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Plate Tectonics
Plate TectonicsWhy is Plate Tectonics important?
• Plate Tectonics explains the distribution of:– Mid-Ocean Ridges,– Deep-Sea Trenches,– Earthquakes,– Volcanoes,– Continents,– Mountains, and– Fossils around the world.
• Plate Tectonics also helps to explain the formation of Earth’s atmosphere and oceans.
Plate Tectonics
Plate Tectonics is the scientific theory that:• The Earth is divided into lithospheric plates (7
major plates, many smaller plates).
• Lithospheric plates are in motion and continually changing in shape and size.
• Movement of lithospheric plates is controlled by movement within the Mantle.
Plate Tectonics: The Early Years
• Pre-1960’s accepted view that ocean basins & continents had fixed locations.
• By the 1970’s a revolution in the form of Plate Tectonics swept the geologic community.
• Where did it all start?- Continental Drift Hypothesis.
Continental Drift
• Alfred Wegener (German meteorologist & geophysicist) like many others since the 1600’s noticed that South America and Africa seemed to “fit together”.– In 1915 proposed a hypothesis, called continental
drift, that the continents had drifted apart.– He suggested that at one time all the continents were
connected together. This super continent he called Pangaea (Greek for “all land”).
– Wegener hypothesized that roughly 200 MYA Pangaea broke apart and the continents “drifted” towards their present positions.
Wegener’s Evidence:
• Wegener’s evidence came from four sources.–Fit of continents.–Fossil distribution.–Rock types and geologic features.–Ancient climates (Paleoclimates).
Fit of Continents
• Coastline of South America and Africa seem to fit well.
• Continental margins fit better!
Fossil Evidence• Identical fossil organisms found on opposite sides of
large oceans.
• Mesosaurus– Fresh water reptile– Elongated head & snout– ~ 0.5 m long
• Lystrosaurus – Mammal like reptile probably an herbivore kind of like a hippo– Tusks– Called “spoon lizard”
• Glossopteris– Seed fern– Large seed kind of like modern day trees.
• Too large to be carried long distances by wind.
Rock types and Geologic Features
• 2.2 billion year old rocks in Brazil closely resemble rocks of similar age in Africa.
• Matching mountain ranges across the North Atlantic.
• Rocks from the Appalachian Mountains matched in age and structure to rocks found in the British Isles and Scandinavia.
Ancient Climates
• Wegener was a meteorologist. Interested in climate changes.
• Evidence came from South America, Africa, India and Australia.
• Distribution of glacial deposits in southern hemisphere from 300-220 MYA.– Glacial till suggested large continental glaciers.
• Direction of ice movement.– Looked like in some areas ice moved from sea onto
land. Very strange!– If land masses were clumped by south pole, that
would explain glaciers!
The Great Debate
• Hypothesis widely criticized.• One major problem was the mechanism of the
movement. – Wegener suggested that possibly tidal forces from
sun & moon caused the slow movement.• Harold Jeffreys (physicist) countered that the magnitude of
the tidal forces would have to so great that they would stop the rotation of the earth in a matter of a few years.
• Wegener had the concept right, but some of the details were wrong. Thus the hypothesis was treated with skepticism.– Some considered the idea plausible & continued the
search for evidence.
The Great Debate
• Mid 1950’s new evidence began to emerge that supported Wegener’s Continental Drift Hypothesis.– Came from exploration of the seafloor and
new field of Paleomagnetism.
Paleomagnetism
• Earth is like a gigantic magnet.– Spinning outer core generates a magnetic
field.
Paleomagnetism
• As iron rich igneous rocks cool, iron in rocks will become magnetized & align w/ current magnetic field.– Once rocks cool enough (below Curie point –
585oC) magnetism becomes “frozen” in place.• Points to the location of the magnetic poles at time
of rock formation.
• Paleomagnetism is the term used for the preserved magnetism.
