The mantle is a 2900km thick layer of the earths structure between the core and the crust. The boundary of the crust and mantle is called the MOHO. The mantle has two main layers the upper and lower mantle. With the earths crust the very top of the uppermantle forms the Lithosphere which is around 200km thick.
The upper mantle is at temperatures of 500-900C but the rocks are solid due to the pressure they are under.However the mantle is said to be "plastic" in nature can flow -like warm plasticine.
The main rock type is called peridotite which mainly consists of the minerals olivine and pyroxene.
MORB is a direct product of the mantle and has a high olivine and pyroxene content.Within the mantle convection currents occur caused by the enormous amount of heat given out from the earths core. Although the mantle is solid it can flow due its" plastic" nature. Convection currents push and pull the tectonic plates over the mantle causing continents to collide and subduction zones.