The upthrusting current would break the crust into two pieces. At other places the cooling mantle would drag the attached crust down as the cooling mantle material being more dense would sink back down into the hotter magma beneath.
The downward movement of the mantle would create subduction zones causing a second break in the crust. The theory of plate tectonics provides a reasonable explanation of how the plates might have formed from the action of convection currents in the hot magma of the mantle.
How the crustal plates first formed in not known for certain however. How did the tectonic plates first form? David Drayer. Feb 4, Explanation: The crust may have started as cooled igneous material from the surface of the mantle. This photo shows an explosion near the summit of the West Mata volcano within the Pacific Ocean; the image area is about 1. Download larger version jpg, 1. Heat within the asthenosphere creates convection currents that cause tectonic plates to move several centimeters per year relative to each other.
If two tectonic plates collide, they form a convergent plate boundary. Usually, one of the converging plates will move beneath the other, a process known as subduction. Deep trenches are features often formed where tectonic plates are being subducted and earthquakes are common. As the sinking plate moves deeper into the mantle, fluids are released from the rock causing the overlying mantle to partially melt.
Convergent boundaries include subduction zones where one plate dives beneath another, as occurs along the coast of South America, and continent-continent collisions, such as where India is colliding with Eurasia and raising the Himalayas. Divergent plate boundaries are where two plates are moving apart and new crust forms as magma rises up to the surface and solidifies, such as at the Mid-Atlantic Ridge. Transform boundaries, where plates slide past one another, occur in places such as the West Coast of North America along the San Andreas Fault.
Also noted on this map are "diffuse boundary zones," where wide swaths of crust are deformed due to a faraway boundary. Credit: K. Earth did not always have plate tectonics.
For millions of years after the planet accreted, its surface roiled with a molten magma ocean. Once the planet cooled enough for a crust to form, the surface may have looked more like modern-day Venus, with the crust and upper mantle — collectively called the lithosphere — forming a single unbroken plate.
Oceanic crust is most commonly formed when basaltic magma rises to the surface at mid-ocean ridges, such as the Mid-Atlantic spreading ridge, and is thinner — typically about 7 kilometers thick — and denser than continental crust. Today, continental crust is formed mainly along subduction zones, where partial melting of descending slabs forms granitic and andesitic magmas at volcanoes on the overriding plate.
This process produces thicker — up to 70 kilometers thick — and more buoyant crust that is not as easily subducted. But it is not known how continental crust formed in the past. Subduction zones form where two plates converge and one begins sliding under the other.
As old lithosphere is recycled back into the mantle at subduction zones and new lithosphere is formed at spreading centers, the balance of lithosphere on Earth remains relatively constant. Today, the planet has eight major plates defined as those with areas over 20 million square kilometers and dozens of minor plates between 1 million and 20 million square kilometers and microplates less than 1 million square kilometers.
While some plates are composed solely of oceanic or continental crust, most major plates contain portions of both. While continental crust that is billions of years old still exists on Earth's surface, most oceanic crust is less than million years old Ma. Older oceanic crust, which is more dense than continental crust, has long since been recycled in the process of subduction. Credit: U. Geological Survey. Mantle convection is driven by temperature differences between the hot interior and the gradually cooling outer layers of the planet.
Cooler, denser material sinking down into the mantle is thought to be the primary driver of circulation, while hotter, less dense material rising to the surface in the form of mantle plumes and upwellings provides a secondary driver.
The forces generated by these vertical movements result in horizontal shifts of the tectonic plates at the surface at rates of about a few centimeters per year. One of the big questions about the onset of plate tectonics is how subduction got started. Geologists think that the lithosphere of the pretectonics Earth existed as a single plate that covered the whole planet. Massive forces would have been needed to break this single lithosphere into multiple plates and to initiate plates descending into the mantle.
Minerals in lithospheric slabs restructure as slabs descend into the mantle, releasing water and increasing the slabs' densities. The dense, downgoing slabs pull on the parts of the plates still at the surface, driving plate tectonics. Some subducting slabs stall at the transition zone, while others descend toward the core-mantle boundary. Credit: both: K. Cantner, AGI. The forces involved are incredible. Modern plate tectonic movement is driven primarily by the descent of the subducting limb of a plate, called a slab, pulling the rest of the plate down behind it.
The momentum of the massive sinking slabs overcomes the friction generated by the upper mantle adjacent to the slabs as they descend. But it raises a chicken-and-egg question.
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