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Weathering and Erosion
Painted Desert,
Arizona
Weathering Vs. Erosion
• Weathering
– The general process in which rocks are
broken down at the Earth’s surface
– Mechanical – rocks broken w/o changing
composition
– Chemical – minerals are altered or dissolved
• Erosion
– Move weathered materials under influence of
gravity on grain-by-grain basis
Intertwined Processes
• Weathering breaks rocks down
• Erosion carries away loose material
exposing new, fresh surface to weathering
Mechanical Weathering: PressureRelease Fracturing
• Igneous and meta rx
generally form deep in
the crust
• Tectonic forces may
cause rocks to rise
forming a mtn
• As mtn erodes, less
pressure on stuff
beneath it
• Rx expand due to lower
pressure--fractures
Mechanical Weathering:
Wedging
• Frost wedging
• Can also occur when
– Salt water seeps into
cracks and
evaporates—haloclasty
• Or when
– Tree roots=wedge—
organic activity
Rocks wedged apart by growing
Tree roots
Honeycomb texture
Characteristic of a rock that
Has undergone haloclasty
Mechanical Weathering:
Abrasion
• Weathering of rocks
by friction and impact
• Actions make jagged
edges round
• Natural sandblasting
in desert
environments—weird
shaped rocks
Mechanical Weathering:
Thermal Expansion
• Occurs only in areas
with dramatic diurnal
temperature variation
– i.e. deserts
• Outside of rock
cools/heats more
quickly than interior—
may cause fractures
to form
Chemical Weathering
• Dissolution: dissolving stuff in water
• Ex: Halite
– Polar water molecule
– Water can pull
apart ionic bonds
Acids and Bases
• Acidic solution
– High concentration of H+
• Basic solution
– High concentration of OH-
• Acids and bases dissolve minerals faster
than water because they have more H+
and OH- ions to pull atoms away from
minerals
• Acid rain
What Governs Weathering?
Properties of the Parent Rock
• Different minerals
weather at
different rates
• Different
solubilities
– The extent to
which their
minerals will
dissolve in water
Properties of the Parent Rock
• Type of rock and
structure
– Determines rate of P
and C. weathering
Ex. Niagara
Falls
Climate
• Amount of rainfall and temperature
• Water is the best agent for dissolving
– Can hold lots of dissolved ions
• Chemical weathering
– Fastest where water abundant, hot (humid)
– Slower in dry, hot climates
– Slowest in cold climates because water
frozen, inert
Soil
• Fragments of rock, clay formed by altering
minerals, and organic matter
– Can trap rainwater, host vegetation, bacteria,
and organisms
– Create an acidic environment which speeds
weathering
– Roots and organisms create fractures
• A positive-feedback process
Length of Exposure
• Obviously, the longer the exposure time,
the more the weathering
– But dependent on other factors
Colorado Rockies:
Short Exposure
Appalachians:
Long Exposure
Surface Area vs. Volume
• The more surface area compared to
volume, the faster that part will weather
Works Much Like Coffee
• Add hot water to
coffee beans
– Water can only
affect the surface of
the beans
– Smaller beans
means more
reactions (Hooray!)