Bay Area Map

Probabilities

 

Loma Prieta 1989

Bay Bridge

Building collapse

 

Hayward Fault

 

The plate boundaries are marked by a concentration of Earthquakes

 

Direct evidence of the plate motions

 

The type and size of earthquake depends on:

• the type of plate boundary
• the rate of plate motion
• the length of the fault that slips

 

Biggest Earthquakes found in subduction zones and continental transform boundaries

 

People like to live near the coast

 

Along the Pacific rim the plate boundary follows the edge of the continents.

 

Billions of people are at risk of earthquakes

 3. Elastic Rebound Theory

 

At geological scale the plates behave like a leaf springs

 

Deformation (strain) is stored as the plates try to move past each other

 

When the plate boundary fails there is an earthquake

 

Strain is released

 

Deformation begins to accumulate again

 

Stick-slip behavior

 

Size of quake depends on the amount of strain stored

 

Earthquake size and frequency variations are systematic

Small quakes are frequent

Big quakes are rare

 

The earthquake cycle may be 100's of years for big quakes

 

Magnitude	Effects	Example	Number per year
<2.5	Not felt		900,000
2.5-6.0	Moderate damage		31,000
6.1-6.9	Destructive	Northridge (1994)	100
7.0-7.9	Serious damage	Loma Prieta (1989)	20
>8.0	Total destruction	Alaska (1964)	1 every 5 years

 

4. Earth quake waves

P-waves

Primary waves

Pressure waves

Fastest (first to arrive)

Sound is a type of P-wave

Transmitted through both solids and liquids

Particles move in the direction the wave travels

 

S-waves

Secondary waves

Second to arrive

Shear waves

Transmitted only by solids

Particles move perpendicular to teh direction the wave travels

 

6. Earthquake Damage

Ground Shaking

Structural collapse

Turkey 99_1

Turkey 99_2

Oakland Freeway

Liquefaction

Fire

Landslides

Tsunamis

Reading: Chapter Chapter 12

Topics:

 

1. Observations

2. The geothermal gradient

3. Physics of heat transfer

4. Second Law of Thermodynamics

5. Lord Kelvin and the age of the Earth

6. Radioactive elements in the crust and mantle

---

1. Observations

In oil wells, the deeper you drill, the hotter it gets

Deed gold mines (Witwatersrand (South Africa))

Temperature increases with depth

At 3200 m depth -> 150 deg. F (65 deg. C)

Areas with permafrost :

If you cover the ground, the permafrost melts

Heat is coming off the ground everywhere

 

 

2. Geothermal gradient

Temperature increaces at 10 to 50 deg. C/km

Average geothermal gradient: 25 deg. C/ km

Rate of increase is less at depth

Otherwise most of the Earth would be molten

Mantle geotherm

 

 

3. Physics of heat transfer

Temperature:

Thermal state of matter: degree of aggitation of atoms

 

Heat:

Energy transfer due to movement of atoms

 

Heat flow:

Amount of heat transfer per unit area

 

Coffee cup experiment 1 :

Leave coffee on counter

 

Heat always flows from hot to cold

This is governed by the Second law of Thermodynamics

It is a fundamental property of our universe

It distinguishes future from past

It gives a definite direction to time

 

Coffee cup experiment 2 :

Drink out of a metal cup

Drink out of a china cup

Why do we burn our lips with the metal cup if coffee is at the same temperature?

 

Thermal conductivity of materials (k)

Iron 80

Rock 2

Wood 0.2

 

Determines how fast heat flows through a material

 

 

5. Lord Kelvin and the Age of the Earth

 

 

 

Reading: Chapter 10 and 24 of Tarbuck and Lutgens, 8th ed.

