Mantle
Like the cores of rocky bodies, the mantle is also relatively inaccessible. We do have samples of the Earth’s mantle, however in the form of xenoliths, which are chunks of rock carried to the surface by volcanic eruptions. The mantles of rocky bodies are composed of peridotite, a rock made of silicate minerals that are rich in magnesium and iron (mainly olivine and pyroxene).
With increasing depth into the mantle, pressure and temperature increase. In response to increasing pressure, mantle minerals undergo phase transitions, which are metamorphic reactions to more dense versions of themselves in order to occupy less volume. Phase transitions are apparent in a plot of density versus depth into the Earth, where there are jumps in density over a short depth interval (looks like stair steps).
And just FYI: The extreme pressures of the Earth’s mantle make it extremely unlikely that there is a crystal cave waiting to be discovered, like in the movie, The Core.
The last main point about the Earth’s upper mantle is that it is subdivided into two layers on the basis of how it deforms. The outermost part of the mantle and including the crust is called the lithosphere. The lithosphere is the hard, brittle shell of our planet that makes up tectonic plates. Underlying the lithosphere is the asthenosphere, which deforms plastically, meaning that it undergoes non-reversible changes of shape in response to applied forces. The asthenosphere is the part of the mantle that is able to flow (albeit very slowly, on the order of centimeters per year). Flow of the Earth’s asthenosphere is coupled with the movement of its tectonic plates (to be discussed in detail in Module 3).
A good analogy is a double stuffed oreo cookie. The chocolate cookie is brittle and breaks like the lithosphere, while the creamy center is plastic and will flow when a force is applied like the asthenosphere.
Here is a rotating view of basalt with xenoliths (pronounced zee-no-lith).
