The diverging volcanic rift system

• We studied the geometry of rift zones on volcanoes describing a new dyke pattern.
• A rectilinear rift on volcano summits passes at the two terminations to a fan shape.
• Geological and structural data and analogue models provide an explanation.
• This new dyke pattern results from tectonic stresses and local gravity stresses.
• This model contributes also towards a better assessment of volcanic hazards.

Eruptions and volcano internal growth are mostly fed by dykes. The comprehension of the control factors on dyke paths is fundamental for the assessment of areas prone to vent formation and to the general understanding of how volcanoes work. We analyse an understudied magma path system; field data of nine volcanoes show they have a rectilinear rift zone in the central part passing into fan-arranged dykes at the two opposite volcano flanks. The geological, geomorphological and structural characteristics of these volcanoes and their substrate suggest that the formation of these “diverging rifts” is not specifically linked to substrate lithology and mechanical behaviour. The studied volcanoes have elongation < 0.88 and V > 10 km3 (mostly > 300 km3). Eight volcanoes have the central rift that is normal to the regional tectonic least principal stress (σ3reg) and in one case it is sub-perpendicular. Field data have been combined with scaled analogue modelling, suggesting that if the σ3reg is oblique to the volcano elongation axis, dyke geometry in the edifice axial zone is controlled by elongation and thus by local gravity σ3, but dyke strike becomes perpendicular to σ3reg when dykes intrude the more external areas of the volcano. If a dyke is injected under the volcano flanks with slope inclination > 50°, it attains a geometry parallel to the slope. At lower slope inclinations at the edifice terminations, magma paths diverge outwards and crosscut slopes at high angle. Our data are in agreement with the assumption that regional tectonic stresses can affect large volcanoes up to the summit area guiding the development of a rectilinear thoroughgoing rift, both in extensional and transtensional regimes. The diverging pattern takes place due to reorientation of the local stress field guided by topography only when dyke inception localizes laterally respect to the edifice axis.