In What Way Does Volcanic Activity Vary in Relation to the Type of Plate Margin Along Which It Occurs?

There is a considerable link between plate boundaries and volcanic activity. According to plate tectonic theory huge slabs of rigid lithosphere, 100-300 km thick, are in constant movement, driven by convection currents originating deep within the Earth. Volcanic activity occurs wherever plates are being pulled apart or pushed together, both circumstances cause magma to be generated.

73% of magma which reaches the earth’s surface occurs along constructive boundaries. Constructive plate margins occur when two plates diverge from each other and new crust is formed. This process, known as sea-floor spreading occurs in the mid-Atlantic where the North and South American Plates are being pulled apart from the Eurasian and African Plates by convection currents. As the plates diverge, the magma rises from the mantle to fill in the gaps, this creates new oceanic crust. The magma initially forms submarine volcanoes which may in time grow above sea-level, e.g Surtsey and Heimaey, Iceland. Another example of a constructive boundary is the Great African Rift Valley, the rift valley extends for 4000km from Mozambique to the Red Sea. Where the land has been pulled apart, sections of land have dropped low enough to be invaded by the sea. On the plateaus, there is volcanic activity in the form of Ruwenzori and Kilimanjaro. The East African Rift Valley system has 14 active volcanoes, with a wide range of magma types depending upon whether it is the asthenosphere partially melting or the overlying continental crust. Fissure eruptions commonly form along constructive plate margin, the elongated cracks in the crust allows lava to spill out over a large area. The eruptions are gentle and persistent, often with a low volcanic explosivity index.

Approximately 80% of the world's active volcanoes occur along destructive boundaries. As the plates collide, part of one plate may be driven beneath another, a process known as subduction. As the denser plate descends into the asthenopshere,...