Earth's Internal Structure
Internally, Earth is composed of concentric rings with different characteristics.
Inner Core
The inner core is the innermost part of Earth’s inner structure. It is made of solid iron. Even though the inner core is well above the melting temperature of iron, it remains solid because of the tremendous pressure exerted on it from all the other layers.
Outer Core
The outer core is made of molten iron. Extraordinarily high temperatures keep the outer core in a liquid state. The flow of this material is what generates Earth’s magnetic field.
Fun fact – due to these flows, Earth’s magnetic north – or what ‘north’ on a compass is pointing to – actually moves! It has moved 1100 kilometers, or 685 miles, across the Canadian Arctic over the past century. It is currently heading towards Siberia at a rate of about 55-60 km (34-37 miles) per year.
Mantle
The mantle is Earth’s thickest layer, representing 80% of the planet’s total volume. The mantle is actually composed of a lower mantle and upper mantle. High temperatures and pressure in the lower mantle make the rock soft and viscous. Temperatures are high enough that rock in the lower mantle are able to flow slowly, deforming over millions of years.
The upper mantle is more rigid with lower temperatures. In this context, ‘rigid’ means the layer will not flow when a force acts on it – rather, it will bend and break.
Crust
Earth’s crust is the outermost part of Earth’s inner structure. It is also the thinnest, making up only 0.38% of Earth’s radius. The crust’s thickness varies widely over Earth’s surface – crustal areas are thickest under mountains (50-60 km/31-37 miles deep) and thinnest under oceans (5 km/3 miles deep).
Only eight natural elements make up over 98% of Earth’s crust by weight! Just two of these – oxygen and silicon – account together for 74.3%.
Lithosphere and Asthenosphere
Technically, the lithosphere is actually Earth’s ‘rigid layer’ (lithos means ‘rocky’ in Greek), going about 70 km (43 miles) deep. The lithosphere is made up of Earth’s crust and uppermost mantle.
The asthenosphere is Earth’s ‘plastic layer’ (asthenos means ‘weak’ in Greek), going about 70-250 km (43-155 miles) deep within the mantle. The asthenosphere is made up of part of Earth’s lower mantle – the soft, viscous rock that flows slowly over time. About 10% of the asthenosphere is molten in uneven patterns. The movement of convection currents in this zone is what causes shifting in the lithosphere.