Unit 4
Lesson 6
Plate Tectonics
Unit 4 Lesson 6 Plate Tectonics
Indiana Standards
• 7.2.1 Describe how the earth is a layered
structure composed of lithospheric plates, a
mantle and a dense core.
• 7.2.4 Explain how convection currents in the
mantle cause lithospheric plates to move causing
fast changes like earthquakes and volcanic
eruptions, and slow changes like creation of
mountains and formation of new ocean floor.
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Unit 4 Lesson 6 Plate Tectonics
Puzzling Evidence
What evidence suggests that
continents move?
• In the late 1800s, Alfred Wegener proposed his
hypothesis of continental drift.
• According to this hypothesis, the continents once
formed a single landmass, broke up, and drifted.
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Unit 4 Lesson 6 Plate Tectonics
What evidence suggests that
continents move?
• Several lines of evidence supported Wegener’s
hypothesis.
• Fossils of the same species are found on continents
on separate sides of the Atlantic Ocean.
• The locations of mountain ranges and rock
formations and evidence of ancient climatic
conditions also supported Wegner’s hypothesis.
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Unit 4 Lesson 6 Plate Tectonics
What is Pangaea?
• Today, scientists accept that the continents were
once joined.
• About 245 million years ago, the continents were
joined in a single large landmass called Pangaea.
• Over millions of years, Pangaea broke into
fragments that drifted and collided with each other.
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Unit 4 Lesson 6 Plate Tectonics
What is Pangaea?
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Unit 4 Lesson 6 Plate Tectonics
What discoveries support the idea of
continental drift?
• Scientists did not accept Wegener’s ideas because
they could not determine how continents moved.
• In the mid-1900s, scientists began mapping the
sea floor and discovered features that supported
some of Wegener’s ideas.
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Unit 4 Lesson 6 Plate Tectonics
What discoveries support the idea of
continental drift?
• Scientists discovered that oceanic crust is young
compared to continental crust.
• They also discovered that sea floor rock contains
magnetic patterns.
• A process called sea-floor spreading explains
the age and magnetic pattern of sea floor rocks.
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Unit 4 Lesson 6 Plate Tectonics
What discoveries support the idea of
continental drift?
• Through sea-floor spreading, molten rock rises at
the ridges and forms new oceanic crust.
• Older crust is pushed away from the ridge, and
the sea floor slowly spreads apart.
• Scientists also discovered huge trenches in the
sea floor where oceanic crust sinks into the
asthenosphere.
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Unit 4 Lesson 6 Plate Tectonics
A Giant Jigsaw
What is the theory of plate tectonics?
• Scientists began to form a new theory to explain
how tectonic plates move.
• Plate tectonics is a theory that describes largescale movements of Earth’s lithosphere.
• It describes why and how continents move and
explains how many of the features on Earth’s
crust form.
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Unit 4 Lesson 6 Plate Tectonics
What is a tectonic plate?
• The lithosphere is divided into pieces called
tectonic plates, which move around on top of
the asthenosphere.
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Unit 4 Lesson 6 Plate Tectonics
Boundaries
What are three types of plate
boundaries?
• The three types of plate boundaries are
convergent boundaries, divergent boundaries, and
transform boundaries.
• Each type is associated with characteristic
landforms.
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Unit 4 Lesson 6 Plate Tectonics
What are three types of plate
boundaries?
• Convergent boundaries form where two plates
collide. This can happen in three ways, depending
on the type of crust that is involved.
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Unit 4 Lesson 6 Plate Tectonics
What are three types of plate
boundaries?
• At a divergent boundary, two plates move away
from each other, and magma forms new
lithosphere at mid-ocean ridges.
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Unit 4 Lesson 6 Plate Tectonics
What are three types of plate
boundaries?
• A boundary at which two plates move past each
other horizontally is called a transform
boundary. At transform boundaries, the motion
of the two plates often produces earthquakes.
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Unit 4 Lesson 6 Plate Tectonics
Hot Plates
What causes tectonic plates to move?
• Scientists have proposed three mechanisms to
explain how tectonic plates move over Earth’s
surface.
• Some evidence suggests that convection, or the
movement of material due to differences in
density, in the mantle drags the overlying tectonic
plates along with it.
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Unit 4 Lesson 6 Plate Tectonics
What causes tectonic plates to move?
• The mechanism called ridge push suggests that as
lithosphere sinks, the plates are pushed away
from the mid-ocean ridge.
• The crust along the mid-ocean ridge is less dense.
As it cools, it becomes denser and sinks into the
mantle, pulling it away from the ridge.
• The force of the asthenosphere below pushes the
rest of the plate away from the mid-ocean ridge.
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Unit 4 Lesson 6 Plate Tectonics
What causes tectonic plates to move?
• The mechanism called slab pull suggests that
plates move because sinking plates pull them.
• The leading edge of a sinking plate is colder and
denser than the mantle, so it sinks. The rest of
the plate follows.
• Many scientists think slab pull is the most
important mechanism driving plate motion.
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