1. Seafloor Spreading
Chapter 7 Section 2
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2. Mapping the Ocean Floor:
Sound waves are used to map large areas of oceans.
Waves echo off the bottom, the longer takes to return, the deeper it is.
Discovered ridges or mountains, and valleys
Rift valleys – volcanic eruptions and earthquakes occur
Mid-ocean ridge – underwater mountain ranges cover most of Earth’s ocean floor.

3. Seafloor Spreading Harry Hess theory
Hot, less dense material below Earth’s crust rises toward the surface at the ridges.
Flows sideways, carrying the seafloor away from the ridge in both directions.
When it spreads, magma’s forced upward and flows from cracks.
Cools, contracts, becomes denser, and turns solid and forms new seafloor.
Forms the ridge

4. Evidence of Spreading Youngest rocks found at the ridges
Rocks become older the farther from ridges
The heat and chemicals from molten material supports exotic life in the deep

5. Magnetic Clues Magnetic field – N and S Pole
Force leaves Earth near S pole and enters at N pole
Magnetic field reversed many times in intervals of thousands or millions of years

6. Magnetic Time Scale
Iron minerals record Earth’s magnetic field direction
Magnetometer – device to detect magnetic field
When poles are normal, rocks form large peaks
Reversals showed new rock was forming, explains how the crust could move.

7. Questions How do we use sound waves to find out how deep an ocean is?
What is a mid-ocean ridge?
Describe the process of seafloor spreading.
How are rocks used for evidence of seafloor spreading?
How is the Earth’s magnetic field used for evidence of seafloor spreading?