Where does magma come from?

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Magma at divergent plate margins

Divergent plate margins are rarely seen at the surface, as mid-ocean ridges (MOR) are on the deep ocean floor. Iceland is one of the few places where rifting and magma can be seen. Oceanic crust is thin below an MOR with mantle rocks just 5km below the surface. Partial melting of the ultramafic peridotite of the upper mantle produces mafic magma.

partial melting of peridotite in asthenosphere

mafic

frequent eruptions submarine, fissure or shield.

mainly non-viscour basalt lava, few pyroclasts

effusive but non-violent eruptions

dolerite dykes and sills below volcanoes

Magma at Hotspots

A similar process is operating at hotspots where mantle plumes rise from deep in the mantle and partial melting of mafic material results. Convection in the mantle slowly transports heat from the core to the Earth's surface. Mantle plumes carry the heat upwards in narrow rising columns of hot material which spreads out when the plume head meets the base of the rigid lithosphere. At this shallow depth lower pressures allow partial melting of mantle peridotite to form enormus volumes of basaltic magma. This basalt may erupt onto the surface over very short timescales to form flood basalts.

If the plume provides a continuous supply of magma in a fixed location it forms a hotspot. As the lithosphere moves over this fixed hot spot due to plate tectonics, the eruption of magma onto the surface forms a chain of volcanoes parallel to the movement of the plate. The Hawaiian island chain in the Pacific Ocean is a good example.

partial melting of peridotite in asthenosphere

mafic

frequent eruptions at shield volcanoes

mainly non-viscous basalt lva

effusive non-violent erptions

dolerite dykes and sills below volcanoes

Magma at Convergent Plate Margins

When two plates collide, magma will be generated. As a plate is forced down, the geothermal gradient increases and partial melting results. The reasons are different, depending on the type of plate margin.

Magma generated at an oceanic-continental plate margins

Where plates converge and the oceanic plate subducts, partial melting of the oceanic plate will produce magma. If the plate subducts and the magma rises quickly at shallow depths then it will be basalt. Often the magma rises through the silicic continental crust and this may partially melt. The rising mafic magma may be at temperatures in excess of 1000 degrees and the silic material melts at just 800 degrees. The result is mixing of the mafic and silicic melts to give intermediate magma. The two components have different viscosities so mixing is difficult. Some of the magma will reach the surface to form intermediate volcanoes. Most will be intruded to form granite batholiths.

partial melting of subducting oceanic plate

partial meting of continental crust and mixing of magmas

intermediate and silicic

irregular and infrequent eruptions

Strato-volcano and lava domes

little lava, mainly andesite with some rhyolite, large amounts of pyroclastics

very violent eruptions of ash, agglomerate and ignimbrites

dolerite dykes and sills below volcanoes

batholiths

Magma generated at continental-continental plate margin

Where two continental plates converge, neither continent will subduct. However, the high pressures and weight of sediments, which have been deformed to form fold mountains, combine to force the base of the crust down. As a result the MOHO is at its deepest below the highest fold mountains. Partial melting at the base of the continental crust will produce silicic magma. As a continental crust is silic and starts to melt at 800 degrees, this is the temperature below the mountains due to the geothermal gradient. As the magma rises it intrudes to form granite batholiths. There are no volcanoes present as the silicic magma is too viscous to rise to the surface.

partial melting of deep continental crust

silicic

no volcanic activity

batholiths

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