The earths interior has a number of layers. The major layers are the earth's crust, earth's mantle and the earth's core. Besides these major layers scientists divide the mantle into the upper and lower mantle. The core is divided into a liquid outer core and a solid inner core. Scientists studying the interior of the Earth use earthquake waves to determine what lies beneath our feet because it is impossible for drilling equipment to drill to the core of the Earth.
Earth's interior layers, NASA
P waves speed up when rocks change density. This is how the mantle crust boundary was discovered. The earthquake waves speed up at the boundary and are slightly deflected by the increased density of the rocks.
S waves, produced by all earthquakes, are stopped when they reach the outer core of the Earth. S waves travel only through solids. They do not travel through liquids and gases.
Isaac Newton's measurements
Three centuries ago Isaac Newton calculated the average density of our planet. He used his study of the planets and the force of gravity to determine the average density of Earth. He discovered that the rocks in the interior of the Earth had to be much denser than crustal rocks found on the surface of the Earth.
Today scientists have many more instruments to study the Earth and have determined that Newton's estimate of the Earth's density was essentially correct. The density of the continental crust is between 2.7 and 3.0 g/cm³. The density of the oceanic crust is between 3.0 and 3.3 g/cm³.The average density of the core is 10.7 to 12 g/cm³. The inner core, which is solid, is approximately 13.5 g/cm³.
The Earth's core has two layers that are mostly iron. It is estimated that 90% of the core is iron with the remaining 10% an alloy of iron that is combined with oxygen, sulfur or nickel.
The outer core has so much heat and pressure that the rocks have melted in this layer. The increasing heat and pressure are so great that the iron and its alloys have become solid in the inner core.
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