Roadmap | Home
HOME MECHANISMS LOCALITIES GENERIC
   The Great Plume Debate

AGU Chapman Conference: The Great Plume Debate

The southern Cuillin Hills, looking NW.

Skye Excursion

Friday 2nd – Tuesday 6th September, 2005

Leader: Dr. Brian Bell

Division of Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK.
Tel: +44 (0) 141 330 6898 (b.bell@earthsci.gla.ac.uk)

The Palaeogene Skye Lava Field covers most of north and west-central Skye (Figure 1). The lavas were erupted predominantly in a terrestrial environment, with only minor occurrences of pillowed flows, hyaloclastites, and related hydromagmatic phenomena. Eruption of the lavas was mainly via fissure systems, now represented by the NW-trending Skye Dyke Swarm and the orthogonally oriented Scalpay and Glen Brittle subswarms. Locally, eruptions of silicic magmas produced accumulations of ignimbrite and other silicic tuffs. The erosional remnants of the lava field have developed a trap (step-like) topography and, in places, give rise to spectacular coastal cliffs, especially in west-central Skye between Talisker Bay and Loch Brittle (Figure 2).

Figure 1: Sketch map showing the distribution of, and inferred relationships between the Palaeogene lava fields of the Hebridean Igneous Province and representative generalized vertical sections through the Hebridean lava fields (from Bell & Williamson 2002). Click here or on map for enlarged PDF version.

The next major period of magmatic activity is represented by the Skye Central Complex. Four phases are recognised, commencing with the ultrabasic and basic rocks that constitute the Cuillin Centre (Figure 3). Layered and unlayered gabbros, troctolites and peridotites form the mountainous area referred to as the Cuillin Hills, rising, in many places, to almost 1000 m above sea level (Figure 4). A number of units have been identified within the complex and are typically ring-shaped in plan. Where layering is present, as defined mainly by modal variation, it dips inwards, towards a focal point at depth in the vicinity of Meall Dearg at the southern end of Glen Sligachan.

Figure 2: The Skye Lava Field, Preshal Beag, looking east towards the Cuillin Hills.

Figure 3: Geological map of the Cuillin and Srath na Creitheach centres (from Bell & Williamson 2002).

 

Figure 4: The southern Cuillin Hills, looking NW.

Inward-dipping, late-stage sheets of relatively fine-grained olivine tholeiite transect the dominant coarse-grained lithologies of the centre, certain of which are thin and have a common focal point at depth and are referred to as cone-sheets. In addition, a large number of dykes from the regional swarm transect the centre. Consequently, due to the effects of differential weathering and erosion, the Cuillin Hills have a very irregular, serrated, topography, unique within Scotland.

The three younger Red Hills granite centres - Srath na Creitheach, Western Red Hills and Eastern Red Hills - constitute the other main phases of subvolcanic activity within the Skye Central Complex (Figures 3 & 5). Each of these three granite centres comprises a number of compositionally distinct intrusions, often with clear evidence of their order of emplacement and their three-dimensional geometry. Because the granite centres were emplaced after the intrusion of the cone-sheets that cut the various units of the Cuillin Centre, and after the intrusion of the majority of the dykes of the regional swarm, they give rise to a contrastingly smooth, generally rounded, although still elevated, topography (Figure 6).

Figure 5: Geological map of the Western Red Hills and Eastern Red Hills centres (from Bell & Williamson 2002).

Figure 6: The Western Red Hills Centre, looking north from the Bla Bheinn ridge.

The relatively shallow emplacement depth(s) of the Cuillin Centre and the granite centres, estimated to have been in the order of 1-2km, into fractured and water-filled rock, led to the development of hydrothermal metamorphic aureoles adjacent (and presumably above) the intrusive units.

Uplift, intense weathering and erosion during the Late Tertiary, in a warm temperate climate, removed a significant proportion of the lava field and exhumed the subvolcanic intrusions. Glaciation over the last few tens of thousands of years have produced the landforms that control the spectacular topography of the island.

Summary Itinerary

Friday, 2nd: Travel from Fort William to Skye (2 – 3 hours). Local geology in the afternoon. Depends upon where we are staying, but most likely in the Royal Hotel, Portree.

Saturday, 3rd: Excursion to investigate the architecture of the flood lava sequence of west-central Skye. Traverse from Talisker Bay, south to the prominent summit of Preshal Beg. Spectacular exposures of: sheet lavas (basalt through to trachyte), examples of palaeo-topography/landscape, inter-lava sedimentary units, biotite-anorthoclase tuffs, intra-canyon flows, pillowed lava facies, columnar joint development, and formation of present-day "inverted topography".

Sunday, 4th: Excursion to investigate the gabbroic and peridotitic layered rocks of the Cuillin Centre. The journey to Loch Coruisk will be by boat (the Bella Jane, leaving from the village of Elgol on the Strathaird peninsula). This 45-minute trip will take us under the impressive Munros (Scottish summits > 3,000 feet) of the Southern Cuillin. From the landing point, it is a short walk to the freshwater Loch Coruisk, one of the most spectacular views in Scotland – in good weather or bad! We will then spend the day either in and around Loch Coruisk and in An Garbh-coire examining the layered rocks and returning by boat to Elgol, or, if desired, undertaking a traverse to the margin of the complex, around the south side of Sgurr na Stri to Camasunary Bay, and then out across the flood lava sequence of Strathaird to the public road.

Monday, 5th: Excursion to investigate the shallow-level silicic plutons, stocks and a ring-dyke of the Western Red Hills Centre. A traverse south along Glen Sligachan will pass over hydrothermally altered, amygdaloidal basalt lava country-rocks. The subsequent traverse up Marsco (summit 736 m a.s.l.) via Harker’s Gully will allow examination of a variety of granites and microgranites, a hybrid ring-dyke comprising basic (ferrodiorite), silicic (porphyritic felsite) and mixed-magma ("marscoite") components. Xenoliths of basement gneiss within the ferrodiorite unit are exposed at the base of the gully and the complete sequence of lithologies is exposed at higher levels, for example at the Shelter Stone. Weather permitting, we will go to the summit of Marsco, where an early gabbro sheet caps the mountain. From here, the views are spectacular in all directions.

Tuesday, 6th: Depart Skye.

Accommodation: Probably at the Royal Hotel, Portree.

Cost: The inclusive cost of the excursion will be announced later.

Literature: Below are listed maps and books relating to Skye 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.)

Maps:

Ordnance Survey topographic maps:
1:50K Landranger series, Sheet 32 South Skye, £6.49
1:25K Explorer series: Sheet 411, Skye Cuillin Hills, £7.49

Geological maps:

1:50,000 British Geological Survey Scotland Sheet 70, Minginish (Solid & Drift) (2000) £10.
1:50,000 British Geological Survey Scotland Sheet 71W, Broadford (Solid & Drift) (2002) £10.
1:25,000 British Geological Survey Scotland Sheet Skye Central Complex (Solid & Drift) (in press; 2005).

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.

BB 31st March, 2005

Further Information:

For further information, please email the fieldtrip leader:

b.bell@earthsci.gla.ac.uk

or the conference conveners:

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

Back to main conference page

:: HOME :: MECHANISMS :: LOCALITIES :: GENERIC ::
MantlePlumes.org