The outer part of the Earth’s body acts as a thermal insulator that prevents intense heat from escaping to the surface. Within the core, the temperature is a few thousand degrees, where the heat is radioactive. Information about Earth’s interior has been gathered by three methods. This first is by the analysis of earthquake waves, and the second method is by analyzing the composition of meteorites; lastly, the earth’s size, shape, and density are taken into consideration. Research indicates that the earth has an interior consisting of concentric shells differing by size, chemical makeup, and density. In addition, the earth is undoubtedly much denser near the center than it is at the surface.
The crust varies from five to twenty-five miles in thickness and consists of continents and ocean basins. Extending to a depth of about 1,800 miles, the mantle probably consists of very dense rock rich in iron and magnesium minerals. The heat energy released in the upper part of the mantle breaks the earth’s crust into plates that slide, setting up stresses along the plate margins that result in the formation of folds and earthquake faults. The core is composed of iron and nickel. Here, the magnetic field undergoes periodic reversals; the last reversal occurred some 780,000 years ago.