Most volcanoes in the Pacific Ring of Fire have an explosive nature. This is due to high concentrations of water in their magma. Obviously, this magma comes from several kilometres below the earth's crust. How is it possible for magma to contain water at such depths?
Brief answer (on tablet)
When an oceanic plate is formed, the crust is hydrated by the hydrothermal vents systems at the ocean ridge where it forms (hydrated means that the minerals are altered to include water in their structure). When this crust is subducted in the ocean trenches around the Pacific, it is taken down towards the mantle; as these hydrated minerals heat up again, they release water. This water then lowers the melting point of the mantle above the descending plate, which melts to produce a basaltic/andesitic melt with a high water content - the water is dissolved in the magma. When this melt reaches the surface again, the high concentration of water leads to explosive eruptions as it comes out of solution.
How is it possible for magma to contain water at such depths?
This can happen precisely because the magma is at such depths. Think of CO2 in soda or coke. When the drink is closed and pressurised, the CO2 is dissolved in the liquid. This means it's part of one physical phase. There is one "thing" that contains the liquid and the CO2 together: the CO2 is not a gas! Only when you open the bottle, the CO2 can escape being dissolved in the liquid, and separates from the liquid as a distinct gas phase.
The same thing happens in magmas. While the magmas are pressurised at depths they have water dissolved in them. Note that this water is not separate liquid water or vapour flowing around the magma. It is inside the magma in one physical entity. Think of dissolving salt in water - you can't see salt grains but it's there. Anyway, once the magma rises it loses pressure. This causes the water to separate out from the magma into a distinct vapour phase that rises and expands rapidly, causing the explosive eruption.