UK Volcanoes

Dykes, Sills, Basalt, Gabbro, Volcanic rocks, Volcanic provinces

A summary of some of the available information on the extinct volcanoes of England, Scotland and Wales

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Introduction

Although there are no living volcanoes on the British mainland, the UK has an interesting volcanic history. For instance, Edinburgh, the Scottish capital is built on an extinct volcano called Arthur's seat, the mountains of north Wales are the remains of a huge volcanic plateau and there are volcanic rocks distributed throughout the British Isles. Even in the Chalk of Southern England, many of the thin marl seams (eg. Newhaven Member) are now considered to be volcanic ash deposits. Volcanoes have therefore played a significant part in the geological history of the UK. None of the volcanoes will become active again in the foreseeable geological future.

What is a Volcano?

A volcano is an opening in the earth's surface through which magma, molten rock, ash and gases are erupted onto the surface. Although some volcanoes are the classic cone-shape, they come in many different shapes and sizes. The differences are due to the type of eruption and the nature of the material erupted by each individual volcano.

Mount St. Helen's erupting. Free public domain photo from the USGS.

What are Volcanic Rocks?

Igneous rocks begin as magma. Intrusive igneous rocks, like granite, form when magma cools inside the Earth. Extrusive igneous rocks, like the basalt lava flow in this photo, form at the Earth's surface.

(Photo after Steve Maddox)

Common igneous rock types found in the UK and worldwide are as follows:

Intrusive: Gabbro, Diorite, Granodiorite, Granite

Extrusive: Basalt, Andesite, Dacite, Rhyolite

The intrusive rocks tend to be coarser grained than the extrusive rocks as they cool at a slower rate, giving the minerals a longer time period grow larger in size.

Many of the extrusive rock types are also found as thin intrusive bodies, called dykes and sills. These are common in the west coast of Scotland Tertiary province where many phases (5+) of dyke intrusion have been recognised.

These rock types can also be found in more explosive deposits such as volcanic agglomerates. Other rock types associated with volcanoes include volcanic ash deposits which are airborne and pillow lava which is formed when a lava flow goes into water and crusts over. It seeps out at the end of the encrusted tubes into 'pillow' shapes:

L.Palaeozoic pillow lava near Scarlett point, Isle of Man (photo after David Kitching)

What can the modern distribution of volcanic rocks tell us?

There are two basic types of plate tectonic boundaries where volcanic rocks occur, migrating from deep in the crust through faults in the Earth's surface:

Divergent Plate Boundaries:

Convergent Plate Boundaries:

The rock types which are associated with these and other types of plate tectonic boundaries are often characterised by certain variations in the chemical composition of the rocktypes, due to the variation in depths at which they are formed within the Earth.

Thus, for example, the variation in Ordovician volcanic rocks of Wales and the Lake District imply a convergent plate boundary with subduction occurring to the north of these areas, southwards under the Lake District and Wales.

In contrast, the volcanic rocks of the Tertiary volcanic province of Scotland are associated with a divergent plate margin, with the opening up and spreading apart of the Atlantic Ocean from Mesozoic times onwards. Modern volcanism associated with this process is well documented on Iceland.

The volcanic rocks in the UK prove that the geological history is complex, as it has been located within many different tectonic settings.

The Ordovician of The Lake District and Wales

As noted above, the variation in Ordovician volcanic rocks of Wales and the Lake District imply convergent plate boundary with subduction occurring to the north of these areas, southwards under the Lake District and Wales. There was a lot of volcanic activity in Wales. Many of the lavas erupted were erupted under the sea and mixed with the sediments on the sea floor, as did the large flows of volcanic subaerial ash, known today as nuées ardentes.

The convergent plates, subduction and volcanic activity resulted in the formation of the Caledonian mountain chain that runs from Wales up through Scotland and into Scandinavia. What we see now is the eroded core of these mountains.

The main centres of volcanic activity were Snowdonia, the Arenigs, (just south of Snowdonia) Cader Idris (just south of the Arenigs) and Prescelly (near the southern bit of Cardigan bay).

