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. The lithosphere is underlain by the asthenosphere , a hot, mobile layer of partially molten rock lying within the earth's upper mantle. The various magma types are described in Physicochemical Controls on Eruption Style .
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. Plates diverge from one another at the site of thermally buoyant mid-oceanic ridges. Oceanic crust is created at divergent plate boundaries. Oceanic crust is destroyed at convergent plate boundaries. Although volcanism is abundant at divergent and convergent plate boundaries, there is a distinct lack of significant volcanism associated with transform plate boundaries.