Volcanoes
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What is a volcano? [Top]
A volcano is a mountain or hill that is formed when there is accumulation of materials that have erupted through openings in the earth's surface. These openings are called volcanic vents. There's more than just one form of volcano, the most recognizable of which include volcanic edifices, calders, and domes of lava. Although there are three main types of volcanoes, each still varies greatly in a number of different ways. Some of these ways include size, shape, composition, and the eruptive history of the volcano; however, most volcanoes that are above see level have steeps sides. Of the volcanoes located above the sea level, 1,511 have been active within the last ten thousand years. 539 of these have erupted more than once, and on average about 55 volcanoes are active every year. Of these 55, half of them are new, the other half are contiuations from years before.
How is a volcano formed? [Top]
A volcano is formed when an accumulation of molten rock is present under the earth's surface. This molten rock, or magma, erupts through volcanic vents in the earth's crust. This magma is formed only when high temperatures and high pressure levels are present. The creation of molten rock usually occurs eighty to one hundred kiliometers below the earth's surface. Because molten rock is less dense than the rock that surrounds it, it rises. Why it rises and the process behind its rising are not understood well by scientists, but for whatever reason, it rises and squeezes between small spaces between minerals in the rocks. Before the volcano erupts onto the surface of the earth, the magma build up in magma reservoirs. Each time the volcano erupts, levels are added to it until it becomes like the mountains one is used to seeing and recognizing as a volcano. Most volcanoes begin beneath an ocean.
What materials erupt from volcanoes? [Top]
The first and most well-known material is lava, or magma. Lava can flow in two different ways. It can either flow in what is known as a pahoehoe, which is where the lava is smooth and has a billowly surface, or it can flow in chunks. The second material is the tephra. Tephra is made of rock fragments formed from magma explosions. As long as this material is airborne, it is given the name of tephra. How is this substance formed? When gases inside the volcano build up and cause the volcano to explode, magma is shot up into the air and forms tephra. Any particles of tephra what are smaller than 2 mm in diameter is called ash, or volcanic dust. The last material released from a volcano are gases. Gas from the eruptions (explosive or not) is usually no more than steam; however, without the release of gas, it is not possible for the volcano to erupt. Because most of the gas is steam, it is primarily made up of water vapor, carbon dioxide, and sulfur dioxide. Chlorine and flourine gases are also present, but in very small amounts.
In what ways does a volcano erupt? [Top]
The way in which a volcano erupts depends on the composition of the magma underneath the earth's surface, as well as a few other major factors. It varies also by the amount of gas in the magma and the type of vent from which the magma is to escape. During a nonexplosive eruption, the magma will simply flow down from the vent. Very little, if any, tephra is ejected in these situations. In an explosive eruption, the magma forms various fragmental materials as it is flung into the air.
What kinds of volcanoes are there? [Top]
The most common and recognizable type of volcano is that which is shaped like a cone. These are called cinder cones and can sometimes create beautiful, symmetric volcanoes. These types of volcanoes are the result of explosive eruptions. Volcanoes that look more like a mound are referred to as shield volcanoes. Because of this shape, it can be assumed that these volcanoes were not created by explosive eruptions, but by simple lava flows from nonexplosive eruptions. Another possible volcanic shape is the caldera. The caldera is a round, low-lying area the forms when the ground sinks from explosive eruptions. Why does it collaspe? Explosive erupts can do one of two things that would cause this: blow of the top of the volcano or expend all of the magma from inside the volcano. Some calders are filled with water instead of magma. It is that that is responsible for creating Crater Lake in Oregon. Last, but not least, there are the volcanic plateaus. These volcanoes don't look anything like what they are. The are giant hills with flat tops. These are formed by eruptions from fissure vents.
What determines where a volcano will form? [Top]
According to the theory of plate tectonics, the earth's surface is made of about a dozen plates, which are made up of materials from both the crust and the mantle. These plates move several centimeters a year. So what does this have to do with volcanoes? Well, most active volcanoes found above see level are located along the borders of these plates.
What kinds of plates are there? [Top]
Divergent plates are plates that are spreading and moving apart. Volcanoes along these plates are usually nonexplosive. Convergent plates are plates that are coming together. This is where most of the world's active volcanoes are located. Transform boundaries are boundaries in which the plates are moving horizontally. These borders are responsible for earthqquakes more than they are for volcanoes.