Other evidence for associated volcanic activity comes form the southern uplands of Scotland and Ireland.

Tectonism, volcanism and Atlantic rifting

Volcanism, is thought to have started to the west of Britain in the Late Jurassic and local sites in the North Sea Basin were active through the Late Jurassic into the Early Cretaceous. At Zuidwal in Holland a volcanic plug of earliest Cretaceous age has been recognised and this and other associated volcanoes at this time may have been responsible for the Fuller's Earth deposits in southern England within the Wealden, for example within the Hastings Beds at Maidstone in Kent. The peak of volcanic activity appears to have been in the Aptian which has extensive deposits of Fullers Earth. Jeans et al.(1977) confirmed the volcanic origin for these deposits with the recognition of glass shards and well preserved pyroclastic material, but the volcanoes responsible for these deposits are still unknown, the airborne deposits possibly originating from to the west of Britain.

During the Aptian the Rockall Trough, which separates the Rockall Bank from the rest of Europe began to form, marking the initiation of sea-floor spreading in this part of the N. Atlantic.

While most of the Cretaceous volcanic activity and indeed tectonic activity may have occurred during the Early Cretaceous, the Late Cretaceous is now known to be a time of both tectonic activity and volcanism in southern England. In particular many of the thin marl deposits of much of the Chalk (such as the Newhaven Beds) are known thought to be volcanic ash deposits.

 The Tertiary Volcanic Province of Scotland

Igneous activity began in the west Greenland area during the Upper Cretaceous, but did not commence elsewhere until the Tertiary. This climax of this activity has been dated from both Scotland and the North Sea (tuffs) as the Palaeocene (earliest Tertiary). In the British area the volcanism seems to have been complete by the early Eocene. The length of time for the activity is relatively short, this being explained by the volcanic activity moving to the newly formed mid-atlantic spreading ridge, part of which was eventually to become Iceland.

In Britain the igneous activity was localised at a number of major centres inclusive:

Lundy (minor), Carlingford, Mourne (N.I.), Ailsa Craig, Arran, Blackstones, Mull, Ardnamurchan, Rhum, Skye, St Kilda and Outer Roag. Each centre consists of a number of overlapping episodes (which in the case of Skye is 5 or more) separated by periods of quiescence. The magma chemistry evolved from the more basic to more acidic. Again in Skye this resulted in major batholiths and complex funnel shaped intrusions being intruded adjacent to the basic Cuillin Hills (= the granitic 'red hills'). Both in Skye (Cuillins) and Rhum the volcanoes eroded so far as to reveal the magma chamber.

The Cuillin's. Also called the Black Hills. Formed of Gabbro. A Tertiary palaeomagma chamber.

(photo from FreeFoto.com)

The style of igneous activity seems to have followed a pattern:

1) the eruption of plateau basalts and tuffs

2) the intrusion of plutonic complexes

3) the intrusion of dyke swarms

In the hebridean basin up to 1.8 Km of basalts have been estimated and there is still 2 Km of lavas left at the Isle of Mull, these representing the eroded remnants of much thicker complexes. The volcanoes were huge shield shape mountains which finally must have connected Ireland to Scotland in a massive plateau. The plateau lavas lie in shallowly faulted basins, and the individual centres are associated with reactivated ancient fault systems such as the Great Glen fault on Mull.

Some volcanoes formed calderas with lakes with rich floras such as lotus (hot & humid climate). Calderas typically form when large volcanoes reach 4,000 metres in height as the lavas then tend to stop erupting and stay in the magma chamber below. Eventually the pressure that all the magma produces caused it to break out suddenly in a huge explosive eruption eruption such as happened at St. Helen's (photo above). This is commonly followed by the crater floor collapsing into the chamber to form a caldera. The presence of calderas confirms the scale of the volcanic activity.

