Plate Tectonics
Chapter 4 Notes
4.1 Continental Drift
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In 1912, Alfred Wegener proposed a
hypothesis called continental drift where
the continents are constantly moving.
All the continents once formed a large super
continent that he called Pangaea which
means “all lands”
He said that there was one large ocean
surrounding it called Panthalassa meaning
“all seas”
4.1 Continental Drift
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As evidence, he found that the
coastlines of continents lined up with
one another.
He also found fossils of the same
animals, that cannot swim, on Africa
and South America
4.1 Continental Drift
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Seafloor spreading also backs up the
theory of continental drift.
The rocks near the middle of the
ocean floor are many million years
younger than rocks on the continents.
4.1 Continental Drift
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There is a break in the plates at the
mid-ocean ridge, where magma from
deep in the earth wells up through the
rift.
The ocean floor moves away from the
ridge and new rocks form as the
magma cools.
This is Seafloor Spreading.
4.2 Theory of Plate Tectonics
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The theory of plate tectonics not only
describes continental movement but
also gives an explanation of why and
how continents move.
Remember: 2 kinds of crust. Oceanic
and Continental
4.2 Theory of Plate Tectonics
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Remember: Lithosphere is rigid upper
mantle and the crust
Remember: Asthenosphere is layer of
rock that flows like putty
There are about 30 plates that are
moving around each other forming our
Earth as we know it today.
4.2 Theory of Plate Tectonics
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Between those plates are boundaries.
Two plates moving away from each
other form a divergent boundary.
The center of a mid-ocean ridge is a
rift valley that is formed from a
divergent boundary.
4.2 Theory of Plate Tectonics
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The direct collision of one plate with
another makes a convergent
boundary
3 types of collisions happen at
convergent boundaries.
1st – Subduction where one plate goes
under another. A deep ocean trench is
usually found here.
4.2 Theory of Plate Tectonics
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2nd – Two plates crash together and
push each other up, crumpling and
producing large mountain ranges.
3rd – Two oceanic plates where one
goes under the other and melts
forming volcanic islands on the other
known as an island arc.
4.2 Theory of Plate Tectonics
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Another boundary is the transform
boundary where two plates grind past
each other.
4.2 Theory of Plate Tectonics
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Scientist think the plates are moving
because of convection currents
moving in the asthenosphere.
This is where the hottest rock moves
up and the cooler rocks fall towards
the core.