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Transcript
LESSON 2
EARTH’S MOVING
CONTINENTS
Chapter 5
Changes Over Time
OBJECTIVES
Discuss evidence for continental drift and
plate tectonics.
Explain seafloor spreading.
MAIN IDEA
Earth’s crust is made of moving plates that
slowly but constantly change its surface.
VOCABULARY
continental drift – the slow movement of
the continents over eons
plate tectonics – the theory that Earth’s
surface is broken into piece’s, or plates,
that slide over the magma in the
mantle
magma – hot, fluid rock below Earth’s
surface
seafloor spreading – the moving apart of
plates on the ocean floor that is caused
by magma flowing up between the
plates and then hardening
ARE THE CONTINENTS
MOVING?
• Alfred Wegener published a book proposing
that the continents had been connected.
• He called this supercontinent Pangea.
• The separation of the continents became
known as continental drift.
• He showed that the continents fit
together like a puzzle.
• Fossil remains, rocks, glaciers, and
mountain ranges matched.
• Also fossils of tropical species were
found on Antarctica.
PLATE TECTONICS
• In Wegener's time, people wondered how the
continents could move through solid rock.
• New evidence in the 1950’s supported
Wegener's proposal.
• Scientists mapping the floor of the Atlantic
Ocean found an underwater mountain chain
in the middle of the Atlantic.
• On both sides of these mountain chains,
the ocean floor was moving.
• Scientists developed a model called plate
tectonics to explain how the continents
and the ocean floor could move.
• According to this model, Earth's surface
is broken into pieces, or plates.
• The plates move over the hot, fluid rock,
or magma, in the mantle.
• Uneven heating in the mantle produces slowmoving currents of plastic-like, fluid rock.
• The cooler, rigid rock of the lithosphere rests
on top of this fluid rock.
• The slow movements in the fluid part of the
mantle drag the lithosphere and its plates
sideways.
• As the lithosphere moves, so do the ocean
floor and the continental plates.
QUICK CHECK
Draw Conclusion
What conclusions did Alfred Wegener draw
about Pangaea?
He concluded that it split apart and that the
continents “drifted” to their present
positions.
Critical Thinking
How does the movement of the ocean
floor support Wegener’s idea of continental
drift?
It provides evidence for the theory of plate
tectonics by demonstrating how plates can
move.
HOW DO OCEANS CHANGE SIZE?
• The processes that move the continents
also help form new crust on the ocean
floor.
• As some crustal plates move apart, magma
enters the cracks and flows outward.
• The magma cools, hardens and builds up
into parallel ridges, or raised structures, on
the ocean floor.
• The new rock exerts a sideways force
called compression.
• Magma continues to flow between the
plates, forcing them farther apart.
• This process is called seafloor
spreading.
• Seafloor spreading explains how plates
move apart and new crust forms.
• At the Mid-Atlantic Ridge in the
Atlantic Ocean, new seafloor crust is
formed at the rate of about 3
centimeters (1 inch) per year.
EVIDENCE OF SEA
FLOOR SPREADING
• Several pieces of evidence support seafloor
spreading.
• For example, the youngest rock on the
ocean floor is found at mid-ocean ridges.
• The older rock is found further away
from the ridge.
• Rock that makes up the continents is generally
older than the seafloor rock.
• The type of rock found on the ocean
floor is also significant.
• Most of the seafloor rock is
volcanic in origin.
• Magma from the mantle cooled
and hardened.
• The magnetism of seafloor rock provides
further evidence for seafloor spreading.
• Earth has a north-south magnetic field;
occasionally it reverses completely,
resulting in a south-north orientation.
• The iron particles in the magma line up
according to the direction of Earth's
magnetic field.
• As the magma cools and solidifies, the
iron particles "freeze" in the direction of
the magnetic field at the time.
• When scientists studied rock along the
seafloor, they found that the
magnetism in the rock altered from
one direction to the other.
• This pattern matched on both
sides of the mid-ocean ridges.
QUICK CHECK
Draw Conclusions
Why would continental rock generally be
older than seafloor rock?
Rock is formed at mid-ocean ridges.
To become continental rock, the
material is pushed away from the ridge
over millions of years.
Critical Thinking
Summarize the evidence that supports seafloor
spreading.
The evidence includes the age of seafloor
rocks (the youngest are at mid-ocean ridges);
the type of rock involved (volcanic);
and the magnetism of the rock (which indicates
that rock formed when poles were aligned in
different directions).
WHAT HAPPENS AT
PLATE BOUNDARIES?
• Plates can move in three ways.
• They can move apart from each
other,
• they can collide,
• or they can slide past each other.
• Locations where plates move apart are
called divergent boundaries.
• Seafloor spreading occurs at
divergent boundaries.
• Divergent boundaries also occur
on land.
• Iceland is located on a divergent boundary at
the northern end of the Mid-Atlantic Ridge.
• The Great Rift Valley in Africa is also located
on a divergent boundary.
• There, the continent of Africa is splitting.
• The split may one day form a new ocean.
• Some divergent boundaries are less visible,
occurring within a continental plate.
• Locations where plates collide are
convergent boundaries.
• If both colliding plates include
continents, the pressure lifts and
crumples the plates, forming
mountains.
• Earthquakes and volcanic activity can
occur at convergent boundaries.
• When one plate carries part of an ocean
floor and the other a continent.
• The oceanic plate slides under the
continental plate into the mantle and
melts, this is subduction.
• Some of the magma beneath rises
through the cracks between the
plates.
• At the surface, a volcano forms.
• When plates move toward each
other on the ocean floor, one plate
sinks under the other.
• This movement forms an ocean
trench.
SLIDING PLATES
• Some plates simply slide past each other.
• The boundary between these plates is a
transform boundary.
• Earthquakes can occur along
transform boundaries as strain on the
rock builds up and then is quickly
released.
• Rock along these boundaries
shatters and breaks.
• Eventually, this rock may pile up and
form narrow ridges and valleys.
QUICK CHECK
Draw Conclusions
If a continental valley begins to widen,
what conclusion might you draw about
the region’s plate movement?
The valley may be on a divergent
boundary and the plates may be moving
apart.
Critical Thinking
Where would it be easiest to drill
through Earth’s crust and reach the
mantle?
The ocean floor is where Earth’s
crust is thinnest and would therefore
be the easiest location in which to
drill.
WHERE ARE THE PLATES?
• Earth's crust has seven major tectonic plates.
•
• Some of the plates are diverging or moving
apart.
• Others are converging, or pushing together.
• Mid-ocean ridges occur along diverging plates.
•
• The Mid-Atlantic Ridge is located where
the South American Plate is moving
away from the African Plate.
• Convergent boundaries can be found at
many large mountain chains.
• The Himalayas are located where the
Indian Plate is colliding with the
Eurasian Plate.
• The Alps are located where the
African Plate is colliding with the
Eurasian Plate.
• Most transform boundaries are located on
the ocean floor.
• On land the most noticeable transform
boundary is the San Andreas Fault
between the North American Plate and the
Pacific Plate.
• This fault is the site of many
powerful earthquakes.
QUICK CHECK
Draw Conclusion
India continues to move into the
Asian continent.
What will happen to the Himalayas?
They will continue to grow.
Critical Thinking
How might the movement of Earth’s
tectonic plates affect you?
Plate movement can cause
earthquakes, which can affect people’s
lives.