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Theory of Plate Tectonics
Plate Tectonics • The theory of plate tectonics states that the
Earth’s crust is divided into broken pieces called
• Earth’s surface has about a dozen rigid plates
• Plates move slowly (a few centimeters per year)
• These rigid pieces of the lithosphere drift around
like “rafts” on the plastic-like asthenosphere
Plates consist of the crust plus the
upper mantle
• This layer is called the
• About 100 km (60 miles) thick
• Composition is Fe silicates
(basalt) except for…
– Continents; mostly Al silicates
– Continents “float” on the rest of
the lithosphere
Below the lithosphere is the
• Same composition but is partly melted
(plastic like)
• Able to flow very slowly
• Flow caused by convection currents
– Heated rock expands and rises
– Cool rock contracts and sinks
– The plate above is carried along on the flow
– Continents ride along on the plates
Convection Currents
• The force that drives the plate movement are
convection currents.