Unit 39


Unit Overview

This unit examines the mass movement of material. The main sections are as follows:

  • Slope processes
  • Mass movement processes
  • Hillslope processes
  • Subsurface processes
  • The importance of slope processes

Mass movement is the movement of material by the force of gravity. If mass movement did not occur or were less effective, the entire degradational system would be slowed down. The unit emphasizes the importance of gravity to gradational forces through an examination of a slope's angle of repose, shearing stress, and friction. The four major types of mass movement are creep, flow, slide, and fall. Creep is an extremely slow movement of soil downslope, and is caused primarily by freezing and thawing. Slides require rock or soil at a high angle on steep slope. A fall is the downslope rolling (and subsequent short transport) of pieces of rock that have become loosened by weathering. A fall can produce a talus cone at the foot of the slope/cliff from which the rocks fell.

Unit Objectives

  • To demonstrate the role of gravity in promoting slope processes in weathered materials
  • To discuss the various types of slope processes and the circumstances under which they usually occur

Glossary of Key Terms

Creep The slowest form of mass movement; involves the slow, imperceptible motion of a soil layer downslope, as revealed in the slight downhill tilt of trees and other stationary objects.
Earth flow A form of mass movement in which a section of soil or weathered bedrock, lying on a rather steep slope, becomes saturated by heavy rains until it is lubricated enough to flow.
Fall The fastest form of mass movement that involves the free fall or downslope rolling of rock pieces loosened by weathering; these boulders form a talus cone or scree slope at the base of the cliff from which they broke away.
Landslide A slide form of mass movement that travels downslope more rapidly than flow movements; in effect; it is a collapse of a slope and does not need water as a lubricant (can also be triggered by an earthquake as well as human activities).
Mass movement The spontaneous downslope movement of earth materials under the force of gravity; materials involved move en masse - in bulk.
Mudflow A flow form of mass movement involving a stream of fluid, lubricated mud; most common where heavy rains strike an area that has long been dry and where weathering has loosened ample quantities of fine-grained material.
Rockslide A landslide-type of mass movement consisting mainly of rock materials.
Slumping A flow type of mass movement in which a major section of regolith, soil, or weakened bedrock comes down a steep slope as a backward-rotating slump block.
Solifluction A special kind of soil creep in which soil and rock debris are saturated with water and flow in bulk as a single mass; most common in periglacial zones.

Unit Outline

  • Slope processes
    • Gravity plays a major role in shaping landscapes
      • angle of repose is the maximum angle at which granular substances can resist the pull of gravity (Fig. 39.2)
      • the steeper the slope, the greater the shearing stress (downslope pull)
      • friction counteracts shearing stress
      • water on a slope adds weight and reduces friction
  • Mass movement processes
    • The movement of materials by gravitational force
      • the term mass wasting is sometimes used
      • important process for breaking down rock
    • Fall movements (Fig. 39.7)
      • in a free fall, pieces of rock that have become loosened by weathering roll downslope
      • material generally not carried far from its origins, forms a talus cone at the base of the slope
    • Slide movements (Fig. 39.6)
      • when a segment of rock or soil at a very steep angle suddenly breaks away, a landslide or rockslide occurs
      • much faster than a flow, a collapse; can be very destructive
      • bedrock also removed, unlike creep and flow movements
    • Creep movements (Fig. 39.10)
      • the slowest type of mass movement, involving the movement of the soil layer downslope
      • soil creep created by alternate freeze/thaw or wet/dry cycles
        • particles expand or rise when wet or frozen, then resettle slightly downslope when dry or thawed
        • solifluction is a kind of soil creep in which soil and rock debris flows downslope with water
    • Rainsplash erosion
      • caused by the impact of raindrops on bare soil surface.
  • Hillslope processes
    • Flow movements (Figs. 39.12)
      • in an earth flow, a mass of soil or weathered rock at a steep angle is saturated and starts to flow downslope
      • slumping is a type of earth flow that involves a large section of soil or rock that stands in a vertical position
      • a mudflow is caused by heavy rains, and carries fine material in a thin, fluid, stream of mud
  • Subsurface processes
    • Eluviation and solution involve the mobilization and movement of material in solution or as small individual minerals through the structure and fabric of the regolith or weathering zone
    • Lessivation is the removal of clay in suspension and its aubsequent accumulation downprofile.
  • The importance of mass movements
    • All streams are constantly being shaped by mass movements, although is not always visible
    • Location and effects of mass movements needs to be considered in planning and policymaking
    • Mass movements expose new bedrock to weathering and eventual removal

Review Questions

  1. Compare and contrast slide movements and fall movements.
  2. Describe the process that occurs during an earth flow.