Structure of the Earth. The Earth's core is composed of two portions, an inner core of solid iron and an outer core of molten iron. Above the core lies the mantle , which is made up of dense rocky materials, and the crust , which floats on the mantle. The oceans and atmosphere are the outermost layers. As we have already mentioned, the Earth must have been largely molten at some early stage in order to have been differentiated into several zones or layers.
Differentiation is perhaps the most significant event in the history of the Earth.
It led to the formation of a crust and eventually the continents. Differentiation probably initiated the escape of gases from the interior, which eventually led to the formation of the atmosphere and oceans. It could have occurred over millions of years or it could have been a more catastrophic event. The earliest Earth was probably an unsorted conglomeration, mostly of silicon compounds, iron and magnesium oxides, and smaller amounts of all the natural elements. Figure X. Three mechanisms that would cause the early Earth to heat up.