12:00 〜 12:15
[SVC30-12] Fracture generation during lava–ice interaction at andesitic stratovolcanoes
キーワード：glaciovolcanism, lava flow, fracture, andesite
Fractures produced in lavas that flowed alongside glaciers provide potentially key data related to the heat transfer properties of lava–ice interactions, which contribute to our understanding of meltwater lahar hazards and paleo-glacier reconstructions. Using field observations, microanalysis, and modelling, we have defined the formation mechanism of a novel type of cooling fracture in ice-bounded andesite lava flows at Ruapehu volcano in Aotearoa New Zealand. For this study, we focused on tensile fractures within the lower 3 m of a 15 m-thick outcrop of ~23,000 year-old andesitic lava that was emplaced against a glacier on the southwest flank of the volcano.
The fractures are defined by opposing planes that typically intersect at angles <20° along horizontal axial valleys, which are located inward of the lava margin at distances of 5–30 cm. The key features on individual fracture planes are: (1) “chisel marks” that are aligned parallel to the axial valley and are regularly spaced on each plane with distances that range from 0.5 to 5 cm; (2) “wall offsets” that strike perpendicular to chisel marks and have mm- to cm-scale relief; and (3) “bridges” that are 1–4 cm-thick slabs of stretched and curved lava that connect opposing fracture planes over lengths <20 cm.
We interpret that chisel marks on each plane reflect the cyclic process of fracture propagation into viscous lava at spacings that depend on the cooling rate at the lava surface and the effective viscosity of the lava. Wall offsets represent the breakage of bridges during opening of fractures along the lava (i.e., margin-parallel directions); therefore, the thicknesses and curvatures of lava bridges provide additional information on lava strain rates. We provide preliminary constraints for the cooling rate and effective viscosity of this andesitic lava during its emplacement, and use textural and geochemical data from electron probe microanalysis to explore how melt composition and crystallinity affected the full rheology.
The fractures are defined by opposing planes that typically intersect at angles <20° along horizontal axial valleys, which are located inward of the lava margin at distances of 5–30 cm. The key features on individual fracture planes are: (1) “chisel marks” that are aligned parallel to the axial valley and are regularly spaced on each plane with distances that range from 0.5 to 5 cm; (2) “wall offsets” that strike perpendicular to chisel marks and have mm- to cm-scale relief; and (3) “bridges” that are 1–4 cm-thick slabs of stretched and curved lava that connect opposing fracture planes over lengths <20 cm.
We interpret that chisel marks on each plane reflect the cyclic process of fracture propagation into viscous lava at spacings that depend on the cooling rate at the lava surface and the effective viscosity of the lava. Wall offsets represent the breakage of bridges during opening of fractures along the lava (i.e., margin-parallel directions); therefore, the thicknesses and curvatures of lava bridges provide additional information on lava strain rates. We provide preliminary constraints for the cooling rate and effective viscosity of this andesitic lava during its emplacement, and use textural and geochemical data from electron probe microanalysis to explore how melt composition and crystallinity affected the full rheology.