Page 485 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

AQuantitativeSolution The Role of Gravity in Landform Development
Endogenic processes produce landforms through uplift and tectonism, while exogenic processes wear the forms away. Gravity has an important role to play in moving mineral matter and water downslope.
Gravity is the mutual force exerted by the masses of objects that are attracted to one another, and is produced in an amount proportional to each object’s mass. It results in potential energy differences between the continents that have been uplifted and the ocean basins.
Mass movement is the downslope movement of material under the influence of gravity. The rate and method of the move- ment depends upon the slope, the cohesion of the material, and the moisture content (Figure AQS 14.1).
When W·sin u exceeds F, movement occurs. F is proportional to W·cos u as:
F = f·W·cos u
where F is the frictional force, f is the coefficient of friction, u is the
slope angle, and W is weight.
Decreasing f, or increasing u, leads to movement.
Water has a dual effect in that it can help stabilize sediments, but it can also contribute to failure. Water adds weight and increases the cohesiveness of soils (although too much water can cause failure). This increases F and stabilizes the slope. Water also lubricates the potential slip faces, decreasing f and, in consequence F, leading to slope failure.
Soil saturated with water experiences increased water pressure that acts to decrease the contact pressure between soil grains, forcing the granular framework apart and decreasing stability.
In Focus Study 14.1, we learned that an estimated 82 million metric tonnes (= 8.2 × 1010 kg) of rock and sediment failed, sliding down the side of Turtle Mountain at speeds nearing 140 km·h−1. The material moved across the valley bottom and up the opposite side. In 90 seconds, the mass of material travelled up to 2 km and covered an area up to 3 km2, with an average depth of 14 m.
The slide started as potential energy on the mountainside. When gravity overcame cohesion, the potential energy was con- verted to kinetic energy as the mass moved downslope. Kinetic
F W·cosθθ
m = mass
g = acceleration due to gravity
W = weight
F = frictional force
f = coefficient of friction W·sinθ
W = mg
▲Figure AQS 14.1 Mass movement.
energy dissipated as heat due to friction as the movement came to an end.
What force was applied by this moving mass?
F=m×g
where F is the force, m is the mass of the object, and g is accel- eration due to gravity (9.8 m · s−2). Filling in the numbers from the Turtle Mountain slide,
F=m×g=8.2×1010 kg×9.8m·s−2 =8.036×1011 n The SI units of force are newtons (N). From this force, we can
determine the work done (in joules, J). W=F×d
where W is work, F is force, and d is distance.
W = F × d = 8.036 × 1011 n × 1500 m = 1.21 × 1015 J
We can now calculate power (P, in Watts, W ) from this measure of work (T is time in seconds). Power is the amount of work done per unit time.
P= W =1.21×1015 J s=1.34×1013 W T 90
This is greater by far than the average rate of consumption of electric power in North America annually. The overwhelming power released by gravity in a slope failure is an illustration of the ability of nature to sculpt the landscape.
θ
concepts review
Key leArning
■ Describe the dynamic equilibrium approach to the study of landforms, and illustrate the forces at work on materials residing on a slope.
Geomorphology is the science that analyzes and describes the origin, evolution, form, and spatial distribution of land- forms. Earth’s exogenic system, powered by solar energy and gravity, tears down the landscape through processes of landmass denudation involving weathering, mass move- ment, erosion, transportation, and deposition. Different rocks offer differing resistance to these weathering processes
and produce a pattern on the landscape of differential weathering.
Agents of change include moving air, water, waves, and ice. Since the 1960s, research and understanding of the processes of denudation have moved toward the dynamic equilibrium model, which considers slope and landform stability to be consequences of the resistance of rock materials to the attack of denudation processes. When a destabilizing event occurs, a landform or land- form system may reach a geomorphic threshold, where force overcomes resistance and the system moves to a new level and toward a new equilibrium state.
Slopes are shaped by the relation between the rate of weathering and breakup of slope materials and the rate of
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