Geologists know that the center of the Earth is composed of various concentric layers of rock with distinct physical characteristics. The way in which they have discovered this is through the study of earthquakes. The observation of seismic wave changes reveals the positions of the main boundaries of the different layers of the Earth. It is believed that the Earth´s core formed 30 million years after the formation of the solar system. During this time, particles of gas and dust accrued together into planetesimals which in turn formed the planets and satellites.
The formation of the Earth took place over a period of 100-200 million years. During this period, small particles of gas and dust collided forming planetesimals which eventually aggregated into larger bodies known as planets. The mass of aggregated planetesimals increased, and so did their gravitational fields, so that the heavier elements, such as iron and nickel descended towards the center of the Earth. The increasing gravitational field attracted meteoroids whose repeated impacts transferred enormous amounts of heat which maintained the Earth in a molten state. Decay of radioactive materials in the Earth´s core added heat, and continue to do so in the present.
During an earthquake, seismic wave velocity increases with depth and density. In geophysics, the reflection and refraction of seismic waves allows scientists to research into the structure of the Earth. During an earthquake, seismographs record P and S waves; however, seismographs located on other parts of the world are not able to record (S-waves) as they cannot pass through liquids, suggesting that that the Earth has a liquid outer core. Richard Dixon Oldman was a geologist who detected the separate arrival of seismic P, S, and surface waves on seismographs, concluding that the Earth had a central core.
Geologists believe that the Earth´s core is made of iron metal along with nickel, surrounded by a liquid outer core. This idea is based on seismic wave studies. By analyzing the speed of the waves at the core, scientists have that the core possesses a density similar to that of iron. In addition to iron, scientists believe that there is a lighter element composing the core. Researchers believe this element might be a left over from the Earth´s formation 4.5 billion years ago. According to a scientific paper published in Nature.com, there is evidence of lighter elements, including sulfur and oxygen at the limits of the Earth´s core.
Scientists think that the lighter elements might be concentrating in the outer liquid core, while the inner core solidifies. In addition, studies suggest that as the light elements ascend to the inner core´s surface, they generate a motion that drives a dynamo giving rise to the Earth´s magnetic field. In the past, scientists speculated about the real causes that were driving the Earth´s internal dynamo; however, the evidence of lighter elements, and the fact that these elements can generate a huge amount of gravitational energy, has provided new viewpoints.