Transcript 17.3 Theory of plate Tectonics

17.3 Theory of Plate Tectonics
Plate Boundaries
• Tectonic plates interact at places called
plate boundaries.
• At some plate boundaries:
1. Plates come together, or converge
2. Plates move away from one another, or diverge
3. Plates move horizontally past one another
Divergent Boundaries
– Divergent boundaries are places where two
tectonic plates are moving apart.
– Most divergent boundaries are found in rifts.
– A rift valley, which is a
narrow depression, is
created when a
divergent boundary
forms on a continent.
Convergent Boundaries
– Convergent boundaries are places where
two tectonic plates are moving toward each
other.
– There are three types of convergent boundaries:
1. Oceanic crust converging with oceanic
crust
2. Oceanic crust converging with continental
crust
3. Continental crust converging and colliding
with continental crust.
Convergent Boundaries
Convergent Boundaries
– Subduction occurs when one of the two converging
plates descends beneath the other.
– A subduction zone forms when one oceanic plate, which
has become denser as a result of cooling, descends
below another plate creating a deep-sea trench.
– Some of the magma forms new oceanic crust at the
ridge or is forced back to the surface, forming an arc of
volcanic islands that parallel the trench.
Convergent Boundaries
– When an oceanic plate converges with a continental
plate, the denser oceanic plate is subducted.
– Oceanic-continental convergence produces a trench
and a series of volcanoes along the edge of the
continental plate.
– Two continental plates collide when an ocean basin
between converging oceanic and continental plates is
entirely subducted.
– Because continental rocks are too buoyant to be forced
into the mantle, the colliding edges of the continents are
crumpled and uplifted to form a mountain range.
Transform Boundaries
– A transform boundary is a
place where two plates slide
horizontally past each other,
deforming or fracturing the
crust.
– Transform boundaries are
characterized by long
faults.
– The San Andreas Fault is
an exception to the fact
that transform boundaries
rarely occur on
continents.
Section Assessment
1. Match the following terms with their
definitions.
B divergent
___
boundary
E rift valley
___
A convergent
___
boundary
D
___
transform
boundary
C
___subduction
zone
A. place where two tectonic plates
are moving toward each other
B. place where two tectonic
plates are moving apart
C. when one of two converging
plates descends beneath
the other
D. place where two plates slide
horizontally past each other
E. result of a divergent boundary
forming on a continent
Section Assessment
2. What happens to an oceanic plate once it
is subducted? What is created with the
material?
The subducted plate melts in the mantle. Some
of the resulting magma is forced to the surface
creating a series of volcanoes that are parallel to
the subduction zone. Some of the magma is
recycled into new oceanic crust at the ridge.
Section Assessment
3. Why does uplift occur when two continental
plates converge? Give an example of this
process.
The rocks that make up a continental plate are
too buoyant to be forced into the mantle. As a
result, the colliding edges of the continents are
crumpled and uplifted to form a mountain range
such as the Himalayas.