Volcanic Processes
Magma from the mantle or lower crust rises through its crust to the surface. If magma reaches the surface, its behavior depends on the viscosity of the molten constituent rock. Viscous (thick) magma produces volcanoes characterised by explosive eruptions, while non-viscous (runny) magma produce volcanoes characterised by an effusive eruptions pouring large amounts of lava onto the surface.
In some cases, rising magma can cool and solidify without reaching the surface. Instead, the cooled and solidified igneous mass crystallises within the crust to form an igneous intrusion.
Read more about this topic: Volcanism
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