Volcano Facts

These are the general facts about Volcanoes in the easily understandable question answer form.

What is a volcano?

Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products -- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

Where did the term "volcano" come from?

The word "volcano" comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan -- the blacksmith of the Roman gods. They thought that the hot lava fragments and clouds of dust erupting form Vulcano came from Vulcan's forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. Today we know that volcanic eruptions are not super-natural but can be studied and interpreted by scientists. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

What is an "eruption" ?

An eruption occurs when magma rises from its source or from a storage reservoir and finally reaches the Earth's surface. As it rises, the magma fractures overlying rocks, which causes earthquakes, and parts of the volcano deform as magma approaching the surface makes room for itself. -- From: Brantley and Topinka, 1984, Earthquake Information Bulletin, v.16, no.2.

How much of the Earth is volcanic?

More than 80 percent of the Earth's surface -- above and below sea level -- is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years formed the Earth's earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced mountains, plateaus, and plains, which subsequent erosion and weathering have sculpted into majestic landscapes and formed fertile soils. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

How many active volcanoes are there in the world?

About 500 active volcanoes are known on Earth, not counting those that lie beneath the sea. -- From: Tilling, 1980, Volcanoes: Earthquake Information Bulletin, v.12, n.4.

What is the "Ring of Fire"?

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". -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

How many people are affected by volcanic eruptions?

Scientists have estimated that at least 200,000 persons have lost their lives as a result of volcanic eruptions during the last 500 years. Between 1980 and 1990, volcanic activity killed at least 26,000 people and forced nearly 450,000 to flee from their homes. -- From: Tilling, 1980, Volcanoes: Earthquake Information Bulletin, v.12, n.4, and Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

What are some positive products from volcanoes?

The Earth's crust, on which we live and depend, is in large part the product of millions of once-active volcanoes and tremendous volumes of magma that did not erupt but instead cooled below the surface. Such persistent and widespread volcanism has resulted in many valuable natural resources throughout the world. For example, volcanic ash blows over thousands of square kilometers of land increases soil fertility for forests and agriculture by adding nutrients and acting as a mulch. Groundwater heated by large, still-hot magma bodies can be tapped for geothermal energy. And over many thousands of years, heated groundwater has concentrated valuable minerals, including copper, tin, gold, and silver, into deposits that are mined throughout the world. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

What was the largest volcanic eruption in the 20th century?"

The largest eruption in the world this century occurred in 1912 at Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6 -- (which is equivalent to 230 years of eruption at Kilauea (Hawaii) or, about 30 times the volume erupted by Mount St. Helens (Washington) in 1980.) -- From: Wright and Pierson, 1992, USGS Circular 1073, and Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication. )

What was the most destructive volcanic eruption in the history of the United States?

The May 18, 1980 eruption of Mount St. Helens (Washington) was the most destructive in the history of the United States. Novarupta (Katmai) Volcano , Alaska, erupted considerably more material in 1912, but owing to the isolation and sparse population of the region affected, there were no human deaths and little property damage. In contrast, Mount St. Helens' eruption in a matter of hours caused loss of lives and widespread destruction of valuable property, primarily by the debris avalanche, the lateral blast, and the mudflows. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS Information Publication.

Where is the largest active volcano in the world?

Mauna Loa (Hawaii) is the world's largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the deep ocean floor. From its base below sea level to its summit, Mauna Loa is taller than Mount Everest. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication, and Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

Where in the United States is there evidence of volcanism?

Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits. A thick ash deposit sandwiched between layers of sandstone in Nebraska, the massive granite peaks of the Sierra Nevada mountain range, and a variety of volcanic layers found in eastern Maine are but a few of the striking clues of past volcanism. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

Where does the United States rank in the number of volcanoes?

The United States ranks third, behind Indonesia and Japan, in the number of historically active volcanoes (that is, those for which we have written accounts of eruptions). In addition, about 10 percent of the more than 1,500 volcanoes that have erupted in the past 10,000 years are located in the United States. Most of these volcanoes are found in the Aleutian Islands, the Alaska Peninsula, the Hawaiian Islands, and the Cascade Range of the Pacific Northwest. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

TRUE or FALSE: Portland, Oregon, has a volcano within its metropolitan area?

