The Earth can be divided into four main layers: the solid crust on the outside, the mantle, the outer core and the inner core. Out of them, the mantle is the thickest layer, while the crust is the thinnest layer.

The Earth’s structure

Artistic depiction of the Earth's structure. Image via Victoria Museum.

Artistic depiction of the Earth’s structure. Image via Victoria Museum.

The Earth’s structure can be defined in several ways, but general, we see the Earth as having a solid crust on the outside, an inner and an outer core, and the mantle in between. The crust’s thickness varies between some 10 km and just over 70 km, having an average of about 40 km. The core has, in total, a radius of 3500 km, but it is generally viewed as two distinct parts:

  • the solid inner core, with a radius of 1220 km
  • the viscous outer core, with a radius of 2300 km

The mantle’s thickness is about 2900 km – so if you consider the Earth’s core as one big thing, then the core is the “thickest layer” (though has a bigger radius is probably a better way of saying it) – but the idea of a separate outer and inner core is generally accepted.

The Mantle – thickness and composition

The mantle comprises about 83% of the Earth’s volume. It is divided into several layers, based on different seismological characteristics (as a matter of fact, much of what we know about the mantle comes from seismological information – more on that later in the article). The upper mantle extends from where the crust ends to about 670 km. Even though this area is regarded as viscous, you can also consider it as formed from rock – a rock called peridotite to be more precise. A peridotite is a dense, coarse-grained igneous rock, consisting mostly of olivine and pyroxene, two minerals only found in igneous rocks.

Peridotite, as seen on the Earth’s surface. Image via Pittsburgh University.

But it gets even more complicated. The crust is divided into tectonics plates, and those tectonic plates are actually thicker than the crust itself, encompassing the top part of the mantle. The crust and that top part of the mantle (going 00 to 200 kilometers below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle. Namely, the rocks in this part of the mantle are more rigid and brittle because of cooler temperatures and lower pressures.

Below that, there is the lower mantle – ranging from 670 to 2900 kilometers below the Earth’s surface. This is the area with the highest temperatures and biggest pressures, reaching all the way to the outer core.

Mantle Trivia: Even though you can consider the mantle as molten rock or magma, modern research found that the mantle has between 1 and 3 times more water than all the oceans on Earth combined.

How can we study the mantle?

Waves propagating from Earthquakes through the Earth. Image via Brisith Geological Survey.

Waves propagating from Earthquakes through the Earth. Image via Brisith Geological Survey.

Pretty much all the practical geology we do takes places at the crust. All the rock analysis, the drilling… everything we do is done in the crust. The deepest drill ever is some 12 km below the surface… so then how can we know the mantle?

As I said earlier, most of what we know about the mantle comes from seismological studies. When big earthquakes take place, the waves propagate throughout the Earth, carrying with them information from the layers they pass through – including the mantle. Furthermore, modern simulations in the lab showed how minerals likely behave at those temperatures and pressures, and we also have indirect gravitational and magnetic information, as well as studies on magma and crystals found on the surface. But the bulk of the information comes from seismic analysis.

Image via Wiki Commons.

Seismic waves, just like light waves, reflect, refract and diffract when they meet a boundary – that’s how we know where the crust ends and where the mantle begins, and the same goes for the mantle and the core. The waves also behave differently depending on different properties, such as density and temperature.

In the mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F) at the upper boundary with the crust; to over 4,000 °C (7,230 °F) at the boundary with the core. Thanks to the huge temperatures and pressures within the mantle, the rocks within undergo slow, viscous like transformations  there is a convective material circulation in the mantle. How material flows towards the surface (because it is hotter, and therefore less dense) while cooler material goes down. Many believe that this convection actually is the main driver behind plate tectonics.

Mantle convection may be the main driver behind plate tectonics. Image via University of Sydney.

Another interesting fact about the mantle: Earthquakes at the surface are a result of stick-slip faulting; rocks in the mantle can’t fault though, yet they sometimes generate similar earthquakes. It’s not clear why this happens, but several mechanisms have been proposed, including dehydration, thermal runaway, and mineral phase change. This is just a reminder of how little we still know about our planet: we’ve only scratched the surface of the thinnest layer, the crust.

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