What is the mantle?
The mantle is the earths biggest sphere and is located between the crust and the core. It is made of mostly melted rock and some times magma seeps up through the mantle and stays at the magma chamber closer to the crust. But, to be more specific on what the mantle is made of it is 44.8% oxygen, 21.5% silicon, 22.8% magnesium, and it also contains iron, calcium, sodium and aluminum (What is the earths mantle made of?, Sept, 14, 2009).
Random fact
The Athenosphere is the term for the lower mantle and the Lithosphere is the term for the upper mantle.