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Plate Tectonics
Plate Tectonics
Why is Plate Tectonics important?
• Plate Tectonics explains the distribution of:
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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.
Apparent Polar Wandering
• 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.
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.
Seafloor Spreading: Evidence
• Magnetic reversals & the seafloorMagnetic 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
• 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, AustralianIndian, 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/t
he_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/t
he_process_of_subduction.htm
Continental-Ocean Collision
Denser 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/anima
tions/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
Plate Tectonics: More Evidence
• 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.
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 coremantle 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.