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Lecture #04 Plate Tectonics Overview
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By plate we really mean a “shell” on Earth’s
surface that is about 100-200 km thick and
includes all of the crust, and the uppermost
part of the mantle.
Tectonics is the term we use in geoscience
to represent the formation of folds, fractures,
faults, etc. in rocks.
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Global Seismicity
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Inter- and Intraplate
Earthquakes
Most earthquakes occur along plate margins
because plate margins are relatively weak,
but about 10% of earthquakes occur within
the interior of plates.
We classify earthquakes depending on where
they are located
Interplate - between plates
Intraplate - within plates
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The Shape of the Continents
As soon as good maps became
available, people noticed the similarity
between the shape of continents, most
notably, South America and Africa.
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Map from 1650
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Map from 1676
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Map from 1893
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Modern
Day Map
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Alfred Wegener
Most people thought the
complementary shapes were
coincidence. Alfred Wegener
was an exception who
sought more evidence that
the continents had been
joined. He found it in the
rocks & fossils and proposed
the hypothesis of Continental
Drift.
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Fossil Evidence
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Wegener’s Problem
The problem with the Continental Drift
Hypothesis was that Wegener had no
mechanism for moving the continents through
the ocean floor.
Powerful physical arguments prevented
continental drift from acceptance by the
majority of scientists, particularly those in the
northern hemisphere.
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Paleomagnetism
Wegener died in 1930 with his hypothesis
largely ignored. But despite the problems and
an overwhelming rejection of his ideas, not
everyone forgot about his work.
In the 1940’s and 1950’s geophysicists
studying the record of Earth’s magnetic field
began to revive some of the ideas to explain
observed variations in their observations.
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Discoveries in Marine Geology
During WWII and with the initiation of the Cold
War, much research was focused attention on
the ocean floor.
– Researchers were surprised to find long, large
mountain chains and deep trenches.
– They also found that the rocks which compose the
ocean floor were young, ranging in age from recent
to 200 Ma.
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Sea-Floor Spreading
Sea Floor Spreading is the hypothesis
in which new ocean floor is created at
the mid-ocean ridges and that plates
move apart at the ridges and the
continents move with them.
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The Northern Mid-Atlantic Ridge
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Sea-Floor Recycling
Since new material is created at the
mid-ocean ridges and Earth is not
expanding, somewhere material must
be removed from the surface.
It turns out that old ocean floor is
“subducted” into the mantle at
subduction zones.
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South America to Africa
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Western Pacific Subduction
Zones
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Geology & The Plates
Plates play an enormous role in
geologic activity.
Much of the action in geology occurs
along the plate edges, or boundaries.
The boundaries are the locus of
– Earthquakes
– Volcanoes
– Heat loss
– etc.
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The Major Plates
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Summary
Plate tectonics is our best model for
how the outer part of Earth contributes
to the planet’s thermal processes.
Plate tectonics is the surface
manifestation of convection (think lava
lamp) that the Earth undergoes to cool
itself.
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Summary (cont.)
A central point to plate tectonics is that
lateral movements are dominant over
vertical movements.
In other words, the “drifting” of
continents is much faster and more
significant than the uplift of mountains.
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Summary (cont.)
Evidence for plate tectonics is
overwhelming:
– Seismicity patterns
– Magnetic lineation on the sea floor
– GPS measurements
– Fossil evidence
– Many,many other lines of evidence
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