Earth’s Spreading Ocean FloorSection 12-2

Rift valleys =

cracks in underwater mountains

Midocean RidgesUnderwater

mountainsEx. Mid-Atlantic

RidgeForm the smallest

mountain chainsThey have a great

amount of volcanic activity

MIDOCEAN RIDGES

Ocean – Floor SpreadingOccurs along midocean

ridgesProcess that explains

continental driftLava erupts from riff

valley causing ocean floor to spread

New ocean floor when it hardens

Rock near midocean ridge is younger

Ocean – Floor Spreading and Magnetic Stripes

Magnetic stripes in ocean floor rocks line up with poles but the Earth’s magnetic poles reverse from time to time giving good evidence.

Ocean Floor Spreading does not make Earth’s

Surface larger because it is being destroyed as fast as it is being made

TrenchesDeepest part of the oceanV-shaped valleys along the ocean bottom near continents or island

Subduction Older ocean floor moves down deep

into the Earth along

trenches

When deep enough, the rock melts becoming part of the

mantle

Denser materials go

down and less dense

float

Transform FaultsThin cracks located between sections of oceanic ridges

Rock on either side of the fault are moving in opposite directions

Block tend to jam rather than slide smoothly past one another which causes stress to built up

When rock does suddenly slip it causes earthquakes