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Natural DisastersPlate Tectonics & Physical Hazards

Current Event--MammothChile Earthquake

Chile Tsunami

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Continental Drift

– Geographic fit of continents– Fossils– Mountains– Glaciation

• Evidence supporting the idea that the continents had Evidence supporting the idea that the continents had drifteddrifted.

• Researchers noted geographic fit of continentsResearchers noted geographic fit of continents• e.g. Africa and S. Americae.g. Africa and S. America• Atlantic formed by separation of Africa from S. Atlantic formed by separation of Africa from S.

AmericaAmerica• Seuss, 1885, proposed super continent by studying Seuss, 1885, proposed super continent by studying

fossils, rocks, mountainsfossils, rocks, mountains

• Wegener and Taylor, early 1900’s, proposed Wegener and Taylor, early 1900’s, proposed continental drift and Pangaeacontinental drift and Pangaea

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Continental Drift – Geographic Fit

•Continents seem to fit like pieces of a puzzle

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Continental Drift - Fossils

•Similar distribution of fossils such as Mesosaurus

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Continental Drift – Mountain Ranges

•Mountain ranges match across oceans

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Continental Drift - Glaciation

•Grooves left by glaciers indicate location of glaciers and direction of movement

•Past glaciation indicates position of paleocontinents

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Problem with Continental Drift

• Alfred Wegener

• Presented hypothesis to other professionals

• Did not provide Did not provide plausible mechanism plausible mechanism to explain how to explain how continents driftedcontinents drifted

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New Theory Developed – Seafloor Spreading

•Harry Hess suggested new seafloor is created at mid-ocean ridges and destroyed in deep ocean trenches

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Seafloor Spreading

• Continental drift reexamined in 1960’s with new information

• Supporting evidence for seafloor spreadingSupporting evidence for seafloor spreading

– EarthquakesEarthquakes– VolcanosVolcanos– Age of SeafloorAge of Seafloor– PaleomagnetismPaleomagnetism

Evidence for Seafloor SpreadingWorld Seismicity

• Earthquake distribution matches plate boundaries

Evidence for Seafloor SpreadingVolcanism

• Volcanoes match some plate boundaries; some are hot spots

Evidence for Seafloor SpreadingAge of Seafloor

• Youngest sea floor is at mid-ocean ridge• Oldest sea floor away from mid-ocean ridge

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Seafloor Spreading - Paleomagnetism

• Earth’s magnetic field reverses approx. every 600,000 years

• As seafloor spreads, normal and reverse polarities are preserved in the rock record

• When rocks cool at the Earth’s surface, they record Earth’s magnetic field

• Earth has a magnetic field

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Seafloor Spreading – Age of Seafloor

• Young rocks (red) found near mid-ocean ridges (MOR)• Away from MOR, age of seafloor gets progressively

older (blue)

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What Drives Plate Motion?

• Convection Currents

• Air heats up, expands, and rises• As air moves away, it cools, contracts, and sinks

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Mechanism for Seafloor Spreading

•Convection Currents– As heat rises, it moves away at spreading centers pulling

plates apart– Plates slide over asthenosphere– The upper mantle then cools and becomes more dense –

sinking at ocean trenches

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Plate Tectonics Theory

• John Tuzo Wilson combined ideas of Continental Drift and Seafloor Spreading into “Plate Tectonics”

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Earth’s Structure

• Earth’s internal structure

– Chemical Composition

– Physical Property

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Earth’s Structure

•Chemical CompositionLayers divided by its composition

– Crust• Continental crust

– Granite (2.8 g/cm3)• Oceanic Crust

– Basalt (3.0 g/cm3)

– Mantle• Composed of Fe & Mg

– Core• Composed of Fe & Ni• Two parts

–Outer core–Inner core

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Earth’s Structure

•Physical Properties Layers divided into rigid, plastic, or liquid

– Lithosphere• Rigid

– Asthenosphere• Plastic

– Mesosphere• Rigid

– Outer Core• Liquid Layer

– Inner Core• Solid Layer

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Lithospheric Plates

•Comprise approx. 12 large plates and 12 smaller plates

•Lithospheric plates are rigid layers that flow over a partially molten (plastic) asthenosphere

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Plate Boundaries

•Divergent•Convergent•Transform

•Where plate boundaries meet . . .

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Divergent Plate Boundary

•Boundaries where plates pull apart•New crust is being formed

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Divergent Examples

•East Africa Rift Zone •Mid-Atlantic Ocean Ridge

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Convergent Plate Boundary

•Plates move toward each other (collide)•Crust is being destroyed•Three types:

– Ocean-continent– Ocean-ocean– Continent-continent

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Ocean-Continent Collision

•Produce volcanic arc system

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Ocean-Continent Examples

•Cascade Ranges – Northern California, Oregon, and Washington (left)

– Form as oceanic crust collides with continental crust

•Mount St. Helens, Washington (right)

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Ocean-Ocean Collision

•Form a chain of island arc volcanoes

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Ocean-Ocean Examples

• Japanese Islands– Form along a trench as

two oceanic plates collide– Mount Fuji (right) active

volcano in Japan– Mount Ontake Erupts- 9\14

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Continental-Continental Collision

• Continental crust have same densities– Neither plates sink (subduct)– Form high mountain chains

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Continental-Continental Examples

• Himalayas – home of Mount Everest (highest mountain in the world)

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Transform Plate Boundaries

•Plates slide past one another•Crust is neither created nor destroyed

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Convergent Boundary Examples

• San Andreas Fault– Runs almost the entire

length of California

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Hot Spot Volcanism

•Hot spot volcanoes around the world•Site of mantle plumes from deep within the core-

mantle boundary•Lithospheric plates slide over hot spots leaving a

chain of dormant (not active) volcanoes

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Hot Spot Examples

•Formation of a volcanic island chain as oceanic plate moves over a stationary hot spot

•The age of the islands increases toward the left•New islands will continue to form over the hot spot

– Loihi will be the next island in the chain