The outermost layer of any planetary surface is called the crust, which is made of solid rocks and minerals. Beneath the crust is a mantle, which is also made of mostly solid rock and minerals. But it is punctuated by malleable areas of semi-solid magma. Finally, there is a hot inner core of iron and nickel. The temperature there is extremely high, but the liquid metals are held in place by intense pressure. The inner core is churning constantly, creating electrical currents that generate Earth?s magnetic field, which reverses about every 200,000 to 300,000 years.
The core is believed to have formed during the early stages of the planet?s formation 4.5 billion years ago. As the material melted, denser substances sank toward the center and less dense elements migrated upward to form the crust. The inner core may record the early stage of this melting process, while the outer core records the final stage.