GEOG 140
Intro Physical Geography
Lecture Notes
Weathering and Mass Movement
Landmass Denudation
-any process that wears away or rearranges landforms
-weathering
-mass movement
-erosion
-transportation
-deposition
Slopes
-agents of erosion must overcome the forces of friction, inertia and cohesion of particles
-if slope angle is steep enough for gravity to overcome friction forces
-or material is dislodged by impact
-raindrops, hail, falling branches, animals, vehicles
-erosion and transport downslope occur
-hillslopes
-curved , inclined surfaces that form boundaries of landforms
-components (vary with rock type and climate)
-waxing (increasing) slope-convex surface curves downward and grade into:
-free face – steep scarp or cliff –outcrop of resistant rock
-debris slope receives rock fragments and materials from above
-humid environment-material carried away by water
-arid environment-material accumulates
-waning slope- concave surface along base of slope
-slopes seek angle of equilibrium
-optimum incline that balances conflicting forces
Physical Weathering Processes
-destruction of rocks through various stresses
-makes more surface area available to chemical processes
-frost action (frost wedging)
-water in cracks and pores of rocks expands with freezing (up to 9% increase in
volume) and contracts with thawing
-rock fragments can remain in place for erosion by water or wind or they can roll
or fall down slope
-scree slope or talus cone
-important process in humid continental, subarctic, polar and highland climates
-crystallization (salt crystal growth)
-pressures similar to ice expansion
-found in arid environments
-water drawn to surface of rocks through evaporation
-dissolved minerals grow crystals
-hydration
-water combining with minerals
-minerals expand and contract forcing grains apart
-works with carbonation and oxidation to convert feldspar to clay minerals
-pressure-release jointing
-removal of overlying layers releases pressure and allows rocks such as granite
to peel apart layer by layer (sheeting and exfoliation)
-organic physical weathering (root wedging)
Chemical Weathering
-hydrolysis
-moistening of minerals causes alteration (not present in hydration) and expansion in
volume contributing to breakdown of rocks
-oxidation
-caused through exposure to oxygen in the atmosphere
-dominant weathering mechanism in the tropics
-removal of iron from rocks disrupts crystal structure and makes rock more susceptible to further
chemical weathering and disintegration
-carbonation and solution
-mineral dissolves into solution
-carbon dioxide and water create carbonic acid
-acid strong enough to react with many minerals especially limestone
-Geography of Weathering
-different climatic zones have different dominant weathering processes
-cold climates
-physical/mechanical weathering (ice wedging) dominates
-humid climates
-abundance of water-chemical weathering dominates
-arid environments
-physical/mechanical more important-chemical less so
-Classes of Mass Movements
-Four basic types of movement
-fall
-slide
-flow
-creep
-Falls and Avalanches
-fall
-downward movement of rock (rarely soil) through the air
-soil doesn't usually form on very steep slopes or cliff faces required for free fall
-exception-toppling of slabs of earth along river banks
-bank calving due to undercutting of banks
-rockfalls (frost action, pressure release)
-common in areas with terrain composed of high, steep rock slopes and cliffs
-size of material varies
-small rock fragments to large blocks
-material spread over extensive area unless confined by topography
-talus slopes
-large rockfalls often transformed into debris avalanches after initial impact
-debris avalanche
-falling and tumbling rock, debris, and soil
-higher rate of speed than debris slide or landslide
-ice and water fluidize debris
-Landslides
-take place along a plane of weakness
-two basic forms
-translational (rockslide)
-rotational (slump blocks)
-rockslide
-rapid movement of rock material along a bedding plane or other plane of structural
weakness
-can move as a large block for a short distance
-often break up into smaller material
-often on steep mountain fronts but also on slopes w/gradients as little as 15°
-among the most catastrophic forms of mass movement
-millions of metric tons in a few seconds
-debris slide-rapid movement soil & loose rock fragments
-slump blocks
-mass that moves down a curved slip face (shear plane-concave-up)
-rotational movement
-upper portion tilted backwards
-can move as a single block
-usually broken into several segments
-debris flow can occur at the end of a slump block
-Flow
-occurs throughout a moving mass; no defined shear plane or rupture where movement occurs
-mudflows
-type of debris flow consisting of large percentage of silt and clay sized particles
-result from heavy rain or sudden thaw
-water content up to 30%
-high water content results in tendency of flow to follow existing drainages
-common in arid and semi-arid regions
-abundant loose, weathered debris
-spread out into a lobe or fan as they reach open areas at mountain fronts
-earthflows
-less water
-material slumps away from top (subsiding motion with backward rotation)
-leaves a step-like terrace bounded by a curved wall-like scarp
-in a slow flow the sluggish movement forms a bulging toe
-Creep
-slow movement of soil and regolith (rock material at base of soil profile)
-repeated lifting (at right angles to slope) and falling (straight down) of individual particles
-causes
-heating and cooling of soil
-growth of frost needles
-wetting and drying
-trampling and burrowing of animals
-vibrations and earthquakes
-effects
-bulges or wavelike swells in soil
-bending of strata
-tilted trees and posts