Plate Tectonics

Definition, Origins

Plate tectonics are explaining Earth's major relief features, based on this idea that continents are drifting relative to each other, being affixed onto geological slabs. Plate tectonics were a sequel to a preceding theory, called the "continental drift", which had been exposed in 1912 by the German scientist Alfred Wegener. He thought that present days continents had fragmented from a unique continent -the "Pangaea"- 225 million years ago. A scientific view of plate tectonics developed during the 1960's. Scientists discovered polar wandering, which led them to find that the magnetism some rocks kept, when they cooled, was showing that they were originating at a same location. The seafloor spreading, on the other hand, was discovered along with the mid-ocean ridge system. Incidentally, this led too to the discovery of the Earth's magnetic field reversals. Each side of the mid-ocean range the polarity of rocks is seen switching regularly. Nearly 4 reversals occurred during the past 5 million years. Such studies led, about 1970, to a single theory called the "New Global Tectonics", or "Plate Tectonics". It's this new theory which is now accounting for the miscalleneous aspects of the geological dynamics and evolution at Earth. Volcanoe ranges, marine trenches, and moutain ranges, all are due to plate tectonics. Considered in relation with the solar system, plate tectonics, on the other hand, are seen at work at the other planets too, shaping and modeling there various aspects of the relief forms

Mechanisms

The Earth's crust is linked to the upper part of the mantle, this part of the interior of our planet which is between the nucleus and the surface. The crust and the upper part of the mantle are forming the lithosphere. Plates are parts of it as a more fluid layer, the "asthenosphere", is lying beneath. The plates of the plate tectonics are just floating upon this second layer. The basics of plate tectonics is that some new Earth's crust is produced a the bottom of the oceans, at the mid-ocean ridges. Such ridges are undersea, volcanic, moutain ranges. This is explaining why the crust on the oceanic bottoms is thinner than on the continents or than under mountains. On the other hand, a part of Earth's crust is disappering in other places, which are called "subduction zones". Plates there are sinking below some other as the crust, thus, is buried and melted back to magma. Such a process is pushing up tall, mountaineous, volcanic ridges on one hand, and is creating oceanic trenches on the other hand. Such locations are very prone to earthquakes which are occurring, logically, deep into the Earth, where one plate is forced under the other. The tremor originates in the upper plate, which is forced up by the subducting one. The subduction process may also occur between oceanic plates, creating submarine trenches, like the Marianas; between an oceanic and a terrestrial plate, creating trench and mountain ranges like along the Pacific coast of South America; or between two terrestrial plates, creating high mountains and plateaus, like in the Himalayas

Tsunamis, like the one in the Indian Ocean by 2004 year's end, are linked to the earthquakes which are generated along two submerged tectonic plates, one subducting under another. As the stress accumulates due to the friction between the two plates, the edge of the overriding one tends to be dragged down along with the subducting one. When a large earthquake eventually occurs -that is when the rocks at the boundary between the plates break- this plate's edge is suddenly released upwards. It's this strong upward motion, violently pushing like a blow towards the ocean's surface, which triggers the circular, gigantic waves of the tsunami. Late data seems to show that those vertical motions however are less important in generation of the tsunami than the horizontal ones in the same region

Forces at work in tectonics are still not well explained. They seem to imply several factors. Earth's gravity makes that once a plate has entered a subduction zone, the gravity naturally pulls it along a gentle gradient. Convective currents in the mantle are yielding a circular pattern: hot magmatic material floats up within the asthenosphere, becomes cooler, and then sinks back again. Another view, now mostly accepted in replacement however, is that it's the plates motion itself which is inducing the convection pattern. At last, "thermal plumes" -which are hot spots, 60-160 miles wide (100-250 km) originating deep in the mantle and rising up through the asthenosphere- are involved in the plate tectonics too

click to a view of the major tectonic plates

Conclusion

Current plate motion is making the Pacific Ocean smaller, the Atlantic larger, and the Himalayas taller. Plates are moving on average at nearly 2 to 3" a year (4 to 7 cm). In some cases, two plates are just sliding along each other. The best example is the St.Andreas fault where California is sliding along the American continent. Might we see the mid-ocean ridges, we might see the longest mountain range on Earth! It's 40,000 miles (60,000 km) long! There are seven major tectonic plates and more than a dozen lesser ones which are still shaping the planet today!