Helena
HelenaA volcano erupts when the pressure of magma within the volcano gets so high that it fractures its way to the surface. When molten rock is below the surface we call it "magma" and once it has reached the surface we call it "lava" if it is flowing or "pyroclastic material" (ash, cinders, etc.) if it is being blown apart and thrown into the air. Many volcanoes have magma chambers that always have a certain amount of magma inside them. At Hawaiian volcanoes for example, where the magma is very fluid, a small increase in supply to the magma chamber from deeper within the earth is often sufficient to cause the magma chamber to expand. Volcanologists at the Hawaiian Volcano Observatory can measure the deformation of the surface due to the magma chamber inflating. Some inflation events result in the ground surface moving upwards by as much as 2 meters! When the pressure becomes too great, the side of the magma chamber breaks and magma can escape into the flanks of the volcano or up to the summit. Sometimes it breaks to the surface to cause an eruption and sometimes the magma just intrudes into the flank of the volcano and stops there. At other volcanoes that are not as active as the Hawaiian examples the magma chamber is not replenished very often because the supply of magma from deeper within the earth is not as great. This allows the magma in the chamber to "differentiate", meaning that many crystals form and sink to the bottom of the chamber and at the same time gas bubbles form and float to the top of the chamber. Sometimes the pressure from all these gas bubbles is sufficient to cause an eruption, and other times it isn't, but is just sitting there waiting for just a little incentive. This incentive is often provided by the arrival of a new batch of magma from deeper within the earth. Even if this new batch is only a small volume, the added heat is enough to trigger an eruption of the magma that has been sitting around for a while.
Sincerely,
Scott Rowland
Other Categories
Other Questions
To VolcanoWorld