1. Seafloor Spreading Chapter 17.2

2. Ocean Floor Discoveries - Through the use of modern technologies such as SONAR and magnetometers, scientists have been able to produce detailed maps of the ocean floor. - Scientists have discovered a underwater mountain range that encircles the Earth. - They call this mountain range the “Mid-ocean Ridge”. - They have also discovered deep-sea trenches many miles deep along the Pacific Ocean rim. - The rock closest to the mid-ocean ridges is younger than the rock further from the ridge. - Scientists also discovered identical patterns of magnetic polarity in the rocks moving out ward from the mid-ocean ridges. Video – Plate Tectonics: Further Evidence (2:11 min)

3. Sea-floor Spreading - All of these discoveries have led scientists to propose the Theory of Sea-floor Spreading. - Rising magma in the center of the Atlantic Ocean is causing the sea-floor to push the continents apart, thus widening the Atlantic Ocean. - As the continents move apart in the Atlantic, they move closer together in the Pacific, causing the Pacific Ocean to narrow.

4. Formation & Destruction of Oceanic Crust Old oceanic crust is destroyed at subduction zone. New oceanic crust forms at mid-ocean ridge. Trench forms at the boundary between two plates.