Chapter 7:
WEATHERING AND EROSION
Objectives
• Describe the two major kinds of rock weathering.
• Identify three end products of weathering.
• Explain the difference between weathering, erosion,
and mass wasting.
• Describe how ice, water, and air transport regolith
across Earth’s surface.
• Define and give examples of mass wasting by slope
failure and/or sediment flow.
Weathering-The First Step in the Rock
Cycle
Weathering-The First Step in the Rock
Cycle
• How rocks disintegrate
– Weathering
• The chemical and physical
breakdown of rock exposed to
air, moisture and living
organisms
– Regolith
• A loose layer of fragments that
covers much of Earth’s surface
– Soil
• The uppermost layer of regolith,
which can support rooted plants
The rock in the photo has weathered in
place with little erosion, forming soil
Weathering: Two Types
• Mechanical engineering
• The breakdown of rock into solid
fragments by physical processes
• Chemical composition of rock
NOT altered
• Chemical weathering
• The decomposition of rocks and
minerals by chemical and
biochemical reactions
• Joints
– A fracture of rock , along
which no appreciable
movement has occurred
– Sheet jointing or exfoliation
– Frost wedging
• Abrasion
– The gradual wearing down of
bedrock by the constant
battering of loose particles
transported by wind, water or
ice
The jointing in these rocks has
exposed new surface area which has
broken and smoothed due to wind,
water and ice.
Weathering
exfoliation and frost wedging
Frost wedging and biomechanical weathering
Chemical weathering
• Dissolution
– The separation of
materials into ions in
a solution by a
solvent, such as water
or acid
– Rainwater acts as
weak solution of
carbonic acid
– Anthropogenic
The marble grave marker has been attacked by
actions influence
acidity of rainwater acidic rain because of the calcite composition. The
grave marker on the right, while old, has not been
dissolved because of its granite composition
Chemical weathering:
ion exchange and the chemical breakdown of feldspar
Factors affecting weathering
• Tectonic setting
– Young, rising mountains
weather relatively
rapidly
– Mechanical weathering
most common
Factors affecting weathering
• Rock composition
– Minerals weather at
different rates
• Calcite weathers
quickly through
dissolution
• Quartz is very
resistant to chemical
and mechanical
weathering
• Mafic rocks with
ferromagnesian
minerals weather
more easily
Factors affecting weathering
• Rock structure
– Distribution of
joints influence rate
of weathering
• Relatively close
joints weather
faster
Factors affecting weathering
• Topography
– Weathering
occurs faster on
steeper slopes
• Rockslides
Factors affecting weathering
• Vegetation
– Contribute to
mechanical and
chemical weathering
– Promotes weathering
due to increased water
retention
– Vegetation removal
increases soil loss
Vegetation can both hold water
And increase weathering. If removed
Rocks may also be vulnerable to abrasion
Factors affecting weathering
• Biologic activity
– Presence of
bacteria can
increase
breakdown of rock
Factors affecting weathering
• Climate
– Chemical weathering is
more prevalent in warm,
wet tropical climates
• Mechanical weathering
less important here
– Mechanical weathering is
more prevalent in cold,
relatively dry regions
• Chemical weathering
occurs slowly here
Note: temperate regions such as at the
center of the chart undergo both
chemical and mechanical
weathering, i.e. New York area
Factors affecting weathering:
color dots on map match colors on chart
Products of Weathering
• Clay
– Tiny mineral particles of any kind that have physical
properties like those of the clay minerals
– Clays are hydrous alumino-silicate minerals
Products of Weathering
• Sand
– A sediment made of relatively
coarse mineral grains
• Soil
– Mixture of minerals with
different grain sizes, along
with some materials of
biologic origin
– Humus
– Partially decayed organic
matter in soil
Erosion and Mass Wasting
Erosion is the removal of weathering products from the
source and most often occurs by water
• Erosion
– The wearing away of bedrock and transport of loosened
particles by a fluid, such as water
– Example: Sediment moved along the bottom of a stream
Erosion and Mass Wasting
• Erosion by wind
Particles of sand are transported close to the surface.
finer particles of silt and clay can be transported great
distances
• Erosion by ice
– Glacier
• A semi-permanent or perenially frozen body of ice, consisting of
recrystallized snow, that moves under the pull of gravity
Wind-blown fine sediments
such as this dust cloud can
Be transported across oceans
Erosion and Mass Wasting
Left: deposits of unsorted glacial till from glacier
Right: rock polished and striated by glacier
Erosion by ice:
glacier removes, breaks and transports rock pieces
glaciers scour valleys and deposit piles of debris as moraines
Erosion and Mass Wasting
• Mass wasting
– The downslope movement of regolith and/or bedrock masses due to the
pull of gravity
• Slope failure
– Falling, slumping or sliding of relatively coherent masses of rock
Erosion and Mass Wasting:
Rock slide, rock fall, and slumping result in downhill transport of broken rock
Erosion and Mass Wasting
• Flow: If water or air combines with the downward
movement, the regolith can “flow” downhill
• Creep
– The imperceptibly slow downslope flow of regolith
• Unstable slopes move very slowly over long periods of time
Erosion and Mass Wasting
Why do major landslides occur near plate
boundaries?
• Tectonics and mass
wasting
– World’s major historic
landslides clustered
near converging
lithospheric plates
• High mountains
undergo rapid
weathering
• Earthquakes near plate
boundaries can trigger
landslides
This massive slide was triggered by
A magnitude 9 earthquake in Alaska
near a subduction zone.
Critical Thinking
• On Earth, clay minerals are the most common
products of weathering. Samples from the
Moon do not contain any clay minerals. Why?
• Why are some granite bodies extensively
jointed, while others are essentially joint free?
Critical Thinking
• On Earth, clay minerals are the most common
products of weathering. Samples from the
Moon do not contain any clay minerals. Why?
• Why are some granite bodies extensively
jointed, while others are essentially joint free?