American Volcanoes

Volcanoes are openings in the earth’s crust through which molten lava, ash, and gases are ejected. There are 169 volcanoes in the United States (U.S.). Eighteen of them have been designated as "very high threat volcanoes" by the United States Geological Survey (USGS). These volcanoes have devastated large areas with volcanic blasts, invaded their surroundings with lava flows, produced large mudflows that have swept over hundreds of square miles, emitted noxious gases that have caused lung ailments and produced ash clouds that have brought down passenger jets and blanketed thousands of square miles. (Geology.com)

The most dangerous U. S. Volcanoes, in descending order are Kìlauea, Hawaii, Mount St. Helens, Washington State, Mount Rainier, Washington State, Mount Hood, Oregon, Mount Shasta, California, South Sister, Oregon, Lassen Volcanic Center, California, Mauna Loa, Hawaii, Redoubt Volcano, Alaska, Crater Lake area, Oregon, Mount Baker, Washington State, Glacier Peak, Washington State, Makushin Volcano, Alaska, Akutan Island, Alaska, Mount Spurr, Alaska, Long Valley caldera, California, Newberry Crater, Oregon and Augustine Island, Alaska. Only three of the most dangerous U.S. volcanoes are sufficiently monitored, according to the report: Kìlauea in Hawaii, Mount St. Helens in Washington State, and the Long Valley caldera in California.

"We do need more monitoring," said Stanley Williams, a volcanologist at Arizona State University in Tempe. "There are few volcanoes that are really being studied at a very close level." Williams, who was not involved with the USGS study, said better monitoring of volcanoes would allow scientists to more accurately forecast eruptions. It would also allow experts to collect detailed information on what causes volcanoes to stir. Such information would help volcanologists to better distinguish routine rumblings from signals of unrest. "People who have hurricanes to study have it nice and easy," Williams said. "They know they have six months to test instruments, to make measurements and six months to work on the data and upgrade things, whereas when volcanoes are erupting is unknown." (National Geographic)

Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory--called plate tectonics--that explains the locations of volcanoes and their relationship to other large-scale geologic features. According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes. Located in the middle of the Pacific Plate, the volcanoes of the Hawaiian Island chain are among the largest on Earth. The volcanoes stretch 2,500 km across the North Pacific Ocean and become progressively older to the northwest. Formed initially above a relatively stationary "hot spot" in the Earth's interior, each volcano was rafted away from the hot spot as the Pacific Plate moves northwestward at about 9 cm per year. The island of Hawaii consists of the youngest volcanoes in the chain and is currently located over the hot spot. (USGS)

The United States is home to 50 active volcanoes (defined as having erupted sometime in the last 200 years). A whopping 80 percent of those are located in Alaska's remote Aleutian Islands chain. The volcanic island chain, which stretches west from the mainland toward Kamchatka on the northwest Asian coast, is the result of the sinking of the Pacific plate beneath the North American plate. The chain has more than 40 active volcanoes, including Mount Spurr, Mount Redoubt, and Mount Augustine.

"We usually have one or two decent eruptions a year," says volcanologist Chris Nye of the Alaska Volcano Observatory, which monitors activity in the Aleutian chain. "On average, we have three or four or five days a year when the eruption columns reach up high enough into the atmosphere to interfere with air traffic." Air traffic may seem inconsequential in such a remote region, but the region is actually an important corridor for international air traffic. "Almost all of the air freight which moves between North America and Asia and Europe and Asia comes through Alaska to refuel. Sixty to eighty thousand wide-body aircraft fly over the Aleutian volcanoes every year." In the Cascade Range in the Pacific Northwest, the home of Mount St. Helens, geophysicists are continuously monitoring a number of other volcanoes that have erupted within the past two centuries, including Mount Shasta, Mount Hood, Lassen Peak, and Mount Rainier. "There is a lot of concern about Mount Rainier because it is so close to Seattle and capable of damaging mudflows," says geologist Mary Reid of the University of California at Los Angeles.

Reid and others have focused their scientific sights on the Long Valley Caldera in northeastern California. The giant depression was formed 760,000 years ago in a massive volcanic explosion that blanketed the entire western United States in ash and volcanic rock. The volcano has been relatively quiet ever since, except for eruptions from some smaller volcanoes inside the caldera (crater).

Back in 1980, a string of three large quakes -- each around magnitude 6 -- rocked the caldera. Since then, swarms of small, imperceptible quakes have regularly lit up seismographs. Those quakes, some researchers suspect, mark the reemergence of volcanic activity in the caldera. There have been other indications too. "The center part of the volcano has been coming up -- doming --- at rates that vary from less than an inch to six inches a year," says Reid. "Different people have different interpretations, but I think that most people would agree that that means there is magma moving beneath the surface."

In 1990, another dramatic sign of resurgent volcanic activity -- vast tree kills from huge amounts of carbon dioxide gas seeping out of the soil -- was first noted at Mammoth Mountain, near the ski resort town of Mammoth Lakes, on the southwestern edge of the caldera. By some estimates, a magma body may be located about seven miles beneath the caldera. Reid's own work suggests that it could be large. She's precisely dated zircon crystals embedded in lava flows from two small eruptions, 115,000 and 625 years ago.

Although Reid thought that the crystals would be about as old as the lava flows, they turned out to be much older -- around 230,000 years old. That means, Reid thinks, that the zircons crystallized in the same magma body over two hundred thousand years ago. The magma then stayed molten until at least 625 years ago. "To keep that magma hot for so long," Reid says, "you'd need to have a pretty big magma chamber below the surface, with perhaps two hundred cubic kilometers of material." Mount St. Helens, for comparison, released only one to two cubic kilometers of volcanic material when it erupted in 1980. (PBS)

Work Cited
Svitil, Kathy. "Savage Earth Out of the Inferno: Volcanoes". PBS. April 3, 2009 .
Watson, John. "Volcanoes and the Theory of Plate Tectonics". USGS. April 3, 2009 .
"The Most Dangerous Volcanoes in the United States". Geology.com. April 3, 2009 .
Roach, John. "18 Most Dangerous U.S. Volcanoes Include Erupting Alaska Peak". National Geographic. April 3, 2009 .