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The Crust and Lithosphere Introduction The Earth's tectonic plates constitute the lithosphere so no proper understanding of plate tectonics can be achieved without reference to the lithosphere, and this requires an understanding of its essential difference from the crust.
There are incorrect uses of both terms in text books - particularly common is the use of 'crustal', as opposed to 'lithospheric' plates - and these have contributed to widespread confusion and misunderstanding. The problem that teachers and, for that matter, authors of school text books have to face up to is that geologists need to employ two different concepts of layering within the outer part of the Earth to understand and explain geological processes - compositional layering crust, mantle , and mechanical layering lithosphere, asthenosphere.
What is the difference between the crust and lithosphere? The crust whether continental or oceanic is the thin layer of distinctive chemical composition overlying the ultramafic upper mantle. The base of the crust is defined seismologically by the Mohorovicic discontinuity, or Moho.
New land is forming here. We'll talk more about that in other videos. You see right over here in the middle of the Atlantic Ocean the African Plate and the South American Plate meet each other, and they're moving away from each other, which means that new land, more plate material I guess you could say, is somehow being created right here-- we'll talk about that in future videos-- and pushing these two plates apart. Now, before we go into the evidence for plate tectonics or even some of the more details about how plates are created and some theories as to why the plates might move, what I want to do is get a little bit of the terminology of plate tectonics out of the way.
Because sometimes people call them crustal plates, and that's not exactly right. And to show you the difference, what I want to do is show you two different ways of classifying the different layers of the Earth and then think about how they might relate to each other.
So what you traditionally see, and actually I've made a video that goes into a lot more detail of this, is a breakdown of the chemical layers of the Earth. And when I talk about chemical layers, I'm talking about what are the constituents of the different layers? So when you talk of it in this term, the top most layer, which is the thinnest layer, is the crust. Then below that is the mantle. Actually, let me show you the whole Earth, although I'm not going to draw it to scale.
So if I were to draw the crust, the crust is the thinnest outer layer of the Earth. You can imagine the blue line itself is the crust. Then below that, you have the mantle. So everything between the blue and the orange line, this over here is the mantle. So let me label the crust. The crust you can literally view as the actual blue pixels over here.
And then inside of the mantle, you have the core. And when you do this very high level division, these are chemical divisions. This is saying that the crust is made up of different types of elements. Its makeup is different than the stuff that's in the mantle, which is made up of different things than what's inside the core. It's not describing the mechanical properties of it. And when I talk about mechanical properties I'm talking about whether something is solid and rigid.
Or maybe it's so hot and melted it's kind of a magma, or kind of a plastic solid. So this would be the most brittle stuff. If it gets warmed up, if rock starts to melt a little bit, then you have something like a magma, or you can view it as like a deformable or a plastic solid.
When we talk about plastic, I'm not talking about the stuff that the case of your cellphone is made of. I'm talking about it's deformable. This rock is deformable because it's so hot and it's somewhat melted.
It kind of behaves like a fluid. It actually does behave like a fluid, but it's much more viscous. It's much thicker and slower moving than what we would normally associate with a fluid like water.
So this a viscous fluid. And then the most fluid would, of course, be the liquid state. This is what we mean when we talk about the mechanical properties. The mantle consists largely of heavy ultramafic rock like olivine.
The crust makes up the upper portion of the lithosphere. It is made up of lighter materials than the mantle and core, comprising mainly mafic and felsic rocks like granite. While it is the thinnest layer of the Earth at only 60 to 70 kilometers thick, it makes up the majority of the lithosphere and is the portion of the Earth that supports life.
The crust surface is shaped by characteristics of the lithosphere that cause formations like mountains and fault lines. The part of the crust that makes up continents is formed of lighter minerals than the part of the crust that makes up the oceanic floor. The lithosphere, unlike the layers of the Earth, is defined not by composition but by behavior. The lithosphere is cold, relative to the fluid asthenosphere at least, and solid. It floats freely on top of the liquid magma of the upper mantle and is divided into discrete sections known as tectonic plates.
The thickness of the lithosphere can be variable, with older portions being thicker, but tends to average a height of kilometers. Young portions of the lithosphere are formed by the downward movement and melting of one tectonic plate beneath another at a boundary known as a subduction zone. These boundaries between tectonic plates have a profound effect on the shape of the surface of the earth. A boundary that moves longitudinally is known as a transform fault line and causes earthquakes.
Volcanic activity occurs at subduction zones and forms continental landmasses, while divergent boundaries cause a magma upwelling that forms the ocean floor.
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