Theory 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. Earth's modern plate tectonic regime developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. 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. Quasi-cyclic surface height changes (from 500,000 to 3 million years in duration) exists at Earth.The lithosphere, generally, acts as the intermediary that transfers the effects of mantle dynamics to the surface as the motion of tectonic plates, generally also helps regulate global temperatures by cycling carbon dioxide between rocks and the atmosphere

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. The tectonic plates, as far the earthquakes, generally, are concerned, have been found, usually, either to creep along steadily -and don't cause any earthquake- while others stick together and then suddely release -triggering, in that case a earthquake. A new type of relative motion might have been found lately, which plates sticking together and then one of them slipping, but only slowly and taking from weeks to a year, hence neither with any earthquake. Earthquakes, further, might well be occurring too deep at a boundary between high- and low-velocity motion of the Earth's crust

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 show that vertical -- and also horizontal -- movement of the seafloor are the factors in a tsunami generation

Shallow regions of the subduction zones found at sea are rich in very-low-frequency and long-duration (up to 100 seconds) seismic waves and home to 'tsunami earthquakes' which generate stronger tsunamis. High-frequency waves which are also extant might be caused by fluid seeping into fractures in the rock, making it easier for parts a plate to slip past each other and generate earthquakes

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. Water participates into plate tectonics as it triggers magma generation beneath volcanoes, lubricates deep fault zones, and fundamentally alters the strength and behaviour of Earth’s mantle. Basalt sinks into the mantle because of its high density, being exposed to rising pressures and temperatures that drive further mineral transformations. The boundary between Earth’s upper and lower mantle is marked by steep gradients in density or seismic-wave velocities at depths of around 410 miles (660 kilometres), where basaltic materials become buoyant beyond. Hydrated rocks in the transition zone are pushed into the lower mantle, where they release water bound in their minerals. In some areas, tectonic plates slide over a mantle plume or a thermal plume -- a 60-160 miles wide (100-250 km) upwelling of magma from deep in the mantle. A typical long-lived mantle plume is the one which gave birth to the Hawai Islands as the chain of those islands are showing how a tectonic place shifted over the plume and yielded each island one by one. Mantle plume are active below the lithosphere. The plume is a channel of hot rock that starts hundreds of miles below the surface and rises through the astenosphere. It rises through the mantle and reaches the bottom of the lithosphere. The heat is then transported up through the lithosphere and alters its chemical composition, which thickens the crust. Such plumes might even be explicative of the tectonics. Otherwise, the inner planet is heated evenly throughout by decaying radioactive elements in Earth’s top layers. There's also primordial warmth left over from the formation of our planet 4.5 billion years ago, and from the meteorites that pummeled it. Recent studies showed that several large blobs of highly compressed, melting rock are part of the Earth's upper interior, as they are lying under the crust and are sinking down deep into the upper part of the mantle. Volcano formation as far as it is concerned, starts where Earth's mantle meets the molten outer core, some 1,800 miles (2,900 kilometers) deep. As the core heat the bottom rocks of it, that turns some into a buoyant blob which rises to the crust, where a few miles short of, it decompresses and eventually erupts out in the form of a volcanoe

->click to a view of the major tectonic plates

Scientists think that something has to decouple the crustal plates from the asthenosphere so they can slide over it. Numerous theories have been proposed, and one of those was that a melt-rich layer lubricates the boundary between the lithosphere and the asthenosphere, allowing the crustal plates to slide. However, since this layer is only present in certain regions it can't be the only mechanism that allows plate tectonics to happen. Where the melt doesn't exist, other possible mechanisms include the addition of volatile material like water to the rock and differences in composition, temperature, or the grain size of minerals. Current data however lacks the resolution to distinguish among them. Most of the melt layers are logically under volcanic regions like Hawaii and various active undersea volcanoes, or around subduction zones

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). The plate Australia is lying upon is the faster moving one, with the island in 2016 5 feet North of where it was in 1994, at a speed of 2.75 inches per year. 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! Strong, 8th-magnitude earthquakes occur, on average, once a year as some scientists think that since the beginning of the 21st century, compared to a period between the mid 1970's and the mid 1990's, the Earth tectonics might have been more active. That might be due to a natural time variation of the stress endured by the lithosphere. A common idea too now among scientists is that seismic waves might travel a long way and play some slight role into trigerring a earthquake far from where another one occurred, that role increasing when the waves hit a region where a fault is close to rupture, pushing further towards that rupture! About 90 percent of the earthquakes worldwide occur along the 'Ring of Fire', the regions surrounding the Pacific Ocean. Ring of Fire is characterized by the presence of active volcanoes and frequent earthquakes associated with the many active subduction and transform boundary zones around the Pacific tectonic plate. 5 to 6 percent of earthquakes occur along a region extending from the Mediterranean area eastwards. Like a example, the subduction between the Nazca and South America's plates dives at a rate of 3 inches (80 millimeters) a year. The larger earthquake ever recorded in the recent times was the 9.5th magnitude one which rocked southern Chile in 1960, killing 1,655 people and triggering a tsunami accross the Pacific, killing 61 people in Hawaii, Japan and the Philippines. As far as the number of casualties is concerned however, the Haiti earthquake, by early 2010, might be now the record holder with more than 250,000 dead