What causes the eruptions at Kilauea?
There is a large plume of upwelling material originating at the core-mantle boundary. This plume is slightly hotter (about 50-100 C, and thus less dense) than the surrounding mantle so it rises. As it rises, the pressure drops until at about 60-80 km below Kilauea, decompression melting begins. The partial melt is less dense so it continues to rise. The lava coming out of Kilauea is a tholeiitic basalt, which is generated by about 10-20% melting of the mantle. The older volcanoes such as Hualalai are not directly above the plume so they are fed by only about 1-5% melt, generating alkali basalts. As the melt nears the surface, the pressure continues to drop, and H2O, CO2 and SO2 gasses dissolved in the magma begin to exsolve. This is like when you open a bottle, the pressure in the bottle drops so the gas is no longer as soluble in the drink, creating bubbles in your soda or beer. These bubbles help drive the magma up to the surface.
