Iceland rift zone

A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear 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.^[2] 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.^[3] Perhaps the best example of this is Mauna Loa, which in Hawaiian means "long mountain",^[4] and which features two very well defined rift zones extending tens of kilometers outward from the central vent.

• http://en.wikipedia.org/wiki/Rift zone
• https://en.wikipedia.org/wiki/Mid-ocean_ridge#/media/File:%C3%9Eingvellir_National_Park,_Bl%C3%A1sk%C3%B3gabygg%C3%B0_(6969755432).jpg