Layers of the Earth

The Earth is often divided into four layers: the crust, mantle, inner core, and outer core. Because this is the ‘simpler’ way of looking at the layers, this is where we shall begin.

The Inner Core: First of all, we have never been to the inner core of the Earth, so this is all a theory based on scientific tests. Scientists believe that the inner core is the hottest layer of the Earth, that it is composed mostly of iron and nickel, and that although it is hot enough to be a liquid, it acts as a solid because of the immense amount of pressure on it.

The Outer Core: Scientists also believe the outer core to be composed primarily of iron and nickel. This layer, although, is believed to be a liquid. Because the Earth is spinning, this layer moves within the Earth. The movement of a layer that contains so much iron generates an electric current within the iron. The electric current produces a magnetic field. The outer core is responsible for the Earth having a magnetic field. This magnetic field protects the Earth (and you and me) from dangerous particles that are emitted from the Sun.

The Mantle: The mantle is the thickest layer of the Earth, making up about 82% of its volume. The mantle is composed primarily of heavy metals, such as iron, nickel, magnesium, and others. Scientists describe the state of the mantle as ‘plastic.’ Many divide the mantle into further such as the upper and lower mantle, as well as the asthenosphere and lithosphere. We will discuss those more in-depth later. The movement of magma within the mantle, called convection currents, is believed to be responsible for the movement of the Earth’s tectonic plates. Layers of the Earth The Crust: The crust is the cooled outer shell of the Earth. There are two types of crust, oceanic crust and continental crust. The oceanic crust is made mostly of basalt- a dark, dense rock made mostly of metals. The continental crust is made mostly of granite- a lighter, less dense rock, composed primarily of silicon and oxygen. The continental crust is much thicker and less dense. The oceanic crust is much thinner and very dense. When the oceanic and continental crusts are pushed together, the oceanic crust is forced below, because of its high density. We will discuss this more later.

These four layers – inner and outer core, mantle, and crust- make up the entirety of the Earth. The mantle and crust, however, are often discussed differently. We will discuss those now.

The Lithosphere: The Earth’s surface is made of tectonic plates. These plates are moved around by the convection currents within the mantle. We often refer to the crust--and the extreme upper mantle, which moves with the crust--as the lithosphere. The Asthenosphere: When discussing tectonic plates and using the term ‘lithosphere,’ you will often hear the term ‘asthenosphere.’ The asthenosphere is the upper mantle, below the lithosphere, on which the plates slide.

*Note: The asthenosphere is part of the mantle. The lower part of the lithosphere is also part of the mantle. Students are often confused when labeling these, because the lower layer of the lithosphere does not have a name. It is a part of the lithosphere, and part of the mantle.*