The Theory of Plate Tectonics
• What is the theory of plate tectonics?
• What are the three types of plate
boundaries?
• Why do tectonic plates move?
The Theory of Plate Tectonics
• plate tectonics
• lithosphere
• divergent plate boundary
• transform plate boundary
• convergent plate boundary
The Theory of Plate Tectonics
• subduction
• convection
• ridge push
• slab pull
The Plate Tectonics Theory
• The theory of plate tectonics,
proposed in the late 1960s, states that
Earth’s surface is made of rigid slabs of
rock, or plates, that move with respect
to each other.
• Plate tectonics suggests that Earth’s
surface is divided into large plates of
rigid rock and each plate moves over
Earth’s hot and semiplastic mantle.
The Plate Tectonics Theory (cont.)
What is plate tectonics?
The Plate Tectonics Theory (cont.)
• Geologists use the word tectonic to
describe the forces that shape Earth’s
surface and the rock structures that
form as a result.
• The cold and rigid outermost rock layer
of the Earth is called the lithosphere.
• The lithosphere is made up of the crust
and the solid, uppermost mantle.
Earth’s Tectonic Plates
The Plate Tectonics Theory (cont.)
• The lithosphere is thin below mid-ocean
ridges and thick below continents.
• Earth’s tectonic plates are large pieces
of the lithosphere that fit together like the
pieces of a giant jigsaw puzzle.
• The layer of Earth below the lithosphere,
called the asthenosphere, is so hot that it
behaves like a plastic material and
enables Earth’s plates to move.
The Plate Tectonics Theory (cont.)
plastic
Science Use capable of being
molded or changing shape
without breaking
Common Use any of numerous
organic, synthetic, or processed
materials made into objects
Plate Boundaries
• A divergent plate boundary forms
where two plates separate.
• When the seafloor spreads at a midocean ridge, lava erupts, cools, and
forms new oceanic crust.
• Divergent plate boundaries can also
exist in the middle of a continent,
where they pull continents apart and
form rift valleys.
Plate Boundaries (cont.)
• A transform plate boundary forms
where two plates slide past each other.
• As they move past one another, the
plates can get stuck and stop moving.
• Stress builds up where the plates are
stuck until they eventually break and
suddenly move apart, resulting in a
rapid release of energy as
earthquakes.
Plate Boundaries (cont.)
• Convergent plate boundaries form
where two plates collide.
• The denser plate sinks below the more
buoyant plate in a process called
subduction.
• The area where a denser plate
descends into Earth along a
convergent plate boundary is called a
subduction zone.
Plate Boundaries (cont.)
subduction
from Latin subductus, means “to
lead under, removal”
Plate Boundaries (cont.)
• When an oceanic plate and a continental
plate collide, the denser oceanic plate
subducts under the edge of the
continent, creating a deep ocean trench
and a line of volcanoes above the
subducting plate on the edge of the
continent.
• When two continental plates collide,
neither plate is subducted, and
mountains form from uplifted rock.
Plate Boundaries (cont.)
What are the three types of
plate boundaries?
Evidence for Plate Tectonics
• Scientists can measure how fast
continents move using a network of
satellites called the Global Positioning
System.
• Because plates are rigid, tectonic
activity occurs where plates meet.
Evidence for Plate Tectonics (cont.)
• Volcanoes form where plates separate
along a mid-ocean ridge or continental
rift or collide along a subduction zone.
• Mountains can form where two
continents collide.
Notice the relationship between
earthquake epicenters, volcanoes, and
plate boundaries.
Evidence for Plate Tectonics (cont.)
How are earthquakes and
volcanoes related to the
theory of plate tectonics?
Plate Motion
• Earth’s plates move because the
asthenosphere moves underneath the
lithosphere.
• Convection is the circulation of material
caused by differences in density.
• Hot mantle material rises upward and
comes in contact with Earth’s crust.
Plate Motion (cont.)
• As the mantle cools, it becomes denser
and then sinks, forming a convection
current.
• Convection currents in the
asthenosphere act like a conveyor belt
moving the lithosphere above it.
• There are three forces that interact to
cause plate motion: basal drag, ridge
push, and slab pull.
Plate Motion (cont.)
What causes convection?
Plate Motion (cont.)
• Basal drag refers to how convection
currents in the asthenosphere circulate
and drag the lithosphere like a
conveyor belt.
• Rising mantle material at mid-ocean
ridges creates the potential for plates
to move away from the ridge with a
force called ridge push.
As a slab, or dense plate, sinks, it pulls
on the rest of the plate with a force called
slab pull.
A Theory in Progress
• Plate tectonics has become the
unifying theory of geology, but several
unanswered questions remain.
• Why is Earth the only planet in the
solar system that has plate tectonic
activity?
• Why do some earthquakes and
volcanoes occur far away from plate
boundaries?
• What forces dominate plate motion?
• What will scientists study next? This is
a 3-D image
of seismic
wave
velocities
from
a new
technique
called
anisotropy.
• Tectonic plates are
made of cold and
rigid slabs of rock.
• Mantle convection—
the circulation of
mantle material due to density
differences—drives plate motion.
• The three types of plate boundaries
are divergent, convergent, and
transform boundaries.
What word do geologists use to
describe the forces that shape
Earth’s surface?
A. semiplastic
B. lithosphere
C. tectonic
D. mantle
Which of these is made up of the
crust and the solid, uppermost
mantle?
A. seafloor
B. lithosphere
C. biosphere
D. hydrosphere
What type of boundary forms
where two plates separate?
A. convergent plate boundary
B. transform plate boundary
C. new plate boundary
D. divergent plate boundary
Do you agree or disagree?
5. Continents drift across a molten
mantle.
6. Mountain ranges can form when
continents collide.