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These layers are the result of processes that formed the solar system and Earth, and are believed to be still occurring, though on a much smaller scale. Shortly after Earth formed, the decay of radioactive elements and heat produced by colliding particles resulting in some interior melting. It is believed that this allowed heavier elements, mainly iron and nickel, to sink while lighter elements moved upwards, creating a solid, iron-rich inner core, a molten metallic outer core, and a solid rocky mantle on which floats (more or less) a relatively light crust.
The mantle, however, is not homogenous. It includes a layer called the upper mantle, which consists of the asthenosphere, a hot, unstable zone capable of flow, above which is the lithosphere, the cool and rigid sphere of rock that includes the crust.
The large amounts of gaseous materials that escaped from the earth’s interior during its formation created the atmosphere, which made life, as we know it, possible. This in turn allowed the formation of water and oceans. The result was the physical environment in which all living things exist, made up of three major parts: the lithosphere (rock), the atmosphere (air) and the hydrosphere (water). Our planet is dynamic because it is characterised by continuous interaction of these major elements. These interactions create the conditions and changing environments that give rise to, and sustain life.
The development of a volcano begins with cracking (also know as a fissure) of the earths crust as the magma forces it’s way to the earth’s surface. As the magma moves upwards it is usually constrained into a circular pipe which reaches the surface as an opening called a vent. Eruptions through this vent of magma and/or pyroclastic flow over time build the volcanic structure. At the apex of the volcano is the crater. A crater is a shallow circular depression around the vent. The crater may be developed by either the deposition of eruptive material around the vent forming a rim or by explosive forces caused by violent eruptions removing material from the crater walls. A very large crater is known as a caldera, while smaller craters may range from meters in width through to hundreds of meters a caldera may extend for over a kilometre up to over 50 kilometres. Throughout the early years of the volcano’s development most volcanic material comes from the vent as discussed earlier. As the volcano matures smaller cones may develop from secondary vents on the sides of the volcano (flanks). These are known as parasitic cones. Those that only emit gasses are known as fumaroles. There are three main types of volcanoes:
Generally shield volcanoes have developed from the ocean floor to form seamounts of volcanic islands. There structure is broad and dome shaped with sides that gently slope. Examples of these include the Galapagos Islands and the Hawaiian Islands.
Cinder cones are amongst the most common of the volcano types. They are characterised by steeply sloping sides which is determined by the angle of repose of pyroclastic material at rest (in other words pyroclastic material can be stable at a high angle). These types of volcanoes are often constructed in a relatively short space of time and frequently within one or two eruptions. Once the magma in the vent and below solidifies there is rarely a subsequent eruption. Consequently these types of volcanoes are not very high with average heights between 30m – 300m. Cinder cones may also be a parasitic cone on a larger volcano.