Compositional Layers of the Earth
A Basic Summary
Our knowledge of the Earth and how it is formed changes with advances in science. In this section we explore the compositional layers of the Earth, which is based on current scientific understanding, and of course is subject to change as new discoveries are made. Much of our knowledge comes from the measurement of the speed of seismic waves (caused by earthquakes, not to mention nuclear testing) as they travel through the Earth. Because the speed of these waves changes at different depths, we are able to determine that the density of the Earth also changes at different intervals. The largest earthquakes can make the Earth “ring like a bell”.
The Earth consists of five layers in an onion-like structure: the Crust, the Upper Mantle, the Lower Mantle, the Outer Core, and the Inner Core. Between the Upper and Lower Mantles there is a transition zone. The compositional differences between the Crust and Upper Mantle are relatively small, and the most abundant elements of both are silicone and oxygen, the compound of which is silica, more commonly known as silicate, or quartz. The composition throughout the mantle is thought to have the chemical composition of peridotite (olivine). This makes the mantle both solid and fluid, through the disruption of the periodotite crystals under heat and pressure. There is a fundamental compositional distinction between the core and the mantle, as it is believed the core is made up mainly of iron and contains no silicate at all. Illustration: peridotite
The Inner Core is solid iron with a small amount of nickel. It is solid, not due to cooling, but due to extreme pressure. The Outer Core is also made up of iron with, possibly, sulphur. The Outer Core is molten and in constant motion through a process of convection, and it is this complex motion that generates the Earth’s magnetic field. Overlying this is the solid Mantle, which is also in constant motion. The outermost level of the Mantle together with the Crust forms the Lithosphere (from lithos, Greek, meaning rock). Rigid plates, called Lithospheric Plates, glide over the weaker parts of the mantle beneath, known as the Asthenosphere (from the Greek word for weak). We will look at this process in more detail in the next section.
The diagram below gives a cross section of the compositional layers of the Earth.