Abstract

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.

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