The Theory of Plate Tectonics
• States that pieces of the Earth’s lithosphere are in constant, slow motion
• This theory explains the movement and subduction of plates on Earth.
The Movement of the Plates
• The convection currents in the asthenosphere causes the plates to move
• No plate can budge without affecting the other plates surrounding it
Review1. The lithosphere is broken in different
sections. What are these sections called?
2. What do the plates float on top of?
3. What causes the plates to move?
4. T/F: When the plates move, they do not affect the other plates in the Lithosphere.
Where The Plates Meet
• The plates meet at boundaries.
• There are 3 kinds of plate boundaries:– Transform boundaries– Divergent boundaries– Convergent boundaries
Transform Boundaries
• At a transform boundary, plates slip past each other moving in opposite directions
• Earthquakes are frequent here
Divergent Boundaries
• At a divergent boundary, two plates move apart
• Most occur at the Mid-Ocean Ridge
Divergent Boundaries
• When a divergent boundary occurs on land, a rift valley forms.
• A rift valley is a deep valley formed where the plates are separating
Convergent Boundaries• At a convergent boundary, two plates
come together, or converge.
• There are 3 types of convergent boundaries:– Oceanic crust collides with oceanic crust– Oceanic crust collides with continental crust– Continental crust collides with continental
crust
And the winner is…
• When two plates collide, the density of the plates determines which one comes out on top
• Remember: oceanic crust is more dense than continental crust
Oceanic VS Oceanic
• Two oceanic plates meet at a trench
• And the winner is: The less dense plate comes out on top– The plate which is more dense dives under
the other one and returns to the mantle (called subduction)
Oceanic VS Continental
• And the winner is: Continental comes out on top
• The oceanic plate is more dense so it sinks and plunges under the continental plate (subduction)
Oceanic
Continental
Continental VS Continental
• Subduction does not occur (neither is dense enough to sink into the mantle)
• This makes mountains
• And the winner is: IT’S A DRAW!
Slow Motion
• The plates move from 1 to 10 centimeters per year!!!
• North America and Eurasia move apart 2.5 centimeters per year
• This is how fast your fingernails grow
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