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Natural Disasters
Plate Tectonics & Physical Hazards
Current Event--Mammoth
Chile Earthquake
Chile Tsunami
1
Continental Drift
• Researchers noted geographic fit of continents
• e.g. Africa and S. America
• Atlantic formed by separation of Africa from S. America
• Seuss, 1885, proposed super continent by studying fossils,
rocks, mountains
• Wegener and Taylor, early 1900’s, proposed continental drift
and Pangaea
• Evidence supporting the idea that the continents had drifted.
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Geographic fit of continents
Fossils
Mountains
Glaciation
Continental Drift – Geographic Fit
• Continents seem to fit like pieces of a puzzle
Continental Drift - Fossils
• Similar distribution of fossils such as Mesosaurus
Continental Drift – Mountain Ranges
• Mountain ranges match across oceans
Continental Drift - Glaciation
• Past glaciation indicates position of paleocontinents
• Grooves left by glaciers indicate location of glaciers
and direction of movement
Problem with Continental Drift
• Alfred Wegener
• Presented hypothesis to
other professionals
• Did not provide plausible
mechanism to explain
how continents drifted
New Theory Developed – Seafloor Spreading
• Harry Hess suggested new seafloor is created at midocean ridges and destroyed in deep ocean trenches
Seafloor Spreading
• Continental drift reexamined in 1960’s with new
information
• Supporting evidence for seafloor spreading
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Earthquakes
Volcanos
Age of Seafloor
Paleomagnetism
Evidence for Seafloor Spreading
World Seismicity
• Earthquake distribution matches plate
boundaries
Evidence for Seafloor Spreading
Volcanism
• Volcanoes match some plate boundaries; some are hot
Evidence for Seafloor Spreading
 Age of Seafloor
• Youngest sea floor is at mid-ocean ridge
• Oldest sea floor away from mid-ocean ridge
Seafloor Spreading - Paleomagnetism
• Earth has a magnetic field
• When rocks cool at the Earth’s surface, they record Earth’s
magnetic field
• Earth’s magnetic field reverses approx. every 600,000 years
• As seafloor spreads, normal and reverse polarities are preserved in
the rock record
Seafloor Spreading – Age of Seafloor
• Young rocks (red) found near mid-ocean ridges (MOR)
• Away from MOR, age of seafloor gets progressively older
(blue)
What Drives Plate Motion?
• Convection Currents
• Air heats up, expands, and rises
• As air moves away, it cools, contracts, and sinks
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
Plate Tectonics Theory
• John Tuzo Wilson combined ideas of
Continental Drift and Seafloor Spreading into
“Plate Tectonics”
Earth’s Structure
• Earth’s internal structure
– Chemical Composition
– Physical Property
Earth’s Structure
• Chemical Composition
Layers 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
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
Lithospheric Plates
• Comprise approx. 12 large plates and 12 smaller plates
• Lithospheric plates are rigid layers that flow over a
partially molten (plastic) asthenosphere
Plate Boundaries
• Where plate boundaries meet . . .
• Divergent
• Convergent
• Transform
Divergent Plate Boundary
• Boundaries where plates pull apart
• New crust is being formed
Divergent Examples
• East Africa Rift Zone
• Mid-Atlantic Ocean Ridge
Convergent Plate Boundary
• Plates move toward each other (collide)
• Crust is being destroyed
• Three types:
– Ocean-continent
– Ocean-ocean
– Continent-continent
Ocean-Continent Collision
• Produce volcanic arc system
Ocean-Continent Examples
• Cascade Ranges – Northern California, Oregon, and
Washington (left)
– Form as oceanic crust collides with continental crust
• Mount St. Helens, Washington (right)
Ocean-Ocean Collision
• Form a chain of island arc volcanoes
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
Continental-Continental Collision
• Continental crust have same densities
– Neither plates sink (subduct)
– Form high mountain chains
Continental-Continental Examples
• Himalayas – home of Mount Everest (highest mountain
in the world)
Transform Plate Boundaries
• Plates slide past one another
• Crust is neither created nor destroyed
Convergent Boundary Examples
• San Andreas Fault
– Runs almost the entire
length of California
Hot Spot Volcanism
• Hot spot volcanoes around the world
• Site of mantle plumes from deep within the coremantle boundary
• Lithospheric plates slide over hot spots leaving a chain
of dormant (not active) volcanoes
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