Effusive volcanoes are more commonly formed at constructive plate margins where plates diverge. Here, lava tends to be basaltic meaning it is low in silica content. The low proportion of silica allows gases to expand and thus
Volcanoes are found mainly in three locations, at constructive and destructive plate margins and at hotspots. The most dangerous volcanoes occur at destructive, convergent plate margins. Here one plate subducts beneath the other, and as it descends, friction, increasing pressure, and heat from the asthenosphere and mantle melt the plate to form an acidic magma chamber. The magma at these boundaries is andesitic and rhyolitic, meaning that they have a high viscosity. Because of this the lava is resistant to flow and often forms blockages in vents.
Adding to the disaster's massive scale, the huge amount of dust blasted into the upper atmosphere by the Tambora eruption contributed to a bizarre and highly destructive weather event the following year. And 1816 became known as The Year Without a Summer. The disaster on the remote island of Sumbawa in the Indian Ocean has been overshadowed by the eruption of the volcano at Krakatoa decades later, partly because the news of Krakatoa traveled quickly via telegraph. Accounts of the Tambora eruption were much more rare, however some vivid ones do exist. An administrator of the East India Company, Sir Thomas Stamford Bingley Raffles, who was serving as governor of Java at the time, published a vivid account of the disaster based on written reports he had collected from English traders and military personnel.
The nature of an event is initially determined by how the cause was stimulated. Volcanic eruptions occur in many different forms determined by the plate margin they are on. The eruptions on Montserrat 1995 show a strong example of the nature of a volcano at a destructive plate margin. The Soufriere Hills volcano had lay dormant for a long period of time. When the eruption did occur it was seen as explosive as it produced large volumes of acidic lava, ash, pyroclastic flows and steam.
These types of lava are very viscous due to its high silica content. This is because the lava rises from the subduction zone through continental lithosphere which has a low density and is filled with air spaces containing gases which become incorporated into the lava. This very viscous lava often blocks off vents of volcanoes and when the pressure building up in the vent is eventually released, the top of the volcano can be blown off leaving a huge crater, such as in the 2002 eruption of Mount Etna in Sicily. When the two plates involved are oceanic, explosions tend to be less violent than this as the melted lithosphere which forms the lava is denser and so contains fewer gases. At constructive boundaries where plates are moving apart from one another, basaltic lava is erupted between the gaps.
What discovery on the ocean floor resurrected Wegener's theory? The discovery that earthquakes in the oceanic floor coincided with submarine mid-oceanic ridges that ran for thousands of miles along the ocean floor. What proposal did Arthur Holmes make in 1928? He proposed that the seafloor may be spreading apart. What did Harry Hess propose in 1963?
Causes When a dense oceanic plate collides with a less dense continental plate the denser oceanic plate subducts under the less dense continental plate. Japan is situated on the Eurasian plate, which is a continental plate, the pacific plate subducts under the Eurasian plate, forming a deep sea trench. The Indian Ocean Tsunami was caused as the Indian continental Plate was subducted by the Burma plate which is considered part of the Eurasian oceanic plate. When the Oceanic plate subducts under the Continental plate friction and pressure build up and the continental plate can snag on the oceanic plate until the tension is finally released sometimes after millions of years. The break in tension causes a jerk and the sea floor to spring upwards.
Haiti is on the boundary of the Caribbean and North American plates and is an example of a conservative plate boundary. Volcanic eruptions are also one the greatest threats among the natural hazards. The active land, volcanoes of the world occupy are only 0.6% of the total land area. Most volcanoes are spread along the coast, in areas that are also earthquake zones. Mid-ocean volcanic ridges are formed where the ocean floors split and move apart.
With reference to two volcanic events that you have studied from contrasting areas of the world, compare the ways in which volcanoes and their impacts have been managed (10 marks) A management strategy for protecting the country against a volcanic eruption involve the 'predict', 'protect' and 'plan' processing. Arguably, it is the management strategies of a country that are the most important factor in minimising the negative impact of a volcanic eruption. These impacts can range from crop failure to power loss all the way to death. Mount St Helens in the USA (HIC) and the Soufiére Hills volcano on the Caribbean island of Montserrat (LIC) are two volcanoes which have both had massive, and fairly recent, eruptions which have caused wide-spread problems for the population however the actual impacts of the eruptions varied intensely and this is possibly due to the variation in management strategies of the two countries. It is possible to roughly predict when a volcanic eruption will happen as, usually, smaller-scale earthquakes will occur as will changes in the shape of the volcano itself.
To what extent can the theory of plate tectonics explain global distribution of seismic and volcanic activity? The global seismic and volcanic activity is concentrated along the margins of the earth’s plates, which are broken pieces of the crust and are moved by convection currents that are caused by heat rising and falling inside the mantle generated by radioactive decay in the core. This movement of the plates and the Earth’s inner activity is called plate tectonics and can cause seismic and volcanic activity. However, earthquakes and volcanoes also occur within the plates rather than their edges, an example of which is the Hawaiian volcanoes that occur above a stationary hot spot beneath the Pacific plate. In the late 1960s the theory of plate tectonics was developed by Alfred Wegener and provides an explanation for the Earths tectonic behaviour, particularly the global distribution of mountain building, earthquake activity, and volcanism in a series of linear belts.