Earth Structure

The earth is split into a number of layers which are defined by either chemical and/or physical characteristics. The crust is the solid outer layer and beneath this is the highly viscous (i.e. thick and sticky) mantle. The junction between the crust and the mantle is called the Mohorovicic Discontinuity. It is called a discontinuity because this is the boundary across which a change in physical and chemical properties occurs.

The crust is split into continental crust and oceanic crust. The crust ranges in thickness between approximately 50 km (continental crust) and 7 km (oceanic crust). The crust and the very top portion of the upper mantle (which is fairly rigid) is collectively known as the lithosphere and this is what constitutes tectonic plates. The lithosphere sits on the asthenosphere, a discrete layer within the mantle which by comparison to the rest of the mantle has relatively low viscosity (i.e. it is more runny, or it can flow more easily). The lithosphere sits on the asthenosphere, a discrete layer within the mantle which by comparison to the rest of the mantle has relatively low viscosity (i.e. it can flow more easily). The mantle contains the bulk of the rock-like material in the Earth. Most magmas that erupt at the surface are derived originally by melting of the mantle, while plates that get subducted are eventually broken up and absorbed back into the mantle.

The core is split into two zones, a liquid outer core and a solid inner core. Both are composed predominantly of iron and nickel. Convection in the outer core gives rise to the Earth's magnetic field, but the core is very slowly solidifying as the planet cools.