These are formed from material ejected as ash and cinders and lava, so they are of mixed composition. Some volcanoes are more complex and are termed composite volcanoes. These usually form when the volcano is coming to the end of its activity. Volcanic peaks formed from ash and other debris ejected by the volcano are called cinder cones. Shield volcanoes can also have secondary vents. So overall, a shield volcano poses much less of a threat to humans. The lava oozes out rather than erupts from a shield volcano. These volcanoes, which are sometimes called Hawaiian-type volcanoes, are not formed from such violent eruptions as the Mount St. Shield volcanoes are low and rounded with gentle slopes. An example of this type is Crater Lake in Oregon. It was formed when the pressure built up to intolerable levels inside the mountain so that there was a tremendous explosion, which blew off the top of the mountain and drained the magma chamber. A caldera is larger than a crater and has steep sides. Some vocanoes have calderas at the peak rather than craters. Most volcanic mountains also have smaller vents or escape outlets on the sides. The crater is connected to this underground chamber by a hollow vent or pipe. This crater has been formed from the expulsion or extrusion of gases and magma, which have escaped to the atmosphere from an underground magma chamber beneath the volcano. Most are conical with at the top a crater. Volcanoes can be classified according to the way in which they erupt. When a volcano erupts, it commonly forms a cone shaped peak and may continue to erupt at periodic intervals for thousands of years. Some pyroclasts can be quite large, up to several centimetres across. The fragmental material ejected from a volcano is referred to as pyroclastic material. On the surface the magma is referred to as lava. Volcanoes, named after Vulcan the Roman god of fire, occur where hot molten rock (magma) and gases escape from deep within the Earth to the surface. However, it it is very true that some volcanoes are both violent and deadly and we need to understand why there are such differences.įirstly let's look at what volcanoes are and what are their characteristics. The word erupt implies violence and so has led to some misunderstanding when we speak of volcanoes. You ask why volcanoes always erupt loudly and violently, but this is not always true. Earthquakes and Volcanoes Why do volcanoes always erupt so loudly and violently?

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