of the regolith above the reservoir was observed and documented. Creep rates increased drastically from a few centimeters per day to as high as 80 cm/day just before the slide occurred. Such high, accelerating, pre-slide creep rates should have alerted public officials to the danger; warnings should have been issued and the valley below the dam should have been evacuated. Nothing was done!

Why was the dam built in the first place? Pressures from groups with financial interests in seeing the dam built, high level political complicity and inertia in the Italian Government, and shameful, white washing of strongly negative, site evaluation studies all contributed. In retrospect, some culpable public officials were tried, convicted, and sent to prison for their roles in the disaster. Without the reservoir, the dangerous, north-facing slope would have eventually failed "naturally", as had happened many times in the past. However, this future, "natural", slope failure would not have caused a flood; and few, if any, lives would have been lost.

3. What is the controlling force of mass wasting?

The Earth's gravitational force or "gravity" acts continuously on all materials and objects. The component of the gravitational force parallel to the slope acts to move objects and materials downslope; this force is resisted by the strength and cohesion of the material and by the friction associated with any downhill motion. The steeper the slope angle, the larger is the fraction of Earth's gravitational acceleration acting to move materials toward the base of the slope.

4. How does water affect mass wasting processes?

In general, water speeds up ordinarily slow, mass wasting processes and greatly increases the chances that faster moving processes will occur. Water in pores and cracks lowers the internal cohesion of most regolithic materials; and, since the gravitational force is always acting, moist or wet material is more likely to move than dry material. Water in pores and cracks displaces air, so water adds to the mass of soil and broken rock on a slope. If pores and cracks are saturated (filled with water), the pore pressure tends to push the material particles apart, further promoting failure and downslope movements.

5. Describe the significance of the angle of repose.

The angle of repose is the steepest angle that a pile of dry, noncohesive (unconsolidated, not cemented) particles of fine-sand or larger size can sustain before sliding, rolling, and avalanching eliminate the oversteepening. For dry sand, the angle is about 34 degrees. Most accumulations of broken rock, such as talus, contain different sizes of fragments, some of which may be quite large; these often have slightly steeper, repose angles than dry sand (35 to 39 degrees).

6. Distinguish among fall, slide, and flow.

Fall refers to the unimpeded, downslope movement of more or less individual rock fragments and particles. Usually, the first stage of a fall is through air as fragments fall off the face or top of a cliff. After the fragments hit the ground surface, they bounce and roll for some additional distance downslope.

Slide refers to a surface mass of rock or soil that moves downhill more or less intact, along a slip surface or fracture plane.

Flow describes movements of materials that deform or flow internally; thus masses of wet soil and debris move mainly through shearing and flow movements inside the mass. Soupy mud and debris move mainly by viscous flow, although a minor component of basal slip is involved.

7. Why can rock avalanches move at such great speeds?

Rock avalanches are triggered when earthquakes shake loose masses of fractured rock from high on a slope or cliff. The first stage of the avalanche involves free fall. As the fast moving mass nears the ground, air is trapped and compressed beneath it, causing the mass to move (run out) over the compressed air layer with virtually no friction. Thus rock avalanches can move long distances at very high speeds; and, of course, they are very dangerous (Figs. 9.1 & 9.7)!

8. Both slump and rockslide move by sliding. In what ways do these processes differ?

Rockslides involve rapid slippage of fracture-bounded blocks along inclined, weak layers and fractures in bedrock. Slump denotes the slow, downhill movement of a block of soil or relatively weak rock along a curved, spoon-shaped, slip surface. As it moves downhill, the block often undergoes rotation, leaving its surface tilted back toward the hillslope.

9. What factors led to the massive rockslide at Gros Ventre, Wyoming?

This famous slide along the south side of the Gros Ventre River occurred in June, 1925. Tilted, sedimentary strata lie roughly parallel to the south slope of the valley; and the surface layer, a fairly hard, resistant sandstone, rests on a much softer, shale stratum. The river had gradually downcut into the shale layer, depriving the inclined, sandstone slab of any lateral (buttressing) support on the downhill side. Water from melting snows and rain seeped into the soil and bedrock, saturating the ground above the shale and weakening the top of the supporting shale layer. These conditions allowed a large, fractured slab of sandstone to break loose and rapidly slide downhill. The slide formed an instant, natural dam and was moving fast enough to climb a short distance up the opposite side of the valley. Two years later the dam burst, causing a tremendous flood on the lower Gros Ventre and upper Snake Rivers (Fig. 9.11).

