Rift zones

A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a linear series of cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed to be primarily caused by internal and gravitational stresses generated by magma emplacement within and across various regions of the volcano, rift zones allow the intrusion of magmatic dykes into the slopes of the volcano itself. The addition of these magmatic materials usually contributes to the further rifting of the slope, in addition to generating fissure eruptions from those dykes that reach the surface. It is the grouping of these fissures, and the dykes that feed them, that serves to delineate where and whether a rift zone is to be defined. The accumulated lava of repeated eruptions from rift zones along with the endogenous growth created by magma intrusions causes these volcanoes to have an elongated shape. Perhaps the best example of this is Mauna Loa, which in Hawaiian means "long mountain", and which features two very well defined rift zones extending tens of kilometers outward from the central vent.

Rift zones are characterized by the close grouping of intrusive dykes and extrusive fissures extending outward along a relatively narrow band from the area of a central vent. The internal extensional forces and isostatic loading generated by intruding magma volumes (either associated with the magma chamber or subsequent dyke and sill formation extending outward from that chamber), in conjunction with accumulation of erupted materials, contribute to the mass and slope of the forming edifice. It is the weight of the edifice exceeding its material strength, with the additional stresses of the magma inflating the internal regions of the edifice, that can generate the initial cracking around a developing volcanic summit. Additionally, tectonic activity such as normal faulting is also commonly associated with formation of rifts along volcanic flanks. Following the path of least resistance, subsequent magmatic dykes form along and within these initial cracks, causing additional stresses to be imparted to the local materials of the edifice, which in turn generate new rifts for the magma to flow towards. In this way, established rift zones can potentially be self-sustaining geologic features along the flanks of the given volcanic vent. The orientation of this rifting is largely dependent on the gravitational and tectonic stresses at play. Basaltic shield volcanoes typically feature two main rift zones, situated with angles of 120° between in ideal situations. On shield volcanoes forming from level seafloor without neighboring vents, flank rifting occurs more evenly distributed around the vent. However, where the flanks of a volcano may be supported on one side by the presence of a pre-existing feature, or burdened with various planes of weakness, rift zone formation promulgates according to down-slope pull of gravity.

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