When we think of a volcanic eruption, rivers of glowing lava or giant columns of ash come to mind. But beneath Earth's surface, a more complex story is unfolding. Magma deep within volcanoes is more than molten rock. It is a thick mixture of melted rock and solid mineral crystals known as "magma mush". To picture it, imagine a semi-frozen granita: a slushy blend of solid ice crystals suspended in sweet liquid syrup.