Topics:

1. Mountain Ranges & Orogenic systems

 

2. Crustal Deformation

Stress

Strain

 

3. Types of deformation

Elastic behavior (brittle deformation)

Plastic Behavios (ductile deformation)

 

4. Faults (brittle)

Tension ->Normal Faults

Compression ->Thrust faults

Horizontal Shear -> Strike-slip faults

 

5. Folds (ductile)

Synclines

Anticlines

 

6. Structures and Tectonic Settings

 

7. Isostacy

 

8. The Appalachian Orogen

 

---

1. Mountain Ranges & Orogenic systems

Alps

Matterhorn

Himalayas

Nepal Himalaya

DEM

Andes

Central Andes Map

United States

Shaded Relief Map

Appalachians

Cordillera

Rocky Mountains

Basin and Range Province

Sierra Nevada

Coast Ranges

 

Folded rocks

 

2. Crustal Deformation

Stress

What causes deformation?

Stress=force/area

Stress acts in all directions

 

Strain =deformation

 

3. Types of deformation

Rock behavior depends on:

Temperature

Strain rate (how fast the deformation happens)

 

Elastic behavior (brittle deformation)

The rocks break

Generates Earthquakes

Shallow in the crust

 

Plastic Behavios (ductile deformation)

The rocks flow under stress

No earthquakes

Deep in the crust

 

 

4. Faults (brittle)

Tension ->Normal Faults

Compression ->Thrust faults

Horizontal Shear -> Strike-slip faults

 

5. Folds (ductile)

Synclines

Anticlines

 

6. Structures and Tectonic Settings

Divergent Plate Boundary:

Rifts

Normal faults

Thin crust

Volcanism

Example: East African Rift

 

Convergent Plate Boundaries

Big Mountains

Thrust Faults

Fold belts

Thick crust

Volcanism

Example: The Himalayas, the Appalachians

 

Transform Boundaries

Complex deformation

Strike slip faults

Example: San Andreas Fault System

 

7. Isostacy

What is down?

 

Gravity Anomalies

 

The crust floats on the mantle

	Density
Mantle	3300 kg/m^3
Continental Crust	2700 kg/m^3
Oceanic Crust	3000 km/m^3

 

Because the continental crust is lighter it is higher than oceanic crust

 

Thicker continental crust results in higher topography

 

Isostacy was one of the paradigms of geology before plate tectonics

 

 

8. The Appalachian orogen

 

Reading: Chapter 6 of Tarbuck and Lutgens, 8th ed.

Topics:

1. Intro to Surficial Process

2. Mechanical Weathering

3. Chemical Weathering

4. Soils

---

1. Intro to Surficial Process

 

Processes that shape the landscape

Landscape appears immutable

Actually it is constantly changing

Normally slow cummulative change

Sometimes catastrophic change

 

 

Geomorphic process

Wear down the mountains

Carve valleys

Create soil -> Permits vegetation to exist

Make the stuff sedimentary rocks are made of

Interaction between lithosphere and hydrosphere-atmosphere

(Interaction between rocks, air, and water)

 

 

Processes:

Weathering- Break down of rocks

Mechanical-disintegration of rock

Chemical-decompositiono f minerals

 

Mass wasting- Down slope movement of rock masses due to gravity

 

Erosion- Removal and transport of material by water, wind, ice

 

Why does West Virginia look so different from Utah or Alaska?

Different Tectonic Setting

West Virginia- Continental interior far from plate boundary

Utah- Continental interior, closer to plate boundary

Alaska - Active plate margin- bigger mountains, young mountains

 

Different Climate

Different Vegetation

Different Geomorphic processes in action

West Virginia -temperate, wet climate

Strong chemical weathering

Erosion- Streams

 

Utah- temperate, dry climate

Strong mechanical weathering - water

Erosion- water

 

Alaska- Arctic, wet climate

Strong mechanical weathering - ice

Erosion - Glaciers and streams

 

 

 

2. Mechanical Weathering

Frost Wedging

Glass bottle in the freezer

 

Water seeps into cracks

Freezes and expands 9%

Pushes the crack open

Very powerful over the centuries

Problem for structural geologists in Alaska

 

Elastic Rebound

Rocks srings back when pressure is released

Enough to crack the rock

Exfoliation of granite

Yosemite:

Half Dome

Glacier Point apron

 

 

 

3. Chemical Weathering

The minerals suffer chemical attack when exposed to near surface conditions

Changes in mineralogy

Changes in composition

 

Controlled by:

Moisture

Temperature

Plants

Geochemical Environment

Rock type

composition

particle size

Time

 

Main agents:

Water, Oxigen, organic acids

 

 

Dissolution

Greatly enhanced by acids

Example:

Limestone (made of calcite) disolves to Ca++ ions and CO2 gas.

