Generally, volcanic eruptions are triggered by the overpressure of dissolved volatiles and/or gases in the rock. These volatiles are typically water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen fluoride. Volcanic eruptions can cause harm to humans and livestock, and may affect food supplies and health services. There are approximately 1,500 potentially active volcanoes in the world, with about 65 in the United States.
Magmatic eruptions are the most commonly observed type of eruption. Magma rises up buoyantly, usually in planar fractures and diapirs. Sometimes it continues to rise, forming a magma chamber. When it begins to cool, it falls as cinders. It sometimes pools at the base of the crust. When it becomes lava, it flows out of the vent at the surface. It may also erupt through a lava tube.
A volcanic eruption may have several different modes, and many aspects are controlled by the magma’s chemistry. The type of eruption and the size of the eruption are dependent on the composition of the magma and the location of the eruption’s center. There are two basic types of eruptions: magmatic and hydrovolcanic.
Magmatic eruptions are the most common and most well-studied type of volcanic eruption. The gas content and viscosity of the magma determines the explosivity and size of the eruption. Magmas of rhyolitic, basaltic, and andesitic composition typically contain dissolved volatiles such as water and carbon dioxide. However, these volatiles may be exsolved from the melt near the surface. This process is similar to the pressure release in a soda pop.
Phreatic eruptions are a subset of magmatic eruptions. These eruptions involve direct interaction of water and hot rock with juvenile magma. The heat of the magma may be the source of thermal energy in the reaction. The temperature of the magma may be influenced by the magma’s position and depth in the earth’s crust.
During a phreatic eruption, the rocks beneath the volcano are granulated and pushed upwards by steam, which is heated by the proximity of the hot rock. This pressure increases as the magma moves upwards. Once it reaches the surface, the granulated rock may fall as tephra. Usually, phreatic eruptions are violent and destroy human structures.
Volcanic eruptions can produce a lava flow, a lava tube, and a pyroclastic flow. Flow fronts can travel up to six miles per hour, while lava tubes on steep slopes can travel up to 30 miles per hour. When the supply of lava is exhausted, the lava tubes dissolve and the lava flow ends. Typically, phreatic eruptions last months, and the eruption phase may last years.
The chemistry of magma also determines the shape of the volcanic edifice. High-silica magmas tend to be highly viscous, and may be very explosive. The greater the concentration of silica in the magma, the longer and more complex the chains that form. These chains tangle together and can impede the flow of the magma. However, these chains are often less dense than the rock below them. This means that the magma can easily push through cracks and craters, which can cause explosive eruptions.