Petrography and field characteristics of the lava, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Petrography of the lavas
The lava fields are made up predominatly of basaltic flows with a lesser quantity of flows showing more evolved compositions, ranging from hawaiite and tholeiitic basaltic andesite to trachyte and rhyodacite. The general petrographical and mineralogical characteristics of lavas of the Skye and Mull fields, which have been summarised by Williamson and Bell (1994), Bell and Williamson (1994) and Kerr (1995a), and those of the Small Isles by Emeleus (1985, 1997), are outlined in Table 10. The geochemistry of the lavas is discussed in Chapter 10.
Field characteristics of the lavas
A variety of field and hand specimen characteristics have been identified for each of the main lava rock types (e.g. Williamson and Bell, 1994) and these generally permit field identification without immediate recourse to microscopic examination.
Flow thickness and extent
Typically, the most compositionally primitive flows are the thinnest. Alkali olivine basalts and transitional olivine basalts, with or without olivine phenocrysts, are usually less than 5 m thick, and commonly less than 2 m. Where seen in cliff sections, they obviously interdigitate and are not laterally extensive. Palaeosols or laterites are thin, poorly developed or absent, testifying to the relatively rapid and near-continuous nature of lava eruption. Plagioclase-phyric olivine basalts are typically more massive and thicker as, commonly, are tholeiitic basalt and tholeiitic olivine basalt flows, although flows of any composition may appear unusually thick where ponded in palaeovalleys. Unusually thick, magnesium-rich lavas occur in north-west Mull. For example, at Port Haunn, a lava, 16 to 30 m thick, contains flat-lying zones rich in amygdales, and has alternating olivine-rich and olivine-poor bands, all suggestive of lava pulses during continuous eruption (Kent et al., 1998).
Individual flows of more evolved composition, such as hawaiite and mugearite, typically have a greater volume and are commonly 8 to 12 m thick with some greater than 20 m (P580461). They have remarkable lateral persistence, despite being derived from more viscous magmas. For example, the outcrops of two mugearites in northern and eastern Eigg are about 12 km in length. The unusual thickness (relative to area covered) of these far-travelled lava flows suggests high effusion rates. The relatively rare benmoreite and trachyte flows are typically thick. In west-central Skye, the benmoreite of Cnoc Dubh Heilla is at least 30 m thick, and the trachyte of Cnoc Scarall in Glen Eynort is in excess of 100 m. It is, however, possible that the thickness of the latter has been accentuated through ponding. Evidence of flow termination is not common; rare examples include the flow fronts observed on Muck, near Torr nam Fitheach and Port an t-Seilich, and on Canna at Cùil a’ Bhainne.
Amygdales of secondary hydrothermal minerals are ubiquitous but are best developed within the thinner, more vesiculated, unevolved lava types, such as the alkali olivine basalts. Amygdale minerals include calcite, quartz, analcime, and the zeolites chabazite, stilbite, mesolite, thomsonite, gyrolite and apophyllite. These flows generally have well-developed, basal flow structures and less common pipe amygdales; the latter are especially concentrated where the flow has over-ridden thin palaeosols and mudstones. Within a lava field there may be a zonal distribution of the amygdale minerals related to depth of burial or the effect of later central complexes, as was demonstrated in the classic study of the Mull Lava Field by Walker (1971) (see p. 71; Figure 12).
Hawaiites and mugearites are generally less vesicular, indicative of a lower dissolved volatile content upon eruption, although some trachytes are distinctly vesicular. The tholeiitic andesites of north-west Rum are noted for their amygdales of bloodstone (green-stained chalcedonic silica flecked with red, oxidised pyrite) and banded agate, both of which occur in the beach deposits at Guirdil.
The development of palaeosols (or boles) to many of the lavas is a particularly useful feature that enables individual flows (both simple and compound) to be distinguished. Such material is indicative of subaerial weathering during the Palaeogene, in a warm temperate climate, with moderate to high rainfall on free-draining ground. Typically, the palaeosols are weathered and obscured inland, but are well exposed in the sea cliffs. They range in colour from dark, chocolate-brown through dull reddish brown to bright ocherous red, and from grey to mauve. They are generally not preserved in the vicinity of the central complexes, where pervasive hydrothermal alteration has commonly reduced lavas and palaeosols alike to dull, grey rocks.
The thickness of the palaeosols varies considerably. Flow tops may be merely stained a reddish brown with no obvious palaeosol between successive flows but, more typically, flows are separated by a few millimetres to several centimetres of apparently structureless, massive material. In a few instances, the thicker beds preserve crude rhizolith-like structures where mineral matter has replaced plant roots. Such features are taken to indicate that the palaeosols formed in situ. of the thickest and best developed palaeosols occur in intimate association with thin mudrocks. The latter are generally preserved as lensoid bodies on the undulating, eroded tops to flows and palaeosols, and point to minor reworking of soil and volcanic ash by running water, most likely rain water run-off rather that organised fluvial systems. Commonly these thicker palaeosols (with or without associated mudstone) are immediately overlain by thick, differentiated flows of hawaiite or mugearite, suggesting prolonged periods between eruptions. Particularly good examples are seen at Talisker Bay, Beinn nan Dubh Loch, Fiskavaig and Biod Mór in west-central Skye.
