The Earth is a dynamic planet. Its rigid outer surface layer is broken into several tectonic plates which are in constant motion relative to one another. Tectonic plates are composed of lithosphere, the rigid outer portion of the earth. With a thickness of about 100 km, the lithosphere is composed of an upper layer of crust (~7 km thick under the oceans, and ~35 km thick under the continents) and a lower, denser layer of the earth's upper mantle. The lithosphere is underlain by the asthenosphere, a hot, mobile layer of partially molten rock lying within the earth's upper mantle. The rigid lithospheric plates are driven by convection within the mobile asthenosphere. Hot mantle rises beneath mid-oceanic ridges, and cold, denser mantle descends at oceanic trenches. Lateral motion of the lithospheric plates above these circular convection cells is analogous to rigid blocks riding above a rotating conveyor belt.

Volcanic eruptions above these lithospheric plates are driven by the ascent of magma (molten rock) from deep beneath the surface. The various magma types are described in Physicochemical Controls on Eruption Style. They vary from mafic, intermediate, to felsic as their silica (SiO₂) content increases. Mafic (basaltic) magmas are generated directly from the mantle, either within the asthenosphere or within the overlying mantle lithosphere. Many mafic-to-intermediate (basaltic-to-andesitic) magmas appear to be derived from the melting of hydrated lithospheric mantle. More differentiated, intermediate-to-felsic magmas, on the other hand, are partly derived from the melting of continental crust by hot, mafic magmas that either pond at the crust-mantle boundary, or intrude into the overlying continents where they reside in magma chambers located at various crustal levels.

Volcanism is typically widespread along plate boundaries. Although volcanism in the interior of plates is less common, these intraplate regions can also generate voluminous eruptive products. The regional volcano-tectonic processes associated with plate-boundary environments and intraplate environments are described in more detail below.

Plate Boundaries

Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. Adjacent plates are delineated by three types of boundaries defined by this relative motion:

Although volcanism is abundant at divergent and convergent plate boundaries, there is a distinct lack of significant volcanism associated with transform plate boundaries. Spreading Center Volcanism occurs at divergent plate margins, and Subduction Zone Volcanism occurs at convergent plate margins. Intraplate Volcanism describes volcanic eruptions within tectonic plates.

Eruption Types

Eruption variability is largely related to magma composition and the amount of water present. The various eruption types are typically associated with particular volcano types. Shield volcanoes, for example, generate low-viscosity basalts associated with calm, effusive eruptions. It is common to find traditional names from classic eruptions to describe other eruptions and volcano forms: Hawaiian, Strombolian, Vulcanian, and Plinian, for example. Other types include: Fissure & Hydrovolcanic.

Height of Eruption Column and Degree of Explosivity

Whereas the height of the eruption column can be measured directly in observed eruptions, it can also be estimated in ancient eruptions by measuring the geographic dispersal of the airfall tephra. The degree to which this tephra is fragmented provides a means to measure the explosiveness of the eruption.

© Dr. Vic Camp, How Volcanoes Work, http://www.geology.sdsu.edu/how_volcanoes_work/ 2004