Seafloor Spreading. By:

Transcription

1 Seafloor Spreading By:

2 Continental Drift Hypothesis! Alfred Wegener proposed the hypothesis that the continents drifted to their present-day locations. He provided evidence of certain fossils, rocks, mountain ranges, and ancient climatic conditions (stored in sedimentary rocks) on continents he once thought were connected (because of their shapes) millions of years ago on a super-continent called Pangaea. His hypothesis was rejected by scientists because he couldn t explain how, when, or why the continents moved. Unanswered Questions:!!How did the continents drift?!!when did they drift?!!what was a force strong enough to make the continents move?

3 Mapping the Sea Floor! During World War 2, enemy U- boats (submarines) would destroy valuable American boats with torpedoes from underwater. These boats needed a way to find these U-boats before they struck. So, sonar was invented. Sonar is the casting off of sound waves. These waves bounce off any surface they touch. Boats would use sonar to find U-boats. Later, sonar was used to map the ocean floor. Harry Hess used sonar to map it and discovered a mile-high underwater mountain chain, known as the Mid-Atlantic Ridge.

4 ! Harry Hess s Hypothesis Harry Hess s discovery led to even greater ones. It was found out that the seafloor was warmer near the Mid-Atlantic Ridge. This was actually heat from the magma underneath it. This led to Harry Hess s even greater theory; seafloor spreading. He hypothesized that magma slowly leaked out of the center of the ridge because of convection in the mantle. It quickly cooled and formed new seafloor. Then, more magma coming out of the ridge pushed the new seafloor away from the ridge. This would make the continental plates move, causing the continents on top of them to move, too. This process explains the unanswered question of Wegener s continental drift hypothesis; how do the continents move?

5 ! Evidence for Seafloor Spreading For the seafloor spreading hypothesis to be accepted by people, Hess would need evidence to support it. First of all, the temperature underneath the Mid-Atlantic Ridge is warmer than lower, surrounding areas. This is because magma is below it, which can come out of the center of the ridge because of mantle convection which would make new seafloor when it cooled. Seafloor drilling led to the discovery that older rocks are farther away from the Mid-Atlantic Ridge and younger ones are closer. This means that the magma coming out of the ridge is pushing old seafloor away. Third, subduction gets rid of seafloor, but it is not slowly disappearing. The only explanation is seafloor spreading. Finally, magnetic polarity alternates between normal and reversed. Igneous rock make tiny crystals that show which one it was at the time. So, since there are stripes of igneous rock with alternating crystals, it means that the seafloor is slowly spreading. The need for evidence in science has not changed. Without evidence, we just have theories and hypothesis, which cannot be supported without evidence to back it up.

6 Link Page (all links in order)

7 The Theory of Plate Tectonics! Alfred Wegener had a hypothesis that the continents drifted to their presentday locations. But, it was rejected by scientists because they couldn t think of a force strong enough to move the continents. Seafloor spreading was a hypothesis created by Harry Hess; this was the theory that the seafloor was slowly spreading. Both of these hypothesis and the information discovered from them led to another theory, called Plate Tectonics. This is the hypothesis that the Earth s crust is broken up into several pieces, called lithospheric plates, and that the plates are slowly moving which changes the geographic appearance of Earth s surface. Here is how plate tectonics works. As discovered during the investigation of seafloor spreading, new seafloor is formed when lava cools after it comes out of a mid-ocean ridge because of convection in the mantle. Once the lava cools, gravity pushes the new seafloor downhill; this process is known as ridge push. Then mantle convection currents carry the seafloor away. When the seafloor becomes cooler and denser than the warm rock in the mantle, the seafloor sinks into the mantle in a subduction zone, also called an ocean trench. The process of the seafloor sinking into the mantle is called slab pull. Once deep in the mantle, it melts back into magma.