MASS WASTING Chapter 11

A. Introduction

1. Definition:  The movement of rock and regolith material downslope due to the direct influence of gravity.

2. Gravity see fig. 11.2, p.247

a. Gravity vs. Shear Strength/Friction ---opposing forces that determine slope stability---whether material moves or not---in the end, gravity always wins, because with time and weathering shear strength decreases, but gravity does not

b. Vectors—magnitude and direction (gravity vector is directed toward the center of the earth

c. Surface component (vector) of gravity parallel to land surface.

3. Factors Decreasing Slope Stability pp. 247-251
a. Weathering
b. Water (acts as a lubricant and also as a “loading factor”)
c. Removal of Support
d. Rock layers dipping toward a valley
e. Loading
f.. Vibrations

4. Stabilizing a Slope (matter of counteracting factors listed above in # 3) pp. 261-265 (mostly photos)

B. Types of Movement see page 252, TABLE 11.2

1. Classification can be based on:
a. Rate of movement
b. Type of material involved
c. Type or form of movement involved

2. Categories of Mass wasting see pages 252-258—pay particular attention to figures and photos
a. FALLS: rock falls, debris falls
b. FLOWS: debris flows, mudflows—requires water, fine-grained material and a slope
c. SLIDES: debris/rock slides or just landslides
d. AVALANCHE: debris/rock---rare but material may travels miles at very high speeds of 150-200 mph
e. SLUMP: slump blocks rotate along concave fractures in episodic fashion.
f. CREEP: Definition: Imperceptibly slow movement of soil and regolith down slope. This is the least spectacular of all type of mass movement, however, with respect to erosion it is the MOST important in terms of volumes of material moved. Creep is pervasive affecting many (most?) landscapes and is active throughout much of the year. Freezing and thawing of soil moisture is one major factor in promoting creep. Evidence for creep includes tilted telephone poles, fence posts, tombstones, etc. (IGNORE solifluction, p.259)

C.            Geologic Work of Mass Wasting

1. Erosion: very important is sculpting most land surfaces.

2. Transportation: large volumes of material transported, but not carried very far.

3. Deposition: there are many recent (modern) deposits but few are preserved as sedimentary rocks; COLLUVIUM is a generic name for all mass wasting deposits. Talus is a name applied to the apron of rock debris that accumulates at the bottom of a cliff or steep slope.