Gravity Takes Its Toll: Mass Wasting - Geology - page 161

Bedrock: Losing its stability
Beneath layers of loose sediments and soil is solid rock, or bedrock. Bedrock can be a
sedimentary, igneous, or metamorphic rock. (See Chapter 7 for descriptions of the rock
types and their formation.) Unlike loose sediments, bedrock is often very stable even on
steep slopes. However, certain conditions make some bedrock susceptible to mass wast-
ing.
Exposure to rain and wind ( weathering ) can break down pieces of sedimentary bedrock
if it is not strongly cemented. Igneous and metamorphic bedrock may have planes (lay-
ers) of weakness that developed during formation due to how the minerals are stacked
together. These areas of weakness will break apart in response to mass wasting triggers
(many of which I describe in the next section), or they may develop cracks. When a bed-
rock has cracks, tree roots can grow into them and further break apart or weaken the
rock.
Triggering Mass Movements
The force of gravity is constantly pulling on earth materials, encouraging them to move
downslope to a position of greater stability. When the conditions are right, large
amounts of material will give in to gravity's pull. What kinds of factors can help to create
the right conditions? In this section, I focus on four of them: water (which is the most
common factor), a change in the slope angle, groundshaking events (such as earth-
quakes and volcanoes), and the loss of vegetation.