10. Compare and contrast mudflow and earthflow.

Mudflows are essentially soupy to more viscous, well-mixed, mud and water masses that behave and flow like liquids. They develop during periods of very intense rainfall or snowmelt in areas with little or no vegetation cover and move faster than earthflows.

Earthflows have higher contents of solids (mud, sand, boulders, etc.) and less water than mudflows, are more viscous than mudflows, and move less rapidly. They are mass movement events in which sheets or slabs of saturated to nearly saturated soil and weathered debris slough off a hillside or roadcut and move downhill to the base of the slope. The leading part (toe) of a slump block often turns into an earthflow when the slump material is wet (Fig. 9.14).

11. Describe the mass wasting that occurred at Mount St. Helens during its active period in 1980 and at Nevado del Ruiz in 1985.

Mudflows developed when hot ash was erupted onto snow on the upper slopes of both volcanoes. The soupy mud moved rapidly (20 mi/hr or so) down the stream valleys and suddenly "appeared" in stream and river valleys at the base of the volcano. Mudflows from both volcanoes caused extensive property damage, and those from Nevado del Ruiz caused the loss of many lives.

The massive, rock avalanche generated by the May 18, 1980, Mount St. Helens eruption is well-documented in Box 4.1. Prehistoric, but geologically recent, mass wasting events on a huge scale are well-documented for Mts. Shasta and Rainier in the Cascades and for large stratovolcanoes elsewhere. Were one of these to occur today, the damage and casualties would be catastrophic.

12. Since creep is an imperceptibly slow process, what evidence might indicate that this phenomenon is affecting a slope? Describe the mechanism that creates this slow movement.

The shearing action produced by the moving soil causes weak, partly weathered, bedrock strata beneath the soil to bend in a downslope direction. As most trees grow, their trunks and rootballs develop in straight line continuity at right angles to a horizontal plane, not at right angles to the slope of the land surface. If a tree sprouts in an area of active soil creep, a bend develops between the base of the trunk and the rootball because the deeper roots are anchored in stationary regolith while the shallow roots are moving slowly downhill with the surface soil layer. Thus the tree seems to be growing laterally (sideways) out from the slope. For similar reasons, manmade features such as posts and utility poles, gradually rotate in the downslope direction from their original, vertical orientations; and linear structures, such as walls and fences built at right angles to the slope direction, may show bending and displacement from their original, straight line construction due to different soil creep rates at different locations along the slope. These relations are well illustrated in Figure 9.16.

Wetting and swelling of clay minerals and frost heaving (when water in open pores in the soil expands as it freezes) push overlying soil particles upward and raise the elevation of the soil surface very slightly. As the soil dries, or the ice melts, the soil shrinks, and the particles move a slight distance downslope, under the pull of gravity. Repetitions of these processes (Fig. 9.15) over long periods of time can produce slow, downhill movement of the entire soil layer. In response to gravity, moist, clay-rich soils deform internally and slowly move downhill over the deeper regolith or bedrock.

13. Why is solifluction only a summertime process?

Solifluction (Fig. 9.18) is the downhill flowage of the water-saturated, surface soil layer above permanently frozen ground (permafrost). In the summer, the soil thaws to some depth below the surface, but the water is trapped in the thawed soil because the permafrost zone is impermeable. Thus solifluction occurs only in the summer when the surface soil layer is thawed. In the winter the surface soil layer is frozen solid.

14. What is permafrost? What portion of Earth's land surface is affected? (Box 9.3.)

Permafrost refers to water-bearing soil, weathered rock debris, and porous bedrock that stay frozen year round. Permafrost is common in northern Canada, in most of Alaska north of the panhandle, and in most of northernmost Russia and northern Siberia. Summer melting saturates the "active", thawed surface layer and makes it susceptible to solifluction movements (Fig. 9.17). Heated structures in contact with permafrost can also induce thawing, causing foundation soils to loose weight-bearing capacity and to undergo solifluction flowage. For these reasons, buildings, highways, railroads, and other engineering projects in permafrost regions are subject to special design and maintenance considerations (Box 9.3).