There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk one of those misconceptions. There will be others. Find them here.
Misconception: Crust and Lithosphere (or plates) are synonymous terms
Most folks intrinsically know that the outer surface of the Earth – that bit that we walk on – is called the crust. Many also know that the surface of the Earth is broken into plates that move around, causing the continents to move with the occasional excitement of earthquakes.
These moving plates are formed of what we in the biz call ‘the lithosphere.’ Lithospheric plates move around though the processes of plate tectonics.
It follows then, intuitively, that crust and lithosphere are the same thing.
Only that they aren’t.
Don’t feel bad. I didn’t get this straight until sometime in graduate school. Unless you’re specifically doing research in plate tectonics, this distinction winds up getting muddled in your head. Until you have to teach it, of course. Which is what I discovered a few years ago the first time I taught introductory geology. But I digress…
Here’s the deal:
The Earth can be split into three main divisions: The crust, the mantle, and the core. The mantle can be further divided into upper and lower mantle, and the core into inner and outer core. The crust itself comes in two types: the thinner oceanic crust which is what forms most ocean floors, and the thicker continental crust which – you guessed it – comprises most continents.
Where the crust and the mantle come together is called the Mohorovičić discontinuity (which I can’t pronounce either), or simply the Moho (which I can pronounce). The crust and mantle are securely connected here, so motion of the plates or the continents cannot happen by slip along this boundary.
Yet plates move.
Below the Moho and down to around 100 kilometers into the Earth is the uppermost part of the upper mantle. This part of the mantle is rigid and formed of dense heavy minerals called peridotite. At about 100 kilometers depth, the rock becomes more plastic and and capable of flow (but it’s still rock, which will be the topic of a coming post).
This more flexible part of the upper mantle is called the asthenosphere. It is in the asthenosphere, and in the rest of the mantle below, that the flow occurs that results in the motion of plates.
The solid, rigid mantle above the asthenosphere, plus the crust above that, is what makes the lithosphere.
Thus the crust is part of the lithosphere, but not the same exact thing. The lithosphere is crust plus the uppermost part of the mantle.