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.
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.
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
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.
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.
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. • Subductionis 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
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