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Journal of the Geological Society; 1998; v. 155; issue.4; p. 599-607;
DOI: 10.1144/gsjgs.155.4.0599
© 1998 Geological Society of London

Article

Emplacement of Hebridean Tertiary flood basalts: evidence from an inflated pahoehoe lava flow on Mull, Scotland

RAY W. KENT1, BONITA A. THOMSON2,3, RAYMOND R. SKELHORN4, ANDREW C. KERR1, MIKE J. NORRY1 & J. NICK WALSH5

1 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK (email: kwr{at}le.ac.uk)
2 Department of Geology, King’s College, University of London, The Strand, London WC2R 2LS, UK
4 Pine Lodge, 43A Abbotswood, Guildford, Surrey GU1 1UY, UK
5 Department of Geology, Royal Holloway College, University of London, Egham, Surrey TW20 0EX, UK

The lower 200 m of the Tertiary lava pile on Mull, western Scotland, consists mainly of small volume (0.01–1 km3), high-magnesian basaltic lava flows. Eruption of these flows probably occurred from fissures or point-source vents, producing pahoehoe-textured sheets averaging about 5 m in thickness. Certain lavas cropping out in northwest Mull greatly exceed this average thickness and may represent inflated pahoehoe flows akin to those described at Kilauea, Hawai’i. Structural features of a 16–30 m thick lava at Port Haunn, northwest Mull, that conform to this proposal include sub-horizontal sheets of amygdales up to 14 cm thick separated by amygdale-poor zones, pipe amygdales at the base of the flow and a low-relief flow top. Alternating olivine-rich and olivine-poor bands in the Port Haunn lava are suggestive of lava pulses during a continuous eruption, also consistent with inflation. Elsewhere in western Mull, there is evidence for filled lava tubes and pahoehoe toes, both of which are characteristic of inflated sheet flows formed on gentle ground slopes (<4° and perhaps <0.5°). Fluid dynamical considerations suggest that each sheet flow in the lower part of the Mull lava sequence probably was active for a short period, permitting the build up of small lava fields over several months or years. This is commensurate with the small volume and limited areal extent of the Tertiary Mull lavas when compared to basalts in igneous provinces such as the Columbia River and Deccan.

Key Words: Mull Island • basalts • pahoehoe • emplacement




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R. T. Single and D. A. Jerram
The 3D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the Palaeogene Skye Lava Field
Journal of the Geological Society, 2004; 161: 911 - 926.
[Abstract] [Full Text] [PDF]