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Plate Tectonics
Brief Structure of the Earth
 Divided into layers:
– Core, mantle, crust
Brief Structure of the Earth
 Lithosphere
– Crust and upper part of the mantle
• About 100 km thick
• Rigid
 Asthenosphere
– Plastic (gooey) layer beneath the lithosphere
• More dense than the lithosphere
Brief Structure of the Earth
 Continental Crust
– Made up of continents and “edges” of the
oceans
– Light
 Oceanic Crust
– Made up of ocean floor
– More dense than continental crust
• Older oceanic crust is more dense than younger
oceanic crust
Theory of Plate Tectonics
 Developed in 1960’s
 Combined continental drift and seafloor
spreading.
Theory of Plate Tectonics
 Stated that:
– The crust and upper mantle (lithosphere) of the
earth are broken into sections called plates.
– These plates sit on top of the plastic (gooey)
part of the mantle (asthenosphere).
– These plates can move apart, converge, or slide
past one another through time.
Divergent Boundaries
 Where plates are moving apart and new
crust is forming.
– Example: Mid-ocean ridges
Convergent Boundaries
 Where two plates collide and crust is
destroyed.
– Subduction zone
• Area where a dense oceanic plate sinks under a light
continental plate or another less dense oceanic plate
– Forms volcanoes.
– When two continental plates collide high
mountains are formed.
Transform Boundary
 Where one plate is sliding past another
plate.
– The plates may be going in opposite directions
or the same direction at different rates.
• Earthquakes are common
Modern Theories for Cause of Plate
Tectonics
 All include
convection
currents in the
mantle.
Convection Current
 Cycle of heating, rising, cooling, sinking
 Caused by differences in density due to
differences in temperature
– Example – boiling pan of water
Forces
 Movement of plates result in three
types of forces:
– Tension – pulls rocks apart
– Compression – pushes rocks together
– Shearing – pushes rocks past each other
Fault
 Surface where rock has broken apart
and moved.
 3 types
– Normal fault
– Reverse fault
– Transform (or strike-slip) fault
Faults
 Normal fault
– Rock above the fault moves down.
– Formed by tension.
 Reverse fault
– Rock above the fault plane moves up.
– Formed by compression.
 Transform fault
– Bodies of rock slide past each other.
– Formed by shearing.
Features of Tension Forces
 Mid-ocean ridge
– Where oceanic crust is being pulled apart.
 Rift Valley
– Where continental crust is being pulled apart.
Features of Compression Forces
 Mountains
– Continental-continental collisions
• Very high mountains (folded mountains) are
formed.
– Oceanic-continental collisions
• Volcanic mountains are formed.
– Oceanic-oceanic collisions
• Island arcs (volcanoes that reach above the surface
of the ocean) are formed.
Features of Shearing
 Earthquakes
– Rocks on each side of a strike-slip fault become
stuck in one place for a time and then move,
releasing energy in the form of an earthquake.
Tracking Plate Movement
 Plate movement is tracked using satellites
and lasers.
– Current rates of movement range from about 1
cm to about 12 cm per year.