Plate Tectonics

1. Plate Tectonics • Plate tectonics is the theory that Earth's outer layer is made up of plates, which have moved throughout Earth's history. The theory explains the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time on what are now widely separated continents.

2. Pangea –A Supercontinent • You probably wouldn't recognize the Earth if you could see it 225 million years ago. Back then, all the major continents formed one giant supercontinent, called Pangaea. • Perhaps initiated by heat building up underneath the vast continent, Pangaea began to rift, or split apart, around 200 million years ago. Oceans filled the areas between these new sub-continents. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. These continents are still on the move today.

3. A Driving Force • Exactly what drives plate tectonics is not known. One theory is that convection within the Earth's mantle pushes the plates, in much the same way that air heated by your body rises upward and is deflected sideways when it reaches the ceiling.

4. Another Idea • Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor. • Whatever drives the movement, plate tectonic activity takes place at four types of boundaries: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where two land masses collide; and transform boundaries, where two plates slide against each other.

5. The Sea Floor SpreadDivergent Boundaries • The Earth's longest mountain chain isn't the Andes in South America, or the Himalayas in Asia, or even North America's Rockies. It's an underwater chain of mountains 47,000 miles long. The chain runs down the middle of the Atlantic Ocean (surfacing at Iceland), around Africa, through the Indian Ocean, between Australia and Antarctica, and north through the Pacific Ocean.

6. The Continental CrushConvergent Boundaries • An ocean floor pushed toward a land mass will always slide under the land mass. This is because the land mass is more buoyant, or lighter, than the ocean floor. When two land masses meet, on the other hand, neither will slide under the other. Instead, the two crush together at what is known as a collisional boundary. They crumple and fold. Some pieces of land are thrust over or under other pieces. The result is a mountain range.

7. Slipping And SlidingTransform Boundaries • Transform boundaries neither create nor consume crust. Rather, two plates move against each other, building up tension, then releasing the tension in a sudden and often violent jerk. This sudden jerk creates an earthquake.

8. Evidence for Plate Tectonics • The shapes of many continents are such that they look like they are separated pieces of a jig-saw puzzle. For example, look in the adjacent map at the shape of the east coast of North and South Americal relative to the shape of the west coast of Africa and Europe

9. 2. Many fossil comparisons along the edges of continents that look like they fit together suggest species similarities that would only make sense if the two continents were joined at some point in the past.

10. 3. There is a large amount of seismic, volcanic, and geothermal activity along the conjectured plate boundaries. This is shown clearly below in the figure labeled "Crustal plate boundaries" where the epicenters of earthquakes above Richter magnitude 5.0 are plotted for a 10-year period. The concentration is striking, and indeed this plot serves to define the plate boundaries extremely well. Here is a clickable map of current volcanic activity on Earth.

11. 4. There are ridges, such as the Mid-Atlantic Ridge (see figures above and below) where plates are separating that are produced by lava welling up from between the plates as they pull apart. Likewise, there are mountain ranges being formed where plates are pushing against each other (e.g., the Himalayas, which are still growing).

12. 5. Magnetic striping and polar reversals .

13. 6. Earthquake patterns also lend evidence to plate tectonics. Most earthquakes occur along convergent plate boundaries. This is only possible if plates are moving and thus subducting.

14. 7. In the Hawaiian Emperor Chain, they have found that in the curve of islands, the largest and least eroded of the islands is at one end of the string, and contains active volcanoes. At the other end, is a small, eroded island, time-dated to be much older than the currently volcanic island. Also, on all the islands is increasingly older volcanic materials, proving that the islands were once volcanic themselves, or actually caused by a volcano. The strip was created by the hot spot under the plate creating volcanoes. As the plate moved by the hot spot, it made volcanoes, and islands, and then the island would be carried away by the plate, and another one would be created. Hot Spots

15. Paleomagnetism • The magnetism of an iron-bearing rock imparted to it by the Earth’s magnetic field when the rock formed. Literally, early magnetism; meaning magnetism formed in a past geologic era

16. 8. Polar Wandering • Based on paleoclimatic and paleomagnetic evidence, to geologists, the words 'polar wandering' have two distinctly different meanings. That's partially because 'pole' can refer to the magnetic pole or to the geographic pole. So what, if anything, moved in the past? The magnetic poles? The geographic poles? Both? 1. CONTINENTAL DRIFT - the poles stayed put but the continents shifted position 2. POLAR WANDERING - the continents stayed put but the poles shifted position • In any case, many geologists go no further than saying that it appears there was some sort of shift in the pole position. But since the magnetic poles and geographical poles are so close together, maybe it happened and maybe it didn't. • Wegener's idea was that the continents did the moving, drifing through the oceanic crust and pushing up mountain ridges ahead of them as they moved.