When examining the cause of a volcanic eruption, it is helpful to first look at the interior of the earth. At the very center of the Earth is a core of mostly iron and nickel, heated well over the melting point of either metal, but kept solid because of tremendous gravitational forces. Around this is the outer core, which has the same basic makeup as the inner core, but with slightly less pressure, this portion is fluid.
Most of the Earth is taken up with the next layer, the mantle. Still enormously hot, enough to vaporize a human being almost instantly, the pressure in this area is just enough to cause the molten rock to be elastic, rather than fluid, sort of like very thick syrup. Like in cooking pudding, heat rises in this area, then sinks again as the rock becomes progressively cooler. (This is convection, the same idea behind convection ovens.)
On top of the mantle floats the crust, only between 10 and 30 miles deep, and deeper under the continents than under the oceans. If the Earth was an apple, the skin would correspond to the crust, while the flesh would be the mantle, and the apple core would be the inner and outer core.
The crust is divided into a number of plates that are in constant movement. These slam into other plates causing mountains, slip past other plates, creating earthquakes when the plates stick, then suddenly release, or one plate will slip under a lighter plate, as happens on the west coast of the US.
When one plate slides under another, it is called a subduction, and the rock that is forced under the other melt, both because of the friction, and because of the heat of the mantle. This molten rock or magma tries to rise, as something hot generally does. If it finds breaks in the rock above it, it will rise through those breaks, often melting the rocks and producing even larger fissures, and vast underground lakes of molten rock. If it breaks through to the surface, it is a volcano. In general, the larger the magma pocket or lake, the larger the eruption will be.
Volcanoes don’t just occur near subduction zones. Sometimes there is an especially hot spot in the mantle that melts through the middle of a plate, and if the magma reaches the surface, it is also a volcano. This is how Hawaii was and is being formed, and it is the basis for the supervolcano of Yellowstone. The crust continues to move, but the hot spot is stationary, so in the case of Hawaii, there is a series of island, and a new one is now being built. It will probably break the surface of the ocean in the next 20 years. (Kilauea, from the base to the tip is actually taller than mount Everest.)
An eruption is nothing more than the molten rock actually breaking the surface, whether in the ocean or on one of the continents. An eruption can be, and often is, very mild. An explosive eruption, on the other hand, such as in the case of Mount St. Helens, Pinatubo, or Vesuvius, occurs when the magma contains a large amount of dissolved gas. As this magma gets closer to the surface, the gas expands as the pressure lessens, until like releasing the cap off a bottle of soda that has been shaken, it bursts for explosively. Explosive eruptions are capable of hurling house sized pieces of rock many miles. The actual composition of the rocks are a great indicator of how much gas is dissolved and how explosive an eruption will be. For instance, in the Hawaiian islands, the lava is low in silicon, so it flows easily and doesn’t have the ability to dissolve a lot of gas. In Mount St. Helens, however, the silicon level was very high, meaning that it could and did have a lot of dissolved gases, so the eruption was quite explosive. Put in another way, the magma was so thick in the latter case that gases could not escape from the magma, except by blowing it’s way out violently.
Here is a very interesting fact: At any given time, there are an average of 13 to 17 volcanoes erupting somewhere on the Earth, most of them under the oceans. The Earth is anything but calm and collected. I have a feeling that she has more surprises in store, too.
