Download Chapter 10 * Plate Tectonics

10-1 Continental Drift
10-2 The Theory of Plate Tectonics
10-3 The Changing Continents
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In 1912, a German scientist named Alfred
Wegener proposed a hypothesis called
continental drift.
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He proposed that the continents once formed part of
a single landmass called a supercontinent.
According to Wegener, this supercontinent began
breaking up into smaller continents during the
Mesozoic Era (250 million years ago).
It has taken millions of years for these continents to
drift to their present locations.
Some mountains may be the result of two plates
colliding.
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Fossil evidence has been used to support this
hypothesis. Fossils of the same plants and
animals have been found in areas that could
have been joined together at one time.
Geologic evidence also supports Wegener’s
hypothesis of continental drift. The ages and
types of rocks in the coastal regions of widely
separated areas, such as western Africa and
eastern South America, match closely.
If the continents are assembled into a model
supercontinent, the mountains of similar age fit
together in continuous chains.
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Climatic patterns suggest that the continents
have not always been located where they are
now. Geologists have discovered layers of
debris from ancient glaciers in southern Africa
and South America. Fossil plants that are
tropical such as ferns have also been found in
much colder climates like Antarctica.
The biggest problem with this theory was that
there was no known way for the continents to
move.
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Mid-Ocean Ridges – the evidence that Wegener
needed to make his theory of continental drift
work was found on the bottom of the ocean.
The undersea mountain ridges are mountains
through which a steep, narrow valley runs
down the middle.
Though observations scientists realized that the
ocean floor closer to the ridge was younger
than the ocean floor farther from the ridge.
They also discovered that seafloor rocks were
younger than rocks on land.
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Seafloor spreading - Harry Hess proposed that
the valley at the center of the ridge was a crack
or rift in the Earth’s crust. At this rift, molten
rock, or magma, from deep inside Earth rises to
fill the crack.
As the ocean floor moves away from the ridge,
rising magma cools and solidifies to form new
rock that replaces the ocean floor.
Seafloor spreading is the mechanism for
causing continents to move. The study of
paleomagnetism showed the magnetic
properties of the rocks.
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Plate tectonics is the theory that explains why
and how continents move and is the study of
the formation of features in Earth’s crust.
The upper part of the mantle forming the
lithosphere are broken up into several blocks,
called tectonic plates.
The asthenosphere is the layer just below the
lithosphere that is “plastic” rock that flows
very slowly under heat and pressure. The
tectonic plates float on top of the asthenosphere
like blocks of wood.
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The earth’s crust is classified into two types:
Oceanic crust-is dense and made of rock rich in iron and
magnesium.
 Continental crust-has a low density and is made of rock that
is rich in silica.
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Tectonic plates can include continental crust, oceanic
crust, or both. The continents are carried along on
the moving tectonic plates.
The Earth is broken into 15 major tectonic plates.
Scientists identify the plate boundaries mostly by
studying the data from earthquakes. Frequent
earthquakes in an area are evidence that there are
two or more plates that meet in the area. The
locations of volcanoes also indicates a tectonic plate
boundary.
The Pacific Ring of Fire is a zone of active volcanoes
that surround the Pacific Ocean.
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3 Types of plate boundaries
Divergent Boundaries-two plates that move away from
each other
 Convergent Boundaries-where two plates collide
 Transform boundaries-two plates slide past each other
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Scientists do not fully understand the force that
drives plate tectonics. They believe it may be due
to convection and ridge push.
Convection is the movement of substances due to
differences in temperature. The hot material rises
and as it cools, the cooler material sinks back
down to the bottom of the layer.
Ridge push is the movement of sinking
lithosphere that pushes downward away from the
mid-ocean ridge.