Transcript WEATHERING, EROSION AND MASS WASTING

WEATHERING, EROSION
AND MASS WASTING
(c) Vicki Drake, 2010
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What is Weathering?


Weathering is the combined actions of all
processes that cause rock to disintegrate
physically or decompose chemically.
Weathering processes include ‘physical’
weathering or ‘chemical’ weathering
– Physical Weathering – breaks rocks down into
smaller and smaller pieces
– Chemical Weathering – completely alters
minerals in rocks; creates new minerals
(c) Vicki Drake, 2010
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Weathering Factors

Factors that affect weathering include:
– Mineralogy of parent rock
– Mafic minerals least resistant to weathering
– Felsic minerals more resistant to weathering
– Climate
 Humid climates = more chemical weathering
 Dry climates = more physical (mechanical) weathering
– Time
 Longer the exposure time, the greater the weathering
opportunity
–
Number of fissures or openings
 More joints, cracks, and openings allow for greater
weathering processes to occur
– Degree of slope
 Steeper slopes encourage greater weathering of exposed
rocks
(c) Vicki Drake, 2010
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.
Why and Where Does Weathering
Occur?
 Weathering
occurs at the Earth’s
surface as rock materials are
exposed to the environment.
 Weathering is possible because all
rock materials – no matter how
‘solid’ - have openings (pore spaces)
that allow air, water and other
materials to do their work.
(c) Vicki Drake, 2010
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PHYSICAL (MECHANICAL)
WEATHERING
 Physical
Weathering does not change
the basic mineralogy of rock
– rocks are disintegrated into smaller and
smaller pieces, ready for transport
 Key
ingredient in physical
weathering: water
(c) Vicki Drake, 2010
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TYPES OF PHYSICAL
WEATHERING

Frost (Ice) Action: the role of water in
weathering when it freezes
– Frost shattering (ice wedging)
– Frost heaving (ice heaving)
 Stone
polygons/rings in high latitude tundra
 Pingos
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
Salt Crystal Growth
Unloading and Exfoliation
Bioturbation
– Vegetation
– Animal
(c) Vicki Drake, 2010
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FROST ACTION
 Frost
Action is the repeated growth
and melting of ice crystals in pore
spaces of soil and within rock
fractures.
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FROST SHATTERING/ICE
WEDGING
 This
type of physical weathering
occurs at high altitudes where there
are definitive cycles of summer and
winter.
 Granites tend to be the most
susceptible to this type of weathering
– Most high altitude mountains are
granitic
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FROST SHATTERING/ICE
WEDGING
During a thaw cycle (summer, for
instance), water from groundwater or
precipitation finds its way into the fissures
of rocks.
 In winter, the water freezes and expands
by up to 9% in volume pushing the fissure
apart even further.
 Over many freeze-thaw cycles, rock will
break into smaller pieces.

(c) Vicki Drake, 2010
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Frost Shattering/Ice Wedging
(c) Vicki Drake, 2010
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FROST SHATTERING/ICE
WEDGING
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FROST/ICE HEAVING
 This
type of physical weathering
occurs in high latitude regions of the
‘arctic tundra’
 Tundra are the extremely high
latitude vast open spaces, covered
with low-growing grasses and sparse
vegetation.
– Some parts of tundra are more ‘bog-like’
– Underlain by permanently frozen soils at depth
- permafrost
– Only upper layers thaw during brief weeks of
‘summer’.
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(c) Vicki Drake, 2010
MAP OF TUNDRA LOCATIONS
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Tundra in summer
Tundra in winter
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PERMAFROST
 Permafrost
is permanently frozen
soil, sediment, or rock.
 Permafrost has a number of different
layers, of which frozen ground is just
one portion
 The 'active layer' is ground that is
seasonally frozen, typically lying
above the perennially frozen
permafrost layer.
– This is the layer involved with ‘frost
heaving’
(c) Vicki Drake, 2010
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PERMAFROST
The Active layer
goes through
repeated cycles of
freezing and
thawing
Frost heaving
occurs in the
‘active’ layer
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HOW DOES ICE HEAVING
WORK?
 Summer
thaw of upper ‘active’ layer
allows water to migrate down
through soil layers under gravity
 Water ‘pools’ against the more
permanently frozen soil layers
 Winter freeze and water expands
vertically, lifting up overlying soil
layers
(c) Vicki Drake, 2010
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ICE/FROST HEAVING
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RESULTS OF ICE HEAVING
Roads can be
affected by ice
heaving by
warping the
surface
As the water freezes, it expands
vertically, pushing up the overlying
layers
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FROST HEAVING: PATTERNED
GROUND AND STONE RINGS
 Under
the right conditions over
hundreds of years, stone and soil
organize themselves into patterns,
through cycles of freezing and
thawing.
 The frost heaving activity found in
the tundra can produce small hills
with center depressions (up to 18
inches tall and 3-4 feet across)
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Patterned Ground: Stone Polygons
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PINGOS

