From the spectacular pyroclastic displays of Mount Vesuvius to the slow spread of pahoehoe on Hawaii’s islands, volcanic eruptions captivate mankind. One of the most intriguing areas of geologic study, volcanoes are simultaneously fascinating and frightening. Not all volcanic eruptions are the same, but they share similar origins. Understanding what causes a volcano to erupt empowers us to respectfully co-exist with earth’s great fire mountains.
Magma is molten rock that lies underneath and within the earth’s crustal layer, and is created when crustal rock is subducted deep into the earth. Plate tectonics is the driving force behind the subduction of crustal rock. The continents of the earth consist of lighter, less dense crustal rock, while oceanic crust is denser and therefore heavier. Oceanic crust is also much thinner than continental crust and as such is susceptible to fractures. The fractures, called rifts, are where magma seeps out onto the ocean floor. The magma cools, becoming denser and heavier, sinking and moving away from the rift opening. This process is sea-floor spreading. As sea floors spread, the oceanic crust farthest out from the rift encounters continental crust. Since oceanic crust is much heavier than the continental crust, it is usually subducted. Subduction is the forced descent of one layer of crust under another. The subducted crust may eventually encounter enough heat that it begins to melt. The steeper the angle of subduction, the earlier the rock melts, which results in volcanoes located close to the subduction zone. An example is the Andes range of South America. Although volcanoes are most commonly found near subduction zones, subduction is not the cause of volcanic eruption.
As crustal rock melts, some of its constituents may dissolve into a gaseous rather than a liquid form. The gas molecules combine to form larger bubbles within the magma. Magma is less dense than solid rock, so when given the opportunity, it will rise. As magma rises, pressure decreases and as pressure decreases, gas expands. The combination of density difference and the force of expanding gas causes magma to erupt from available vents. A volcanic vent is any opening, usually a crack or fissure in the crust, through which magma can reach the earth’s surface.
As density, gas content and opportunity decide whether or not a volcano will erupt, viscosity and temperature dictate how a volcano erupts. Magma consists primarily of only ten elements: oxygen (O), phosphorous (P), iron (Fe), sodium (Na), aluminum (Al), magnesium (Mg), potassium (K), titanium (Ti), silicon (Si) and calcium (Ca). The two most common elements are oxygen and silicon. When the two bond they form silica (SiO2). According to the San Diego State Department of Geological Sciences, the Si-O bond is the single most important factor in determining the degree of a magma’s viscosity.
Images of volcanic eruptions could be of magma fountains jetting into the sky, great pyroclastic shows of hot gasses and lava bombs, or of the slow creeping of syrupy rivers oozing across the landscape. The basic cause of the eruptions does not vary, only the type of eruption. Magma that is superheated, contains less silica and has lower gas pressure has higher viscosity and will erupt quietly. An example of an area where quiet eruptions are common is Hawaii with its large, sloping shield volcanoes. Oppositely, a cooler magma with high silica content and great internal gas pressure has much lower viscosity and will erupt spectacularly. The stratovolcanoes Mount St. Helens in Washington State, USA and Mount Fuji of Japan are excellent examples.
Most people comprehend the danger and threat posed by volcanoes, yet many choose to live within close proximity of them. Understanding the causes of volcanic eruption has enabled us to better predict them and save lives. Whether a small cinder cone in the American southwest or a giant stratovolcano in the Pacific, the basic mechanics behind volcanic eruptions remains the same. Part of the great geologic cycle, rocks that return to the depths can melt and rise again, a process unchanged for millions of years.
http://vulcan.wr.usgs.gov/Proj ects/Mastin/Publications/OFR93 -445/OFR93-445.html
Decker, Robert & Decker Barbara, Volcanoes. San Francisco: W.H. Freeman & Company, 1981.