Metropolitan Portland, Oregon, like Auckland, New Zealand, includes most of a Plio-Pleistocene volcanic field. The Boring Lava includes at least 32 and possibly 50 cinder cones and small shield volcanoes. From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, Contribution by John E. Allen

What GOOD is a volcano?

Over the long term and geologic time, volcanic eruptions and related processes have directly and indirectly benefited mankind. Volcanic materials ultimately break down and weather to form some of the most fertile soils on Earth, cultivation of which has produced abundant food and fostered civilizations. People use volcanic products, the internal heat associated with young volcanic systems has been harnessed to produce geothermal energy, and most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with magmas found deep within the roots of extinct volcanoes. -- From: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS General Interest Publication, and Tilling, 1985, Volcanoes: USGS General Interest Publication

What is the extent of the Cascade Range?

Several lofty volcanic peaks dominate the Cascade Range of the Pacific Northwest; the principal part of the range extends from Mount Garibaldi in British Columbia, Canada, to Lassen Peak in northern California, a distance of about 1000 miles. -- From: Tilling, et.al., 1990, Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication, and Preparing for The Next Eruption in the Cascades: USGS Open-File Report 94-485

What are the major peaks of the Cascade Range?


How many eruptions have there been in the Cascades during the last 4,000 years?

Eruptions in the Cascades have occurred at an average rate of 1-2 per century during the last 4000 years, and future eruptions are certain. -- From: Preparing for The Next Eruption in the Cascades: USGS Open-File Report 94-485



How many eruptions have there been in the Cascades during the last 200 years?

Seven volcanoes in the Cascades have erupted since the first U.S. Independence Day a little more than 200 years ago. -- From: Preparing for The Next Eruption in the Cascades: USGS Open-File Report 94-485

1. Mount Baker, Washington
2. Glacier Peak, Washington
3. Mount Rainier, Washington
4. Mount St. Helens, Washington
5. Mount Hood, Oregon
6. Mount Shasta, California
7. Lassen Peak, California

Which is the tallest volcano in the Cascades? Is it also the largest?

Mount Rainier, in Washington State, is the tallest (4,392 meters, 14,410 feet) volcano in the Cascade Range but it is only the third most voluminous volcano after Mounts Shasta and Adams . -- From: Swanson, et.al., 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: American Geophysical Union Field Trip Guidebook T106, p.8.

Which Cascade Range volcano erupted through a glacier?

Mount Garibaldi (British Columbia, Canada) is a composite cone and domes built on a glacier. It is one of the larger volcanoes (6.5 cubic kilometers) in a chain of small Quaternary volcanic piles -- the Garibaldi Belt -- within the southern Coast Mountains of British Columbia. Mount Garibaldi is noteworthy both for the excellent exposures of its internal structure and for its striking topographic anomalies, which can be attributed to the growth of the volcano onto a major glacial stream, part of the Cordilleran Ice Sheet, and the subsequent collapse of the flanks of the volcano with the melting of the ice. -- From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.144-145, Contribution by William H. Mathews

Which is the most glaciated volcano in the Cascades? Which is the second?

Mount Rainier (Washington) at 14,410 feet (4,393 meters), the highest peak in the Cascade Range, is a dormant volcano whose load of glacier ice exceeds that of any other mountain in the conterminous United States. Mount Rainier has 26 glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. Mount Baker (Washington) at 10,778 feet (3,285 meters), is an ice-clad volcano in the North Cascades of Washington State about 31 miles due east of the city of Bellingham. After Mount Rainier, it is the most heavily glaciated of the Cascade volcanoes: the volume of snow and ice on Mount Baker (about 1.8 cubic kilometers; 0.43 cubic mile) is greater than that of all the other Cascades volcanoes (except Rainier) combined. -- From: Hoblitt, et.al., 1995, Volcano hazards from Mount Rainier, Washington: USGS Open-File Report 95-273, Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication, and Gardner, et.al., 1995, Potential Volcanic Hazards from Future Activity of Mount Baker, Washington: USGS Open-File Report 95-498.

How dangerous is Mount Rainier?

