Origins

Formation of Layers within Earth

 Where we left off . . .

Slowly, our solar system is taking shape!  Planets are starting to cool and, as they do, shrink in size.  This eventually leads to the formation of layers within each planet.

Radioactive elements + Thermal contraction = Density stratification!

As our planet grew larger with each impact event, so to did its gravitational attraction. Gravity compressed the Earth into a smaller spherical shape. This smaller size converted gravitational energy into thermal energy. Rocks, however, are a poor conductor of heat. So there was a buildup of heat on the inside of the planet and not enough cooling on the outside. There was also a greater concentration of radioactive elements than there is today. Radioactive elements are unstable, and use various decay mechanisms in order to achieve stability. One by-product of this decay process is heat.

Basically, heat from impact events + heat from gravity + heat from radioactivity = melting of the planet. As the Earth cooled, heavier elements moved to the center of the planet, while lighter ones stayed on the outside. Thus our planet changed from a homogeneous composition to a three-layered structure.

Image source:  Sonjia Leyva, own work, © 2020.

The outer layer of our planet, the crust, was comprised of a volcanic rock called basalt.  Today, we know that the mantle is mostly composed of a rock called peridotite (or something similar), and the core is made up mostly of iron.