SEA FLOOR SPREADING MidOcean Ridge the longest chain

SEA FLOOR SPREADING

Mid-Ocean Ridge – the longest chain of mountains in the world---these are divergent plate boundaries.

Mapping the mid-ocean ridge:

Sonar: • A device that bounces sound waves off underwater objects and then records the echoes of these sound waves. • Sonar mapped mid-ocean ridges.

Harry Hess: • Studied the Mid-Ocean Ridges • Proposed the seafloor spreading theory in 1962. • The Sea floor spreading theory states that new ocean crust is being created at mid-ocean ridges and destroyed at deepsea trenches. .

Sea-Floor Spreading? ? • The process that continually adds new material to the ocean floor.

The force responsible for driving or moving the plates is ___________. convection currents Convection Currents occur within the mantle of the earth when hot magma rises and cool magma sinks

• Magma, because it is hotter and less dense, is forced up towards the ocean floor cooling off when it touches water. • When the magma hardens, a small amount of new ocean floor is added to the Earth’s crust. As more magma rises and cools it pushes the new sea floor away from the ridge. This process is called sea floor spreading.

Evidence # 1: Molten Material 1. Rocks shaped like pillows. - molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly

Evidence #2 - Magnetic Stripes • Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. • They hold a record of reversals in Earth’s magnetic field.

Evidence #3 - Drilling Samples • The Glomar Challenger did a drilling sample and found rocks that the farther away from the ridge the older the rocks were. The younger ones were in the center of the ridge.

Subduction at Deep-Ocean Trenches • Deep-ocean trenches • Ocean floor plunges into deep underwater canyons are deep-ocean trenches. • Subduction- is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

Subduction • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into mantle, over tens of millions of years. • Subduction and Earth’s Oceans • Earth’s ocean floor is renewed about every 200 million years.

Subduction in the Pacific & Atlantic • Deep ocean trenches are swallowing more oceanic crust than the mid-ocean ridge can produce. Thus, the width of the Pacific will shrink. • The Atlantic is expanding. It has short trenches. In some places, the oceanic crust is attached to the continental crust which moves the continents.

Animation of Sea Floor Spreading and Ages… 2. 5