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Entablature: fracture types and mechanisms

Forbes, A. E. S.; Blake, S. and Tuffen, H. (2014). Entablature: fracture types and mechanisms. Bulletin of Volcanology, 76(5), article no. 820.

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Entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows. It is thought to form when water from rivers dammed by the lava inundates the lava flow surface, and during lava-meltwater interaction in subglacial settings. A number of different fracture types are described in entablature outcrops from the Búrfell lava and older lava flows in Þjórsárdalur, southwest Iceland. These are: striae-bearing, column-bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types—master fractures with dimpled surface textures and subsidiary fractures with curved striae. The interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature. Cube-jointing is a more densely fractured version of entablature, which likely forms when more coolant enters the hot lava. The entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling. Master fracture surfaces show a thin band with an evolved composition at the fracture surface; mineral textures in this band also show evidence of quenching of this material. This is interpreted as gas-driven filter pressing of late-stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot, partially crystallised lava. This melt is then quenched by an influx of water and/or steam when the master fracture fully opens. Our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation.

Item Type: Journal Item
Copyright Holders: 2014 Springer-Verlag
ISSN: 1432-0819
Project Funding Details:
Funded Project NameProject IDFunding Body
Not SetNot SetNERC (Natural Environment Research Council)
Not SetNot SetThe Open University (OU)
Extra Information: 13 pp.
Keywords: entablature; columnar jointing; pseudopillow fracture systems; ductile fracture; thermal fracturing; quenching; basaltic lavas
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 41318
Depositing User: Stephen Blake
Date Deposited: 14 Nov 2014 09:10
Last Modified: 07 Dec 2018 10:26
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