Explosive Volcanoes

Where are the explosive volcanoes?All these volcanoes occur at convergent plate margins and particularly around the edge of the Pacific Zone where the oceanic plate is subducting. The chains of volcanoes above the subduction zone are called the 'Ring of Fire' and there is usually at least one erupting somewhere in the world.Strato-volcanoes are so named becaused they are composed of layers of lava and pyroclastic deposits. The lava has a wide range of compositions with silica between 71 to 51% and the rocks will mainly be andesite with some basalt and rhyolite. Rhyolite lava flows are rare because the silicic magma is so viscous that most of it cools below the surface as intrusive granites.Intermediate and silicic volcanoes have many features in common, even though the shape and size of these volcanoes varies: the viscosity of the magma is high because the silica content is high. The andesite and especially rhyolite lavas are thick and sticky and move slowly. This means that the angle of the slope is quite steep, at about 30 degrees. The gas content is high but the high viscosity does not allow the gas to escape so that eruptions are explosive when the gas finally does break through. These volcanoes range from 2 to 8 on the VEI scale at least 50% of the material erupted is pyroclastic, which can include the incandescent gaseous cloud containing droplets of magma and ash called nuee ardente. Eruptions are infrequent, sometimes hundreds of thousands of years beween erptions, or more often tens of years. Their activity is described as explosive and classified as Strombolian, Vulcanian or Plinian - all named after Italian volcanoes They occur at convergent plate margins where subduction is taking place Strato-volcanoesStrato-volcanoes form the largest percentage, 60%, of the Earth's individual volcanoes. These volcanoes ideally have a conical shape but in many cases the outline is irregular due to slope failure, caldera collapse or explosive damage. New cones build up near the site of an older cone and create complex volcanoes. Each eruption can last from hours to years and through the viscous lava rarely flows much further than the volcano base, but the ash will cover vast areas.When the lava is silicic it may be so viscous that it hardly flows at all, piling up in the vent to form volcanic domes. Lava domes are bulbous and very steep sided and form slowly with new lava pushing up from below. Silicic magmas have high gas content and this can result in nuee ardente pyroclastic flows.Each volcano has an eruptive cycle: a period of no activity where the volcano may appear dormant. During this time the magma chamber is filling with magma, differentiation may be taking place and pressure is building up. These viscous magmas allow gas pressures to build up to high levels. Once the pressure exceeds the weight of the overlying rock, including old lava plugging the vent, there will be an explosion. This blasts away part of the top of the volcano where the vent was blocked and allows the gas-rich, pyroclastic material to escape. This creates layers of ash (tuff), blocks (agglomerates) and nuee ardente pyroclastic flows (ignimbrites). the final stage is when lava reaches the surface and forms a layer on top of the pyroclasts .As gas pressure decreases, the lava supply reduces until it cools in the vent, plugging it ready for the next cycle. CalderasA caldera is a large circular volcanic depression caused by violent explosions followed by the collapse of the top of the volcanic cone. a series of violent explosive Plinian eruptions of pyroclastic flows, ash and pumice take place, removing large volumes of magma the magma chamber starts to empty as the magma is erupted and the top of the volcano begins to collapse down into the weakened area below. This compresses the remaining magma so that eruptions are even more violent finally the entire cone of the volcano collapses and this may cause tsunamis if the volcano is coastal or is an island, such as Krakatoa. Where is the magma chamber?The size of a magma chamber can affect the eruption. If the chamber where the magma is stored is small then the eruption is likely to be short. Most magma chambers are probably areas where the crust or mantle is partially melted rather than than a lake of magma below the surface. Geysers and hot springsGeysers are hot springs from which a column of hot water and steam is explosively discharged at intervals. The largest geysers erupt to a height of 200m. This happens if the vent is constricted allowing the pressure to build up. Hot springs are common in volcanic areas as groundwater is heated by the magma at depth. Convection causes the gas-rich, superheaed water to rise. As it rises, the pressure decreases and flash boiling results and the water may explode upwards. The water drains back into the ground and is reheated for the cycle to begin again.

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