Roughly speaking, this is the part of the Earth which lies below its crust, and above its metallic core.
   It is thought to be made up mostly of the mineral olivine (also known in gem variety as peridot).  Though white-hot and seemingly solid, it is actually a very slow moving fluid.  Motion within it, caused by heat convection, results in movement of the continents above, which are made of less dense rock, and are literally floating on the mantle.


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