Geophysicists believe that hotspots originate at one or two major boundaries deep in the Earth, either a shallow interface in the lower mantle between an upper convecting layer and a lower non-convecting layer, or a deeper D″ ("D double-prime") layer, approximately thick and immediately above the core-mantle boundary. A mantle plume would initiate at the interface when the warmer lower layer heats a portion of the cooler upper layer. This heated, buoyant and less-viscous portion of the upper layer would become less dense due to thermal expansion and rise towards the surface as a Rayleigh-Taylor instability. When the mantle plume reaches the base of the lithosphere, the plume heats it and produces melt. This magma then makes its way to the surface, where it is erupted as lava.