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Why do volcanoes erupt? It seems like a pretty simple question when you first pose it, so let’s look into the answer, shall we. The simplest answer would seem to be that magma, being less dense than the rock surrounding it, tries to float to the surface. If the magma contains water and dissolved gasses, when the magma reaches the surface the water and dissolved gasses will suddenly expand into steam and gas, causing a violent eruption. There are several factors that will trigger a volcanic eruption, but three predominate: the buoyancy of the magma, the pressure from the exsolved gases in the magma and the injection of a new batch of magma into an already filled magma chamber.
When rock inside the Earth melts, its mass remains the same, but its volume increases, producing magma that is less dense than the surrounding rock. This lighter magma rises toward the surface because of its buoyancy. When the density of the magma between the zone of its generation and the surface is less than that of the surrounding and overlying rocks, the will magma reach the surface and erupt.
Magmas made of andesitic and rhyolitic components contain dissolved volatiles like water, sulfur dioxide and carbon dioxide. The amount of a dissolved gas in magma at atmospheric pressure is zero, but rises with increasing pressure. So, in an andesitic magma saturated with water and six km below the surface, about 5 percent of its weight is dissolved water. When this magma moves toward the surface, the solubility of the water in the magma decreases, and so the excess water separates from the magma as bubbles. The closer it gets to the surface, more water exsolves from the magma, increasing the gas/magma ratio in the magma tube. When the volume of bubbles reaches about 75 percent, the magma turns into partially molten and solid fragments and will erupt explosively.
The last major reasons that a volcano will erupt is when new magma enters a chamber that is already at capacity. The new magma can be of the same or different composition. Either way, the new magma causes some of the existing magma to move to the surface and erupt. Well, there you have the answer to ‘why do volcanoes erupt?’.
We’ve also recorded an episode of Astronomy Cast all about volcanoes. Listen here, Episode 141: Volcanoes, Hot and Cold.