Magma mixing triggered during volcanic eruptions

Numerous volcanic eruptions deposit mixtures of different magma which originate from a layered magma reservoir. It has been argued that, in some cases, the mixing acts as the trigger for eruption. In contrast to this, here we show that, in principle, in some situations the eruption may act as the trigger for mixing or overturn of the magma in the chamber. In essence, if the lower layer of magma contains more exsolved volatiles, then it will expand more on decompression of the chamber during the eruption. This can lower its density below that of the upper layer, and in some cases may lead to overturn during the eruption. We present some new laboratory experiments to illustrate the process as well as some theoretical calculations which identify that relatively crystalline basalts in shallow chambers in the crust are the possible candidates for such overturn.