1. Chapter 2: Plate Tectonics Lesson 1 Objective #3

2. Ch.2, CRO #3 Describe the process of sea-floor spreading. Describe where sea-floor spreading is occurring. Describe how sea-floor spreading was discovered and the evidence used to support it.

3. A huge discovery While mapping the sea floor using technology developed in WWII (SONAR), oceanographers found a huge chain of underwater mountain ranges and volcanoes. A huge rift (crack) ran down the center of these ranges, suggesting the crust was being pulled apart.

4. Mid-Ocean Ridges

5. The theory of sea-floor spreading In the 1950’s a theory known as “sea floor spreading” emerged. (Harry Hess) Sea-floor spreading – the process in which the ocean crust is pulled apart (spreading) and widening the ocean as new sea-floor is formed from volcanic eruptions of mantle lava along the ocean ridges.

6. Harry Hess

7. Sea-floor spreading

9. Sea-floor Spreading (Hess)

10. Bill Nye greatest discoveries clip Sea-floor spreadingSea-floor

11. Mid-ocean ridges The sea floor is spreading away from formations known as mid-ocean ridges. These are long mountain ranges on the sea floor divided by a rift valley and active volcanoes. The mid-Atlantic ridge runs right down the exact center of the Atlantic ocean. It is here that volcanoes create new oceanic crust to “grow the ocean”.

12. Evidence Sea floor spreading was discovered based on: –The observation that magnetic reversals were found recorded in the ocean crust in a symmetrical pattern on both sides of the ridges. –The observation that ocean crust gets older on each side of the ridges the farther away you get from them.

13. Magnetic Reversals

15. Connection to Continental Drift The ages of the rock on the Atlantic sea floor as well as the rate of spreading suggests that the Atlantic ocean began spreading about 180-200 million years ago. This matches the continental drift evidence that suggests that Pangaea broke up about this same time. Sea-floor spreading appears to be the force behind continental drift (causes continents to move); Alfred was right after all !!! an ode to alfred wegener