Earth's Interior

At a depth of 1,800 mi (2,900 km) there is another abrupt change in the seismic wave patterns, the Gutenberg discontinuity or core-mantle boundary (CMB). The density change at the CMB is greater than that at the interface of air and rock on the Earth's outer surface. At the CMB, P waves decrease while S waves disappear completely. Because S waves cannot be transmitted through liquids, it is thought that the CMB denotes a phase change from the solid mantle above to a liquid outer core below. This phase change is believed to be accompanied by an abrupt temperature increase of 1,300°F (704°C). This hot, liquid outer core material is much denser than the cooler, solid mantle, probably due to a greater percentage of iron. It is believed that the outer core consists of a liquid of 80–92% iron, alloyed with lighter element. The composition of the remaining 8–20% is not well understood, but it must be a compressible element that can mix with liquid iron at these immense pressures. Various candidates proposed for this element include silicon, sulfur, or oxygen.

The actual boundary between the mantle and the outer core is a narrow, uneven zone that contains undulations on the order of 3–6 mi (5–8 km) high. These undulations are affected by heat-driven convection activity within the overlying mantle, which may be the driving force for plate tectonics. The interaction between the solid mantle and the liquid outer core is also important to Earth dynamics for another reason; eddies and currents in the iron-rich, fluid outer core are ultimately responsible for the Earth's magnetic field.

There is one final, deeper transition, evident from seismic wave data: Within Earth's core, at a depth of about 3,150 mi (5,100 km), P waves encounter yet another seismic transition zone. This indicates that the material in the inner core is solid. The immense pressures present at this depth probable cause a phase change, from liquid to solid. Density estimates are consist with the hypothesis that the solid, inner core is nearly pure iron.

The heat that keeps the whole interior of the Earth at high temperatures is derived from two sources: heat of formation and radioactive metals. As the Earth accreted from the original solar nebula, impacts of new material delivered sufficient energy to melt most or all of the forming planet's bulk. As most of the new Earth's iron sank its center through its bulkier, lighter elements (silicon, oxygen, etc.), further energy was released, sufficient to raise the temperature of the core by several thousand degrees Centigrade. Radioactive elements such as uranium and thorium, mostly resident in the mantle, have continued to supply the Earth's interior with heat in the billions of years since its formation; however, the Earth's interior continues to cool, steadily losing its primordial heat to space through the crust. As the core cools, its inner, solid portion grows at the expense of its outer, liquid portion. The current rate of thickening of the inner core is about 0.04 inch (1 mm) per year.