Traditionally, when we talk about the layers of our planet we talk about the inner core, outer core, mantle, and crust.
But when we talk about Plate Tectonics we have to deal with another concept – that is the lithosphere.
What is the Lithosphere?
The lithosphere is defined as the outermost rigid rocky layer of the Earth and it is made up of the crust and the top brittle section of the mantle. The Earth’s tectonic plates are ‘slabs’ of the lithosphere.
The upper part of the lithosphere is the crust – which can be divided up into oceanic crust or continental crust. Oceanic crust is made up of the rock types basalt and gabbro and the continental crust is made up of a mix of all the igneous, sedimentary and metamorphic rocks. (You can find out more about the crust here)
Underlying the crust is the brittle part of the mantle which is made up primarily of the rock type peridotite. This is a very dense ultrabasic igneous rock made up of the minerals olivine and pyroxene. Sometimes pieces of peridotite are brought to the surface as chucks of materials in lava during a volcanic eruption. This layer is found under both oceanic and continental crust making up those tectonic plates.
The boundary between the crust and the mantle layers of the lithosphere is called the Mohorovičić discontinuity – or just Moho for short. This boundary was recognized by the difference in density and how it affected seismic waves. In some amazing places on Earth where tectonic plates have collided so that the lower lithosphere has been thrust up to the surface and you can see the ancient Moho exposed.
So what about the Asthenosphere?
The Asthenosphere is the semi-molten weak layer of the upper mantle that lies directly below the lithosphere. It may be around 2-4% molten and the rest is a ‘plastic’ layer that is ductile and weak. It is on this layer that the tectonic plates ‘slide’.