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The outermost skin of our planet (about 25 miles thick) is the Earth's crust. The crust is composed of igneous, metamorphic, and igneous rock, but primarily igneous rock. The crust is chemically divided into two types. Continental crust is the less dense of the two, granitic in composition, and the thicker of the two. Oceanic crust is more dense, basaltic in composition, and relatively thin. Oceanic crust is being recycled in roughly 200 million year intervals as new crust is being created at mid-ocean ridges and destroyed by the processes of subduction at underwater trenches. The more buoyant continental crust is on average much older, the oldest continental rocks being over 4 billion years in age. Both the oceanic and continental crust are floating on top of the asthenosphere, the physical layer of the upper mantle in which convection currents of heat push and pull the individual segments of crust (known as plates) around. The crust is the thinnest of Earth's layers, comprising a tiny fraction of the planet's total mass.

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