Chapter 5: “Weathering, Soil,
and Mass Movements”
5.1: “Weathering”
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Mechanical Weathering
• Occurs when physical forces break rock into
small pieces without changing its mineral
composition.
– Each piece has the same characteristics as the
original rock.
• There are three physical processes in nature
that are responsible for mechanical weathering.
– Frost Wedging
– Unloading
– Biological Activity
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Frost Wedging
• Water seeps into the cracks of
rocks and eventually it will
freeze and expand.
• This processes causes the
cracks to enlarge.
• After many freeze-thaw cycles,
the rock breaks into pieces.
• Most common in mountainous
regions in the middle latitudes.
• Sections of rock that are
wedged loose may tumble into
large piles called talus.
– These typically form at the base of
steep, rocky cliffs.
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Unloading and Exfoliation
of Igneous Rocks
• Large masses of igneous
rock can be exposed to uplift
and erosion of overlying
rocks.
• Pressure on the igneous rock
is reduced, called unloading,
and this causes the outer
rock layers to expand.
• These outer layers separate
from the rest of the rock
mass which creates
exfoliation.
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Weathering and
Biological Activity
• Activities of plants,
burrowing animals, and
humans can cause
mechanical weathering.
– Plants: roots grow into
cracks & are wedged apart
as the plant grows.
– Burrowing animals: move
rocks to surface to become
exposed to weathering.
– Humans: blasting areas in
search of minerals or
creating new roads;
deforestation.
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Chemical Weathering
• Chemical weathering is the transformation of
rock into one or more new compounds.
• The most important agent in this type of
weathering is water.
– Water absorbs gases from the atmosphere and the
ground.
• Chemical Weathering of Granite
– Weathering of potassium feldspar produces clay
minerals, soluble salt (potassium bicarbonate), and
silica in solution.
– Quartz remains substantially unaltered.
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Chemical Weathering
• Weathering of Silicate Minerals
– When silicate minerals undergo chemical weathering,
the sodium, calcium, potassium, and magnesium they
contain dissolve and are carried away by
groundwater.
– Produces insoluble iron oxides and clay minerals.
• Spheroidal Weathering
– When water enters the joints in a rock, it weathers the
corners and edges most rapidly.
• This causes the corners and edges of the rock to be more
rounded.
• The rock takes on a spherical shape.
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Spheroidal Weathering
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Rate of Weathering
• Two other factors that affect the rate of
weathering are rock characteristics and climate.
– 1. Rock Characteristics
• Mineral composition and solubility
• Physical features such as joints
– 2. Climate
• Temperature and moisture are the most crucial factors.
• Chemical weathering is most effective in areas with high
tempeartures and abundant moisture.
– 3. Differential Weathering
• Caused by variations in composition
• Creates unusual and spectacular rock formations and
landforms
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