Chapter 5, Lesson 4

Sea Floor Spreading

Chapter 5, Lesson 4

Pages 196-201

What are Mid-Ocean Ridges?

When scientists observed the ocean floor, they discovered that some places looked "stitched" together like the seams of a baseball

These seams formed mountain ranges along the middle of the ocean floor called mid-ocean ridges

Mid-ocean ridges form long chains of mountains that rise up from the ocean floor

A steep-sided valley splits the tops of some mid-ocean ridges

​Draw this diagram.

What is Sea-Floor Spreading?

Sea-floor Spreading is the process of new material continually being added to the ocean floor at mid-ocean ridges

Along the mid-ocean ridge, new molten material from inside earth rises, erupts, cools, and hardens to form a solid strip of rock

Sea-floor spreading adds more crust to the ocean floor. At the same time, older strips of rock move outward from either side of the ridge

In the central valley of mid-ocean ridges, scientists have found rocks shaped like pillows. Such rocks form only when molten material hardens quickly after erupting under water.

Evidence for Sea-Floor Spreading from Ocean Materials

Magnetic minerals inside the rock line up in the direction of earth's magnetic poles. Each stripe defines a period when molten material erupted and hardened.

Stripes on each side of the ridge are usually mirror images

Evidence from Magnetic Striping

By drilling into the ocean floor, scientists found that the farther a rock sample was from the ridge, the older it was

Evidence from Drilling Samples

What happens at Deep-Ocean Trenches?

The ocean floor doesn't just keep getting wider - eventually it plunges into deep underwater canyons called deep-ocean trenches

In a process that takes tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches

The Process of Subduction

Changes to density affect the ocean floor - as the new crust moves away from the mid-ocean ridge it cools and becomes more dense

Gravity will pull the more dense, oceanic crust deeper into the mantle below the less dense continental crust

Subduction - the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle

Multiple Choice

Label the location of the sea floor at #1.

• Deep-Ocean Trench

• Mantle

• Mid-Ocean Ridge

• Subduction Zone

Multiple Choice

Label the location of the sea floor at #2.

• Deep-Ocean Trench

• Mantle

• Mid-Ocean Ridge

• Subduction Zone

Multiple Choice

Label the location of the sea floor at #3.

• Deep-Ocean Trench

• Mantle

• Mid-Ocean Ridge

• Subduction Zone

Multiple Choice

Select the correct order to complete the statements for location A.

• newly formed; colder; less

• newly formed; hotter; less

• older; hotter; less

• older; colder; more

Multiple Choice

Select the correct order to complete the statements for location B.

• newly formed; colder; less

• newly formed; hotter; less

• older; hotter; less

• older; colder; more

Subduction and Earth's Oceans

The sizes of Earth's oceans are dependent on mid-ocean ridges and deep-ocean trenches. If more crust is erupting at a mid-ocean ridge than is subducting at a deep-ocean trench, the ocean will be larger than if more is subducting than erupting

Multiple Choice

Which features form the longest mountain ranges on Earth?

• the mid-ocean ridges

• the deep-ocean trenches

• the Rockies

• the Andes

Multiple Choice

Which process adds more crust to the ocean floor?

• suction

• sea-floor spreading

• subduction

• magnetic stripe

Multiple Choice

Where does subduction occur?

• along the middle of some ocean floors

• down the middle of mountain ranges

• on continents

• at deep-ocean trenches

Multiple Choice

Which process or processes change the size and shape of the oceans?

• subduction only

• seafloor spreading only

• both subduction and seafloor spreading

• both drilling for samples and subduction