Many sedimentary rocks are held together by calcite cement

 

Sources of acid:

Plants

Acid rain (polution)

Volcanoes

 

Oxidation

Geochemical conditions in the subsurface reducing (low oxygen)

Minerals are stable under those conditions

Near surface the environment is oxidizing, some minerals react with oxygen and break down to oxides and hydroxides

Oxigen is a very active chemical agent

Most living things are burning continously (breathing=controlled oxidation)

 

4. Soils

 

Surface of the Earth is covered by a layer of decomposed rock

Regolith:

Chemically Weathered rock (soil)

Rock fragments (mechanical ly weathered)

Soil:

Organic Layer (humus) (0)

Zone of leaching (A)

Zone of accumulation (B)

Parental material (C)

 

Soil formation takes place from the top down

Thickness and nature depends on:

climate (rainfall, temperature),

vegetation

rock type

slope

 

Tropical soils:

Very thick

Amazon:

High rainfall during rainy season

Little rainfall in dry season

Hot climate

Organic layer-very thin

Very active chemical weathering

Difficult to find solid rock

Zone of leaching- very thick (depleted in nutrients)

Poor soil for sustained agriculture

Cutting the forest leads to rapid erosion of Organic Layer

Forest is replaced by grassland

Amazon natives have to abandon gardens after a few years and clear a new patch of forest

 

Andes Highlands

Moderate rainfall through the year

Thick Organic layer

Very good for sustained agriculture

 

Laterite soils:

Mostly in the tropics

Complete leaching

Only the most insoluble componds are left behind:

Aluminum hydroxide

Iron Oxide

Clay

Aluminun and Nickel mineral deposits are laterites

Start with an igneous rock with no quartz (basalt, syenite, pyroxenite, etc)

Initially:

Al -- 15% (in plagioclase mostly)

 

Mafic minerals are easily decomposed chemically

After Leaching :

Al --50% (in Aluminum hydroxides (bauxite))

---

 

Lecture 18 - Mass Wasting

Reading: Chapter 15 of Tarbuck and Lutgens, 8th ed.

 

Topics:

1. Intro Mass Wasting and Video

2. Driving and Resisting forces

3. Angle of Repose

4. Causes

6. Mitigation of slope failure (not covered in lecture)

5. Types of Mass Wasting Process

Rock Fall

Slides

Flows

Creep

---

1. Intro Mass Wasting and video

Mass wasting = mass movement, downslope movement

Movement of Earth materials (rock, regolith, soil, debris, snow) downslope as a result of the pull of gravity.

Examples:

Landslides

Avalanches

Debris flows

Rock Falls

 

Mass wasting can be very slow or catastrophic

In the US alone, landslides cause about $1.5 billion in economic losses and 25-50 deaths each year.

In West Virginia because of steep slopes and wet climate landslides are an important problem.

 

2. Driving and Resisting forces

 

Gravitational force acts vertically down

Slope angle causes a component of the force to act down-slope

Magnitude of down-slope component depends on steepness of slope

 

G= gravitational force

F= down-slope component

a= 90 - slope angle

F= G Cos a

 

 

Resisting resisting forces:

Friction

Cohesion

Cohesion between material and substrate

Internal cohesion of material

 

SHEAR STRENGTH - measure of the ability to with stand a change in shape.