Apparent Polar Wander
• 1950’s – Paleomagnetic data from lava flows of different ages indicated that the magnetic poles shifted over time.– W/ respect to Europe, magnetic north moved
from near Hawaii to it’s present location.• This strongly suggested that either the
magnetic poles migrated, or Europe had drifted.
• Comparing the polar wander paths for multiple continental bodies helped determine which explanation was correct.– If continents stayed stationary, then polar
wander paths would be the same for all continents.
– If continents moved, polar wander paths would be different.
Apparent Polar Wandering
Polar wander path in current continent configuration
Polar wander path with reassembled continents.
Seafloor Spreading
• Submarines mapped the longest mountain chain in the world.– It wrapped around the entire planet and it was in the
middle of the oceans.
• Before this time, people thought the oceans were mostly flat and boring.– Found that the deepest areas were not in the middle
of the oceans, but along the edges (deep-ocean trenches)
– The shape of the Mid-Ocean Ridges also matched the shape of some continental margins!
Seafloor Spreading
• Early 1960’s – Harry Hess hypothesized that the mid ocean ridges were above areas of upwelling in the mantle.– Convection in the mantle caused the motion
of the seafloor.– Crust was continuously moving away from the
ridge areas, allowing new crust to be formed.– Trenches were areas where the crust dipped
back down into the mantle to be “recycled”.
Seafloor Spreading: Magnetic Reversals
• Early 1960’s – geophysicists discovered evidence of magnetic reversals in iron rich igneous rocks.– Magnetic reversal – magnetic north & south switch
places.– Normal polarity – same magnetism as present.– Reverse polarity – opposite magnetism than present.
• Age dated each lava flow & recorded the direction on the magnetic field.
Seafloor Spreading: Magnetic Reversals
• Magnetic time scale established the general timing of reversals.– Chron – major division of the magnetic time
scale where the magnetic field is dominantly normal or reverse.• Short periods of the opposite polarity can occur
during chron – generally around 200,000 long.
• Magnetic reversals & the seafloor- Magnetic surveys of ocean floor revealed alternating bands of high and low intensity magnetism – often called “zebra stripes”– The bands correspond to times of normal and
reverse polarity. – The bands are parallel to mid-oceans ridge &
are remarkably symmetrical on either side of the ridge.
Seafloor Spreading: Evidence
Seafloor Spreading: Evidence
• If oceanic crust was created all at one time, then the entire seafloor would have the same magnetic polarity.
• “Zebra stripes” exist because new crust is continuously added at the ridge slowly, pushing the older crust away from the ridge making the seafloor spread.
Plate Tectonics
• 1968 - Seafloor spreading merged with continental drift giving rise to plate tectonics.– The Earth is divided into lithospheric plates.– Lithospheric plates are in motion and
continually changing in shape and size.– Movement of lithospheric plates is controlled
by movement within the Mantle.
7 major plates: North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic.
Plate Tectonics
3 types of plate boundaries• Divergent• Convergent• Transform
Plate Tectonics
Divergent boundaries are where two pieces of crust move apart.
• Hot mantle material wells upwards creating new seafloor.
• Volcanoes and earthquakes occur at divergent boundaries.
• Geologic Features: Mid-Ocean Ridges, Continental Rift Valleys.
http://www.wwnorton.com/college/geo/animations/the_process_of_rifting.htm
The East African Rift Valley is an example of a divergent plate boundary.
Plate Tectonics
Convergent boundaries where two plates move towards each other.
• Oceanic lithosphere is subducted & destroyed.– Continental lithosphere uplifted to create mountains.
• Volcanoes and earthquakes occur here.• Geologic Features: Deep-Sea Trenches,
Continental Volcanic Arcs, Volcanic Island Arcs.• Three types of convergence.
http://www.wwnorton.com/college/geo/animations/the_process_of_subduction.htm
Continental-Ocean CollisionDenser oceanic crust is being subducted beneath (carried down below) the less dense continental crust.
