Earth's continents may be the only crust of their kind in the Solar System
Planetary geologists sort rocky crusts into three types. Almost every world has the second kind, basalt melted from the mantle. But the continents we live on count as tertiary crust, repeatedly reworked, and no other planet is known to have any, probably because only Earth has plate tectonics.
Crust is the outermost solid shell of a planet or moon, usually distinguished from the mantle beneath by its chemistry, though on icy moons the difference may simply be solid over liquid. On Earth it is remarkably thin, less than 1 percent of the planet's volume, and forms the top of the lithosphere, the rigid layer broken into moving plates that let internal heat escape.
Primary crust is the original skin that froze out of a magma ocean near the end of a planet's formation, likely smashed and remade many times during the late heavy bombardment. None of Earth's survives: erosion and plate recycling have erased every rock older than about 4 billion years, so its makeup remains unknown. The best surviving example is on the Moon, whose pale anorthosite highlands formed as plagioclase crystals floated to the top of a lunar magma ocean. Mercury's highlands and the Martian meteorite ALH84001 may also be primary crust, though both are debated.
Secondary crust, made by partial melting of the mantle and usually basaltic, is the most common kind in the Solar System. It covers most of Mercury, Venus, Earth and Mars and fills the lunar maria. On Earth it is produced mainly at mid-ocean ridges, where rising mantle partly melts. Tertiary crust is more heavily altered still, formed by remelting secondary crust or by piling up sediments, and continental crust is the only confirmed example.
The Moon offers a detailed case study. It is thought to have formed from debris after a protoplanet called Theia struck the young Earth. Its crust runs from about 20 to 120 km thick, averaging perhaps 50 to 60 km, and is about 12 km thicker on the far side. The upper part is around 88 percent plagioclase, and the bulk of it solidified within a couple of hundred million years of the Moon's birth, roughly 4.5 to 4.3 billion years ago. Mare basalts added later erupted between roughly 3.9 and 3.2 billion years ago.
Source: Crust (geology)
