The Theory of Plate Tectonics

Earth’s Internal Layers

• The crust varies in thickness (4-60 km) oxygen, silicon, magnesium and iron

• The mantle (2885 km) silicon & oxygen

• Outer core (2270 km) iron & nickle – liquid

• Inner core (1218 km) iron & nickle - solid

The Rock Cycle

• Three types of rock found in the crust are:– Igneous: cooled magma/lava– Sedimentary: particles deposited by water

flow. Organic/inorganic matter (fossils)– Metamorphic: as layers build up, this rock is

formed when pressure and heat become great enough to change the rock chemically• The rock cycle is completed through the tectonic

process. Rock returns to the mantle, remelt, become magma, return to the crust as igneous rock.

Pangaea

• Pangaea is the name given to the single giant continent (Wegener 1912)

• Panthalassa (single ocean)• The theory of continental drift states

that the continents were once a single landmass that drifted apart and are still doing so.

The Theory of Seafloor Spresding

• New crust emerges from the rift valley in a mid-ocean ridge.

• Magma from the mantle pushes up through the rift and solidifies into new crust.

• New seafloor forms at the rift valleys and mid-ocean ridges, spreading away from the ridges until it returns as part of the rock cycle at subduction zones (trenches)

Magnetic polarity

EM field reverses from time to time

Seafloor spreading and continental drift combined by theory of plate tectonics

• More than 12 separate plates– Divergent boundary: two plates moving apart– Convergent boundary: two plates push

together– Transform boundary: two plates moving past

each other (earthquakes)

Boundary Interactions• Divergent boundary (normal)

– Tectonic plates moves in opposite directions– Mid-oceanic ridge forms as seafloor spreading creates new crust and

seafloor as magma fills the gap created over geologic time.

• Convergent boundary (reverse)– Tectonic plates move towards each other– Oceanic subduction under continental: volcanoes, earthquakes

• Andes

– Continental /continental convergence: Mountains• Himaylayas – Mt. Everest• Oceanic/oceanic: Trenches – Mariana trench, tsunamis

• Transform boundary (strike-slip)– Tectonic plates move past each other: Earthquakes– San Andeas fault in San Francisco

Bridge across the Álfagjá rift valley in southwest Iceland, the boundary

between the Eurasian and North American continental tectonic plates.

Hot Spots

• The hot spot theory states that hot spots are small melting areas within the mantle where thermal plumes cause magma columns to push up, breaking the crust

• Hot spots do not move with tectonic plates because they originate in the mantle

• Volcanic isalnd chains are the result of the plate moving over a hot spot (Hawaii, Galapagos, etc.)

Hot Spots

Plate movement

• Convection is the primary force driving seafloor spreading– Convection currents form as hot material rises

and cold materials sink• A second driving force comes from the

seafloor spreading– As new seafloor forms, the plates tend to slide

away from the elevated mid-ocean ridge

The end