Transcript Tearing Down Mountains I: Weathering, Mass Movement

Tearing Down Mountains I:
Weathering, Mass Movement, & Landslides
GEOSC 10: Geology of the National Parks
Presented by Dr. Sridhar Anandakrishnan
The Pennsylvania State University
Erosion
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Erosion
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Weathering
Transportation
Deposition on the Earth’s surface
Landscapes created and destroyed
Involves atmosphere, water, gravity
GEOSC 10 - Geology of the National Parks
Erosion Agents
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Agents:
‣ Mass wasting
‣ Running water (streams)
‣ Glaciers
‣ Wind
‣ Ocean waves
‣ Ground water
GEOSC 10 - Geology of the National Parks
Outline
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Mass wasting
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Tetons National Park
Soils
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Badlands National Park
Rain
‣ Redwoods National Park
GEOSC 10 - Geology of the National Parks
Selected National Parks
Tetons
Badlands
Redwoods
GEOSC 10 - Geology of the National Parks
Mass Wasting
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Masses of debris or bedrock moving
downhill
Landslides and slower movements
Driven by gravity
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Classification of Mass Wasting
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Rate of movement
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Soil creep to slumps to mudflows
Extremely slow (~1mm/year) to very rapid
(>100 km/hour)
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Soil creep is slowest but ultimately most
important - it happens everywhere
Others happen in wet and/or steep places
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Controlling Factors
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Most important: Water
Climatic controls
‣ Ice
‣ Vegetation
GEOSC 10 - Geology of the National Parks
Controlling Factors
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Water adds weight
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BUT a little water makes soil stronger (damp sand
castle)
- “the water holds the grains together”
Too much water makes soil weak
- “floats the grains apart”
Vegetation
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Absorbs excess water from soil (increasing strength)
Binds soil together (increasing strength)
GEOSC 10 - Geology of the National Parks
Controlling Factors
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Overgrazing
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Reduces vegetation
- Weakens soil
- Allows water to enter soil
Overloading
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Dumping of mine debris
Heavy buildings on steep, denuded hillsides
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Mudflow
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Flow of watery debris
Occurs where lack of vegetation:
- Dry climates
- Volcanoes
- After forest fires
‣ Very fast up to 80 km/hr
‣ Triggered by heavy rainfall
‣ Los Angeles
GEOSC 10 - Geology of the National Parks
Most Common Mass Wasting
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Soil creep
‣ gentle slopes
‣ vegetation slows movement Freeze
‣ very slow flow (< 1 cm/year)
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- facilitated by water in soil
- by freeze-thaw in colder climates
Freezing of water expands soil.
Thawing drops the grains downhill.
Thaw
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Outline
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Mass wasting
‣
Tetons National Park
Soils
‣
Badlands National Park
Rain
‣ Redwoods National Park
GEOSC 10 - Geology of the National Parks
Selected National Parks
Tetons
Badlands
Redwoods
GEOSC 10 - Geology of the National Parks
Weathering
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Breakdown of rocks at the Earth’s surface
Factors controlling weathering
‣ Presence of water
– Gasses
‣ Changing temperatures
‣ Biological organisms
– Pressure changes
GEOSC 10 - Geology of the National Parks
Definitions
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Parent material is removed by erosion
Transport by wind, water, glaciers
In-place weathering produces soil
Differential weathering for different rock types
GEOSC 10 - Geology of the National Parks
Mechanical Disintegration
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Physically break rocks into smaller pieces
Frost wedging and heaving
Pressure release
Thermal expansion
Biological organisms
Salt Crystal growth
GEOSC 10 - Geology of the National Parks
Frost Action
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Water expands when it freezes
Necessary conditions
‣ Temps that go below freezing
‣ Free water
‣ Cracks for the water to seep into
Rocks spall off and form a talus slope
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Figure 5.4,
p. 113
GEOSC 10 - Geology of the National Parks
Pressure Release
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Rocks that form under high pressure
‣ Deep in the earth
‣ Subsequent uplift to the surface
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Thermal Contraction/Expansion
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Rocks expand when heated and shrink when
cooled
Interior remains at a steady temperature
Resulting in fracturing
Important during fires
On the Moon
GEOSC 10 - Geology of the National Parks
Biological Organisms
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Tree roots and trunks
Burrowing insects, worms, animals
Earthworms
Help to expose deeper layers to water
Mix soils and circulate them to the surface
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Chemical Weathering
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Rainwater with carbon dioxide (CO2)
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Forms a weak acid
Acid attacks the rock
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Results depend on what type of rock
Temperature, acidity, water, etc.
GEOSC 10 - Geology of the National Parks
Granite and Acids
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Granite in the presence of carbonic acid will
weather and break apart
The Iron (Fe) will rust (oxidize)
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It usually remains in the soil
The Aluminum (Al), Potassium (K), and Silica
(SiO2) turn into clays and remain in the soil
GEOSC 10 - Geology of the National Parks
Some of the Products Wash Away
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The Calcium (Ca), sodium (Na), and
Magnesium (Mg) wash away
‣ The Ca goes into the sea and is used by sea
creatures to make their shells
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The Na makes the sea salty
The Mg reacts with rocks at the spreading centers to
make new rocks
GEOSC 10 - Geology of the National Parks
Soil
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What is left behind is soil
More heat, more water --> more stuff is
washed away and the soil has fewer minerals
in it.
Very dry areas, the Ca, Mg, and even Na
remain and the soil is very salty
GEOSC 10 - Geology of the National Parks
Badlands
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Erosion from the Rockies, deposited in the
great plains.
Soil is very clay rich
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The clays create a hard surface that channels the
water, washes away plants
The clays are unstable, slopes slump down,
washing away plants
Water doesn’t soak in well.
GEOSC 10 - Geology of the National Parks
Wind and Weather
GEOSC 10 - Geology of the National Parks
Wind and Weather
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Hot @ equator, cold @ N/S Pole
‣ Because the sun shines straight down @ equator
‣ Curve of earth makes sunlight spread out @ Poles
Hot air rises @ equator, forms convection cell
‣ Cools off in upper atmosphere, is shoved N/S
‣ Comes down in the mid-latitudes
‣ Returns to Equator
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
World’s Deserts
mage courtesy USGS
GEOSC 10 - Geology of the National Parks
Rain Shadow Desert
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Death Valley dry and hot because it is in the
rain shadow of the Sierras.
Warm, wet air rises from the CA coast and
cools.
‣ Cool air holds less water, so it rains.
‣ Water vapor to rain releases heat
- opposite of the way evaporation of sweat takes in heat...
GEOSC 10 - Geology of the National Parks
Rain Shadow Desert (2)
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So, the air that gets to the top of the
Sierras is dry and “warm”
‣ It only cools by 0.6C/100m because of the rainfall
‣ If it had been dry to begin with, it would have cooled
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by 1C/100m.
When that dry, “warm” air goes down the other
side it warms by 1C/100m
Death Valley is HOT AND DRY
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks