When discussing the onion-like nature of the Earth, it is important to mention there is some debate as to what constitutes a layer of the planet. This is particularly true when one starts discussing the areas around the core of the Earth. Nonetheless, there are readily accepted general categories regarding the makeup of the Earth.
The lithosphere is the rocky ground upon which we live. Technically, it is comprised of the crust of the planet and the upper, brittle mantle below the crust. It is approximately 100 km in thickness on average and includes all the oceans, mountains, deepest canyons and tectonic plates.
The mantle is the next layer below the lithosphere. The mantle consists of hot, molten rock and is the birthplace of new crust and recycling point for old crust. The mantle is hot enough and far enough below the crust of the Earth that there is little empirical evidence supporting scientist's claims on the exact makeup. Instead, deductions are made from studying new material coming from the mantle in areas where the tectonic plates are separating.
To make things a bit more confusing, a layer of the upper mantle is known as the asthenosphere. This area is considered highly volatile because of severe pressure and heat factors. The asthenosphere is believed to be the first area where the crust of the Earth actually becomes molten.
The outer core lies a few thousand miles below the crust. While this may sound like a vague estimation, the simple fact is there is no clear evidence supporting a particular depth. Regardless, the outer core is a highly pressurized, liquid material roughly 2,200 km thick. It is believe the outer core is comprised of a nickel-iron alloy mix, which is also believed to be the originating source of the magnetic field of the Earth.
At the very center of the planet is the inner core. Little is known about the inner core other than basic principles. The core is believed to be primarily iron and massively dense. Due to gravity and extremely high pressures, the inner core is believed to be solid.
Regardless of how you categorize the makeup of the Earth, it is clear our planet is unique in the known galaxy.