Pingos are ice-cored hills forming in the
tundra
In time, the expanding ice forms an
isolated mass
– its volume increases and it pushes up
the overlying tundra
 Pingos grow at a rate of approximately
one-half inch per year
 The tallest pingo in the world (in the
western Arctic) is 16 stories (192 feet)
high.

(c) Vicki Drake, 2010
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PINGOS
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PINGO IN NW ALASKA
Salt Crystal Action
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Similar to ice-crystal growth – salt
crystals grow instead
Susceptible rocks:
– Sandstones in dry and arid regions
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Process:
– Groundwater moving down under
gravity through permeable
sandstone, naturally high in salts.
– Water hits an impermeable layer
(shale, for instance)
– Water flows along impermeable
shale to an opening
– Water exits rock leaving salt
crystals behind.
– Salt crystals grow at base of cliff,
producing niches (caves)
– Base of cliff wears away, rest of
cliff collapses and process begins
again
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WEATHERED SANDSTONE
Cliff Dwellers
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UNLOADING AND EXFOLIATION
Large sections of granitic rock, formed at
great depth under pressure, brought to
surface through plate tectonics.
 At the molecular level, the granite
expands in lower pressure environment
 Develops fractures in form of thick shells
that peel away from rock
 Forms rounded features: domes, for
instance

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EXFOLIATION
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OTHER PHYSICAL WEATHERING
 Surface
Heating and Cooling
– Expansion and contraction of rock over
time
– Fire fracturing
 Bioturbation
– Plant roots
– Animal burrowing
(c) Vicki Drake, 2010
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BIOTURBATION
Fire
Roots
Burrowing
animals
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CHEMICAL WEATHERING
 Chemical
weathering alters the
minerals of rocks – in some cases,
the minerals are dissolved.
 Types of Chemical Weathering
– Hydrolysis
– Oxidation
– Dissolution: Carbonic Acid Action
(c) Vicki Drake, 2010
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Hydrolysis
 The
addition of water at the
molecular level to silicates.
 Creates a grain-by-grain breakup of
the minerals in Granite into a clay
called Kaolinite
– Kaolinite used in manufacturing of spark
plugs and ceramic casings for lights.
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Oxidation
 The
addition of oxygen molecules (a
hydroxyl radical) to metallic minerals
(such as iron) (think: RUST)
 Results in decay of igneous and
metamorphic rocks down to 100
meters or more in tropical areas
(c) Vicki Drake, 2010
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Dissolution: Carbonic Acid Action
 The
mixing of CO2 and water creates
carbonic acid – a weak acid
 Carbonic acid attacks limestones and
marbles: rocks composed of calcium
carbonate (CaCO3)
 In regions underlain by limestone,
removal of CaCO3 results in
development of karst topography
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Karst Topography
Regions with limestone bedrock being
weathered out.
 Results in landforms such as caverns,
sinkholes, disappearing streams, and low
elevation.
 Karst is a German name for an unusual
and distinct limestone terrain in Slovenia,
called Kras.
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Karst Topography
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EROSION AND MASS WASTING
 Erosion:
Movement of
weathered rock over long
distances by water or wind.
 Mass Wasting: Downslope
movement of weathered rock
over short distances due to
gravity
(c) Vicki Drake, 2010
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MASS WASTING
 Main
force moving weathered
materials down slope is gravity.
 Factors that control mass wasting:
– Steepness of slope
– Water content of materials
– Presence (or absence) of native
vegetation
– Human activities
(c) Vicki Drake, 2010
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SLOPE STABILITY: STEEPNESS
W = Weight of total mass of earth material (at center of mass).
D = Vector component of W parallel to potential movement.
N = Vector component of W normal to slip plane.
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SLOPE STABILITY: WATER
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SLOPE STABILITY: VEGETATION
Native vegetation (such as chaparral) tend
to grow on steep slopes.
 Root structures act as binders and
stabilizers of loose unconsolidated
materials.
 Removal of native vegetation through fire
or clearing reduce stability of weathered
materials on a slope.

