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Tectonic Plate Theory
Understanding Basic Principles of
Earth Science Related to Geology
Earth’s Crust
 Earth’s crust is the outermost layer.
 The rigid crust “floats” on the molten part of
the mantle called the asthenosphere.
Pangea
 Pangea was the supercontinent from which all
continents have split.
 In 1915 Wegener proposed Earth’s continents looked
this way about 225 million years ago.
 The theory of continental drift was used to explain the
current positions of the continents.
 Wegener could not come up with a mechanism for
continental drift.
Evidence that Pangea
Existed
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Fit of the continents
Fossil similarities
Rock similarities
Mountain chains
*Paleoclimatic evidence
*Paleomagnetism
Theory of Plate Tectonics
Fit of the Continents
 North and South America match almost
perfectly with Africa and Europe
Fossil Similarities
 *Mesosaurus, a reptile similar to an alligator
once lived in the shallow waters of both
South America and Africa.
Rock Similarities
 Rocks of same age are found in locations of
South America and Africa that were once
joined in Pangea.
Termination of Mountain
Chains
 The map shows location of Pangean
mountain chains (dark blue) that correspond
with current mountain chains.
Paleoclimatic Evidence
 Glacial deposits at the equator
 Coral reefs in Antarctica
 Therefore the landmasses must have
been in different locations in the past.
Magnetism
 Earth is a bar magnet
 Magnetic north and south poles
 These are not the same location
as the geographic north and south
poles
 Exact location varies year-to-year
Paleomagnetism
 This is magnetism
“frozen” in the rock at
the time it was
formed.
 Geologists can
determine pole
reversals by studying
rock all over the
globe.
Sea Floor Spreading
 Surveys of the ocean basins showed a
system of ridges and trenches.
 This led geologists to come up with the
idea of Sea Floor Spreading.
 Symmetrical magnetic “stripes” were
discovered on either side of the Atlantic
Mid-ocean ridge.
Sea Floor Spreading
Magnetic Reversals along
a Mid-Ocean Ridge
(“stripes”)
Youngest,
newest rock
is along the
ridge.
Oldest
rock is
farthest
from
ridge.
Theory of Plate Tectonics
 A rigid piece of lithosphere (outer
crust) is floating on a partially
plastic (molten, soft)
asthenosphere
 Seven Major Plates
 3 types of Plate Boundaries
Plate Boundaries
 *Divergent-Spreading CenterOcean ridges and seafloor
spreading (Atlantic Ocean)
 *Convergent-Creates trenches and
island arcs (Pacific Ocean)
 *Transform-Plates move past one
another (San Andreas Fault in
California)
Divergent Boundaries
 Magma from deep within Earth forces
its way between two plates, pushing
them apart.
 The magma cools and forms new
seafloor.
 More magma emerges and pushes
apart the cooled rock.
 Seafloor Spreading/mid-oceanic
ridges
Rift Valley
 A rift valley forms when a
continent is slowing splitting
apart along a divergent boundary.
 The African Rift Valley is an
example.
Locations of Seafloor
Spreading
Convergent
 Plates move together
 Oceanic crust slides under
continental crust.
 Subducting crust slides under.
 Andes Mountains Chain in South
America is an example.
 Mountains formed from volcanoes
along the convergent boundary.
Convergent Boundaries
 The Pacific Ocean by several
continental plates.
 The ocean basin is sliding under the
continental plates along the plate
boundaries.
 As a result, the Pacific Ocean is
growing smaller.
Why the Pacific is Shrinking
and the Atlantic is Growing
Hawaiian Islands are an
island arc.
More Examples of
Convergent Boundaries
 Japanese Islands (Pacific Ocean)
 Cascade Mountains (Northwestern
U.S.A.)
 Himalaya Mountains (Nepal-Asia)
Transform Plate Boundary
 Plates move past one another along
strike-slip faults
What Drives the Tectonic
Plates??
 Convection Cells
 Hot Spots