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   The Great Plume Debate

AGU Chapman Conference: The Great Plume Debate

Ben More (966 m), the highest mountain on Mull, is composed of lava flows of the Plateau Group.

Mull Excursion

Friday 2nd – Tuesday 6th September, 2005

Leader: Prof. Godfrey Fitton

School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK,
Tel: +44 (0) 131 650 8529

Figure 1. Location of Mull at the SW end of the Great Glen.
The island of Mull, located on the Great Glen strike-slip fault (Figure 1), is the site of one of the Scottish Tertiary central volcanoes which form part of the North Atlantic Igneous Province. Volcanic activity on Mull started at about 61 Ma with the eruption of fissure-fed lava flows that built a plateau-lava sequence over 1 km thick (Figure 2).
Magmatism then focused into a central volcanic system, the remains of which are represented by the intrusive central complexes. Three such intrusive centres are recognised on Mull (Figure 3). The locus of activity migrated northwards with time and the magma composition changed from dominantly basic in Centre 1 to dominantly silicic in Centre 3. The plutonic complexes were emplaced at high levels largely through caldera collapse and ring-dyke formation. The youngest of these ring dykes (Loch Ba) was emplaced at about 58 Ma.

Figure 2. The Mull plateau-lava sequence exposed in The Wilderness, seen here looking northwards from the W end of the Ross of Mull peninsula.

Figure 3. Outline geological map of Mull (from Emeleus, C.H. & Gyopari, M.C. (1992). British Tertiary Volcanic Province. Geological Conservation Review Series. Chapman & Hall, 259 pp.).

Basement rocks are well exposed on Mull and give a clear picture of earlier geological events. Proterozoic Moine schists are the oldest rocks exposed on the island though Archaean Lewisian Gneiss is exposed on the island of Iona (Figure 1). In SW Scotland the Great Glen fault separates Moine schists from the younger Dalradian metasediments, and small outcrops of Dalradian rocks are found in the SE part of Mull. At the end of the Caledonian orogeny (~410 Ma) a small granite pluton was emplaced into the Moine rocks on what is now the tip of the Ross of Mull peninsula (Figure 1). Erosion of the Caledonian mountain belt led ultimately to the deposition of Triassic conglomerates and marls in an arid environment in intermontane basins. These are well exposed on the coast of Mull at Gribun (Figure 3). The formation of these basins marks the beginning of an extensional episode that led to the deposition of the marine Jurassic and Cretaceous sediments exposed at Gribun and Carsaig Bay (Figure 3). Regional uplift preceding the onset of Tertiary magmatism brought marine sedimentation to an end. The earliest Tertiary lava flows were erupted on a land surface composed of the eroded remains of Cretaceous chalk deposits.

Summary Itinerary

Figure 4. The route from Fort William to Mull.

Figure 5. The locations to be visited during the field trip.

Friday, 2nd: Travel from Fort William to Craignure (where we will be staying) in Mull (Figure 4). We shall drive SW along the Great Glen and cross it at the Corran Ferry; then over Morvern and across to Mull on the Lochaline Ferry.

Afternoon: Grass Point and Craignure. Grass Point (locality 1 on Figure 5) provides excellent exposures of a steeply dipping succession of Mesozoic and Tertiary sedimentary rocks and Tertiary lava flows. The folding responsible for the dip was caused by the intrusion of the Centre 1 magma. The timing can be demonstrated at Craignure (2) where a gently dipping and undeformed silicic cone sheet belonging to Centre 2 can be seen cutting deformed Mesozoic sedimentary rocks.

Saturday, 3rd: A trip to the end of the Ross of Mull Peninsula in the SW part of the island to examine some of the basement rocks at Ardalanish Bay (3). Here Proterozoic Moine schists are cut by a Caledonian (410 Ma) granite (Figure 6). To Ardtun (4) in the afternoon to see columnar jointed basalt flows with fluvial interbeds low in the plateau sequence. The interbeds consist of shales with fossil plants, and conglomerates containing Cretaceous flint pebbles. The presence of flint pebbles proves uplift and erosion of marine Cretaceous chalk shortly before eruption of the first plateau lava flows.

Figure 6. Rafts of Moine metasediment in margin of Ross of Mull granite

Sunday, 4th: Morning: Mesozoic sedimentary rocks at Gribun (5). The succession at Gribun consists of Triassic conglomerate and marls deposited on Moine metasediments (Figure 7). Afternoon: exposures at Carsaig (6) extend the succession through the Jurassic and into remnants of Cretaceous marine sandstone and chalk overlain by Tertiary plateau lava flows. Return to Craignure across the island and stop to see the Centre 1 gabbros (7). These are layered in places and show remarkable “sedimentary” structures.

Figure 7. Triassic conglomerate on Moine metasediments at Gribun

Monday, 5th: The Centre 3 silicic ring dyke at Loch Ba (8), which represents the last major igneous event in the evolution of the Mull central volcano (Figure 8). Centre 3 is dominantly silicic (granophyre and rhyolite). This part of the excursion will be fairly strenuous and requires good walking boots and a reasonable degree of fitness.

Tuesday, 6th: Depart Mull. The minibus will return to Edinburgh via Glasgow and we will be able to stop at the airports.

Accommodation: Isle of Mull Hotel, Craignure. The inclusive cost of the excursion will be announced later.

Figure 8. The Loch Ba ring dyke (between dashed lines) cutting centre 3 granophyre (in foreground).

Literature: Below are listed maps and books relating to Mull and its geology. Topographic maps can be obtained through most booksellers or from Edward Stanford Ltd., 12-14 Long Acre, London WC2E 9LP. Publications (maps, memoirs etc) of the British Geological Survey can be purchased from BGS Keyworth, Nottingham NG12 5GG, BGS, Murchison House, West Mains Road, Edinburgh EH9 3JW, the Natural History Museum, Earth Sciences Galleries, South Kensington, London or through approved stockists. (NB. BGS publications required for educational purposes and ordered through an educational establishment may attract a considerable discount.)


Ordnance Survey topographic maps:
1:50,000 Sheets 48 and 49 cover Mull.

Geological maps:
1:50,000 British Geological Survey Scotland Sheet 44 (Solid) covers most of Mull.

Geological publications:
Bell, B. R. and Williamson, I. T. 2003. Tertiary igneous activity. In Trewin, N H (editor) The Geology of Scotland (4th Edition). The Geological Society: London

Emeleus, C. H. and Bell, B. R. (publication due 2005). British Regional Geology: the Palaeogene volcanic districts of Scotland (4th Edition) London: HMSO for the British Geological Survey.

JGF 29th March, 2005

Further Information:

For further information, please email the fieldtrip leader:

or the conference conveners:

Ian Campbell
Gillian R. Foulger
James H. Natland
W. Jason Morgan

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