Figs. 1: Photograph of a "fresh" (well, freshly-broken) sample of craignurite from Mull, showing well-defined medium-grey spots in a pale matrix. Sample 118.194, in situ, 755 grams, 12 x 8.5 x 6 cm, magnetic susceptibility 4.72x10-3 SI units. Collected 27 March 1979, from a point roughly 100 metres southwards on the coast from the Caledonian MacBrayne (Craignure-Oban) ferry pier at Craignure, at circa NM717371 (Ordnance Survey, 1971), near the south end of Craignure Bay. The craignurite outcrop is extensive, extending on both sides of the pier. Craignure is on the coast of southeastern Mull, just northeast of the complex yet well-exposed remnants of the central volcano of Mull, and 20 km E.N.E. of Ben More, the highest peak on the island, which is found on the opposite, west flank of the volcano, south of Loch Na Keal (Bailey et al., 1924; McKirdy, 2017).
"Rock of the Month # 245, posted for November 2021" ---
Unlike many of the samples in these pages, this outcrop is on the beach in a small town, beside the local ferry terminal! I'll have to be careful with this one, as so many good geologists and observant locals have had a chance to ponder it at leisure.
By chance, this month sees the publication of a lavish work by Littlewood and Jones (2021). Author and photographer offer a wonderful guide to the natural history of Mull and some of the smaller isles nearby, including a brief introduction to the local geology (ibid., pp.13-33).
This story begins in the field: the plot thickens under the microscope. At the risk of giving away the conclusion, this taley involves the glassy volcanic rock pitchstone (see Rock of the Month 242, below). Such rocks may accumulate on the surface as volcanic deposits, or the magma may cool below the paleosurface, forming part or all of a "minor intrusion" such as a dyke or sill.
The outcrop at Craignure is extensive. The volcanic aspect is hardly surprising, given that a good 90% of Mull is composed of Tertiary igneous rocks. Some near-vertical dykes of fresh black dolerite (diabase) postdate the local volcanics, craignurite included. The dolerite is rapidly cooled, with columnar jointing, and an absence of the alteration evident in the host rocks.
Figs. 2-3: Photographs of three samples of craignurite from Mull. Samples 118.193 and 118.194 (left) and 118.088 (right). These rocks are rather enigmatic: the dark, cm-sized rounded masses appear similar to pitchstone, and are possibly relics of the original lava, within a largely devitrified groundmass. 118.194 was described in the caption to Figure 1. 118.193 was collected on the same day, nearby, in situ, roughly 100 m northwards on the coast from the pier, at circa NM715374. Weight 426 grams, 9 x 7 x 3 cm, magnetic susceptibility 10.06x10-3 SI units. Sample 118.088 was collected on an earlier visit, 16 March 1976, in the same area, in situ, near the pier at NM715374, weight 213 grams, 6 x 5 x 2 cm, magnetic susceptibility 17.7x10-3 SI units. The samples are superficially similar, but the variation in magnetic properties (most probably a function of magnetite content) reveals appreciable variation in the extensive outcrop.
Craignurite is another strange local term applied to a Scottish igneous rock. It seems to have a strictly Scottish provenance: Emeleus and Gyopari (1992, pp.124,179) refer to occurrences on Mull and nearby Ardnamurchan. The very definition is rather fuzzy: Le Maitre et al. (1989, p.58) rafer to "A local Hebridean name for a series of volcanic rocks occurring as cone sheets and ranging in composition from intermediate to acid. They are characterised by acicular and skeletal crystals of plagioclase, augite and hornblende in a felsic groundmass". The arrested development of the larger silicate crystals (microcrysts or microphenocrysts) suggest a rapid cooling (quenching) of the magma.
At Craignure, this rock extends over a wide area around the ferry pier. The cm-scale spots in a fine-grained matrix suggest devitrification of a glassy igneous rock (glass, of just about any composition, is metastable on even modest geological time scales --- in time, the atoms suspended in the glass reorganize themselves and form discrete crystals of one or more minerals). This recrystallization eventually causes the rock to lose the lustre and translucency that we expect of a glass. The pale colour is consistent with a felsic to intermediate protolith. The outcrop shows flow banding roughly parallel to the shore. Apparent grain size, distribution of spots and bands vary widely. Genuine xenoliths (microdiorite?) are rare. The craignurite outcrop is cut by later dolerite (diabase) dykes. The craignurite exposure is large, and the original fresh rock, presumed to be pitchstone, could have been a flow (as seen in the spectacular Sgurr of Eigg, found to the north in the "Small Isles" between Mull and Skye) or a minor intrusion.
This Craignure outcrop and another by the shore to the northwest, near Altcrich Cottage, are described as examples of composite intrusions, composed of relatively thin basic (basaltic) margins and a more felsic (felsite / craignurite) core. Actually, the upper basic margin at Craignure is obscured by a raised beach (Skelhorn, 1969, in Skelhorn et al., pp.32-35) but this does not invalidate the interpretation, though it does postpone a definite proof. Skelhorn's work on these rocks suggests that the rounded inclusions in craignurite are remnants of a now largely devitrified pitchstone. Skelhorn noted the prior existence of the high-temperature polymorph of silica, tridymite, now inverted to quartz. It appears that the glassy rock known as pitchstone may be emplaced as shallow, rapidly-cooling intrusions or as volcanic flows, whether as lava or welded tuff. The example in the Sgurr of Eigg is hard to beat, and shows columnar jointing (Gillen, 2013, pp.141,144,145), further evidence of rapid cooling of magma, as seen elsewhere in the world, including on nearby Staffa and the famous Giant's Causeway, near Bushmills on the Antrim coast of Ireland. You can read more of pitchstones from the Isle of Arran and of younger samples from Popocatepetl volcano in Mexico.
In prep.: petrographic notes and photomicrograph(s) from a new thin section.
Bailey,EB, Clough,CT, Wright,WB, Richey,JE and Wilson,GV (1924) Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban. Memoir for parts of geological sheets 43, 44, 51 and 52 (Scotland). British Geological Survey Memoir, HMSO, Edinburgh, vii+449pp., reprinted 1987 [with contributions by E.M. Anderson, H.B. Mauffe, G.W. Lee, B. Lightfoot, T.O. Bosworth and G.A. Burnett, plus petrology by H.H. Thomas and E.B. Bailey, chemical analyses by E.G. Radley and F.R. Ennos, and palaeobotany by A.C. Seward and R.E. Holttum].
Emeleus,CH and Gyopari,MC (1992) British Tertiary Volcanic Province. Chapman & Hall, 259pp.
Gillen,C (2013) Geology and Landscapes of Scotland. Dunedin Academic Press Limited, Edinburgh and London, 2nd edition, viii+246pp. [an update of the Terra Publishing 1st edition of 2003].
Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.
Littlewood,S and Jones,M (2021) Wild Mull: A Natural History of the Island and its People. Pelagic Publishing, Exeter, 300pp.
McKirdy,A (2017) Mull, Iona and Ardnamurchan: Landscapes in Stone. Birlinn Ltd, Edinburgh., printed and bound by Latimer Trend, Plymouth. 48pp.
Ordnance Survey (1971) Sound of Mull. Ordnance Survey one-inch map 45, 1:63,360 scale, fully revised 1954.
Skelhorn,RR, Macdougall,JDS and Longland,PJN (1969) The Tertiary Igneous Geology of the Isle of Mull. Geologists' Association Guide 20, 35pp.
Graham Wilson, posted 11 October, 2021, additions 27 October- 01, 10,30 November 2021
See the Rock of the Month Thematic Index
or, visit the Turnstone "Rock of the Month" Chronological Archives!
|Class/Group/Family||11 rocks from Scotland|
|The "Rock of the Month"|
|Granite||--- #30 --- Ross of Mull granite, Isle of Mull, Scotland|
|Troctolite||--- #31 --- Allivalite, Isle of Rhum, Scotland|
|Diorite||--- #32 --- Appinite, Strontian area, Scotland|
|Marble||--- #51 --- Forsterite marble, Glenelg, Scotland|
|Marble||-- #112 --- Brucite ophicalcite (marble), Isle of Skye, Scotland|
|Monzonite||-- #239 --- Kentallenite, a dark monzonite, Ballachulish area, Scotland|
|Articles ready now, and with new thin sections in prep. (late 2021):|
|Volcanic glass||-- #242 --- Pitchstones from the Isle of Arran, Scotland|
|Diabase||-- #243 --- Crinanite (analcime olivine dolerite / diabase), Isle of Arran, Scotland|
|Gabbro||-- #244 --- "Old Gabbro" of Ardnamurchan, Scotland|
|Volcanic / minor intrusive pitchstones||-- #245 --- Craignurite, Isle of Mull, Scotland - "YOU ARE HERE"|
|Minor intrusive||-- #246 --- Lamprophyre, Iona, Scotland|