Structure of the Earth obducted onto the continental crust and preserved as ophiolite sequences. Many rocks now making up Earth's crust formed less than 100 million (1) years ago; however, the oldest known mineral grains are about 4.4 billion (4.4) years old, indicating that Earth has had a solid crust for at least 4.4 billion years. Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The mantle is divided into upper and lower mantle. The upper and lower mantle are separated by the transition zone. The lowest part of the mantle next to the core-mantle boundary is
Geology of the Moon radius. The crust of the Moon is on average about 50 km thick (though this is uncertain by about ±15 km). It is widely believed that the far-side crust is on average thicker than the near side by about 15 km. Seismology has constrained the thickness of the crust only near the "Apollo 12" and "14" landing sites. Although the initial "Apollo"-era analyses suggested a crustal thickness of about 60 km at this site, recent reanalyses of this data suggest that it is thinner, somewhere between about 30 and 45 km. Compared with that of Earth, the Moon has only