A compass magnet aligns with the magnetic field anywhere on the Earth, but other bodies like the Moon and Mars have no magnetic field. Almost all of the earth is hotter, and the only other way to make a magnetic field is with a circulating electric current . Circulation and convection of electrically conductive molten iron in the Earth's outer core produces the magnetic field. Because the Earth's magnetic field arises in the unstable patterns of fluid flow in the core, it changes direction at irregular intervals. In recent geologic history it may have switched direction about every 20,000 years.
Geophysicists can measure the changes in direction to make a magnetostratigraphy for the deposit.
At oceanic spreading centers new ocean floor is being created constantly and slowly moved away from the rift. The farther the rock is from the rift, the older it is, and it will also show the magnetic reversals like a tape recording. This map of the Pacific Plate at various stages of geologic history could be constructed from the tape recording. Such maps show how the tectonic plates have re-arranged themselves over the last 200 million years. Exploring the Earth's Mantle Convection and the release of heat from the Earth's core drives further convection to release heat from the mantle.