Seafloor Spreading Chapter 7 Section 2 Pgs. 186 - 188

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.

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

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

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

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.

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?