Entablature: fracture types and mechanisms
- 655 Downloads
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.
KeywordsEntablature Columnar jointing Pseudopillow fracture systems Ductile fracture Thermal fracturing Quenching Basaltic lavas
We would like to thank Susanne Schwenzer and Andy Tindle for help with the electron microprobe, John Watson for help with XRF analyses, and Andy Wilson for assistance with drafting figures. AESF was supported by a NERC PhD studentship and The Open University, and HT is supported by a Royal Society University Research Fellowship. Many thanks to Atilla Aydin and an anonymous reviewer for their comments which have improved this article.
- Costa A, Blake S, Self S (2009) Segregation processes in vesiculating crystallizing magmas. J Volcanol Geotherm Res 153(3):287–300Google Scholar
- Hull D (1999) Fractography: observing, measuring, and interpreting fracture surface topography. Cambridge University Press, CambridgeGoogle Scholar
- Lescinsky D, Fink J (2000) Lava and ice interaction at stratovolcanoes: use of characteristic features to determine past glacial extents and future volcanic hazards. J Geophys Res 105(B10):711–723Google Scholar
- Peck DL (1978) Cooling and vesiculation of Alae lava lake. US Geological Survey professional paper, Hawaii, 935Google Scholar
- Sæmundsson K (1970) Interglacial lava flows in the lowlands of southern Iceland and the problem of two-tiered columnar jointing. Jökull 20:62–77Google Scholar
- Thordarson T, Höskuldsson A (2002) Iceland. Terra, HarpendenGoogle Scholar
- Vilmundardóttir E, Guðmundsson Á, Snorrason SP (1985) Jarðfræði Búrfells og nágrennis (In Icelandic, summary in English: Geology of the Búrfell area). Náttúrufræðingurinn 54:97–113Google Scholar
- Wright TL, Okamura RT (1977) Cooling and crystallization of tholeiitic basalt, 1965 Makaopuhi lava lake, Hawaii. USGS Professional Paper 1004. US Government Printing Office, p 78Google Scholar