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 crust is the thinnest layer of the Earth, amounting for less than 1% of our planet’s volume.

The Earth’s structure

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

The Earth’s structure can be classified in two ways – based on mechanical properties, and based on the chemistry. Here, we’re just going to discuss a basic classification, without going into most details. The main focus here is to understand how the Earth’s crust really is, and why it is the thinnest layer.

  • The crust ranges from 5–70 km (~3–44 miles) in depth and is the outermost layer. The thinnest parts are oceanic crust, while the thicker parts are continental crust. Most rocks at the Earth’s surface are relatively young (less than 100 million years old, compared to the Earth’s age, which is approximately 4,4 billion years), but since we found some rocks which are much older, we know that Earth has had a solid crust for at least 4.4 billion years.
  • The mantle extends from where the crust ends to about 2,890 km, making it the thickest layer of Earth. The mantle is also composed of silicate rocks, but the mantle as a whole is very viscous –  the high temperatures there cause the silicate material to be sufficiently ductile that it can flow (in a very long time). The mantle is generally divided into the upper and the lower mantle.
  • The core, typically divided into the outer core and the inner core. The outer core is regarded as viscous, though much less so than the mantle, while the inner core is solid.

Schematic view of the interior of Earth. 1. continental crust – 2. oceanic crust – 3. upper mantle – 4. lower mantle – 5. outer core – 6. inner core – A: Mohorovičić discontinuity – B: Gutenberg Discontinuity – C: Lehmann–Bullen discontinuity.

The Earth’s crust

Our planet’s crust is on average about 40 km deep – which is much thinner than the mantle, the outer core and the inner core – you can think of it like the peel of an apple. The crust here has been generated through igneous processes, which explains why the crust has much more incompatible elements than the mantle.

Oceanic vs continental crust. Image via USGS.

At the bottom of the oceans and some seas, there is oceanic crust. Oceanic crust is very thin (usually under 10 km), and is composed of dense, typically dark (mafic) rocks: basalt, gabbro, diabase. The continental crust is thicker than that – usually it’s around 40 km deep, but can go up to 70. The two types of crust are also sometimes called granitic (continental) and basaltic (oceanic).

The crust is not one rigid layer, but is broken into fifteen tectonic plates, all in relative movement one to the other. This is called global tectonics. The plates themselves are thicker than the crust alone, and also consist of the shallow mantle beneath the crust – this together is called the litosphere. The crust is where rocks interact with the hydrosphere and more importantly, the atmosphere. New rocks, minerals and materials are formed here. Here’s the important part: all of the variety and phenomena that we can see with our own eyes take place in the crust. Everything, from mining ores to oil to forming mountains to thick deposits, faults and whatever you ever heard about geologists observing directly takes place inside the crust (or at the very surface). The deepest drill ever is just over 12 km, and we won’t be seeing the bottom of the crust with our own eyes for a very long time.

How we know

OK, so there’s a crust, it’s thin, there’s also a mantle and a core… but if we can’t go there, how do we know?

That’s a very good question – but this is where science comes in. We know all of this (and we know it with a very high degree of confidence) through indirect observation.

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

A century ago, people didn’t know the Earth had a crust. Some theoretized it did, but there was very little proof. The first clues came from astronomic indications, but most of what we know today about the Earth’s structure comes from seismological observations. Seismic waves from large earthquakes pass throughout the Earth, and they carry with them information from the environments they passed through. Just like rays of light, seismic waves can reflect, refract and diffract. Because the speed of the seismic waves depends on density, we can use the travel-time of seismic waves to map change in density with depth. Also, because some waves only propagate through solid environments, we know that some environments (like the the outer core) are viscous – because the waves don’t propagate through them.

In 1909, the brilliant seismologist Andrija Mohorovicic found that about 50 kilometers deep in the Earth there is a sudden change in seismic velocity – and knew that it must be a very significant discontinuity. He also observed that seismic waves reflect and refract at that depth, which confirmed his ideas. That discontinuity, named today the Mohorovicic discontinuity (or simply “Moho”) is regarded today as the limit between the crust and the mantle.

 

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