Palaeosol-like layers with indistinct bedding and rudimentary pisoidal structure, commonly containing pristine minerals such as sanidine, biotite and pyroxene, are interpreted as the product of contemporaneous weathering of crystal and vitric tuffs and not a consequence of deep-weathering of in-situ lava. Examples are quite common and include those found close to Lochan nan Dunan in northern Trotternish, Skye, and in south-east Muck (see p. 60) (Bell et al., 1996; Emeleus et al., 1996a; P580462).
These structures are best, but not exclusively, developed within the more evolved lavas, and the flow banding is generally parallel to the dip of the flow. The commonest type is due to the alignment of groundmass and microphenocryst feldspars, which imparts an obvious fissility and banded appearance to the rock, and is a useful aid to field identification (e.g Harker, 1908); flow-alignment of plagioclase macrophenocrysts is not generally noted. Where the base of the flow is irregular, flow folding of the foliation is common (P580463). Large-scale examples include the mugearite at McFarlane’s Rock, west-central Skye, whereas smaller-scale folds are present in the tholeiitic andesite on Fionchra, Rum. Systematic and abrupt changes in the inclination of flow-banding and jointing are seen at several localities. Usually, an otherwise flat to low-angle pattern is replaced by a near-vertical one. This could indicate either a flow dipping in an intrusive manner under its own surface, or reflect the possibility of the lava having flowed over uneven topography. There are many examples, including those seen at Loch Dubh, Cnoc Scarall and Coir’ an Rathaid, in west-central Skye.
Flow direction may be established on a local scale where the base of a flow has over-ridden unconsolidated material such as sediment. There are good examples in flows overlying the Minginish Conglomerate Formation in the Allt Mór south of Loch Eynort, west-central Skye. There, the flows have both ‘nosed’ into soft sediment, incorporating strings of sandy material, and also have inclined ‘flame-like’ masses of sediment along their bases. Flow direction may also be indicated by inclined or deflected pipe-amygdales and by discoid amygdales.
Some basalts, hawaiites and mugearites have brecciated tops, typical of a’a-type lava flows, and this autoclastic material can easily be misidentified as monolithological pyroclastic breccia. Typically, such a’a-type lavas are indicative of cooler, volatile-depleted magma at some distance from its point of eruption. A mugearite at Rubha Cruinn on the north side of Talisker Bay, west-central Skye, illustrates this very well and also shows a basal breccia carpet.
The extent to which columnar jointing is developed in lava flows appears to depend, amongst other things, on the composition of the magma. The best examples of columnar jointing are usually found in tholeiitic basalt or tholeiitic olivine basalt lava flows, and the most celebrated example of a columnar jointed flow in the Hebridean Igneous Province is the tholeiitic basalt at Fingal’s Cave on Staffa (P580464). Details of the columnar structure are well exhibited on Preshal More and Preshal Beg in west-central Skye, where tholeiitic olivine basalt flows are over 120 m thick. Each of these flat-lying flows has a lower ‘colonnade’ in which vertical columns are typically six-sided and between 0.3 and 1 m across. The overlying ‘entablature’ shows a complex, highly irregular jointing pattern, and a thin zone characterised by a subhorizontal flow-foliation separates the colonnade from the entablature. Curved ball and socket joints commonly divide the columns of the colonnades along their lengths. Inclined columns at several localities indicate the proximity of the sidewalls of the valley in which the magma was ponded, but which have been subsequently removed by erosion. Spectacular jointing also occurs elsewhere in lavas of the Staffa Lava Formation in south-west Mull, at Ardtun and on the Ardmeanach peninsula, where there are clusters of radiating columns in lavas overlying hyaloclastite deposits on the foreshore south of the MacCulloch’s Tree locality.
True columnar jointing is less common within the unevolved alkali olivine basalt flows, which are generally characterised by a more irregular ‘blocky’ or prismatic jointing. Columnar jointing is commonly present in hawaiite, mugearite and tholeitic andesite flows, and in flows of more evolved composition such as trachyte and silicic pitchstone. The Sgurr of Eigg rhyodacitic pitch-stone flow contains superb columns, smaller in diameter than those of the basalts and highly irregular in places (P580457).
Weathering style is also a useful field characteristic of lava type. The minerals in basalt lavas rich in olivine tend to alter more readily than those in the more evolved types, such as hawaiite and mugearite. Coupled to the more obviously amygdaloidal and usually coarser grained nature of the olivine basalts, this results in the degradation of rock faces and a tendency for the basalt lavas to form hollows or lower ground (‘slacks’) or terraced slopes between more resistant flows. Within individual flows, the amygdaloidal top and base weather more readily than the massive centre, resulting in typical ‘trap-featuring’ (P580465).