In Which Way Does Volcanic Activity Vary in Relation to the Type of Plate Margin Along Which They Occur

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In which ways does volcanic activity vary in relation to the type of plate margin along which it occurs? (10 Marks) Volcanic activity can occur at constructive or destructive plate margins, but it can also occur at hotspots in which no plate margin is involved. At destructive margins two plates which are moving together can be either both oceanic plates or one continental and the other oceanic. In the case of one continental plate and one oceanic plate, volcanic eruptions are very violent and emit andesitic or rhyolitic lava. 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. This type of lava is not very viscous due to its low silica content. This is because no subduction of crust is involved so the lava is not made of melted lithosphere but has risen from the mantle itself. The low viscosity of this lava causes it to flow very far before cooling and solidifying. This forms shield volcanoes with very gentle slopes and a much wider base than more conical shaped volcanoes involved with convergent plate margins. The low viscosity of the basaltic lava also causes very gentle eruptions, so shield volcanoes have less violent

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