Sea-Floor Spreading Chapter 3 Lesson 4 http://www.youtube.com/watch?v=gyT8Xs6Ab-k

Mid-Ocean Ridge A mid-ocean ridge is an underwater mountain range where new ocean floor is produced. Scientists used sonar to discover mid-ocean ridges. Most are hidden, but some like Iceland rise above the surface.

Sea-Floor Spreading…Was the continental drift theory true? Sea-floor spreading continually adds new material to the ocean floor. In sea-floor spreading, the sea floor spreads 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 the continents along with them. Along the ridge, molten material (lava) rises and erupts. Older rock moves outward on both sides of the ridge. As the molten material cools, it forms new crust in the center of the ridge.

Sea-Floor Spreading Video https://www.pearsonsuccessnet.com/snpa pp/iText/products/0-13-364590- 8/phs_GAsci6_main.html (Firefox, p. 99)

Evidence for Sea-Floor Spreading Eruptions of molten material Magnetic stripes in the rock of the ocean floor The ages of the rocks themselves https://www.pearsonsuccessnet.com/snpa pp/iText/products/0-13-364590- 8/phs_GAsci6_main.html p. 100

Deep-Ocean Trenches A deep-ocean trench is a deep underwater canyon where the ocean floor bends downward towards the mantle. This is where the ocean floor sinks back into the mantle.

Subduction at Trenches The process by which ocean floor sinks beneath a deep-ocean trench and back into the mantle is called subduction. New crust is hot. As it moves away from the mid-ocean ridge, it cools and becomes more dense. Gravity pulls the denser crust beneath the trench. Convection currents in the mantle cause subduction and sea-floor spreading. This results in the ocean being changed in size and shape.

Subduction at Trenches Conduction allows the oceanic crust to melt back into the mantle. (Heat transfer by particles touching as the crust touches the mantle.) The Pacific Ocean is shrinking because subduction through the many trenches that ring the ocean is occurring faster than new crust can be added. The Atlantic Ocean is expanding because it only has a few short trenches. The spreading ocean floor has nowhere to go.

Subduction Video https://www.pearsonsuccessnet.com/snpa pp/iText/products/0-13-364590- 8/phs_GAsci6_main.html p. 102

Review and Reinforce #1 Name and describe the feature of the ocean floor shown at A. Mid-ocean ridge: mountain range that extends along the sea floor

Review and Reinforce #2 Describe the process shown occurring at B, and explain what results from this. Sea-floor spreading: continually adds new material to the ocean floor

Review and Reinforce #3 What happens to old oceanic crust as new molten material rises from the mantle? The new material splits apart the old material and pushes it aside.

Review and Reinforce #4 What process is shown occurring at C, and why does it occur? Subduction at a deep-ocean trench: Oceanic crust becomes denser the farther it moves away from the mid-ocean ridge. Gravity pulls the dense crust down beneath the trench.

Vocabulary 6. A device that scientists use to map the ocean floor is SONAR. 7. The feature on the ocean floor at C is called a(n) DEEP-OCEAN TRENCH. 8. The process that continually adds new material to the ocean floor is called SEA-FLOOR SPREADING. 9. The process by which the ocean floor sinks into the mantle is called SUBDUCTION. 10. A chain of underwater mountains along which sea-floor spreading occurs is a MID-OCEAN RIDGE