Prior to Earth formation, the solar system was a giant cloud of dust and gas that originated from previous explosions of stars. Due to the force of gravity, this cloud of material began to collapse in on itself, resulting in the material clumping together. This material is ultimately what makes up our planet and was composed of many different elements like carbon, hydrogen, nitrogen, oxygen, calcium, and phosphorous, which happen to be the components that make up almost 99% of the mass of the human body. But, these are only a few of the elements that were around upon earth’s formation. The extremely heavy, or dense elements sank to the center of the earth while the lighter elements rose towards the surface of the earth.

You can imagine this process to be the same as if you were to mix water and oil for example; if left untouched for a few minutes, the less dense oil will rise above the more dense water creating two separate layers. The formation of Earth’s layers is the same process. The earth has a very dense inner core made up of heavy metal elements, like iron and nickel, and a much less dense crust (the crust is the layer of earth that we are in direct contact with as this is the layer we live on) that “floats” on top of the dense layers beneath it. If you notice in the figure below (click on the link), the Earth is actually made up of five layers * structure of the earth ; the reason for so many layers is that each one is compositionally unique, meaning that each layer is made up of slightly different elements. The composition of each layer is directly related to the density, or how heavy, each layer is.

The core is made up of the heaviest elements within the earth and is therefore at the center of all of the layers. The outer core is made up of slightly less dense material compared with the inner core, the lower mantle is less dense than the outer core, the upper mantle is less dense than the lower mantle, and so on with the crust being the least dense and lightest portion of our Earth. This is what we call compositional stratification of Earth and this is why the earth has different layers!

*Note from ScienceLine Moderator: The picture above was taken from the following link: here