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Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface.

The Chapter Magma Chambers explains the behavior of these magma chambers, the geological and geophysical evidence for their size and dimensions, and the processes that occur within them. The Chapter Rates and Timing of Magma Ascent describes how petrologic studies and geochemical research on short-lived isotopes in magma are providing information on the rates of magma ascent.

The melt-producing region is believed to be on the order of 100 km wide at its base and narrows at shallower depths as mantle flow lines become horizontal (and thus cease melting) producing a triangular melting region. Ultimately, melt accumulates beneath the ridge in shallow crustal magma reservoirs that are elongated parallel to the ridge and generally located beneath the axis of spreading. The processes by which melts focus from a 100 km wide region to narrow melt lenses less than a few kilometers in width are debated. Among the proposed mechanisms are coalescence of ascending melt channels driven by instabilities and positive feedbacks of a reactive melt flowing through a porous media and lateral flow in high-porosity decompaction layers located along the sloped base of the oceanic lithosphere.

Furthermore, Kuno discussed the origin of such lateral variations based on his earlier work on the Hawaiian basalts8); tholeiitic basalt magmas are generated at relatively shallow levels in the mantle and alkali basalt magmas at deeper levels by partial melting of mantle peridotite. Kuno1) also took into account the seismic studies in the Japanese island arc. He realized that the boundary between the zones of tholeiitic basalt and alkali basalt nearly coincides with the 200 km depth contour of the deep seismic zone found by Wadati and Imai.9) Kuno1) proposed, based on this evidence, that tholeiitic basalt magmas are formed at depths shallower than about 200 km and alkali basalt magmas at depths greater than 200 km.

In this work, high-precision hypocentre locations from onshore and offshore seismic data, reconciled with modelling of surface deformation observations, reveal that a crustal fault can serve as both a pathway and a barrier to magma ascent.

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When runny basaltic magma erupts as lava, it pours out of the ground along long surface cracks or through volcanic vents and may be sprayed into the air as spectacular lava fountains.

They frequently form from explosive eruptions that crack the area around a volcano with the magma filling the cracks forming a dike. A concordant igneous rock body runs parallel to the pre-existing bedrock. Laccoliths and sills are examples of concordant igneous rock bodies.

Prof. Stephen A. Nelson · Volcanoes, Magma, and Volcanic Eruptions