Welcome Page   Lab Images of Volcanoes Hazards of Volcanoes Links to other Volcano Sites NASA Homepage US Geological Survey Bibliography			WHAT ARE VOLCANOS AND WHY DO THEY ERUPT? Volcanoes occur naturally and can form in many different places. Some volcanoes form under water, others in the polar areas, and still others on islands. Volcanoes can be found on almost any land mass. Volcanoes are actually openings that lay on the surface of the earth. This opening is what allows gases and molten rock (magma) to escape. This is what you see when a volcano erupts. These substances come from the mantle layer of the earth where temperature are extremely hot! This heat builds up pressure and pushes the molten rock and other substances through the top layer of the earth and erupts through the surface. When molten rock reaches the surface, it is then called lava. The lava then cools when it escapes from underneath the earth’s surface where it hardens and becomes rock. This rock piles up and forms the mountain-like object that we commonly call the volcano.  ARE THERE DIFFERENT TYPES OF VOLCANOS? Yes! There are many different types of volcanoes. These different types of volcanoes are characterized by the different shapes that they form. There are shield volcanoes, cinder cone volcanoes, composite volcanoes, dome volcanoes, and volcanoes with calderas peaks.  Shield volcanoes were given their name because after erupting, they take the shape of a shield. These types of volcanoes have a wide summit, or peak, and sloping sides. This happens because the lava that is released through the volcanic opening has a low viscosity. A well-known shield volcano is the Island of Hawaii.  Cinder cone volcanoes, or scoria cones, are formed by the gases that erupt from underneath the earth. As these gases are propelled throughout the air, they carry particles which solidify in the air and fall to the ground. This means that the longer the eruption lasts, the bigger and higher the cinder cone becomes.  Composite volcanoes, or sratovolcanoes, are composed from a type of magma that forms delicate, brittle lava. When this brittle lava builds up, it can reach enormous heights and make very steep slopes. Sometimes the lava will reach such a great height that it collapses because of the force of gravity. This particular type of volcano is formed over hundreds of years and many eruptions.  The dome volcanoes are constructed when magma containing massive amounts of silica erupts slowly. When this magma erupts, it forms thick layers that can be either dome or spiral shaped.  Calderas are circular depressions that are formed when arcuate fractures collapse. This process occurs because of the products that are given off when the volcano erupts. Similar to the cinder cone, the bigger the eruption is, the larger the calderas will be.  WHAT GASES ARE EMITTED WHEN A VOLCANO ERUPTS? There are four main types of gases that are emitted when a volcano erupts, carbon dioxide, sulfur dioxide, chlorine, and fluorine. These gases each contribute negatively to our environment. Excess carbon dioxide is bad for the environment because it contributes to global warming which is a current concern. Sulfur dioxide is another gas that is emitted by an eruption. Sulfur dioxide affects the short term local weather conditions by lowering temperatures as the sulfur dioxide gas is changed into sulfuric acid by sunlight. This sulfuric acid remains in the atmosphere for a few years and can decrease temperatures. Another gas that affects our atmosphere is the chlorine gas that is emitted from volcanoes. Chlorine is emitted as hydrochloric acid. These chlorine atoms can then harm the ozone layer by breaking down its components. The last major gas that is emitted from a volcanic explosion is fluorine. This gas condenses in the rain and on ash particles which then fall on grasses and water bodies. This pollutes lakes and streams and may poison animals who eat the contaminated grasses.