When this kind of eruption happens the original vent sometimes becomes blocked, so the volcano forms a new vent. An example of this can be seen on Ardnamurchan. When these volcanoes became extinct the volcanic activity changed to dykes.

The Giant's Causeway and Fingal's cave are perhaps the most famous worldwide locations of the British Tertiary volcanic province. These locations are famous for their hexagonal columns of basalt lava, the columns forming during cooling.

The Giant's Causeway

(Photo after N.I. Tourist Board)

When the lava cools down and it crystallises and as it does so it loses volume. The lava usually cools on the surface first and the most economical way for it to shrink is to shrink into hexagonal shapes, leaving a crack between each hexagon. Eventually the lava will cools right through and leaves deep hexagonal columns.

(photo from FreeFoto.com)

Ardnamurchan

Ardnamurchan was active as a volcanic centre for about 1 million years. The early activity was heralded by the eruption of volcanic ash through numerous fissures scattered across the landscape. It is likely that this ash covered much of the peninsula. After the ash came huge volumes of basalt lava, which again erupted through fissures creating an extensive and thick lava plateau.

Today, only fragments of the lava plateau remain with the thickest section (100 metres) at Ardslignish. Once the lava plateaux had formed, the volcanic activity became more focused and additional lava and volcanic ash was erupted through the vents and craters of large, traditional cone-shape volcanoes. Volcanic ash would have been blown clear of the vent, but coarser lava or agglomerate, comprising pebbles, cobble or boulder sized blocks of rock would have remained within the confines of the crater, which may have been several kilometres in width. The presence of large areas of agglomerate on the peninsula are testament to both the size of the craters and the prolonged and violent nature of the eruptions.

Intrusions called cone-sheets and ring-dykes were produced at this time. The Ardnamurchan peninsula is, renowned for its well developed examples of these ring-structures.

It was by studying the distribution of Ardnamurchan's cone-sheets and ring-dykes that geologists were able to identify the three separate centres of volcanic activity on the peninsula.

The first volcanic centre was around Ben Hiant. From here the focus of activity moved several kilometres westwards to around Aodann. Here, only the volcano's eroded roots and a small volcanic vent (west of Kilchoan) remain along with sections of earlier plateau lava and agglomerate erupted from the previous volcanic centre.

From around Aodann the volcanic activity centre shifted north-eastwards to Achnaha, where the third centre developed. Large ring-structures are visible here and an obvious and dominating one is called the Great Eucrite.

Also associated with the volcanic activity was the development of dykes and dyke swarms. Dykes form when magma is intruded upwards into long, linear, vertical cracks in the earth's crust, typically created through crustal stretching. Dyke swarms represent particularly large numbers of these features occurring close together and parallel to one another.

Arthurs's Seat, Edinburgh

Edinburgh is situated on top of a series of extinct volcanoes, the tallest of which now measures, at 825ft, is Arthur's Seat. This is also the best preserved of these ancient volcanoes. However, erosion over millions of years has dug deep into the core of the volcano and exposed the vents, lava flows and ash falls, which are all today clearly visible. The vents, which carried the lava to the surface, still reach deep into the earth, although today are cold, solidified and completely blocked.

(Photo after Scottish tourist board)

The volcano at Arthur's Seat has been extinct for over 350 million years. It erupted between 350 and 400 million years ago. The best preserved of these ancient volcanoes is Arthur's Seat. However, erosion over millions of years has dug deep into the core of the volcano and exposed the vents, lava flows and ash falls, which are all today clearly visible. The vents, which carried the lava to the surface, still reach deep into the earth, although today are cold, solidified and completely blocked.

 The volcanic rocks present include a sill, and a volcanic vent which includes a polygenetic volcanic agglomerate.

Mafic (basalt) lava flows are present which exhibit columnar structure, vesiculation, gas brecciation and rock grain-size variation, and water-laid volcanic tuff can be observed. The local sandstones exhibit cross-bedding and ripple marks.

