why do tectonic plates float on the mantle
mantle A zone of hot rock in the interior of the Earth, located between the outer crust and the molten core. are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called
continental drift. The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth. Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may вcrumpleв to form mountain ranges. It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places known for having earthquakes.
The theory of plate tectonics and continental drift were proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed. It took more than 50 years for Wegenerвs theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully. The continents float around on the surface of the earth in super-slow-motion. They float on rafts of rock called tectonic plates (white lines on the picture below). There are about fifteen major plates, and many small plates.
All the land on earth floats, but not on water. It floats on the Mantle of semi-liquid rock just beneath Earth's crust. The mantle is an 1,800 mile deep sphere beneath the crust. It is solid in its center, but soft on its upper boundary. Like a thick liquid, the upper mantle has convection currents--which make the plates move. When the mantle oozes out of a plate boundary, or out of a volcano, we call it lava. Earth's crust beneath the land is different from the crust beneath the ocean. The continental crust is thicker but lighter than the ocean crust, on the average 20-40 miles thick, and as a result it floats higher in the outer mantle than the oceanic basins. The oceanic crust consists of a number of igneous rocks among which is basalt--a hard black rock, also nickel, magnesium, and iron. The oceanic crust is much thinner than the continental crust (between 2 and 5 miles thick)--thinner, but heavier, which means it settles deeper into the semi-liquid outer mantle than the continents do.
About 225 million years ago (mya)--during the Jurassic Period, the plates were clustered together forming a connected land mass supercontinent we call Pangea. By 200 mya Pangea had split into Gondwanaland and Laurasia. By 135 mya things begin to look familiar. India is floating north; Antarctica is floating south. By 65 mya, Australia has broken off Antarctica and is floating northeast. The continental plates are still creeping apart--in the middle of the Atlantic Ocean at the plate boundary, the plates are moving apart, and new crust is being formed. Today, if we look at the shapes of the various continents on a map or a globe, we can see how the shapes appear to fit together like puzzle pieces.
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