Volcanic eruptions are difficult to predict. Scientists can monitor unusual seismic activity, a.k.a. earthquakes, occurring in the area surrounding a volcano, but even that is not a precise method of prediction. This article is the last in a series of charts providing information on tornadoes, hurricanes, wildfires, earthquakes, blizzards, and volcanoes. The risk of volcanoes is relatively low in the United States as a whole, but there are some areas with an exposure, mostly along the Pacific Coast.
Stages of Volcanoes
Volcanoes had three “life stages”: active, dormant, and extinct. The boundaries between these distinctions are not set in stone, or even considered uniform by scientists, but they are generally categorized as follows: Active volcanoes, according to the USGS, are those that have erupted within the last 11,650 years, which was the end of the last ice age; active volcanoes, even if they are not currently erupting, may erupt some time in the future. Dormant volcanoes are not currently erupting or showing any signs of unrest, but still have the potential to erupt in the future. Extinct volcanoes are those that will, most likely, never erupt again based on a lack of magma; for example, Mount Thielsen, located near Crater Lake in Oregon, last erupted 300,000 years ago, and time has worn away a great deal of the top of the mountain. Based on this data and other scientific observations, Mount Thielsen will never erupt again.
Measuring Volcanic Eruptions
The Volcanic Explosivity Index (VEI) accounts for the magnitude and intensity of a volcanic eruption. According to the National Parks Service (NPS), the numerical VEI rating, on a scale of 0 to 8, is on par with the scales used to measure earthquakes, like the Richter and Modified Mercalli scales. As the VEI increases, so does the volume of magma exuded by the volcano and the height of the ash column. Each step up the scale is a tenfold increase in the size of an eruption, with the exception of the step from VEI 7 to VEI 8.
The VEI also includes a classification based on the physical explosiveness (“style”) of an eruption, and a single adjective descriptor of it. There are six classifications of eruptions—Hawaiian (lowest), Strombolian, Vulcanian, Sub-Plinian, Plinian, and Ultra-Plinian (highest)—named after eruptions of famous volcanoes. The adjective description, on the other hand, has nine levels—effusive (smallest), severe, explosive, catastrophic, cataclysmic, paroxysmal, colossal, mega-colossal, and apocalyptic (largest).
Take, for example, the eruption of Mount St. Helens on May 18, 1980. It was a 5 on the VEI scale, spewed 2 miles (10 km) cubed of magma, and had an ash column taller than 16 miles (25 meters). It is classified as Plinian and was described as paroxysmal.
Types of Volcanoes - The Big Three
The USGS describes three “classic” types of volcanoes: cinder cones, composite volcanoes (stratovolcanoes), and shield volcanoes.
Cinder cones are the most abundant type and usually resemble the stereotypical volcano – a roughly triangular shape, with steep sides and a pronounced top. They have a wide range of heights, from a few hundred feet (100 to 150 meters) to 600 or 900 feet (200 to 300 meters), with some taller exceptions. For example, Capulin Volcano, a cinder cone located in New Mexico, is roughly 1,000 feet (305 meters) tall. Eruptions from a cinder cone are usually between VEI 1 and VEI 2, with a rare eruption reaching VEI 4.
Composite volcanoes, sometimes called stratovolcanoes, are cone-shaped and become much steeper at the top. They can be thousands of feet (or meters) tall and typically erupt around VEI 4 or VEI 5. Mount Rainier, near Seattle, Washington, is a composite volcano that last erupted in the 19th century.
Shield volcanoes, however, do not have peaks like cinder cones or composite volcanoes, but they are far larger in terms of physical size. Shield volcanoes have broad, gentle slopes with a flatter summit at the top, so they are wider than they are tall. The NPS website compares Mauna Loa, a shield volcano in Hawaii, with Mount Rainier, the tallest composite volcano in the lower 48. At its base, Mauna Loa is 100 miles (160 kilometers) wide, while the base of Mount Rainier is barely one-tenth (10 miles/16 kilometers) that wide. However, for all their size, shield volcanoes usually erupt between VEI 0 and VEI 1.
Other Types of Volcanoes
Though cinder cones, composite volcanoes, and shield volcanoes are the most well-known types of volcanoes, there are six further sub-types:
- Volcanic domes – form from lava that is too thick to “flow” across the ground.
- Maar and tuff rings – Pyroclastic cones with craters below (maar) or above (tuff ring) the surface of the ground.
- Fissure volcanoes – Erupt along a single fissure instead of through a vent.
- Monogenetic volcanic fields – Form in areas with a low volume of magma, from “monogenetic” volcanoes, which are volcanoes that, typically, have a single eruption, such as cinder cones, maars, tuff rings, and volcanic domes.
- Calderas – form during massive eruptions when a volcano’s magma chamber is partially emptied and then collapses.
- “Super volcanoes” – a not-entirely-scientific term referring to any volcanic center that has erupted at VEI 8.
The USGS conducted its most recent national volcanic threat assessment in 2018 based on a series of hazard and exposure factors. Of the 161 volcanoes assessed, eighteen pose a “very high” threat if an eruption occurs. Those eighteen volcanoes are spread across five states: Alaska, California, Hawaii, Oregon, and Washington. Of the top fifty-seven threats, which represents all volcanoes listed as “very high” and “high” threats, only three are not located in one of those five states: the Yellowstone caldera (no. 21, Wyoming); Pagan Island (no. 44, Northern Mariana Islands); and Agrigan Island (no. 57, Northern Mariana Islands).