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Plate tectonics is the driving force for most volcanic activities. At plate boundaries, the crust is either created or destroyed. Divergent plate boundaries, create new crust material from the rising of magma. This new crust slowly pushes the old crust until it gets destroyed by subduction. The sinking of this crust also causes volcanic activities above it. On earth, they are five volcanic environments in relation to plate tectonics and this is ocean-ocean island arc, oceanic hot spot, mid-ocean ridge, ocean-continental arc, and intercontinental rift as shown in Figure: 1.
Ocean-ocean island arcs form at subduction zones between two oceanic plates. The oceanic crust contains water, carbon dioxide, and other fluids. Since both oceanic crusts are dense, the crust that is subducted is the one that is older and colder making it denser. A trench forms at this subduction zone. The submerged oceanic plate is exposed to great temperatures that drive off most of its volatile components (water and gases). This volatile component stimulates partial melting of the mantle plate above it. This is because the volatile component reduces the melting point of the mantle. The melting of this plate produces magma, which is basaltic in nature. This type of magma tends to have more iron and magnesium than silicon. This magma has low viscosity and its eruption is not explosive (Lumen Learning, 2021). On the surface, this rising magma produces a curved line of islands, known as an island arc parallel to the trench. This is the case of the Mariana Islands, which run parallel to the Mariana trench in the western Pacific Ocean (Encyclopedia Britannica, 2021).
Oceanic hot spots occur in areas that are distant from plate boundaries. Volcanoes form above a column of super-heated magma called a mantle plume. As the plume rises, pressure drops, while the temperature remains high. This causes decompression melting of the hot mantle material which is more mafic and results in the generation of large volumes of low-viscosity magma (Volcano Discovery, 2021). The heat from the mantle plume causes melting and thinning of the crust, which leads to volcanic activity at the surface. A plume that feeds hot spot volcanoes is stationary relative to the mantle, whilst the plate above it usually moves. This result in a chain of volcanoes being created on the overlying plate. Moving away from the hot spot, towards the direction of plate movement, the age of the islands increase. The Hawaiian Islands are an example of a chain of volcanoes, which have formed due to the presence of a hotspot or mantle plume in the middle of the Pacific Plate as shown in Figure: 2 (British Geological Survey, 2021).
The spreading of the oceanic plate results in magma generation and the formation of a mid-oceanic ridge. This spreading causes the mantle underneath to decompress, thus partially melting the upper mantle and producing basaltic magma which is low in silica, and less viscous. As the magma rises, it is quickly quenched at the bottom of the ocean to create bulbous shapes called pillow basalts which are not explosive. The mid-oceanic ridge produces low-lying elongated volcanoes from the combined effect of fissure eruptions from the low viscous magma (Rubin, 2016). This erupting basalt magma can generate vast submarine lava fields. This magma is constantly attached to the edges of the spreading plates creating a new oceanic crust. Thus, oceanic crusts are younger near the ridge but get older away from the ridge (Gamp, 2021).
Ocean continental arc forms at subduction zones between oceanic and continental plates. The denser oceanic crust sinks into the mantle and is exposed to great pressure and temperature. The oceanic crust contains water, carbon dioxide, and other fluids, which are released into the overlaying mantle. This addition of fluids lowers the melting point of the mantle. As a result, the mantle rocks overlying the subducting slab melt to produce magma. This magma is less dense than the mantle and it rises above the subduction zones. The best examples are of subduction zones around the Pacific Ocean, often referred to as the.
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