Life has created some of Earth's outer crust, the limestones. Only in the twentieth century did science verify that Earth's crust is in constant movement.
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.
plate boundaries in white
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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