inner layer of the earth (The Core)

Earth’s Core
The outer Core The boundary between Earth’s mantle and outer core, called the core–mantle boundary, is significant because of its varied material properties. P wave velocities drop dramatically as
they cross the core–mantle boundary, and S waves do not penetrate the outer core. Because S waves do not pass through liquids, their absence in the outer core indicates its liquid state. The change in density at the core–mantle boundary from 5.6 to 9.9 g/cm3 is even
larger than the air–rock difference observed at Earth’s surface.

Based on our knowledge of the composition of meteorites, geologists expect Earth to contain a significantly higher percentage of iron than is observed in rocks found in the crust and mantle. This fact, coupled with the great density of the core, indicates that the outer core consists mostly of iron, with lesser amounts of nickel, which has a density similar to that of iron.

Density and seismic studies suggest that in addition to iron and nickel, about 15 percent of the outer core consists of lighter elements. Based on mineral physics experiments, these are likely to include sulfur, oxygen, silicon, and hydrogen. For instance, an iron
sulfur mixture melts at a much lower temperature than pure iron. As Earth was forming, high-velocity impacts of nebular debris and the decay of radioactive elements caused the temperature of our planet to increase  When heated sufficiently, iron in the presence of sulfur would begin to melt before pure iron. Melting produced liquid blobs of an iron–sulfur alloy that sank to form the core. In a similar manner, other light elements were dragged down into the                                                      core.
The core accounts for about one-sixth of Earth’s volume but one-third of its mass because it is composed mostly of iron, which is the most dense of the common elements. In fact, iron is Earth’s most abundant element when measured by mass.
The Inner Core:
At the center of the core is the inner core, a solid dense sphere (around 13 g/cm3) of iron with trace amounts of other elements. Because the inner core is a sphere whereas Earth’s other layers
are shells, drawings make the inner core appear much larger than it really is (see Fig1).

The inner core is actually relatively small, only 1/142 of the volume of Earth (less than 1 percent). The inner core did not exist
early in Earth’s history, when our planet was hotter. However, as Earth cooled, iron began to crystallize at the center to form the solid inner core. Even today, the inner core continues to grow larger at the expense of the outer core—as the planet cools. Separated from the mantle by the liquid outer core, the solid inner core is free to rotate independently from Earth’s outer layers. Recent studies suggest that the inner core rotates faster than the crust and mantle, lapping them every few hundred years
(Fig12).

 The inner core’s small size and great distance from the surface make it the most difficult-toexamine region of Earth. Discovering Boundaries: The Inner Core–outer Core Boundary The boundary between the solid inner core and liquid outer core was discovered in 1936 by Danish seismologist Inge Lehman. By examining seismograph records, Lehman discovered that some P waves were strongly refracted (bent) by a sudden increase in seismic velocities at a boundary within Earth’s core. The waves she observed were bent enough to arrive within the P-wave shadow zone, as shown in Fig11A.

Because of the work of Lehman and other scientists, it is now understood that the inner core is an integral part of the activities of Earth’s interior.