CoachMagma is molten rock underground. It is a hot liquid which may also carry crystals and, dissolved in it, gases such as water vapour and carbon dioxide. Magma that comes out at the surface is called lava; as it comes out, it loses most of its gas.
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Example 19.6 (Pinatubo, 1991: the flat soda and the shaken one)
In June 1991 the first magma reached the surface of Mount Pinatubo, in the Philippines. On its way up it had lost most of its gas, “like a bottle of soda pop gone flat”, in the words of the scientists who were watching. It oozed out slowly and piled up into a mound over the vent: no explosion. Then, on 12 June, magma still full of gas reached the surface and exploded — the column in the photograph above. On 15 June, even more gas-rich magma arrived, and the volcano had one of the largest eruptions of the twentieth century: it threw out more than five cubic kilometres of rock and ash, its ash cloud rose high, and the top of the mountain collapsed into a hollow across. The same volcano, the same week: the gas made the difference.