Pseudopillow fracture systems in lavas: Insights into cooling mechanisms and environments from lava flow fractures

Detailed field observations of structures within the flow front of a Holocene trachyandesite lava from Snæfellsnes, Iceland, are presented. The lava provides exceptional three-dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water.The flow front interior is characterised by structures consisting of a large (metre-scale) curviplanar master fracture with many smaller (centimetre-scale) subsidiary fractures perpendicular to the master fracture. Such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures. We propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together. All documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant (i.e. liquid water, ice or snow).We use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures. Master fracture surface textures used to identify fracture mechanisms include chisel marks (striae), cavitation dimples, river lines and rough/smooth fracture surface textures. These indicate both brittle and ductile fracture happening on different types of master fracture. Chisel marks on subsidiary fractures indicate comparative cooling rates, cooling directions and isotherm orientations at the time of fracture. We propose a model for pseudopillow fracture system formation taking into account all the various fracture types, textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants.

► We study a trachyandesite lava that has interacted with seawater.
► Complex fractures in the lava show evidence of brittle and ductile modes of fracture.
► Fracture styles are related to the ingress of seawater while the lava was still hot.
► Fracture propagation directions enable the type of coolant to be deduced.