Geomorphic Processes

The forces that build up and wear down the land: diastrophism and volcanism, weathering, mass movements, erosion, and how soil forms.

Key ideas

Endogenic forces from inside the earth build up the land; exogenic forces from the atmosphere wear it down; the sun’s energy drives all exogenic processes.
Diastrophism includes orogeny (mountain building by folding), epeirogeny (uplift of large areas), earthquakes and plate tectonics.
Weathering breaks rocks in place (in situ): chemical (solution, carbonation, hydration, oxidation, reduction), physical and biological.
Mass movements carry debris downslope by gravity alone: creep, slump, debris slide, rockslide and rock fall.
Erosion removes and carries material by agents such as running water, glaciers, wind and waves; denudation is all of these together.
Soil forms from parent material, topography, climate, biological activity and time.
Climate and biological activity are active factors; parent material, topography and time are passive.

Remember

Gradient of a slope	rise ÷ horizontal distance × 100 (in %)
Soil loss	years = depth lost (in mm) ÷ loss per year (in mm); 1 cm = 10 mm
Soil formation time	years for 1 cm × number of cm
Kankar	calcium carbonate nodules in soils of intermediate rainfall
Desilication	silica washed out of soil in wet equatorial areas
Rhizobium	bacteria in legume root nodules that fix nitrogen
Waves	the only agent of erosion not controlled by climate

Watch out

Calling exfoliation by unloading chemical weathering (it is physical).
Thinking mass movement needs weathering first: gravity moves any material.
Mixing up orogeny (folding, mountains) and epeirogeny (broad uplift).

Worked example

Slope A rises 8 m over 100 m and slope B rises 40 m over 200 m. What is the gradient of each as a percentage, and on which are mass movements more likely?

Working: 8 ÷ 100 × 100; 40 ÷ 200 × 100

Answer: A 8%, B 20%; slope B, being steeper