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Earth’s External Processes
• Weathering – disintegration of rock at or near the earth’s surface
• Mass wasting – transfer of material down slope in response to gravity
• Erosion – transportation of material by a mobile agent such as water, wind, or ice
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These are non–stop, never-ending processes!
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Weathering must happen before erosion can take place!
Which comes first
Erosion or weathering?
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Mechanical Weathering
•Breakdown of bigger pieces into smaller pieces
•This is also referred to as physical weathering
•Only size changes occur in this process•No chemical composition change occurs
in mechanical weathering
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WeatheringMechanical weathering – the physical breaking or
fragmentation of rock into smaller pieces
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What things can affect mechanical weathering?
-Temperature
-Most of the temperature influence comes from the freezing of H2O
-Ice expands by 9% over the volume of liquid H2O
-Freezing H2O cracks rocks
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-Repeated freezing and thawing of H2O in rocks
-Can form potholes in cold climate areas-Can be responsible for splitting trees as well
Frost wedging
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Biological ActivityPlant roots in
search of minerals and
water grow into fractures, and as the roots grow, they wedge the
rocks apart. This is physical
weathering.
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Pressure
-Reduced pressure is what we are talking about
-Bedrock moves upward as overlying rock is removed
-Bedrock expands upward-Cracks form as pressure is reduced
-Cracks are called joints
-Joints/cracks form parallel to the ground
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Mechanical Weathering
Exfoliation:Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.
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WeatheringMechanical Weathering
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Sheet Joints(Exfoliation)
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Stone Mountain, GA
Half Dome, Yosemite, CA
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Exfoliated Domes, Yosemite
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Stone Mountain, GA
Stone Mountain, Georgia, showing the product of exfoliation due to unloading
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Chemical Weathering• Oxygen dissolved in
water will oxidize some materials like
nails and rocks!
(Rust)• Carbon dioxide dissolved in water forms a weak acid
called carbonic acid, which helps dissolve
rock material.
Chemical Weathering of Statues, Bath, UK
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The Result of Chemical Weathering
-New Minerals Are Formed and Dissolved Particles Are Released
-The Amount of Chemical Weathering Is Determined by the Composition of the
Parent Rock-Temperature increases leads to an
increase chemical reactions
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How Granite WeathersChemical weathering – the alteration of the internal structure of minerals by chemical reactions
Potassium Feldspar + carbonic acid + water
Clay mineral + Potassium bicarbonate + Silica
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Spheroidal Weathering
Fluids enter rock along joints or
fractures and the edges become
rounded.
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Rates of Weathering
Granite versus Marble
Factors influencing weathering:
-Mineral makeup
-Climate
-Differential Weathering
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Differential WeatheringRocks weather at different rates because of different
mineral makeup, degree of jointing, and exposure to the elements.
More resistant rock protrudes as ridges and pinnacles.
Bryce National Park, Utah
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What Is Soil?
The four components of
soil:
Decomposed rock
Humus
Air
Water
Soil is the layer of rock and mineral fragments along with organic matter, water and air that supports the growth of
plants.
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What is Dirt?
Dirt is what gets on your clothes and into your house
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Controls on Soil Formation• Parent material – Bedrock
vs. Unconsolidated• Time – longer time, more
soil• Climate – temperature and
precipitation• More plants and animals =
more organic matter• Poorly developed soils on
steep slopes
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Soil Texture and StructurePoint A:
10% silt
40% clay
50% sand
Clay loam has no single
particle size percentage
that dominates!
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Controls of Soil FormationIf the parent material is
bedrock, then we get residual soils
On unconsolidated sediments, we get transported soils
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Soil composition
-Soil forms in layers-Larger materials are found on the bottom
How quickly is soil formed?
-It may take several hundred years for 1cm to form
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Soil profile
A vertical column showing the
sequence of layers of particles in the
soil
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Soil Horizons-A distinct layer or zone within a profile-There are four major zones (O, A, B, C)
O zone -Leaf litter, high decomposition
A Zone
-High organic matter
-Generally the darkest in color
-High humus content
B Zone
-Less developed
-Lighter in color
-Zone of accumulation (highly soluble mineral build-up)
-Red or brown in color
-Clay accumulated (Hard pan formation)
C Zone
-Contains weather parent material
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Soil HorizonsO zone
-Leaf litter, high decomposition
A Zone-High organic matter-Generally the darkest in color-High humus content
B Zone-Less developed-Lighter in color-Zone of accumulation (highly
soluble mineral build-up) -Red or brown in color-Clay accumulated (Hard pan
formation)C Zone
-Contains weather parent material
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Rill and Gully Erosion
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Eroded channels ranging in size from rivulets to small gullies caused mainly by runoff waters rather than raindrop dislocation.
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Gully erosion in Tanzania. What is the main factor for it’s cause?
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Who cares about soil erosion?
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Mass Wasting: The Work of Gravity
Mass wasting refers to the down slope
movement of rock and soil
under the influence of
gravity.Hurricane Mitch in 1998 caused severe mud
flows in Central America
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Triggers of Mass Wasting
• The Role of Water
• Oversteepened Slopes
• Vegetation
• Earthquakes
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Flows, Slides, and Falls (All types of Mass Wasting)
• Type of material, the kind of motion, and the velocity of movement dictate kind of mass movement
• Falls involve the free-fall of debris or rock• Slides occur when the material remains coherent
and moves along a well-defined surface• Flow occurs when material moves down slope as
a viscous fluid (liquidy)
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Mass Wasting Processes
A debris flow involves a flow of soil
and regolith containing a large amount
of water.
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Mass Wasting ProcessesEarthflows
form on hillsides in
humid areas during times of heavy rainfall.
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Slow Movements
Creep involves the gradual
downhill movement of
soil and regolith.
Freeze and thaw cycles
contribute to creep.
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Slow MovementsSlump
Landslide movement along a curved understructure
Occur on thick soils with moderate slopes
Common after rainfalls
Mixing of debris
Crescent shaped scar on the landscape
Triggered by rain or EQ
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Solifluction
Solifluction occurs in regions of
permafrost when the thawed upper region flows down
slope.
The frozen bottom layer stays in
place.
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Rock AvalanchesThe high
speed of rock avalanches
suggests that trapped air has helped them attain velocities of
125 miles per hour.
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What does this coin have to do with
Rock Falls?
"The Old Man of the Mountain" is a rock formation that can be found on Mt. Cannon in the Franconia Notch
gateway to Northern New Hampshire. From the right view, this unique rock formation, comprised of
five layers of Conway red granite, depicts the distinct profile of an elderly man gazing eastward.
Geographers believe that the layers of granite were positioned by the
melting and slipping away action of an ice sheet that covered the
Franconia Mountains at the end of the glacial period - some 2,000 to
10,000 years ago. Today, the formation, measuring over 40 feet
high with a lateral distance of 25 feet, is held in place by cables and turnbuckles to prevent further
slipping and possible destruction.