Plate Tectonics

Brief Structure of the EarthDivided into layers:

– Core, mantle, crust

Brief Structure of the EarthLithosphere

– 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’sCombined 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, sinkingCaused by differences in density due to

differences in temperature– Example – boiling pan of water

ForcesMovement of plates result in three

types of forces:– Tension – pulls rocks apart– Compression – pushes rocks together– Shearing – pushes rocks past each other

FaultSurface 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 ShearingEarthquakes

– 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.