(fig. from the University of Edinburgh)

The combination above shows how today's landscape related to the ancient volcano. The volcano has been tilted approximately 20 degrees to the East due to the immense forces which have acted on its in the millions of years since it was active. The volcanic pipe, which was the conduit through which the lava was erupted, forms the bulk of Arthur's Seat itself, and the lava flows which form the flanks of the volcano are exposed on the eastern side of the park (left hand side of the ilustration above, which is looking South).

The volcano sits on top of sedimentary rocks which were deposited before the volcano existed within a shallow sea. These sedimentary rocks contains some fossils, but more fossils are to be found in thin layers of younger sedimentary rocks which are found between the lava flows. This shows that the volcano was erupting into a sea, and between eruptions, these sediments were depositing. It is these rocks which include the plant fragments and fish remains.

The location comprises sedimentary, igneous and glacial geology. It was one of the crucial localities at which the science of geology had its origins during the late Eighteenth Century through the work of James Hutton and others.

The Isle of Skye

Volcanic rocks of Tertiary age

On Skye, the earliest activity was of an explosive nature resulting in deposits of basaltic ash up to 30m thick in northern Skye. The ashes range from very fine grained material to blocks over 20cm across. Much of this material was deposited in shallow lakes and associated minor lava flows show the globular "pillow" structures characteristic of lavas which have cooled under water. Associated lake sediments include plant fragments which indicate that the climate was of a Mediterranean type. Seams of lignite ("brown coal") up to 1m thick within these sediments have been worked locally.

The overlying main "plateau lava series" was probably over 1,200m thick prior to erosion. Continuous outcrops cover most of northern and western Skye and extend south-westwards on the sea bed to beyond Canna. Remnants are also preserved around the Black Cuillin and the Red Hills and on the Strathaird Peninsula. The eruptions are thought to have been from a series of fissures, generally elongated in a NW-SE direction. The lowest groups of flows are predominantly basaltic, but higher groups include an increasing proportion of paler-coloured, more silica-rich flows with a range of compositions between basalt and the more silicic, "trachyte". Once particularly abundant type of intermediate composition is called "mugearite" after the settlement of Mugeary, between Portree and Bracadale, a name which has been adopted worldwide for this type of lava. The individual lava flows are commonly 10 to 15m thick but some up to 37m have been recorded. The basaltic flows in particular, commonly have well developed columnar jointing in their central section, as in the Giants Causeway and Fingal's Cave flows. The upper and lower sections of the flows are usually more broken and slaggy, with many "amygdales" (former gas bubbles filled with later minerals). It is this contrast between the massive, hard central parts of flows and the softer, rubbly tops and bottoms which has produced the characteristic stepped "trap" topography. Where a significant time interval occurred between successive flows, red, clay-rich lateritic soil horizons (termed "boles"), developed on the tops of flows due to leaching. Although these show concentration of iron oxide of up to 38% they have not been exploited commercially like their better-developed equivalents in Co. Antrim, which were extracted for both iron ore and bauxite (aluminium ore).

The presence of the boles indicates that terrestrial conditions prevailed during the eruptions and the rarity of intercalated ash bands suggests that the eruptions, although voluminous, were quiet with little explosive activity. Numerous local accumulations of sedimentary rocks within the main lava sequence imply that sufficient time elapsed between flows for river systems, lakes and vegetation to develop. The sediments include conglomerates, sandstones, siltstones, shales and a few seams of lignite.

Major intrusive igneous centres of Tertiary age

Following the eruption of the plateau lavas, igneous activity became concentrated around complex intrusive centres in central Skye. These consist of a series of arcuate or ring-like intrusions, up to ten or fifteen kilometres in diameter, arranged concentrically around four major centres. It is believed that these represent the root zones of large central volcanoes which built up on top of the earlier lava plateau, but which have subsequently been removed by erosion. What we see now are the solidified remains of the magma chambers beneath these volcanoes which, because they cooled slowly at depth, now consist of a wide variety of spectacular and beautiful coarse-grained rocks. The basaltic magmas solidified into coarse-grained gabbroic rocks which form the earliest Cuillin Centre. The more silicic magmas solidified into granitic rocks which form the bulk of the other centres; in order of formation the Strath na Creitheach, the Western Red Hills and the Eastern Red Hills centres.

