Volcano

V

Types of Volcanoes

Volcanoes come in different shapes and sizes, depending on the makeup of the magma, the style of the eruption, and how often they erupt. The major types of volcanoes, roughly in order of increasing size, are cinder cones, composite volcanoes (also called stratovolcanoes), shield volcanoes, calderas, and plateaus. Calderas and plateaus are shaped differently than traditional volcanoes—neither has a mountain-like shape.

A

Cinder Cones and Composite Volcanoes

Cinder cones and composite volcanoes have the familiar conelike shape that people most often associate with volcanoes. Some of these form beautifully symmetrical volcanic hills or mountains such as Parícutin Volcano in Mexico and Mount Fuji in Japan. Although both cinder cones and composite volcanoes are mostly the results of explosive eruptions, cinder cones consist exclusively of fragmental lava. This fragmental lava is erupted explosively and made up of cinders. Cinder cones are typically much smaller than composite volcanoes for two reasons: (1) they involve only weakly explosive, small-volume eruptions of basaltic cinder that does not travel far from the vent; and (2) they usually have a short life—often only a single eruptive burst before becoming extinct. In contrast, composite volcanoes can grow much larger because they represent the accumulated products of repeated eruptions from the same vent(s) over a long time.

Composite volcanoes are composed of explosively erupted pyroclastic materials layered with nonexplosively erupted lava flows and deposits of volcanic debris. They are mostly built from materials that come from andesitic or dacitic lava. In some composite volcanoes that undergo a major explosive eruption, such as Mount Saint Helens, nonexplosive extrusions of lava within the summit crater can later construct a bulbous mound of accumulated lava. This mound is called a lava dome or a volcanic dome.

B

Shield Volcanoes

Shield volcanoes (also called volcanic shields) get their name from their distinctive, gently sloping mound-like shapes that resemble the fighting shields that ancient warriors carried into battle. Their shapes reflect the fact that they are constructed mainly of countless fluid basaltic lava flows that erupted nonexplosively. Such flows can easily spread great distances from the feeding volcanic vents, similar to the spreading out of hot syrup poured onto a plate. Volcanic shields may be either small or large, and the largest shield volcanoes are many times larger than the largest composite volcanoes. The classic examples of shield volcanoes are the Hawaiian volcanoes Mauna Loa and Kilauea.

---

---

C

Caldera

A caldera is a round or oval-shaped low-lying area that forms when the ground collapses because of explosive eruptions (see Crater). An explosive eruption can explode the top off of the mountain or eject all of the magma that is inside the volcano. Either of these actions may cause the volcano to collapse. Calderas can be bigger than the largest shield volcanoes in diameter. Such volcanic features, if geologically young, are often outlined by an irregular, steep-walled boundary (a caldera rim), which reflects the original ringlike zone, or fault, along which the ground collapse occurred.

Some calderas have hills and mountains rising within them, called resurgent domes, that reflect volcanic activity after the initial collapse. Some calderas are filled with water, forming lakes such as Crater Lake in Oregon. Good examples of calderas can be seen at Yellowstone National Park (Wyoming) and Long Valley (eastern California). These were formed by explosive eruptions in the geologic past that were thousands of times larger than any historical eruption.

A number of geologists have used the term supervolcano for especially massive caldera eruptions such as one that occurred at Yellowstone about 2.1 million years ago or the Mount Toba eruption in Indonesia about 74,000 years ago. Such powerful caldera-forming eruptions, whose ash deposits can be traced thousands of kilometers from their sources, potentially pose the greatest volcanic hazards to society. Evidence suggests that such giant explosions can cause dramatic cooling of the climate. Among other effects, sulfur dioxide gas sent high into the atmosphere turns to sulfuric acid and forms long-lasting sulfate particles. These sulfate particles reflect sunlight back into space, cooling temperatures and potentially interfering with the growth of plants. Volcanoes that exist as islands pose additional hazards. The explosion and collapse of the Krakatau volcano in the Indonesian island chain in 1883 generated huge tsunamis that killed thousands of people when the ocean rushed into the deep caldera left by the main blast. Luckily, such dangerous eruptions are very rare geological events.

D

Volcanic Plateaus

Some of the largest volcanic features on Earth do not actually look like volcanoes. Instead, they form extensive, nearly flat-topped accumulations of erupted materials. These materials form volcanic plateaus or plains covering many thousands of square kilometers. The volcanic materials can be either very fluid basaltic lava flows or far-traveled pyroclastic flows. The basaltic lava flows are called flood or plateau basalts and are erupted from many fissure vents. The Columbia Plateau in the states of Oregon, Washington, and Idaho is an example of flood basalts. The pyroclastic flows, or ash flows, are from huge explosive caldera-forming eruptions. The Yellowstone Plateau of Wyoming and Montana is built of pyroclastic flows.

Some plateau eruptions have been associated with mass extinctions during the history of life on Earth. Many scientists think that a gigantic eruption in Siberia at the end of the Permian Period about 251 million years ago released enormous amounts of carbon dioxide and other volcanic gases, resulting in catastrophic changes to Earth’s climate and atmosphere. Nearly 94 percent of species in the oceans and about 70 percent of species on land died out. Another giant eruption known as the Deccan Traps occurred around 65 million years ago in India and may have contributed to the extinction of the dinosaurs, along with effects of an asteroid impact.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain