WEATHERING
WEATHERING
• Nature of weathering and erosion
• Weathering
• chemical and/or physical breakdown of a
rock or mineral material
• weathering involves specific processes acting
on materials at or near the Earth’s surface
• Erosion
• removal or transportation of material by
agents as running water, glacial ice, wind, etc.
• different degrees of weathering and erosion
working together can form interesting
geologic features as Bryce Canyon
Bryce Canyon National Park, Utah
WEATHERING
• Kinds of weathering
• Physical (Mechanical)
• breakdown of a substance by
disintegration
• frost (ice) wedging
• alternating freezing and thawing of
moisture in rock openings causing rock
disintegration
• single most abundant form of physical
weathering--potholes form from wedging
• a slope of talus can form at base of a cliff
Frost wedging
Talus slope at the base of mountain
WEATHERING
• unloading (exfoliation)
• erosion of upper rock causes underlying
rock to expand resulting in cracking and
peeling of rock in slabs
Half Dome
in Yosemite
National
Park
WEATHERING
• organic activity
• burrowing worms or animals and roots of
trees can cause rock disintegration
WEATHERING
• Chemical weathering
• breakdown of substance by decomposition
resulting in formation of new minerals
• water and acids especially carbonic acid
are important agents
• hydrolysis
• chemical reaction between water and
substance in which (OH) in water (HOH)
becomes part of the new substance formed
• often silicate minerals will weather by this
process to form clay
WEATHERING
• solution (leaching)
• dissolving of substances by acids in water
often leaving nonsolubilized remains
• carbonate minerals are highly affected by
leaching
• oxidation
• reaction of a substance with oxygen
causing a rust material to form
• manganese and especially iron minerals
are affected by oxidation
• often weathering of materials includes a
combination of the 2 kinds of weathering
WEATHERING
• Factors which affect the rate and/or extent of
weathering
• Total surface area of mineral or rock
• chemical and physical weathering increases
proportionately with amount of open space
(cracks, holes, etc.) at surface and extending
through the material
• finer grained rocks weather greater than a
coarser grained equivalent in composition and
size
• basalt and gabbro; rhyolite and granite
WEATHERING
• Climate
• temperature and amount of moisture can
influence the kind and magnitude of
weathering
• in a humid hot climate there would be a lot
of hydrolysis, leaching, oxidation and
certain types of physical weathering but
little or no frost wedging
• in a dry cold climate there would be a
sparce variety and lower magnitude of
chemical weathering
WEATHERING
• Cleopatra’s Needle was in Egypt for nearly
3500 years and shows minimal weathering
compared to the same in New York for
about 80 years
in Egypt
in
New York
WEATHERING
• Composition of rock or mineral substance
• as mentioned earlier certain
composition materials are affected by
specific kinds of chemical weathering
• Silicate mineral weathering series
• based on composition alone silicate
minerals higher in Bowen’s mineral series
will decompose more than those lower in
the series
• a gabbro should weather at a greater rate
and more extensively than a granite if all
other weathering factors are the same
WEATHERING
• which would weather faster and a greater
rate, a basalt or a rhyolite if all other
weathering factors are the same?
WEATHERING
• Time
• obviously time is a factor in all geologic
processes including weathering rate and
magnitude or extent of weathering
• Soil
• Definition and basic nature of soil
• unconsolidated material formed from
chemical and/or physical weathering of
substances as bedrock or outcrops
• contains at least some organic matter
• can be in situ (in place) or transported
WEATHERING
• Soil profile and soil horizons
• typical soil profiles include 3 main horizons
• 2 upper horizons, A and B are known as
solum
• A horizon is concentrated in organic
matter and acids and represents an area
of high leaching or solution activity
• B horizon is concentrated in clay and
rich in iron
• eluviation is the removal of clay and
solubilized materials from A horizon
WEATHERING
• illuviation is the precipitation of
eluviated materials in the B horizon
• B horizon forms and thickens as the
degree of eluviation and illuviation
increase
• C horizon
• comprised of fragmented bedrock
A
WEATHERING
Mass Wasting Examples Related to Soils
• Soil creep
• a form of mass wasting of earth materials
• creep is often initiated if ground becomes
saturated with water allowing gravity to pull
the water logged soil downslope
• freezing and thawing in soil can also cause
heaving thus loosening the soil allowing
creep
Soil Creep
Debris Slide
Rock and Debris Flow
Landslides in La Conchita, California
WEATHERING
• Solifluction
• a form of mass wasting and soil creep that
occurs in regions underlain by perafrost in
arctic climates
• creep occurs in active layer above
permafrost--active layer becomes saturated in
water during summer months but not the
impermeable permafrost layer--this results in
a creep downslope of the active layer
WEATHERING
Solifluction—diagram and picture