Ocean floor is young; some continental rock is over four billion years old
The ground beneath us is a skin thinner than one percent of Earth's radius, floating on denser mantle rock. Seafloor is constantly made and swallowed, so none of it is older than about 200 million years, while scraps of continent in Western Australia and northern Canada date back more than 3.7 billion.
The crust is the uppermost part of the lithosphere, the rigid shell that also takes in the top of the mantle and is broken into moving tectonic plates. It rests on peridotite, which is considerably denser, and the dividing line, the Mohorovičić discontinuity, is recognised by a sudden change in how fast seismic waves travel. Heat rises with depth: locally up to 30 °C per kilometre near the surface, reaching roughly 700 to 1,600 °C where crust meets mantle.
There are two kinds. Continental crust is usually 25 to 70 km thick, built largely from lighter felsic rocks like granite, and swells to 50 to 80 km under the Tibetan Plateau and the Altiplano. Oceanic crust is only 5 to 10 km thick and made of heavier mafic rock such as basalt and gabbro. Both float, a balance called isostasy, and the thicker, more buoyant continents ride high above deep ocean basins. Feldspars make up about 41 percent of continental crust by mass, with quartz at 12 percent.
Earth formed around 4.6 billion years ago and melted completely from the heat of accretion. Its first, primordial crust was repeatedly smashed by impacts and remelted, and not a trace of it survives. What exists now is secondary crust, the ocean floor produced at mid-ocean ridges, and tertiary crust, the continents, assembled at subduction zones by recycling old seafloor.
Because new seafloor pushes outward from ridges and old seafloor sinks back into trenches, the ocean crust is continually renewed. Continents last far longer: their average age is estimated at about 2.0 billion years, and the most ancient pieces sit in cratons, whose light, buoyant roots resist subduction. Zircon grains from the Narryer Gneiss terrane reach about 4.3 billion years. Bursts of new continental crust coincide with mountain building during the assembly of supercontinents such as Rodinia and Pangaea.
Source: Earth's crust
