Transcript Ch. 4.2 The Theory of Plate Tectonics

Ch. 4.2 The Theory of
Plate Tectonics
The theory of plate tectonics explains the
what, how and why of continental
movement.
Two types of earth’s crust: oceanic crust
(makes up the ocean floor), and
continental crust (makes up the
continental landmasses).
Lithosphere—the oceanic and continental
crust and the rigid upper mantle.
Asthenosphere—a layer of slowly flowing,
deformable rock under the lithosphere.
Lithosphere is broken into separate plates
that “float” on the denser asthenosphere.
The continents and oceans are carried
along on the moving lithospheric plates.
There are about 30 identified
lithospheric plates, most consisting
of both types of crust.
Plates moving in relation to each other
have created earth’s major surface
features.
Plates can either move towards each
other, away from each other (mid ocean
ridges), or they can slide past each other.
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Lithospheric plate boundaries don’t
necessarily correspond to the outlines of
continents. They may extend out into
oceans, or lie within continents.
Divergent Boundaries
When two plates move away from each
other.
Melted asthenosphere rises to fill the void.
It forms mid-ocean ridges, with a narrow
rift valley in the center.
Rift valleys are wide where continents are
separating (Example: the Red Sea).
Convergent Boundaries
When one plate collides with another.
Three possible types of collisions,
depending on if the plate has oceanic
crust or continental crust as its leading
edge.
When oceanic crust collides with
continental crust, the denser oceanic crust
is subducted (forced under) the less dense
continental crust.
This area is called a subduction zone, and
results in deep ocean trenches and
volcanic mountains.
When two plates of continental crust come
together, neither is subducted. Instead,
the leading edges are crumpled and
uplifted, forming high mountains.
When two plates of oceanic crust come
together, the result is again a subduction
zone, with deep ocean trenches and a
chain of volcanic islands called an island
arc.
Transform Boundaries
Where two plates are grinding past each
other.
Not smooth movement. Series of sudden
movements follow by long periods of
inactivity (Ex. San Andreas Fault is on a
major transform boundary)
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Causes of Plate Motion
Convection—the transfer of heat through
the movement of heated material.
Convection Current—the cycle of warm
material rising and cool material sinking.
Rising asthenosphere material at
divergent boundaries forms new
lithosphere.
As it moves away from the divergent
boundary, it eventually cools and sinks.
The downward pull of the sinking plate is
believed to be the main driving force for
tectonic plate movement.