The interior of the Earth, similar to the other terrestrial planets, is chemically ivided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Many of the rocks now making up the Earth's crust formed less than 100 million (1×108) years ago; however the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that the Earth has had a solid crust for at least that long. Most of the Earth's structure is theoretical, being based on extrapolations of physical evidence which has come from the first few kilometres of the earth's surface, samples brought to the surface from deeper depths and analysis of signals that pass through it in the Electromagnetic spectrum.