Active volcanoes can be observed directly in many parts of the world. However, in order to study their underlying structure and hence understand more fully their origin, make-up and evolution, it is necessary to turn to older volcanic fields, such as those of Skye and elsewhere in the inner Hebrides, where their exhumed roots can be seen. Since Alfred Harker first published the results of his mapping for the Geological Survey in 1904, Skye has been a constant focus for such studies, which have gained worldwide recognition and importance.

The Cuillin Centre is responsible for what is, without doubt, the most spectacular mountain range in the British Isles. The arc of jagged peaks, almost 1,000m high, which constitutes the main Cuillin ridge, together with Sgurr na Stri and Bla Bheinn. The centre is some 15 x 10 km in extent and is centred upon the upper part of Glen Sligachan. It is intruded into the earlier Tertiary lavas and into Mesozoic and Torridonian sediments, which have all suffered intense baking, reconstitution and even melting. The gabbroic rocks form a series of concentric, arcuate intrusions. Many show spectacular layering due to the settling-out of minerals which crystallised early during the cooling of the magmas. The layering shows many of the features most commonly exhibited by sedimentary rocks due to grains settling out of water. In general the dip of the layering is inwards towards the centre of the intrusions, which consequently resemble a stack of saucers of three-dimensional shape. Where early-crystallising olivine (a magnesium silicate) has accumulated to such an extent that it forms 40% or more of the rock, the rock is termed a peridotite and is distinguished by its pale brown weathering and coarse sponge-like appearance, well-seen on much of the main Cuillin ridge.

There are also numerous sheets of fine-grained basaltic rock which cut through the gabbroic intrusions. Although they are responsible for much of the jagged outline of the mountains on account of their differential rate of weathering compared with the gabbroic host rocks, they tend to be brittle, with many close-spaced joints and are notoriously smooth and slippery in the wet. Many of these basaltic sheets are in the form of vertical dykes, either radial or tangential to the main arcuate intrusions. Others take the form of inclined sheets which, on a large scale, map out as inverted cones dipping inwards towards the intrusive centre. The renowned Inaccessible Pinnacle on the summit of Sgurr Dearg consists of two parallel, vertical dykes forming a thin blade which rises above a surface formed by an inclined sheet.

In recent years areas containing significant amounts of "ultrabasic" igneous rocks, such as peridotite have been the focus of much mineral exploration, since they have potential for concentrations of such elements as magnesium, nickel, chromium, gold and platinum. So far, there has been no major exploration of the Cuillin, and a recent offshore reconnaisance sampling programme in Loch Scavaig failed to reveal any significant concentrations in seabed sediments. (The same survey found good concentrations of magnesium-rich olivine and chromium off Rhum, which has a much higher proportion of ultrabasic rocks than the Cuillin).

The small granitic centre of Strath na Creitheach and the succeeding large centres of the Western Red Hills and Eastern Red Hills are a marked contrast to the Cuillin. The granitic rocks weather in a uniform manner and are less dissected by later intrusive sheets, so the hills tend to have smooth outlines with few steep rocky faces. Apart from minor gabbros, intruded early in the evolution of both the Western and Eastern Red Hills centres, the rocks are all varieties of granite. They were intruded as cylindrical bodies, rising through the earth's crust by displacing pre-existing country rocks and earlier granite which foundered into the advancing magma ("block subsidence"). The result is a series of arcuate concentric steep-sided intrusions known as "ring-dykes". Some of the intrusions probably rose quite high in the crust and on Glamaig a "roof" of basalt is preserved on top of the granite.