Although Mount Rainier (Washington) has not produced a significant eruption in the past 500 years, it is potentially the most dangerous volcano in the Cascade Range because of its great height, frequent earthquakes, active hydrothermal system, and extensive glacier mantle. Mount Rainier has 26 glaciers containing more than five times as much snow and ice as all the other Cascade volcanoes combined. If only a small part of this ice were melted by volcanic activity, it would yield enough water to trigger enormous lahars. Mount Rainier's potential for generating destructive mudflows is enhanced by its great height above surrounding valleys. -- From: Scott, et.al., 1990, Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier, Washington: USGS Open-File Report 90-385, and Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

What is the greatest hazard presented by Mount Rainier?

Debris flows pose the greatest hazard to people near Mount Rainier. A debris flow is a mixture of mud and rock debris that looks and behaves like flowing concrete. Giant debris flows sometimes develop when large masses of weak, water-saturated rock slide from the volcano's flanks. Many of these debris flows cannot be predicted and may even occur independently of a volcanic eruption. Giant debris flows can also form during an eruption as hot rock fragments tumble down the volcano's slopes, eroding and melting snow and glacier ice. Although they happen infrequently, giant debris flows have the potential to inundate much of the southern Puget Sound lowland. Scientists estimate that debris flows can travel the distance between Mount Rainier and the Puget Sound lowland in as little as 30 minutes to a few hours. About 100,000 people now live in areas that have been buried by debris flows during the past few thousand years. -- From: Walder and Driedger, 1995, Living With a Volcano in Your Backyard - Volcanic Hazards at Mount Rainier: USGS Open-File Report 95-421.

How often do giant debris flows occur at Mount Rainier?

During the past 10,000 years, about 60 giant debris flows from Mount Rainier have filled river valleys to a depth of hundreds of feet near the volcano, and have buried the land surface under many feet of mud and rock sixty miles downstream. Seven debris flows large enough to reach Puget Sound have occurred in the past 6,000 years. -- From: Walder and Driedger, 1995, Living With a Volcano in Your Backyard - Volcanic Hazards at Mount Rainier: USGS Open-File Report 95-421.

How would an eruption of Mount Rainier compare to the 1980 eruption of Mount St. Helens?

Eruptions of Mount Rainier usually produce much less volcanic ash than do eruptions at Mount St. Helens. However, owing to the volcano's great height and widespread cover of snow and glacier ice, eruption-triggered debris flows at Mount Rainier are likely to be much larger -- and will travel a greater distance -- than those at Mount St. Helens in 1980. Furthermore, areas at risk from debris flows from Mount Rainier are more densely populated than similar areas around Mount St. Helens. -- From: Walder and Driedger, 1995, Living With a Volcano in Your Backyard - Volcanic Hazards at Mount Rainier: USGS Open-File Report 95-421.

How old is Mount St. Helens?

The eruptive history of Mount St. Helens (Washington) began about 40,000 years ago with dacitic volcanism, which continued intermittently until about 2,500 years ago. This activity included numerous explosive eruptions over periods of hundreds to thousands of years, which were separated by apparent dormant intervals ranging in length from a few hundred to about 15,000 years. The range of rock types erupted by the volcano changed about 2,500 years ago, and since then Mount St. Helens repeatedly has produced lava flows of andesite, and on at least two occasions, basalt. Other eruptions during the last 2,500 years produced dacite and andesite pyroclastic flows and lahars, and dacite, andesite, and basalt airfall tephra. ... Major dormant intervals of the last 2,500 years range in length from about 2 to 7 centuries. -- From: Mullineaux and Crandell, 1981, The Eruptive History of Mount St. Helens: IN: The 1980 Eruptions of Mount St. Helens, Washington: USGS Professional Paper 1250.

Where did the name "Mount St. Helens" come from?

Some Indians of the Pacific Northwest variously called Mount St. Helens (Washington) "Louwala-Clough," or "smoking mountain." The modern name, Mount St. Helens, was given to the volcanic peak in 1792 by Captain George Vancouver of the British Royal Navy, a seafarer and explorer. He named it in honor of a fellow countryman, Alleyne Fitzherbert, who held the title Baron St. Helens and who was at the time the British Ambassador to Spain. Vancouver also named three other volcanoes in the Cascades -- Mounts Baker, Hood, and Rainier -- for British naval officers. -- From: Tilling, et.al., 1990, Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

How high was Mount St. Helens before the May 18, 1980 eruption. How high was it after?

