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Mid-Ocean Ridges

• Where new oceanic crust is forming from magma coming up through the valley in the mid-ocean ridge

Sea-floor spreading

• Occurs at mid-ocean ridges

• Happens because molten material and magma from the mantle is coming up, erupting, hardening, and cooling to form new oceanic crust at the mid-ocean ridges

Evidence for sea-floor spreading

Rock material - pillow rocks Magnetic strips - matching patterns on either side of the mid-ocean ridges Rock samples - the age of rocks farther from ridges is older and the closer to ridges is younger

Deep-Ocean Trenches

• Located where dense oceanic crust meets less dense continental crust

• More dense crust will subduct under the less dense crust

• A trench or deep valley forms

• The crust that subducts goes back down into the mantle where it melts

Subduction

• Occurs at deep-ocean trenches

• Happens because more dense crust subducts or sinks below less dense crust

Subduction and Oceans

Oceans can get bigger or smaller based on the relationship of how fast new crust forms versus sea-floor spreading

What is the theory of plate tectonics?

BLANK STATES that Earth’s plates are in slow, constant motion , driven by convection currents in the mantle

What are the three Plate boundaries

• Divergent

• Convergent

• transform

What happens at each type of boundary?

Convergent - all will have some incidence of small earthquakes Continental/Continental - Collision occurs, Very high mountains form Continental/Oceanic - Subduction occurs, deep-ocean trench forms, volcanoes and small mountains form on the continental plate Oceanic/Oceanic - subduction of the denser/older oceanic plate occurs, a deep-ocean trench forms, volcanoes and small mountains will form on the non-subducting plate Divergent Underwater - a mid-ocean ridge forms, new crust created here, forms a long mountain range because of the build up new crust/rock Above water - rift valley forms because the plates are pulling apart; if you go into the valley, at the bottom there will likely be water filling in and new crust forming underneath Transform - no features created here; earthquakes occur here.

Faults

Where there is a break in the Earth’s crust

Plates

• Earth’s plates meet at boundaries