Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types:
Planetary geologists divide crust into three categories, based on how and when they formed.
This is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of
The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are the igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today. Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed
However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this is debated. The
This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the
Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways:
The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics is needed to create tertiary crust, and Earth is the only planet in our Solar System with plate tectonics.