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A volcano is a conical landform or mountain built from lava and ash. The lava and ash erupt from the volcano through a vent that connects with reservoirs of hot molten rock or magma deep within the Earth.
More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. The most volcanically active regions of the Nation are in Alaska, Hawaii, California, Oregon, and Washington.
There are more than 1500 volcanoes on the Earth which have the potential to become active because they have erupted within the past 10,000 years. Approximately 10% of the Earth's population live close enough to a volcano to be at risk of volcanic hazards.
Volcanoes produce a wide variety of natural hazards that can kill people and destroy property. This simplified sketch shows a volcano typical of those found in the Western United States and Alaska, but many of these hazards also pose risks at other volcanoes, such as those in Hawai`i. Some hazards, such as lahars and landslides, can occur even when a volcano is not erupting.
Mauna Loa Volcano, Hawaii, a shield volcano, as viewed from the summit of Kilauea, about 33 miles to the southeast.
Mauna Loa, on the Big Island of Hawaii, is the largest active volcano in the world. It last erupted in 1984.
Mauna Loa erupted 14 times in the 20th Century, and 37 times since 1832.
Mauna Loa is the most massive mountain on Earth, rising to an elevation of 13,677 feet above sea level, or 31,677 feet above the sea floor.
Its volume is 10,000 miles3.
The tallest mountain on Earth is located nearby, also on the Big Island of Hawaii. It is Mauna Kea, rising to an elevation of 13,796 feet above sea level, or 31,796 feet above the sea floor.
Both Mauna Loa and Mauna Kea are shield volcanoes.
In comparison, Mt. Everest (in the Himalayas), the highest point on Earth above sea level, rises to an elevation of 8848 m (or 29,028 ft). Mt. Everest is NOT a volcano, however.
The largest volcano in the Solar System is also a shiled volcano. It is located on the planet Mars. Its name is Olympus Mons (or Mount Olympus), and it is three times as high as the largest volcanoes on Earth (nearly 27 km high). It is about 100 times as massive as one of the Hawaiian volcanoes.
Cinder cone, Puu Puai, created by eruption in 1959, Devastation Trail, Kilauea, Hawaii Volcanoes National Park.
The volcano Paracutin, in Mexico, is a well-known example of a cinder cone.
Eruption of Mt. St. Helens
large (1 - 10 km across)
layered structure, consisting of alternating layers of lava and pyroclastic material
high silica content (sialic or intermediate) with composition of andesite, dacite, and occasionally rhyolite
These volcanoes make up the largest perentage of the Earth's volcanoes (about 60%)
Examples: Mt. Vesuvius, Cascade Range volcanoes such as Mt. St. Helens and Mt. Ranier
Shapes of volcanoes are due to the viscosity of the magma (or lava).
Runny basaltic lava will not form a steep cone; forms relatively flat shield volcanoes.
Mafic lavas are low in silica (only about 50% SiO2)
Sialic lavas may have more than 70% SiO2.
Explosivity of the volcano is also controlled by the viscosity (and chemistry) of the lava or magma.
Gases are easily released from low viscosity (runny) lavas.
Ex. = vesicular basalt.
Looking into Caldera of Kilauea Volcano, which is about 2 to 2.5 miles in diameter, and about 400 feet deep. A road around the crater rim is 11 miles long. Steam is rising from the inner crater, Halema'uma'u, near the center of the left photo.
Kilauea is the world's most active volcano, and it has been erupting continuously since January 3, 1983, with lobes of lava threatening (and destroying) housing subdivisions, and entering the sea through lava tubes.
Kilauea rarely erupts from its summit. Instead it erupts from vents on its flanks, particularly along its east and southwest rift zones.
The summit of Kilauea is about 4000 feet above sea level.
See explanatory diagram of Kilauea caldera on sign at Volcanoes National Park.
Pahoehoe flows (from circa 1993) near Kalapana Black Sand Beach.
Pahoehoe at the end of Chain of Craters Road (from circa 1995).
