Chapter 9
Plate Tectonics
Section 9.2
Plate Tectonics
Plate tectonics
• By the 1960’s, accumulated evidence supporting
the hypothesis of continental drift and seafloor
spreading led to the formation of a more farreaching theory.
• This theory is called plate tectonics.
• The theory of plate tectonics not only describes
continental movement but also proposes a possible
explanation of why and how continents move.
• The term tectonics comes from the Greek word
tekonikos, meaning “construction.”
• Tectonics is the study of the formation of features in
the earth’s crust.
Plate tectonics
• According to the plate tectonics theory, the
uppermost mantle, along with the overlying crust,
behaves as a strong, rigid layer.
• This layer is known as the lithosphere.
• This outer shell lies over a weaker region in the
mantle known as the asthenosphere.
– The lithosphere floats upon the asthenosphere and
permits plate motion.
• The lithosphere is divided into segments called
plates, which move and continually change shape
and size.
– There are 7 major plates and many smaller plates.
• Seven Major Plates: Eurasian, African, Australian-Indian,
Antarctic, Pacific, North American, and South American.
Plate tectonics
Plate tectonics
• The largest plate is the Pacific plate, which
covers most of the Pacific Ocean.
• Several of the plates include an entire
continents plus a large area of the seafloor.
– Continental and Oceanic lithosphere.
• This is a major departure from Wegener’s
hypothesis of continental drift, which
proposed that the plates moved through the
ocean floor, not with it.
Plate tectonics
• The lithospheric plates move relative to each
other at a very slow but continuous rate that
averages about 5 centimeters per year.
• This movement is driven by the unequal
distribution of heat within Earth (Earth’s
convection cycle).
– Hot mantle rises, cooler mantle sinks, setting
Earth’s outer shell into motion.
• This grinding movement generates
earthquakes, creates volcanoes, and deforms
large masses of rock into mountains.
According to the theory of plate
tectonics,
A. The asthenosphere is divided into plates.
B. The lithosphere is divided into plates.
C. The asthenosphere moves over the
lithosphere.
D. The asthenosphere is strong and rigid.
Which of the following statements
correctly describes the
asthenosphere?
A.
B.
C.
D.
It is a thin, cold, and rigid layer.
It is the source of Earth’s heat.
It permits plate motion.
It occurs only near subduction zones.
In the plate tectonics theory, the
lithosphere is divided into
A.
B.
C.
D.
100 major plates.
7 major plates and many smaller plates.
Many small plates, but no large plates.
50 major plates and many smaller plates.
The lithospheric plates move an
average of
A.
B.
C.
D.
5 inches per year.
50 inches per year.
5 centimeters per year.
50 centimeters per year.
A tectonic plate consists of
A.
B.
C.
D.
The crust and uppermost mantle.
The oceanic and continental crust only.
The crust and entire mantle.
The asthenosphere only.
In the plate tectonic theory, a plate
can be made up of
A.
B.
C.
D.
Continental lithosphere only.
Oceanic lithosphere only.
Both continental and oceanic lithosphere.
Both continental and oceanic
asthenosphere.
Plate tectonics
•
•
All major interaction among individual plates
occur along their boundaries.
There are three main types of boundaries.
– Convergent.
– Divergent.
– Transform.
Plate tectonics
1.
•
•
•
•
Convergent:
Form where two plates move
together.
This results in oceanic
lithosphere plunging beneath an
overriding plate, and
descending into the mantle.
Scientists refer to the region
along a plate boundary where
one plate moves under another
as a subduction zone.
A deep ocean trench generally
forms along a subduction zone.
–
Ex: The Andes in South
America.
Plate tectonics
2. Divergent:
•
Occur when two plates
move apart.
–
•
Also called spreading
centers.
Results in upwelling of
material from the mantle
to create new seafloor.
This formation is called a
rift valley.
•
–
Ex: The Mid-Ocean Ridge
and the East African Rift
Valley.
Plate tectonics
3. Transform Fault:
• Margins where two
plates grind past each
other without the
production or
destruction of
lithosphere.
– Ex: San Andreas Fault
Plate tectonics
• Each plate contains a combination of these three
types of boundaries.
• The total surface area of the Earth does not change,
but the plates may shrink or grow.
• This shrinking/growing depends on the locations of
convergent and divergent boundaries.
– Ex: The Antarctic plate is getting larger while the Phillipine
plate is becoming smaller.
• New plate boundaries can be created because of
changes in the forces acting on these rigid slabs.
What kind of plate boundary occurs
where two plates grind past each
other without destroying or producing
lithosphere?
A.
B.
C.
D.
Divergent boundary.
Convergent boundary.
Transitional boundary.
Transform fault boundary.
A divergent boundary at two
oceanic plates can result in a
A.
B.
C.
D.
Rift valley.
Volcanic island arc.
Continental volcanic arc.
Subduction zone.
What type of boundary occurs where
two plates move together, causing
one plate to descend into the mantle
beneath the other plate?
A.
B.
C.
D.
Transform fault boundary.
Divergent boundary.
Convergent boundary.
Transitional boundary.
Which of the following is a
geographic example of a transform
fault boundary?
A.
B.
C.
D.
The East African Rift valley.
The San Andreas Fault.
The Mid-Atlantic Ridge.
The Andes Mountains.
New ocean crust is formed at
A.
B.
C.
D.
Divergent boundaries.
Convergent boundaries.
Continental volcanic arcs.
Transform fault boundaries.