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The Dynamic Earth
Chapter 17: Plate Tectonics
17.1 Drifting Continents
Some of the first people to consider the possibility that continents moved over time were early
____________________________________who noticed the apparent ______________between
continents on opposite sides of the Atlantic Ocean.
A German Scientist named Alfred _____________________________proposed the hypothesis of
continental drift in________________________. He suggested a supercontinent had existed about
200 million years ago he named________________________________.
Wegener proposed several lines of evidence to support this hypothesis including:
a. similar __________________formations and __________________ranges on opposite sides
of the Atlantic Ocean
b. fossils of the same species of _________________________animals on opposite sides of
oceans.
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c. fossils of the seed fern ___________________________on several continents that are now
separated by thousands of miles.
d. Similar _________________________and ___________________deposits on different
continents.
Despite this evidence most scientists _______________________________the idea that continents
moved over time because Wegener could not __________________________________caused the
continents’ to move. It wasn’t until the ________________________these questions were
answered.
17.2 Seafloor Spreading
Advances in ________________________lead to the first pieces of evidence suggesting that
the continents did indeed move.
__________________________________equipment on (first used on submarines during WW2)
allowed for the first detailed maps of the ocean floor to be made.
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Vast underwater mountain ranges (__________________________ridges) were found that
experienced frequent ________________________eruptions and_______________________.
__________________________much deeper than the Grand Canyon were also discovered.
____________________________________were used to show that the ocean floor had bands of
_______________________________(a volcanic rock rich in iron) with alternating magnetic lines.
As rock rose from inside the Earth, the iron in the molten rock would align itself in the Earth’s
magnetic field before the rock__________________________.
The study of the magnetic record is called __________________________and the record
shows the earth’s magnetic field has reversed itself on a regular basis (magnetic reversal).
In addition, the basalt closest to a mid-ocean ridge was _________________________than
basalt further from the ridge. These results suggested the seafloor was _________________apart
along the mid-ocean ridge (seafloor spreading).
Seafloor spreading showed how the continents were carried by the expanding ocean
crust.
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17.3 Theory of Plate Tectonics
The theory of _________________________states that the Earth’s crust and upper mantle
are broken into about a ___________________large plates and several smaller ones all of which
move at different rates.
Much of the supporting evidence that the plates move comes from _______________events
that occur where the _____________________of the plates meet (plate boundaries).
There are three important types of plate boundaries.
a. ______________________________Plate: plates are moving away from each other. Midocean ridges are the most common feature at divergent boundaries.
The ________________________________in Africa is a unique divergent boundary because it
occurs in the middle of a continent.
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b. _______________________________Boundary: where two plates come together.
Mountain ranges (_______________________________), volcanoes (Mount St. Helen’s),
trenches (Mariana Trench), earthquakes, and __________________zones all occur at convergent
plate boundaries.
c. _______________________Boundary: where two plates slide horizontally past each other.
The ___________________________________is a good example. __________________are very common
along transform plate boundaries.
17.4 Causes of Plate Motions
Evidence collected using technology and geologic events that occur at plate boundaries
was enough to convince most scientists that continents did move (__________________
becomes______________________________). Scientists are still not sure what force causes this
motion.
The transfer of heat by the movement of heated matter is called ________________________.
A good example of convection occurs when hot air rises and cool air sinks. This is what is
responsible for turbulence during airplane flights.
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Today most scientists believe _________________________Convection plays a major role in
the movement of continents.
When magma is heated (near the Earth’s core) it becomes ______________dense and
begins to rise. Cooler magma near the crust begins to ________________deeper into the Earth
because it is more dense.
These _____________________________cells (currents) carry the continents causing them to
interact at plate_______________________.
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