By A.R. Royo
What Are Volcanoes?
Volcanoes are vents in the Earth’s surface where gases and molten rock – magma – escape from the planet’s interior. When magma and large amounts of gas build up pressure under the surface, volcanic explosions can occur, propelling lava, rocks, gas and ash from the vent, sometimes quite violently. An erupting volcano can trigger tsunamis, flash floods, earthquakes, mudflows and rockfalls.
Lava flows can reach temperatures greater than 2,000° F and set ablaze everything in their path including grass, trees, forests, houses, people and whole towns. Rocks and even boulders sometimes expolode from these vents and rain down on farms and villages. Mud flows from melting snow can denude mountains and valleys in minutes, entombing farms, villages and towns. Ash can bury crop, livestock and highways, clog vehicles and othert machinery; toxic gases can asphixiate or cause lung damage, especially to the very young and old. (Photo below © G. Brad Lewis)
The Ring of Fire
Most volcanoes exist along the edges of tectonic plates, the gigantic rock slabs that float on liquid rock below, comprising the continents of the Earth’s surface. About 90% of all volcanoes exist within the Ring of Fire along the eastern and western edges of the Pacific Ocean.
About 1,900 volcanoes on the planet are considered active, meaning they are likely to explode again. Many other volcanoes are dormant, showing no current signs of exploding but will likely become active at some future point. Others are considered extinct.
About 500 million people currently live close to active volcanoes, most of whom do not realize that the earth is capable of exploding and decimating their way of life. It is estimated that more than a quarter million people people have died in the past 300 years from volcanic eruptions.
The 3 Types of Volcanoes
The lower 48 states in the U.S. have about 40 active volcanoes while Alaska has another 60. Most of these are Composite Volcanoes, which tend to be large symetrical cones with steeps sides. They usually occur where one tectonic plate slides beneath another (subduction zone) and can be especially violent when a plate beneath the ocean slides beneath a continental plate.
Cinder Cones are also common, built from as gas-charged lava is blown violently into the air, then breaks into small fragments that solidify and fall as cinders around the vent to form a relatively small circular or oval cone.
The Hawaiian Islands are composed of linear chains of Shield Volcanoes built almost entirely of thick, slow moving lava flows that pour out in all directions from a central summit vent, or group of vents, building a broad, gently sloping flattened dome. They are built up slowly by thousands of basalt lava flows, which cool as relatively thin, sloping sheets. Some of the world’s largest volcanoes are shield volcanoes, including two of the world’s most active volcanoes: Mauna Loa and Kilauea Volcanoes on the Big Island of Hawaii. (Photo bellow by Priit Vesilind, National Geographic)
The Most Dangerous Volcanoes in the U.S.
The most dangerous volcanoes in the U.S. are all composite volcanoes along the American Ring of Fire on the U.S. West Coast and Alaska, except for the two shield volcanoes on the Big Island of Hawaii. Mauna Loa and Kilauea are ranked #8 and #1 respectively. Here are the Ten Most Dangerous U.S. Volcanoes in reverse order, according to the United States Geological Service (USGS).
10. Crater Lake Volcano, Oregon is the result of a blast that expelled 50 times the volume of magma as the Mount St. Helens eruption in 1980. This clear-blue, water-filled caldera is also the U.S.’s tenth most dangerous volcano. (Photo below courtesy R. Clucas, Alaska Volcano Observatory/USGS)
9. Redoubt Volcano, Alaska, which last erupted in 2009, is listed as a high risk because of its proximity to the city of Anchorage, the most populous in the 49th state, with its international airport, and its flight paths overhead. Ash from this volcano temporarily shut down a 747′s engines in December 1989. (Photo below courtesy NASA)
8. Mauna Loa Volcano, Big Island of Hawaii, is the largest volcano (and the highest mountain on Earth) rising 30,080 feet from the Pacific Ocean floor. At 60 miles long and 30 miles wide, Mauna Loa comprises half of the entire Big Island. A shield volcano, it has been erupting for at least 100,000 years. The lava flow of 1855-1856 is said to be one of the greatest flows ever seen by modern observers. In 1868, an eruption caused the largest recorded earthquake in Hawaii, a magnitude 8.0. Such eruptions and flows are not near any large population centers so there is virtually no immediate danger to humans.
7. Lassen Volcanic Center, California. Lassen Peak’s last eruption occurred during 1915 – 1917 when it obliterated a patch of forest, but on a much smaller scale than Mt. St. Helen’s.
6. South Sister Volcano, Oregon. The Three Sisters area has a volcanic area of about 115 square miles just west of Bend, in northeastern Oregon. A dozen years ago an portions of ground west of the Middle and South Sister began a deformation called “the Bulge,”which has since then subsided. (Photo below by James P. Blair, National Geographic)
5. Mount Shasta Volcano, California can be seen in all its magnificense from I-5 which runs by it just to the west. The USGS gives it a “very high threat” rating because of the more than 1,000 homes on its slopes. The last reported eruption of Mt. Shasta was seen from the Pacific Ocean in 1786, and it was very active in geologic time before that.
4. Mount Hood Volcano, Oregon was very active at the end of the last ice age 10,000 years ago. The last eruption occurred just before Lewis and Clark arrived in the early 1800s. But many people live on the slopes of the volcano, and two state highways cross its flanks. (Photo below courtesy R. Clucas, Alaska Volcano Observatory/USGS)
3. Mount Rainier Volcano, Washington poses great danger because it’s covered by more snow and ice than all the other Cascade Range volcanoes combined, presenting a high risk of lahars, volcanic mudflows, threatening the Puget sound’s highly developed areas. Eruptions occur on a 500-year cycle, and its last big explosion was just about 500 years ago! (Photo below courtesy Lyn Topinka, USGS)
2. Mount St. Helens Volcano, Washington erupted only 30 years ago on May 18, 1980. It was the most devastating volcanic explosion in U.S. history, killing 57 people and blasting 520 million tons of volcanic ash into the atmosphere, which darkened the skies of Spokane, more than 250 miles away. Mount St. Helens has erupted more often than any other volcano in the Cascade Range, during the past 10,000 years. (Photo below © Ron Dahlquist)
1. Kilauea Volcano, Big Island of Hawaii is the the world’s most active volcano. Kilauea began erupting on May 24, 1969, and ended on July 22, 1974. It began erupting again on January 3, 1983 and has been erupting continuously ever since – for almost 30 years! Lava flows in 1987 buried much of the Royal Gardens subdivision, which was near the boundary of Hawaii Volcanoes National Park. The 1990 lava flows destroyed the towns of Kalapan, Kaim and Kaim Bay, which were totally destroyed, as was Kalapana black sand beach, and a large section of Route 130, which now abruptly dead-ends at the lava flow. To date, 189 structures have been destroyed by Kilauea.
Despite its relative safety for researchers and visitors, Kilauea is regarded by the USGS as the most dangerous volcano in the U.S. Located within Hawaii Volcanoes National Park, Kilauea is visited by millions of tourists each year, making it the most visited attraction in Hawaii and the most visited volcano in the world. A number of Kilauea Volcano Tours are available for visitors to enjoy.