a. Description of Property

Natural Heritage

The rocks of the St Kilda archipelago comprise the St Kilda Central Complex. This complex was intruded into Lewisian gneiss, which represents some of the oldest rocks in the world. The contact between the Lewisian basement and the St Kilda Central Complex lies offshore and is not exposed anywhere on the archipelago. The Lewisian ‘country rocks’ date back some 3,000 million years and crop out over a structural horst block known as the Outer Hebrides Platform. They form the metamorphic basement in much of the Foreland Province to the Caledonian mountain belt or orogen, which includes much younger, metamorphic rocks designated as Dalradian and Moine. By contrast with the ancient Lewisian rocks, the St Kilda Central Complex was formed only around 55 million years ago during an intense, but relatively short-lived, phase of igneous activity. This igneous episode occurred when the northern remains of the continent of Pangaea were breaking apart, with the formation of the North Atlantic Ocean.

Figure 3.1: Solid geology of St Kilda and the surrounding area generalised from BGS Special Sheet (1984) and BGS 1:250 000 Series (1991a).

The North Atlantic Ocean formed as the continental crust between the western British Isles (and the rest of the European landmass) and Greenland (together with North America) was stretched and thinned. When an upwelling of hot mantle, known as a plume, impinged upon the thinned continental crust, vast outpourings of lava erupted through fissures that opened in the crust. As continental rifting continued, the fissure-style eruption ceased and volcanism became centred upon several discrete volcanic foci. The present-day East African Rift Valley, with its huge volcanic peaks, represents modern continental splitting (or rifting), which is analogous to the earliest stages of the rift between the western British Isles and Greenland in latest Cretaceous and Paleogene times, around 65 million years ago.

Figure 3.2: Regional solid geology of NW Scotland and adjacent offshore areas (modified after BGS 1:1,000,000 Geology of the UK, Ireland and the adjacent continental shelf north sheet, 1991c).

The most intense volcanic activity in the northwestern British Isles occurred during the Paleocene and early Eocene (c. 63-52 million years ago). Volcanism was most vigorous in western Scotland (the Hebridean Province) but also extended to north-east Ireland, north-east England, North Wales and the Bristol Channel. The region encompassing this activity has been commonly referred to as the British Tertiary Volcanic (or Igneous) Province (BTVP).

A few kilometres below the central volcano that probably once existed above the St Kilda Central Complex, large bodies of magma accumulated in chambers. The magma ultimately rose and fed the volcano at the surface, with explosive eruptions of lava and ash. More often than not, the rising magma was forcibly emplaced, or intruded into fissures and voids within the crust below the volcano and did not reach the Earth’s surface. Within the magma chambers, the composition of the liquid magma was altered as the first-formed mineral crystals settled from the melt. This gave rise to magmas of varying composition from basic (silica-poor rocks such as gabbro and dolerite) to acid (silica-rich rocks such as granite, microgranite and felsite). It is the eroded remains of these intrusions and magma chambers that are magnificently exposed today on the cliffs of this archipelago.

Figure 3.3: Regional structural framework of NW Scotland and adjacent offshore areas (modified after Stoker et al., 1993).

Of all the islands of the St Kilda archipelago, Hirta and Dun have the most varied geology. The other islands of Soay, Boreray and surrounding stacks, and Levenish appear to be made up of breccias (fragmented and reconsolidated masses) of gabbro and dolerite cut by a few composite felsite-dolerite sheets (thin sheet-like intrusions).

The earliest (or oldest) intrusion in the archipelago, is the Western Gabbro, which is a coarse-grained layered intrusion, forming the western edges of Hirta and Dun. The layering, which is a reflection of varying modal proportions of ferromagnesian minerals (principally olivine and diopsidic augite) and plagioclase, dips towards a focus c. 2 km ENE of Hirta, which probably represents the centre point of the original mass of molten magma. The gabbro also contains minor orthopyroxene, amphibole, spinel and iron-titanium oxides. This gabbro became crushed and sheared and then veined by numerous sheets of basalt and dolerite on the west cliffs of the Cambir. Similar breccias of fine-to-coarse grained basic rocks, form the outlying islands and north Hirta.