Ocean-Ocean Collision
Continental-Continental Collision
Plate Tectonics
Transform boundary where two plates slide past each other.
• Lithosphere neither created nor destroyed.• Earthquakes.• No volcanoes (usually).• Geologic Features: Fracture Zones.
http://www.wwnorton.com/college/geo/animations/transform_faulting.htm
The San Andres Fault is a transform plate boundary created by the Pacific plate moving past the North American plate.
Plate Tectonics: More Evidence
• Deep Sea Drilling Project (1968-1983)– Hundreds of holes drilled into seafloor through
sediments & into the crust.• Used fossils to date the sedimentary layers.
– Found that youngest sediments & crust were located near the ridges & oldest parts of seafloor near the continents.• Also found that sediment layers grew in thickness
with distance from the ridge.
Plate Tectonics: More Evidence
• Mapping projects of volcanoes & sea mounts in the Pacific Ocean revealed linear chains of volcanic structures.
• Well know example is Hawaii– Radiometric dating showed that the ages of
the islands increased with increasing distance from the Main island.• Main island ~ 1 MY old• Midway island ~27 MY old• Suiko seamount ~ 65 MY old
• Hot Spots – formed by a plume of hot mantle material.– Creates an area of volcanism & high heat flow
in the crust that is independent of the plate boundaries.• Mantle plumes do not move, they are in fixed
locations.– As plates move over a hot spot, it leaves be
hind a chain of volcanic structures (hot spot track).• Age of volcanic structure indicates how long it has
been since that area of the plate was located over the hot spot.
Plate Tectonics: More Evidence
Hot spots and hot spot tracks
Plate Motion
• Plates move at a slow but continuous rate relative to each other.
• Average rate is about 2-3 centimeters (1 inch) per year.
• Can track plate motion in several ways:– Take age of seafloor and divide if by distance from
spreading zone.– Hot Spots– Width of magnetic strips in seafloor.– Satellites
Current rates motion of selected surface points collected from satellite data.
What Drives Plate Motion?
• Evidence for mantle motion seen in Seismic tomography.– Tomography measures density of materials
from velocities of seismic waves.• More and less dense regions in Earth’s
interior can be linked to higher and lower velocities.
• Has been used to locate hot spots and mantle plumes.
Seismic-velocity image of earth’s interior. Blue regions are cooler (faster moving) areas and red-brown are warmer (slower moving areas).
What Drives Plate Motion?
• Convection (=heat circulation) in asthenosphere is a major force that drives plate motions.
• Upwelling of heat and magma occurs at divergent plate boundaries.– New oceanic crust formed.
• Cooler, denser slabs of oceanic lithosphere descend into the mantle at subduction zones.
• Asthenosphere that does not be come crust spreads outwards along the bottom of the lithosphere, cooling and eventually sinking.
What Drives Plate Motion?
• Convection not the only force.• Gravity also drives plate motion.
– Slab Pull• Cold dense slabs of subducted crust sink & pull on
the trailing plate. – Ridge-push
• Rising magma pushes new crust created at mid ocean ridges higher that older crust. Thus pushing the whole plate down & away from the ridge.
Mantle – Plate Convection
• Two models for convection in the mantle– Need to account for compositional differences
in rocks created from mantle material
Mantle – Plate Convection
• Layering at 660 km– 2 zones of convection w/ line between at
660km (bottom of the transition zone).– Hot spot source is from deeper convection
cell & mid-ocean ridge from upper convection cell.
– Subducting slabs incorporated into upper convection cell.• Recent data shows that some subducting oceanic
lithosphere makes it down below 660km to the core-mantle boundary
Mantle – Plate Convection
• Whole-Mantle Convection– Subducted oceanic material sinks to core-
mantle boundary & helps stir the entire mantle.
– Models show that whole-mantle convection would cause the entire mantle to be mixed w/in a few MY, eliminating composition differences.• Doesn’t fit with what we observe w/ hot spots.
