VIII: Volcanic Eruptions

1. Where do volcanoes occur?

Active volcanoes tend to be located in linear volcanic mountain chains thousands of kilometers long on the edges of continents, in the middle of oceans, or as island chains. The locations of these volcanic chains are located along the boundaries of the massive tectonic plates where they spread apart or collide. The Pacific Ring of Fire contains two-thirds of the Earth's volcanoes.These volcanoes are located along convergent plate boundaries. The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. But some of the world's most active volcanoes, like Kilauea Volcano on the Island of Hawaii, are found in the middle of these massive plates above hot spots in the Earth's interior. As the plate moves over the hotspot, the volcano that was formed moves with the plate. Therefore, there are extinct volcanic chains extending in the direction of plate movement from the hotspot, such as the Emperor Seamount Chain running northwest of the Hawaiin Island Chain.

[Pacific Ring of Fire]

[Volcanic country of Iceland]

[Hot spots]

[Extinct volcanic chains]

2. How often do they erupt?

More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. U.S. Geological Survey (USGS) scientists are assessing hazards at many of the almost 70 active and potentially active volcanoes in the United States. They are closely monitoring activity at the most dangerous of these volcanoes and are prepared to issue warnings of impending eruptions or other hazardous events.


[ Map of United States Volcanoes ]

3. What happens when a volcano erupts?

[ Internal & External Diagram - based on USGS diagram]

4. What are the different types of volcanoes?

(From USGS)

In order of increasing violence of explosion:

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 of a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. 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. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.


[ Photo of Shield Volcano ]

Cinder Cones
Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.


[ Photo of Cinder Cone Volcano ]

Strato Volcanoes/Composite Volcano
Some of the Earth's grandest mountains are composite volcanoes -- sometimes called strato volcanoes. 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. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.


[ Photo of Strato Volcano ]

Lava Dome
Lava (usually dacite or rhyolite) that is too sticky to flow far from its vent forms steep-sided mounds called lava domes.

5. Examples of volcanic activity:

[ Colima, Mexico ]

An active volcano in the East African Rift Zone erupted explosively in 1966:

[ Oldoinyo Lengai ]

Mt. St. Helens erupted on May 18, 1980:

[ Mt. St. Helens ]

On 26 October Sicily was hit by a swarm of earthquakes - a sign that Mt Etna was about to blow. Sure enough at 4 am the next day the volcano split its sides and spewed fire fountains from fresh gashes in Etna's north and south flanks.


[ Mt.Etna ]