The Glen Bay Gabbro intrudes and is chilled against the basic breccias. The chilled contact on the east of Glen Bay is most unusual among the gabbroic intrusions of the Hebridean Province in possessing a complete textural gradation from a 10 mm border zone of splintery, glassy basalt (evidence of very rapid cooling) to coarse-grained gabbro, exposed on the east side of the bay. Normally, these gabbros are not conspicuously chilled against other basic bodies and presumably the molten intrusion was emplaced into cold solidified rock. Fine, vertical banding occurs in the marginal zone parallel to the contact and the effects of chilling are estimated to extend for 100-120 m into the gabbro. Gabbro on the west of the Bay is much sheared and granulated and is separated from the eastern outcrops by the oldest granite on Hirta, the Glen Bay Granite. This granite is chilled against the earlier gabbro, showing that it was intruded after it.

The pervasive shattering of many of the St Kilda gabbros and dolerites to form breccias, is a striking feature of the St Kilda Central Complex and suggests that explosive release of water may have occurred towards the end of their solidification, followed by rapid injection of quickly cooled basaltic magma. It is also possible that the highly unusual glassy, quenched contact of the Glen Bay Gabbro may owe its origin to high-temperature, hydrothermal quenching.

The next intrusion phase involved four pulses of mixed basic and acid magmas that formed the Mullach Sgar ‘complex’ that crops out between Glen Bay and Village Bay. This group of rocks includes dolerite, microdiorite, microgranite and rocks of hybrid (mixed dolerite and granite) aspect. Angular and lobate masses of marginally chilled basic rocks occur in more acid matrices and are veined by felsic (acid) material. A large amount of shattering of the dolerite and basalt has occurred, giving areas of complex net-veining. It has been suggested that initial intrusions of basaltic magma were followed successively by granitic magma and further basalt. The Mullach Sgar ‘complex’ provides a superb example of the coexistence of acid and basic magmas and their near simultaneous intrusion.

Figure 3.4: Field sketch from Geikie (1897) of the sea cliff below Conachair, showing basic dykes in granophyre.

The final major intrusion is the Conachair Granite that forms the high ground north-east of Village Bay. This granite intrudes the Mullach Sgar ‘complex’ without notable chilling along the contact. The granite typically contains the minerals quartz, perthitic alkali feldspar and albitic plagioclase, with minor amounts of biotite and amphibole. Other accessory minerals include zircon, sphene, rutile, anatase, titanium-rich magnetite, fluorite and needle-like, deep-brown crystals of the rare-earth-bearing silicate chevkinite. The Conachair Granite characteristically has a microgranitic texture, commonly with a considerable content of intergrown quartz and feldspar. Some of the larger quartz crystals are interpreted as corroded, inverted high-temperature quartz. Radiometric age determinations on this granite give a date of c. 55 million years ago, indicating a Paleocene age.

Village BaySeveral generations of small (or minor) intrusions with compositions ranging from basalt to rhyolite have been recognised. Commonly these occur as dykes and also as inclined sheets whose disposition suggests that they once formed a classic set of cone-sheets. Many of the inclined sheets cut the Conachair Granite and are therefore the latest intrusions on St Kilda.

Later during the Paleogene and Neogene, over a period of some 50 million years, the St Kilda volcano was reduced and fragmented through processes of subaerial weathering and erosion, glaciation and marine erosion. Today we see only the drowned remnants of the former landscape.

The testimony of the rocks of St Kilda to the posteriority of the granophyre to the gabbros and basalts is thus clear and emphatic. ‘It entirely confirms my previous observations regarding the order of sequence of these rocks in Mull, Rum and Skye. But the St Kilda sections display, even more strikingly than can be usually seen in these islands, the intricate network of veins which proceed from the granophyre, the shattered condition of the basic rocks which these veins penetrate, the remarkable liquidity of the acid magma at the time of its intrusion, and the solvent action of this magma on the basic fragments which it enveloped.

Quotation from Geikie’1897. (Vol. 2, p. 416)

The geology of the islands was first examined in the 19th century by the early Scottish geologist, John McCulloch, who was the pioneer of the first detailed geological map of Scotland. McCulloch was the first to recognise the presence of both basic and acid rocks on St Kilda. In the late 19th century, the renowned Scottish geologist, Sir Archibald Geikie, who wrote some of the earliest textbooks, described the rock types of St Kilda and compared them with those of the Hebridean central complexes. The first detailed map was published by A.M.
Cockburn in 1935 and the islands were remapped by the Geological Survey in 1979 and 1980, with a full account published in 1984.