Before May 18, 1980, Mount St. Helens' summit altitude of 9,677 feet made it only the fifth highest peak in Washington State. It stood out handsomely, however, from surrounding hills because it rose thousands of feet above them and had a perennial cover of ice and snow. The peak rose more than 5,000 feet above its base, where the lower flanks merge with adjacent ridges. On May 18, 1980, the volcano lost an estimated 3.4 billion cubic yards (0.63 cubic mile) of its cone (about 1,300 feet in height), leaving behind a horseshoe-shaped crater (open to the north), with the highest part of the crater rim on the southwestern side being at 8,365 feet elevation. -- From: Foxworthy and Hill, 1982, Volcanic Eruptions of 1980 at Mount St. Helens, The First 100 Days: USGS Professional Paper 1249.

How much ash was there from the May 18, 1980 eruption of Mount St. Helens?

During the 9 hours of vigorous eruptive activity, about 540 million tons of ash fell over an area of more than 22,000 square miles. Tot total volume of the ash before its compaction by rainfall was about 0.3 cubic mile, equivalent to an area the size of a football field piled about 150 miles high with fluffy ash. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

How far did the ash from Mount St. Helens travel?

The May 18, 1980 eruptive column at Mount St. Helens fluctuated in height through the day, but the eruption subsided by late afternoon. By early May 19, the eruption had stopped. By that time the ash cloud had spread to the central United States. Two days later, even though the ash cloud had become more diffuse, fine ash was detected by systems used to monitor air pollution in several cities of the northeastern United States. Some of the ash drifted around the globe within about 2 weeks. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

How did Mount Hood get its name? How tall is it?

Mount Hood, 3428 meters high, is the fourth highest peak in the Cascades and the highest in Oregon. It was named after a British admiral and first described in 1792 by William Broughton, member of an expedition under command of Captain George Vancouver. The first geologic reconnaissance primarily described the existing glaciers. -- From: Swanson, et.al., 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: American Geophysical Union Field Trip Guidebook T106.

Are there earthquakes associated with Mount Hood?

Felt earthquakes on Mount Hood (Oregon) occur every 2 years on the average. Seismic monitoring, in effect since 1977, indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level. A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 kilometers deep with a maximum bodywave magnitude of 2.8) recorded in a 5-day period, prompted development of an emergency response plan to coordinate local authorities in the event of future eruption. -- From: Swanson, et.al., 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: American Geophysical Union Field Trip Guidebook T106.

What are the "Three Sisters"?

The Three Sisters area (Oregon) contains 5 large volcanic cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. From: Hoblitt, et.al., 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297

Which is the oldest of the Three Sisters?

North Sister is the oldest of the Three Sisters (Oregon), and is deeply dissected and probably has been inactive for at least 100,000 years. Middle Sister is intermediate in age between North and South Sister, and was active in late Pleistocene but not postglacial time. South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater. Most of South Sister predates late Wisconsin glaciation and is therefore older than 25,000 years, however, eruptions of rhyolite from flank vents have occurred as recently as 2000 years ago. -- From: Scott and Gardner, 1990, Field trip guide to the central Oregon High Cascades, Part 1: Mount Bachelor-South Sister area: Oregon Geology, September 1990, v.42, n.5, and Hoblitt, et.al., 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297.

Which Cascade volcano has a "volcano in a lake in a volcano"?

Crater Lake Caldera, Oregon, with Wizard Island Cinder Cone -- From a probable altitude of roughly 12,000 feet, the top of former Mount Mazama was lost to eruption and collapse that left the present huge crater and the deepest lake ( Crater Lake - 1,932 feet) in North America. Explosive eruptions built Wizard Island (at least 800-900 years ago) and two other cones (submerged) on present crater floor. From: Foxworthy and Hill, 1982, Volcanic Eruptions of 1980 at Mount St. Helens, The First 100 Days: USGS Professional Paper 1249


How often does Mount Shasta erupt?

Mount Shasta (California) has erupted, on the average, at least once per 800 years during the last 10,000 years, and about once per 600 years during the last 4,500 years. The last known eruption occurred about 200 radiocarbon years ago. -- From: Miller, 1980, Potential Hazards from Future Eruptions in the Vicinity of Mount Shasta Volcano, Northern California: USGS Bulletin 1503.

When did Lassen Peak last erupt?

