Chapter 4.2 and 5.3
Plate tectonics →
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Causes and Effects of Plate Tectonics
The hypothesis of continental drift and seafloor spreading led
scientists to the theory of plate tectonics
Plate tectonics- the lithosphere is made of plates that float on the
asthenosphere, moved by convection currents in the asthenosphere
Lithosphere →
The lithosphere is made up of oceanic crust (Earth’s surface with the
ocean on it) and the upper mantle, and the continental crust
(Earth’s surface with the continents on it) and the upper mantle
The lithosphere forms the thin outer shell of Earth
Asthenosphere →
The asthenosphere is located under the lithosphere
The asthenosphere is made of rock that flows like tar, or putty
Within the asthenosphere are convection currents
Causes of plate motion →
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Convection currents within the magma cause the plates to move
Convection is the transfer of heat through material (like a pot of
water on the stove). Warm rock close to the core rises, and cooler
rock sinks. This process of rising and cooling material is a
convection current. Scientists believe that this process occurs
in the asthenosphere, and causes the plates to move
Plates →
The lithosphere is broken into plates that ride on the denser
asthenosphere like wood floats on water. All of the plates are
pressed up against each other, and move in different directions
There are about 30 lithospheric plates (7 large, the rest are small)
Plate boundary- location where the plates meet
Three types of plate
boundaries →
1) Divergent boundary- plates move away from each other
 Produces rift valleys and the mid- ocean ridge
 Seafloor spreading is a result of a divergent boundary

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Most divergent boundaries are on the ocean floor
2) Convergent boundary- plates move towards each other
Oceanic-Continental= oceanic plate is subducted as they move
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towards each other because it is more dense (it is heavier)
 produces deep ocean trenches & volcanoes
Continental-Continental= continental plates are uplifted as they
move towards each other
 produces folded mountains
Oceanic-Oceanic= one oceanic plate is subducted as they move
towards each other which melts the plate causing magma to rise
to the surface
 produces a chain of islands called an island arc
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3) Transform boundary (also called transform fault boundary and
strike slip fault)- plates grind past each other horizontally
as they move
 produces earthquakes and volcanoes
 San Andreas Fault is the fault in California that causes our
earthquakes