Weathering andErosion
Weathering is the breakdown of a rock mass into smaller fragments (clasts).
This can be caused by physical action, for example an earthquake fracturing
rocks with the release of elastic energy or the cracking of a rock when
water freezes and expands. Weathering also happens in response to chemical
changes like the dissolving of calcite in limestone or the reaction feldspars
with slightly acid rain to produce clay minerals in igneous and metamorphic
rocks. From the reading and lecture, you will want to become familiar with
the different kind of weathering and how you might be able to recognize
them if you see them in the field.
Once the rock is broken, it can begin its downslope movement that will
eventually take it to a sedimentary basin. Often the initial movement occurs
when gravity pull material down a steep slope. Landslides are rapid movement.
Creep is the slow gradual downslope movement of soil and the material in
the soil. Eventually much of the material will find its way into a stream
and begin to be carried more rapidly. Once in the stream transport system,
the movement of eroded material becomes dependent on the energy of the
stream, but that's another story.
I. Chemical and physical weathering
A. Definitions and examples
1.
Recognizing the effects of freeze-thaw
2.
Chemical weathering and soil formation
B. Environmental control of weathering
1. Temperature
2. Moisture
3. Elevation
C.
Differential weathering
1. Devil's Tower
2. Grand Canyon
II. Downslope movement -- initial stage of erosion
A. Landslides
B. Creep
C. Slump
D. Rock Rivers
Objectives:
1. Describe different means of physical and chemical weathering
2. Describe the effect of chemical weathering on a granite boulder buried
in soil.
3. Describe the effect of chemical weathering on a marble tombstone.
4. Describe the effect of physical weathering on a sandstone cliff.
5. Describe the effect of frost wedging on a hillside underlain by shale.
6. Explain why frost wedging is unlikely to effect a granite.
7. Explain why mountainous regions tend to have very high rates of physical
weathering.
8. Explain why a tropical rainforest will have high rates of chemical
weathering.
9. Explain why there is usually little soil development is a desert
region.
10. Describe the process that often leads to slanted telephone poles
or fence posts on slopes.
11. Discuss several mechanisms that might trigger a landslide or rockfall.