The most recent eruptive activity occurred at Lassen Peak (California) in 1914-1917 A.D. This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year. By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 meters over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. A lahar was generated that reached more than 18 kilometers down Lost Creek. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 kilometers northeast of the summit. The eruption also generated lahars that traveled more than 20 kilometers down Lost Creek and floods that went down Hat Creek. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 kilometers above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 kilometers to the east-northeast. The fall of fine ash was reported as far away as Elko Nevada, more than 500 kilometers east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917. -- From: Hoblitt, et.al., 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297

Where is Dante's Peak located?

Dante's Peak is a man-made Cascade volcano created for the 1997 movie Dante's Peak, starring Pierce Brosnan and Linda Hamilton. The peak itself was a 100 square foot by 35 foot high wood and steel structure built on a sound stage in Los Angeles and wheeled onto a tarmac to be shot against the sky and later composited with live action footage shot on location (Wallace, Idaho). Computer-generated smoke, ash and lava were created as special effects. (Info courtesy: Universal Pictures Marketing)

Is Mount Wilshire a real volcano?

Mount Wilshire is a fictitious volcano which erupted in the La Brea Tar Pits in Los Angeles, California, in the 1997 movie Volcano, starring Tommy Lee Jones.

How many volcanoes are there in Alaska?

The Alaska Peninsula and the Aleutian Islands have about 80 major volcanic centers that consist of one or more volcanoes. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

How often do Alaskan volcanoes erupt?

Alaskan volcanoes have produced one or two eruptions per year since 1900. At least 20 catastrophic caldera-forming eruptions have occurred in the past 10,000 years; the awesome eruption of 1912 at Novarupta in the Katmai National Monument is the most recent. Scientists are particularly concerned about the volcanoes whose eruptions can affect the Cook Inlet region, where 60 percent of Alaska's population lives. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.

What are the "Hawaiian Islands"?

The Hawaiian Islands are the tops of gigantic volcanic mountains formed by countless eruptions of fluid lava over several million years; some tower more than 30,000 feet above the sea floor. The Islands are composed of linear chains of shield volcanoes including Kilauea and Mauna Loa on the island of Hawaii -- two of the world's most active volcanoes. -- From: Tilling, et.al., 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS General Interest Publication, and Tilling, 1985, Volcanoes: USGS General Interest Publication.

How many major Hawaiian Islands are there? What are their principal volcanoes?


1. Niihau
2. Kauai
3. Oahu
4. Molokai
5. Lanai
6. Maui
7. Kahoolawe
8. Hawaii (Big Island)

How big are the Hawaiian volcanoes?

The Hawaiian shield volcanoes are the largest mountains on Earth. Mauna Kea Volcano rises 13,796 feet above sea level but extends about 19,700 feet below sea level to meet the deep ocean floor, its total height is nearly 33,500 feet, considerably higher than the height of the tallest mountain on land, Mount Everest (Chomolungma) in the Himalayas (29,028 feet above sea level). Mauna Loa stands not quite as high as Mauna Kea but is much larger in volume. -- From: Tilling, et.al., 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS General Interest Publication.

What is "composite" or "strato" volcano?

Some of the Earth's grandest mountains are composite volcanoes -- sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, Mount St. Helens and Mount Rainier in Washington. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

What is a "shield" volcano?

Shield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that a warrior's shield. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii -- two of the world's most active volcanoes. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

What is a "caldera"?

The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth's surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep. The caldera now filled by Oregon's Crater Lake was produced by an eruption that destroyed a volcano the size of Mount St. Helens and sent volcanic ash as far east as Nebraska. -- From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication and Wright and Pierson, 1992, Living With Volcanoes, The U.S. Geological Survey's Volcano Hazards Program: USGS Circular 1973.

What causes earthquakes?

An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault. Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years. ... Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. -- From: Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washing State Department of Natural Resources, Washington Division of Geology and Earth Resources Information Circular 85

Can I outrun a debris avalanche?

On May 18, 1980, at Mount St. Helens, data shows that an estimated 7-20 seconds elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles per hour. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

What is a "pyroclastic flow"? Where did the term come from?

The term "pyroclastic" -- derived from the Greek works pyro (fire) and klastos (broken) -- describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Pyroclastic flows -- sometimes called nuees ardents (French for "glowing clouds') -- are hot, often incandescent mixtures of volcanic fragments and gases that sweep along close to the ground. Depending on the volume of material, proportion of solids to gas, temperature, and slope gradient, the flows can travel at velocities as great as 450 miles an hour. Most volcanic ash is basically fine-grained pyroclastic material composed of tiny particles of explosively disintegrated old volcanic rock or new magma. Larger sized pyroclastic fragments are called lapilli, blocks, or bombs. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

How dangerous are pyroclastic flows?

Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility. During the 1902 eruption of Mont Pelee (Martinique, West Indies), for example, a nuee ardente demolished the coastal city of St. Pierre, killing nearly 30,000 inhabitants. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.

What is the difference between "magma" and "lava"?

Scientists use the term magma for molten rock underground and lava for molten rock (and contained gases) that breaks through the Earth's surface. Originating many tens of miles beneath the ground, magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Lava is red hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty, blocky masses. -- From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS General Interest Publication and Tilling, 1985, Volcanoes: USGS General Interest Publication.

Can volcanic rocks float?

The violent separation of gas from lava may produce rock froth called pumice. Some of this froth is so light -- because of the many gas bubbles -- that it floats on water. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

What is a "lava dome"?

Volcanic domes are masses of solid rock that are formed when viscous lava is erupted slowly from a vent. If the lava is viscous enough, it will pile up above the vent to form a dome rather than move away as a lava flow. The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement. Most domes are composed of silica-rich lavas that have a lower gas content than do the lavas erupted earlier in the same eruptive sequence; nevertheless, some dome lavas still contain enough gas to cause explosions within a dome as it is being formed. From: Miller, 1989, Potential Hazards from Future Volcanic Eruptions in California: USGS Bulletin 1847.

What is "volcanic ash"?

During many volcanic eruptions, fragments of lava or rock are blasted into the air by explosions or carried upward by a convecting column of hot gases. These fragments fall back to earth on and downwind from their source vent to form a pyroclastic-fall or "ash" deposit. Pyroclastic-fall deposits, referred to as tephra, consist of combinations of pumice, scoria, dense-rock material, and crystals, that range in size from ash (<2mm)>64mm). Eruptions that produce tephra range from short-lived weak ones that eject debris only a few meters into the air, to cataclysmic explosions that throw debris to heights of several tens of kilometers. -- From: Miller, 1989, Potential Hazards from Future Volcanic Eruptions in California: U.S. Geological Survey Bulletin 1847

What is a "lahar"?

Lahar is an Indonesian word describing mudflows and debris flows that originate from the slopes of a volcano. Both types of flows contain a high concentration of rock debris to give them the internal strength necessary to transport huge boulders as well as buildings and bridges and to exert extremely high impact forces against objects in their paths. Debris flows are coarser and less cohesive than mudflows. As lahars become dilute in downstream direction they become hyperconcentrated streamflows. Lacking internal strength, the mixture of rock debris and water takes on different flow properties. The coarser debris in this type of flow is no longer held in suspension by matrix strength and therefore settles to the bottom of the flow. -- From: Brantley and Power, 1985, Reports from the U.S. Geological Survey's Cascades Volcano Observatory at Vancouver, Washington: Earthquake Information Bulletin, v.17, n.1, January-February 1985.

How big are lahars?

Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second. Steep, unvegetated slopes during a heavy rain are often good sites to observe such small flows. At the other extreme, they can be a few hundred meters wide, tens of meters deep, flow at several tens of meters per second, and travel over 100 kilometers from a volcano. Such catastrophic lahars are triggered by volcanic eruptions or by massive landslides such as the one that occurred on May 18, 1980, at Mount St. Helens volcano. -- From: Brantley and Power, 1985, Reports from the U.S. Geological Survey's Cascades Volcano Observatory at Vancouver, Washington: Earthquake Information Bulletin, v.17, n.1, January-February 1985.

How are lahars formed?

Lahars are commonly initiated by: 1) large landslides of water-saturated debris, 2) heavy rainfall eroding volcanic deposits, 3) sudden melting of snow and ice near a volcanic vent by radiant heat or on the flanks of a volcano by pyroclastic flows, or 4) breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions. Since 1980, lahars have formed by all of these processes at Mount St. Helens. -- From: Brantley and Power, 1985, Reports from the U.S. Geological Survey's Cascades Volcano Observatory at Vancouver, Washington: Earthquake Information Bulletin, v.17, n.1, January-February 1985.