Earth and Life Science Practice Test 2026 - Free Practice Questions and Study Guide

The outer core is liquid because the temperatures there are high enough to melt iron and nickel, and the pressure, while intense, isn’t sufficient to keep them in a solid state. This molten iron-nickel layer can flow and convect, driving the geodynamo that generates Earth’s magnetic field. In contrast, the inner core remains solid because the pressures at the center are so extreme that they force the metal into a crystalline solid despite the high temperature. The crust is solid, and the mantle is mostly solid too (though the mantle can behave plastically over long timescales and has partially molten zones).