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CHAPTER 10
PLATE TECTONICS
CONTINENTAL DRIFT
• Early explorers noticed similarities in
coastlines.
• Alfred Wegener, 1912 (German scientist)
• Hypothesis – Continental Drift
– The continents once formed a single land mass called
a supercontinent.
– The “crumpling” of the crust may have formed
mountain ranges.
– The single land mass broke up and continents drifted
to their present locations.
– He thought that the continents plowed through the
rock of the ocean floor … on this point he was wrong.
Continental Drift
Fossil Evidence for Continental Drift
• The same fossils are found on two continents,
suggesting that the continents were once connected.
– Mesosaur, small reptile: S. America & W. Africa (Lived 270 mya)
Not likely the
small reptile
swam the ocean
nor was there
any evidence of
land bridges.
Link
Evidence From Rock Formations
• The ages and types of rocks in the coastal regions of
widely separated areas matched closely.
• S. America and Western Africa
• Mountain chains that ended at the coastline of one
continent seemed to continue on other continents
across the ocean.
– The Appalachian Mountains extend northward
along the eastern coast of North America, and
mountains of similar age and structure are found
in Greenland, Scotland, and northern Europe.
– When reassembled into one continent, the
mountains of similar age line up and fit together.
Similar Rocks
Mountain Ranges
Climatic Evidence
• Changes in climatic patterns also suggest that
the continents have not always been located
where they are now.
– Layers of glacial debris from ancient glaciers in
S. Africa and S. America.
– Fern fossils in the Antarctica suggest that tropical or
subtropical climates once covered areas that are now
much colder. In other words, the continents have
moved.
Climates
Mid-Ocean Ridges
• Undersea mountain ranges through the center of
which run steep, narrow valleys.
– Sediment covering the sea floor is thinner closer to the
ridge than farther away from the ridge.
– The closer the sediment is to the ridge, the younger
the sediment.
– Ocean floor rock is very young (> 175 million years)
while rocks on land are as old as 3.8 billion years.
– Reason:????????????????????????????????????????
Mid-Ocean Ridges
• Undersea mountain ranges through the center of
which run steep, narrow valleys.
– Sediment covering the sea floor is thinner closer to the
ridge than farther away from the ridge.
– The closer the sediment is to the ridge, the younger
the sediment.
– Ocean floor rock is very young (> 175 million years)
while rocks on land are as old as 3.8 billion years.
– Reason: new ocean crust is being formed at the ridges.
Mid Atlantic Ridge
Sea-Floor Spreading
• Harry Hess, 1950’s (proposed a new hypothesis)
• The valley at the center of the ridge was a crack
or rift, in the Earth’s crust.
• At the rift, magma rises and moves away from the
rift, cools and solidifies, and forms new rock that
replaces the rock of the ocean floor (Sea-floor
spreading).
• It suggested that if the ocean floor is moving,
then the continents might also be moving.
• Explained Wegener’s hypothesis.
Mechanism
Evidence from Paleomagnetism
• The study of the magnetic properties of rock.
• Earth has North and South geomagnetic poles (it acts as a magnet).
• As magma solidifies to form rock, iron-rich minerals in the magma
align with Earth’s magnetic field.
• When the rock hardens, the magnetic orientation of the minerals
becomes permanent.
• Geologic evidence suggests that Earth’s magnetic field has not
always pointed North (normal polarity).
• There are periods of time when the Earth’s magnetic field points
South (reversed polarity).
• There is a pattern of alternating normal and reverse polarity
(geomagnetic reversal time scale).
• Supports Hess’ hypothesis of sea-floor spreading and Wegener’s
hypothesis of continental drift.
Earth’s crust is classified into:
Oceanic crust:
• Higher density
• Rock rich in iron and
magnesium (Mafic)
Continental crust:
• Lower density
• Rock rich in silica (Felsic)
Theory of Plate Tectonics
• 1960’s
• Earth’s crust and the rigid, upper part of the
mantle = lithosphere (thin outer shell of Earth)
– Broken into several blocks (tectonic plates)
– Plates ride on a deformable layer of the mantle
(asthenosphere)
o
o
“plastic” rock just below the lithosphere
“plastic” = solid rock that flows slowly
• When tectonic plates move, sudden shifts can
occur along their boundaries (earthquakes).
