Sea-Floor Spreading Key Concepts What is the process of sea-floor spreading? What is the evidence for sea-floor spreading? What happens at deep-ocean trenches?

Key Terms Mid-ocean ridge Sonar Sea-floor spreading Deep-ocean trench subduction

Mapping the Ocean Floor Sonar was used in the mid 1900’s to map the ocean floor Sonar bounces sound waves off objects and records the echoes of these waves. Time it takes for echo to come back indicates the distance from the object Sonar discovered the mid-ocean ridges

Mid-Ocean Ridges Mid-ocean ridge system is a chain of underwater mountains that extend into all of Earth’s oceans. They are more than 50,000 km long Most are under hundreds of meters of water They reach the surface in a few places such as Iceland. A steep sided valley splits the top of some mid-ocean ridges.

MID-OCEAN RIDGES

Mid- Ocean Ridges

Sea Floor Spreading In 1960 Harry Hess, an American geologist, suggested that the sea floor spread apart along both sides of a mid-ocean ridge as new crust is added. As a result the ocean floors move like conveyor belts carrying continents with them. He called this sea-floor spreading Molten material erupts through the valley in the center of the ridge. Hardens and cools to form new sea floor.

Sea Floor Spreading

Evidence for sea floor spreading From Molten Material Alvin, The world’s first deep ocean submersible used for deep sea exploration, found strange rocks shaped like pillows in the mid-ocean ridge. This was evidence that molten material came out of the ridge and hardened quickly

Bulbous Pillow Lava

Evidence from Magnetic Stripes Scientists studied the rock patterns in the ocean floor . They found that the rock on the ocean floor lies in a pattern of magnetized stripes Magnetic stripes in ocean floor rocks are formed by the reversal of the Earth’s magnetic poles. The last reversal happened 780,000 years ago. The pattern of stripes matches on each side of the ocean ridge.

Magnetic stripes

Evidence from drilling samples Glomar Challenger gathered samples of rocks by drilling into the ocean floor Drilled through 6 km of water into ocean floor Scientists then determined the age of the rocks They found that the rocks next to ocean ridges are younger than rocks farther away. As the ocean floor spreads the older rocks move farther away.

Glomar Challenger

Is the Earth’s surface getting larger from sea-floor spreading?

Subduction at Trenches Deep ocean trenches are deep underwater canyons In a process that takes tens of millions of years, the ocean floor sinks back into the mantle at deep ocean trenches. Subduction is the process where the ocean floor sinks beneath a deep ocean trench.

Subduction zones Oceanic crust and Continental crust Oceanic and Oceanic Crust

The Process of Subduction Sea-floor spreading and subduction work together like a conveyor belt. New oceanic crust is hot. As it moves away from the ridge it cools, and becomes more dense At a trench, the older, more dense crust is pulled by gravity and sinks down beneath the trench

Sea floor spreading and subduction

Subduction and the Earth’s Oceans Sea-Floor spreading and subduction can change the size and shape of the oceans. Ocean floor is renewed every 200 million years – the time it takes for the floor to travel from ridge to trench. The Pacific Ocean is shrinking. More crust is being subducted than is being formed. The Atlantic Ocean is expanding. More crust is being formed than is being subducted.

References http://www.whoi.edu/page.do?pid=8422 http://ocean-ridge.ldeo.columbia.edu/courses/subgeol/mid_ocean_landscape.html http://oceansjsu.com/105/exped_commotion/8.html