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Vegetation’s role in
slope stability:
(A, B) Roots support
and stabilize soils
near surface and at
depth
C) Upslope soils
stabilized by stems
and roots close to
surface
SLOPE STABILITY: HUMAN
ACTIVITIES
(c) Vicki Drake, 2010
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SLOPE STABILITY: MIINING
The Frank Slide: rock avalanche
and is composed of limestone
blocks mainly.
At 4:30 am on April 29, 1903,
the face of Turtle Mountain,
Alberta, Canada, collapsed onto
the coal-mining town of Frank,
killing at least 70 people.
This landslide has a volume of
30 million cubic meters, and an
equivalent weight of 90 million
tons
(c) Vicki Drake, 2010
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TYPES OF MASS WASTING
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Rock fall
– Talus slope development
 Angle of Repose
– Bedrock failure
Slides
– Material remains coherent and moves along
defined surface: joint, fracture, bedding planes
Slumps
– Downward rotation of rock/regolith along
concave-upward curved surface
Flows
– Materials flow down slope – mixture of water,
rock and other materials (slow to fast movement)
 Slurry: Lahar, Mud Flow, Debris Flow,
Solifluction
 Granular: Debris avalanche, Earth Flow, Soil
Creep
(c) Vicki Drake, 2010
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ROCK FALLS
Fastest form of mass
wasting
Hundreds of tons of
rock free-falling to
surface
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Granitic rock failure
TALUS SLOPES
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The pile of rocks that accumulates at the base of
a cliff, chute, or slope.
Movements occur whenever the talus slope
exceeds the critical angle: “angle of repose”
– ‘angle of repose’ is the steepest angle unconsolidated
material may remain stable
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The exact angle at which failure takes place
depends upon the materials, rock size, and
moisture content
Dry homogenous materials in a pile experience
slope failure when the angle of repose (the
resting slope angle) exceeds 33–37°
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TALUS SLOPES
Talus (loose, weathered bedrock) falls to
base of mountain building up a ramp that is
very unstable
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TALUS SLOPES AND
EXFOLIATION
SLIDES: ROCK AND DEBRIS
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PORTUGUESE BEND
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Landslides have been active in this region of the Palos
Verdes Peninsula for thousands of years beginning in the
Holocene
The 1956 landslide has been attributed to human activities.
– Human activities introduced ground water beneath the
homes, lubricating a layer of bentonite clay formed by
the subsurface weathering of volcanic rock called tuff.
Landslide encompassed an area of approximately 270 acres
and involved over 160 homes.
Rates of slippage have varied, initially moving between 2
and 12 cm/day for the first two years, and diminishing to
less than 1 cm/day during the next four years.
The slide mass has continued to move for over 40 years
and the cumulative displacement exceeds 30 meters in
some areas.
(c) Vicki Drake, 2010
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PORTUGUESE BEND, PALOS
VERDES PENINSULA
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POINT FERMIN LANDSLIDE
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The Point Fermin landslide originally consisted of
about 10.5 acres that began sliding in 1929.
More movement in the early 1940’s was
discovered when broken water pipes appeared.
Movement was slowed during the early 1960’s,
however, damage to houses was enough to force
evacuation of area
(c) Vicki Drake, 2010
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POINT FERMIN LANDSLIDE
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SLUMPS
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La Conchita, Ventura, CA
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La Conchita Slide
FLOWS: LAHAR, DEBRIS,
SOLIFLUCTION, AND EARTHFLOW
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Lahar: combination of volcanic ash, mud and
water flowing down a stratovolcano during an
eruption
Debris Flow: combination of weathered rock,
water and mud flowing out of canyons during
extreme rain events
– Alluvial Fans: formation of ramps along at mouths of
canyons in arid areas
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Solifluction: slow down slope movement of
upper layers of weathered tundra soils; form
large lobes on slope
Earthflow: downslope viscous flow of finegrained materials saturated with water under the
influence of gravity
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LAHAR
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DEBRIS FLOW
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CONTROLLING DEBRIS FLOW
 Los
Angeles County Flood Control
developed two types of basins to
attempt control of material flowing
out of San Gabriel Mountains into
foothill communities
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DEBRIS and CATCHMENT BASINS
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HOW DEBRIS BASINS WORK
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WHEN DEBRIS BASINS FAIL
Winter,
2010, La
CañadaFlintridge
Debris Basin
failure
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SOLIFLUCTION
SOLIFLUCTION: TUNDRA SOILS
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SOIL CREEP
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