Geology: The Crust and Upper Mantle

Continuing on with the structure of the Earth, just because I feel like it (and I like rocks).

The Earth’s crust is a patchwork of distinct plates which lie on top of the mantle and move slowly relative to each other. Different plates move in different directions and at somewhat different speeds; however, the usual comparison is that the plates move at roughly the same speed that your fingernails grow.

The plates underneath the ocean are significantly different than the ones underlying the continents. Ocean plates are relatively thin—about seven kilometers thick—and quite young, geologically speaking. The oldest rocks in the oldest ocean plate are only about 200 million years old. The youngest were made two seconds ago; new rock is constantly being added to several of the ocean plates.

The plates underlying the continents are much thicker. The average thickness is about 35-40 kilometers, but some parts of some plates are as much as 70 kilometers thick. Continental plates are usually much older than ocean plates. For example, some rocks in the Canadian shield and in Australia are more than 4000 million years old (i.e. four billion).

Ocean plates are mostly made of basalt, a black rock with a high iron and magnesium content. Continental plates are much more varied, with many different types of rocks and minerals. On average, however, continental plates are less dense than ocean plates. Loosely speaking, this is why continents are (mostly) above sea level: ocean plates are dense and “ride low” on top of the mantle, while continents are lighter and “ride high”.

There’s some weirdness about the dividing line between the crust and the mantle. The line is determined by the Mohorovicic discontinuity, named after a Yugoslavian scientist and generally shortened to “the Moho” because no one wants to type “Mohorovicic” repeatedly. The Moho is a layer where the composition of the rocks changes significantly from the basalt of ocean plates and the more varied continental plates, to a relatively uniform rock called peridotite. This change in composition is quite noticeable in seismological readings, so the Moho was officially taken as the crust-mantle boundary.

However, there’s a different boundary you might also care about. As I noted last time, most of the mantle has the consistency of thick peanut butter: solid but still able to flow very slowly. However, the topmost section of the mantle is cool enough that it doesn’t flow. It’s hard, like peanut brittle.

So the crust is mostly unflowing solid rock, and the top part of the mantle is also unflowing solid rock. This has led geologists to define the lithosphere as the crust plus the part of the mantle that doesn’t flow. Below that is the asthenosphere which is the part of the mantle where stuff starts to flow (and below that is the mesosphere which is a stiffer part of the mantle but it’s still a bit flow-y).

Confused? I certainly was when I was first taught this stuff. But if you think crust/hard mantle/soft mantle, you get the idea. The difference between the crust and mantle is what the rock is made of. The difference between the hard mantle and soft mantle isn’t the composition—they’re made of the same stuff—but the texture.

So those are the basic layers of the Earth. Some of these are divided into sub-layers, but let’s not complicate things. Instead, if I decide to keep going, I’ll move on to plate tectonics, the key to modern geology.

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