The origin of the granites has been a matter of great debate. Some probably evolved from a basaltic magma, similar to that which formed the Cuillin Centre, by collecting together the silicic magma left over after the earlier crystallisation of the minerals which make-up the gabbros. Others may have been formed by the melting of Lewisian and/or Torridonian country rocks by direct heat emanating from the Cuillin intrusions. One feature of great interest over the years has been the so-called "hybrid" rocks, particularly well-developed on Marsco, which resulted from the direct intermixing of two magmas, one siliceous and one more basic. This leads to rocks with strange mixtures of minerals and distinctive textures. Simultaneous intrusion of two different magmas is also seen in the so-called "composite" sills which crop out in a 15km-long arc around the south-east of the Eastern Red Hills Centre, from Corry and Harrapool to Loch Slapin. These inclined sheets have margins of basalt, but the central parts are of fine-grained granitic material.

Intimately associated with the main intrusive centres, are a host of intrusive bodies containing disaggregated rock fragments. These are interpreted as volcanic `vents', originally connected to surface eruptions of a highly explosive nature. Over 40 such vents have been recognised in and around the Cuillin Centre and large vents also occur in the Strath na Creitheach Centre, on Belig in the Western Red Hills and around Kilchrist in the Eastern Red Hills. The fragments range in size from fine ash particles to huge blocks. They consist predominantly of rocks from the intrusive centres and the earlier lavas, but also include Torridonian and Jurassic sediments and fragments of Lewisian gneiss, which must therefore be present in the basement beneath the centres. Some of the vents contain bedded ash deposits which must have accumulated at the surface during the eruptions, probably in large, circular volcanic depressions termed calderas. The presence of these surface deposits alongside relatively deep-seated intrusive rocks suggests that considerable subsidence of up to 1,000 m may have occurred, probably within calderas bounded by the cylindrical "ring faults" which also controlled the rising magma in the intrusive centres.

Outwith the area of the Cuillin and the Red Hills, the most voluminous group of igneous intrusions are the basic sills of northern Skye. These sills are a prominent feature of the eastern and northern coastal areas of the Trotternish Peninsula, but they are even more extensive on the sea bed of the southern Minch and the Little Minch and form the Shiant Islands, the Ascrib Islands, Fladda-chuain, Eilean Trodday and the islands of Staffin Bay. They are intruded mainly into Jurassic strata, below the lava plateau, and form gently inclined sheets up to 90m thick. Several sills are commonly stacked together to form a sill complex with a near-constant total thickness of about 250m. The rocks are all varieties of dolerite, intermediate in grain size between the basalts of the lava flows and the gabbros of the major intrusions, but similar in composition. Vertical columnar jointing is usually very well developed and is well seen, for example, at the popular `Kilt Rock', south of Staffin.

The regional dyke swarm of Tertiary age

Vertical dykes of Tertiary age are present throughout the whole of the Isle of Skye and adjacent islands. Apart from those closely associated with the major intrusive centres, the dykes generally trend between NNW-SSE and NW-SE and constitute a major regional dyke swarm which extends over an area of over 4,000 km2 from Lewis and Harris, through Skye to the Great Glen. Most of the dykes are composed of dolerite. Most are less than 2m wide, but some are up to 40m and locally they are so numerous that they occupy up to 20% of the rock outcrop. The dykes probably rose from elongate ridge-like magma chambers which developed in deep crustal fractures related to the opening of the North Atlantic Ocean. Some undoubtedly represent solidified feeders to the fissure eruptions which produced the plateau lavas. However, they continued to be emplaced during the development of the central complexes and for some time afterwards, until the igneous activity in the Hebrides finally waned between 55 and 52 million years ago. The whole of the igneous activity, from the earliest volcanic eruptions to the latest dyke intrusions was therefore completed in a period of about 8 million years, short by geological standards and entirely within the early Tertiary Period.

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 Acknowledgements:

This work is a compilation from various sources inclusive Edingburgh University, the Scottish Tourist Board, the Ardnamurchan Estate, the Skye Data Atlas, the NI Tourist Board, Freefoto.com, the USGS and a web publication by Steve Maddox.