Weathered aa lava flow, Kalapana Region, Big Island of Hawaii
1974 Aa lava flow, Chain of Craters Road, Hawaiian Volcanoes National Park.
The word "pyroclastic" refers to any fragmental material released from a volcano. It is derived from the Greek, pyro, meaning "fire" and klastos, meaning "fragments". Pyroclastic fragments range from dust-sized up to blocks that weigh several tons.
Image of ash-fall tephra deposit about 9 cm thick at former U.S. Clark Air Base, Philippines, about 25 km east of Mount Pinatubo. The pumice and ash fell to the ground on June 15, 1991. Photograph by R.P. Hoblitt on June 16, 1991. Image courtesy of U.S. Geological Survey.
Pyroclastic material from Kilauea volcano, Hawaii.
Olivine crystals (green) and Pele's Tears (black oval) in pyroclastic debris along Devastation Trail, Hawaii Volcanoes National Park. (In the palm of a hand.)
Between 100 and 2000 metric tons of sulfur dioxide (SO2) are released per day from Kilauea. The rain is so acidic that a desert has formed downwind from the summit of the volcano.
Sulfur Banks, Hawaii Volcanoes National Park. Sulfur and other minerals are being deposited here from gases rising from the hot magma below. The steam contains mostly water vapor, with lesser amounts of carbon dioxide and hydrogen sulfide.
Warning signs about gases in Hawaii Volcanoes National Park.
A nuee ardente from Mount Pelee, on the Caribbean island of
Martinique, destroyed the town of St. Pierre in 1902, killing almost all of its 28,000 inhabitants at once
(a prisoner in a dungeon, a shoemaker, and a few people on ships in the harbor survived).
Volcanoes can occur in a number of different tectonic settings.
Map showing distribution of active volcanoes around the world, along with the tectonic plate boundaries. The Ring of Fire is labeled.
Volcanoes at Convergent Plate Boundaries - The Ring of Fire
Most volcanoes are located at convergent plate boundaries (such as around the rim of the Pacific Ocean, called the Ring of Fire), where the Earth's tectonic plates move toward one another. Approximately 80% of the Earth's volcanoes are in the Pacific Rim region. The volcanoes that occur around the Pacific rim are the result of subduction occurring at convergent tectonic plate boundaries, where one tectonic plate moves beneath another at a deep sea trench. When an oceanic plate is overridden by another plate, the oceanic plate begins to melt, and the molten rock begins to rise, eventually providing the magma source for a volcano. These volcanoes may be in island arcs (such as the Aleutian Islands off Alaska, or Japan, New Zealand, or Indonesia), or they may be a chain of volcanoes along the edge of a continent (such as the volcanoes in the Cascade Range of the northwestern US and Canada, or the Andes Mountains in South America).
Volcanoes at Divergent Tectonic Boundaries
Volcanoes also occur at divergent tectonic plate boundaries or spreading centers (such as Iceland along the Mid-Atlantic Ridge or Mt. Kilamanjaro along the East African Rift Valley in Africa), where the plates are pulling apart and magma wells up into the gap, producing volcanoes and lava flows.
Intra-plate Volcanoes - Volcanoes at Hot Spots
Still other volcanoes occur within a tectonic plate (i.e., "intra-plate" volcanoes), as a result of mantle hot spots or mantle plumes (such as the Hawaiian hot spot).
Map of the Pacific basin showing the location of the Hawaiian Ridge-Emperor Seamount Chain. Base map reprinted by permission from World Ocean Floor Panorama by Bruce C. Heezen and Marie Tharp, Copyright 1977. Image courtesy of U.S. Geological Survey.
Artist's conception of the northwestward movement of the Pacific Plate over the fixed Hawaiian "Hot Spot" to illustrate the formation of the Hawaiian Ridge-Emperor Sea- mount Chain. The distinctive linear shape of the Hawaiian-Emperor Chain reflects the progressive movement of the Pacific Plate over a deep immobile hot spot. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes. The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Kauai, 5.6 to 3.8; Oahu, 3.4 to 2.2; Molokai, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawaii, less than 0.7 and still growing.
Volcano diagrams used with permission of Bruce E. Herbert, Texas A & M University, Big Bend Virtual Field Trip
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Georgia Perimeter College,
Page created March 8, 2005
Image links updated June 12, 2008