– San Andreas fault
• Volcanoes can also form along plate
boundaries.
– Pacific Ring of Fire
Types of
Plate Boundaries
• Divergent Boundaries
– Two plates move away from each other.
– Magma from the asthenosphere rises to the surface
at these boundaries to form new oceanic crust.
• The Atlantic ocean is moving at a rate of 1 to 2 cm per
year.
• The Pacific ocean is spreading at a rate of 3 to 8 cm per
year.
• Forms mid-ocean ridges and rift valleys
– Mid-Atlantic ridge/rift
• Most divergent boundaries are on the ocean floor
– Red Sea: separates African plate from Arabian plate
Convergent Boundaries
– Two plates move towards each other; collide
– Three types of collisions
• Oceanic lithosphere collides with Continental lithosphere
– Oceanic crust is more dense and sinks (subducts) under the less dense
continental crust.
– Called a subduction zone
– Forms a deep ocean trench, mantle melts to form magma
– Magma rises to form volcanoes.
– Continental crust does not subduct
• Continental lithosphere collides with Continental lithosphere
– Both of the same density
– The colliding edges crumple and thicken which causes uplift that forms
mountain ranges.
» Himalaya mountains
• Oceanic lithosphere collides with Oceanic lithosphere
– One plate subducts under the other plate and a deep ocean trench
forms
– Mantle melts and forms magma
– Magma rises to the surface to form an island arc (chain of volcanic
islands). An example is Japan
Transform Boundaries
– Two plates slide past each other, horizontally.
• Move in opposite direction
• Move in same direction at different speeds
– They do not slide smoothly, they scrape against
each other in a series of sudden jerks that are felt
as earthquakes.
• San Andreas Fault ….
N. American plate and
Pacific plate.
– Do not produce lava
Causes of Plate Motion
• Mantle Convection
– Rising hot material and sinking colder material.
Plate Movement
Reshaping Earth’s Crust
• Slow movements of the tectonic plates change
the size and shape of the continents over
millions of years.
• All continents contains large areas of stable
rock called cratons ( > 540 million years old)
• Cratons represent ancient cores that the
continents formed around.
• Rocks containing Cratons exposed at the
surface are called shields.
Cratons
• Rifting causes crust to break apart.
• The East African Rift Valley is splitting a continent.
– Terranes are pieces of land that get added to a
continent at convergent boundaries… becomes part of
a continent.
• Accretion (added)
– Sea mounts, atolls, islands
– Himalaya Mtns. formed when India collided with Asia 45 mya.
• Some continents were once closer to the poles.
– Ice once covered Earth’s continents, including the
Sahara in Africa (once closer to the South Pole).
– As continents rift or as mountains forms, populations
of organisms become separated (new species evolve).
• Madagascar separated from Africa and India
Terranes
The Supercontinent Cycle
• Several periods of supercontinents … break apart and reform.
– Rodina formed 750 mya
– Pannotia formed 450 mya
• Supercontinent PANGAEA formed about 300 mya (Paleozoic).
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Appalachian (North America) and Ural (Russia) mountains formed.
Tethys Sea cut into the eastern edge of Pangaea.
Single large ocean = Panthalassa
250 mya, Pangaea broke into two continents = Laurasia and
Gondwanaland
North Atlantic ocean forms and Tethys Sea becomes the
Mediteranean Sea.
Gondwanaland broke into Africa and South America.
Further break ups formed India, Australia, and Antarctica.
Slowly the continents moved into their present positions.
Future Movements
• In 150 million years
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Africa will collide with Eurasia
Mediterranean Sea will close
East Africa will separate from the rest of Africa
New ocean will form
North American and South American plates will move
westward.
Eurasia and Africa move eastward
Atlantic ocean will become wider
Australia will continue to move northward, and will
eventually collide with Eurasia.
Western California will move towards Alaska
In 250 million years, a new supercontinent will form.