 

 

3. Angle of Repose

Demo with sand and flour

Steepest angle a slope can sustain

Driving and resisting forces are perfectly balanced

Dynamic equilibrium

Critical State

Angle of repose is 25 to 40 degrees depending on internal cohesion

For sand it is about 30 degrees

 

 

 

4. Causes

 

Change in slope

Undercutting of slope

by rivers

by ocean waves

by ground water

by humans

 

Weakening of material

Chemical weathering

Mechanical weathering

Water

Water saturated soil is weaker

Water adds to the load on the slope

 

Thawing

Frozen water holds the soild together

Example: Today there is rapid erosion of the Arctic coastlines due to melting of the permafrost

 

Changes in vegetation

Plants holds the soil together

Plants absorb water

Factors that influence slope stability:

1. Shear strength of the material (rock, soil, regolith, sediment)

2. Gradient, steepness of the slope

3. Weathering and Climate

4. Water content

5. Undercutting

6. Overloading

7. Vegetation

8. Geology and Slope Stability

9. Slope modification by human activities -- roadcuts, quarries, etc.

Factors that can trigger a slope failure or landslide:

1. Excessive rain or melting snow

2. Earthquakes, seismic activity

3. Volcano eruption

4. Rapid undercutting by waves, streams, or humans

5. Mitigation and Prevention of Slope Failures

Do not build on or near steep slopes.

Not always possible: Highways need roadcuts: there will always be slope failures.

People like a view, they will build on slopes or sea cliff despite the high risk of slope failure.

 

Engineering techniques:

Retaining walls, rock bolts, weep holes, drainage pipes, diversion walls, slope shallowing, etc.}

 

 

6. Types of Mass Wasting Process

Rapid Mass Movements

Rockfalls

Rocks falling from near-vertical cliffs, the rockfall is the smallest, most common, and most rapid from of mass wasting.

Yosemite Rock Fall

Talus

Slides:

Rapid transport of mixture of soil, rock, and vegetation down a moderate to steep slope.

Rock and snow avalanche, Mount Huascaran, Peru. In 1970, an earthquake-induced rock and snow avalanche on Mt. Huascaran, Peru, buried the towns of Yungay and Ranrahirca. The total death toll was 66,000. The avalanche swept about 11 miles to the village of Yungay at an average speed of more that 100 miles an hour. Photograph by G. Plafker, U.S. Geological Survey.

Sinkhole

Debris slides (Rather dry)

Rock Slide

Avalanches (Snow and Ice)

Landslides

La Conchita, CA

Colorado

Thisle, UT

• Rock and snow avalanche, Mount Huascaran, Peru. In 1970, an earthquake-induced rock and snow avalanche on Mt. Huascaran, Peru, buried the towns of Yungay and Ranrahirca. The total death toll was 66,000. The avalanche swept about 11 miles to the village of Yungay at an average speed of more that 100 miles an hour. Photograph by G. Plafker, U.S. Geological Survey.

 

Flows:

Debris flow (Wetter than slides): Mix of water, rock, soil, mud moving rapidly downslope.

Debris Flow, Colorado 1994

Mudflows:

Fluid, rapid flow of mud, mixed with rocks and other debris.

 

Occur following heavy rainstorms in arid regions

Associated with volcanic eruptions

Usually follow preexisting channels ( streams or valleys).

Mt. St. Helens

  

Earthflows: Slow flowage of water saturated soils down moderate to steep slopes.

More fluid, shallower, and smaller than slumps.

 

Earthflow, OH

 

Slow Mass Movements

Slumps - Intermittent movement of a mass of earth or rock along a curved plane.

Usuallyo occur after a heavy rain, on a steep slope with deep, clay-rich soils.

 

Creep - The slowest but most widespread of the slow mass wasting.

Creep involves the entire hillside.

Evidence: tilted trees, fence posts, utility poles, tombstones

Soil behaves like a very viscous fluid. It slowly flows down hill.

 

Frost heaving - Earth material pushed outward due to ice expansion during cold episodes then downward during thawing.

 

Solifluction - Slow downslope movement of soil and regolith in tundra and alpine areas due to thawing of permafrost