Volcanoes and the Making of Scotland

• Editorial: Dunedin Academic Press
• Año: 2015
• ISBN:
  9781780465418
• Categoría: Scienze fisiche
• Categoría: Sismologia e vulcanismo

Scotlands mountains and glens retain the secrets of the long and frequently violent geological history that has gone into their making. Volcanoes have played a major role in the creation of Scotland and while the youngest, a mere sixty million years old, were responsible for much of the scenic splendour of the Inner Hebrides, the rocks composing many of the famous Scottish landforms as, for example, those of Glencoe and the Edinburgh district are also the direct result of volcanism.Volcanoes and the Making of Scotland explores back in time from the most recent examples to volcanoes of the obscure Precambrian times which left their signature in the ancient rocks of the far north-west. Geographically the book ranges across all of Scotland from Shetland to the Borders. Reflecting current research into Scotlands geology, the author also speculates as to the climate, geography and ecology of the long-gone landscapes in which the volcanoes of differing ages were created and destroyed.The book is extensively illustrated with maps, sketches, cross-sections and photographs and relates what can currently be seen in the worn-down remains of Scotlands old volcanoes to active analogues around the world. This book vividly brings life and meaning to what the layman would otherwise regard as cold and incomprehensible rocks.

Chapter 1

Introduction

Scotland today presents a peaceful landscape. Earthquakes are rare and mercifully small, and the nearest active volcanoes lie far to the north-west in Iceland, or away to the south-east in Italy. Admittedly we do not have to travel quite so far to see landscapes that are easily recognizable as volcanic; in central France the Chaîne des Puys consists of volcanic hills that have been little modified by erosion since they last erupted several thousand to tens of thousands of years ago. The Romans, who recognized them as old volcanoes, were, of course, thoroughly familiar with such features at home. The Eifel district in western Germany, like the Chaîne des Puys, remains a potentially active volcanic region. Scotland, however, is volcanologically inert, offering no possibility of an eruption at least within the next few millennia. And yet a record of volcanism over a huge period of geological time is preserved in the Scottish rocks, and this book attempts to present a guide for students and amateur geologists to Scotland’s dramatic and fiery past. Evidence of the former existence of volcanoes, in more or less ruinous and fragmentary state, is to be seen scattered in a thousand sea-cliffs, road-cuts, quarries, hillsides and mountains from Shetland to the English border.

Although there is general public awareness that Scottish landscape features such as Arthur’s Seat, Staffa, Glencoe and the Cuillins are of volcanic origin, there is a conceptual difficulty in relating what one actually sees to the perception of an active volcano. Nowhere does one actually see a volcano, just their relics. If a city had been destroyed by earthquake or war almost every building might have been flattened and the rubble removed and yet, from the basements, foundations and plumbing, an archaeologist might well be able to reconstruct a tolerable picture of what it had been like. In Scotland, whilst the lavas might be clearly seen, the old volcano constructs are invariably destroyed, and it is from their foundations, together with knowledge of recent and active volcanoes, that the volcanologist and igneous petrologist can piece together a model of the former volcano.

Terminology is one of the main stumbling blocks. Lack of understanding of the relationship between intrusive igneous rock bodies formed within and beneath volcanoes and the extrusive phenomena with which they may have been associated is another. This book is written as a non-technical account of the volcanic history of Scotland. In general, the older the volcanic rocks the more the accidents and crises throughout geological time (e.g., faulting, folding and erosion) have made the obvious links between their outcrop and a modern volcano ever more tenuous. Consequently, whereas it is traditional in geology texts to start with the oldest rocks and work ‘upwards’ towards younger formations, I am deliberately reversing this convention. I shall start with the youngest rocks and go back in time to features in the oldest rock formations that date from roughly 3000 million years ago. It is easier to understand how features such as the Cuillins, a mere 58 million years old, might represent the sawn-down ruins of a once great volcano than it is to try to do the same with, for example, a much older and highly contorted suite of volcanic rocks such as are seen on the south-west coast near Ballantrae. It is consequently my intention to tell history backwards, inviting the reader to join me in my metaphorical time-machine to consider the origins of the remarkable variety of volcanic rocks that contribute to the Scottish landscape. This account of the ancient volcanoes is intended neither as an academic treatise nor as a field guide. My choice of volcanoes is to a large extent idiosyncratic while, at the same time, including the better known topographic features such as Ben Nevis, Glencoe and Castle Rock (Edinburgh) that are of volcanic origin.

The tectonic plate (or plates if we go back far enough) within which the Scottish rocks formed have, as a generality, migrated on a northerly journey from polar latitudes far south of the Equator to their current position in the northern hemisphere. Thus, if we are considering the more recent evidence (from the Hebrides), we must think of these being some hundreds of kilometres further south, in a distinctly milder climatic zone. I write here of plates ‘migrating’. It is of critical importance to bear in mind that things are not now as they were then. With the passing of time, the geography of the world changes. Just in the past decade, for instance, as a consequence of the earthquake responsible for the December 2004 tsunami, the map of parts of the Indian Ocean, like the coastlines of the Nicobar and Andaman Islands off the Burmese coast, changed irrevocably. On average, the plates move (and change shape) only by millimetres per year, so that over a thousand years the Atlas maps change a little. Over ten million years, however, they change quite a lot. In this book I shall be considering the history of the rocks that now constitute Scotland over hundreds, and indeed thousands, of million years. If we could watch the geography of the world change using time-lapse photography over such immense timescales the shape, sizes and positions of the plates would appear to scuttle around like cockroaches on royal jelly. Continents amalgamate to form ‘super-continents’. In course of time these disintegrate, with new oceans forming between the separating portions. The bits and pieces that currently compose the mosaic that we call Scotland have been components of up to five such super-continents in the past. So, the point I wish to emphasize is that the familiar coastlines and topography of today’s map of Scotland change beyond all measure as we journey back in time. Volcanic processes were closely associated with many of these changes.

A volcano

It is necessary first to define what a volcano is, and to give some background as to how and why a volcano happens. A volcano is generated when molten material is vented from the depths (typically less than 100km) onto the surface. The molten material is called magma. Release of gas may pre-date and/or post-date the eruption of magma, and invariably accompanies its eruption. Eruption of magma may take place through a conduit, which may be roughly pipe-like or one that has the form of an elongated crack or fissure. Hence volcanoes may be crudely subdivided into those of ‘central-type’, where the plan is roughly circular, and those of ‘fissuretype’, where the plan is elongate. The erupted (extrusive) products are gases, lavas (which flow out) and fragmental materials (which are blown out). The sub-volcanic intrusive products are formed from magma that fails to reach the surface. Whilst the gases consist mainly of water and carbon dioxide, they have many other components. At depth the gases are held in solution in the magma. If we consider a champagne bottle, the gas (carbon dioxide) is kept in solution for as long as the pressure in the bottle is maintained. Uncorking the bottle and thereby releasing the pressure allows the gas to come out of solution to produce the sparkling bubbles. We may use this analogy to appreciate that gases (or ‘potentially volatile materials’), dissolved in the magma while the pressures remain sufficiently high, will separate from the melt in near-surface, low-pressure environments.

The lavas represent the congealed magmas that have been degassed to varying degrees. The fragmental deposits may be exclusively derived from the magma, dispersed into particles or droplets, or from the solid rocks that form the sides of the magma conduits (so-called ‘country-rocks’) that have been broken off by the explosive release of escaping gas. Commonly both sources contribute to the fragmental deposits, conveniently termed ‘pyroclastic’ from the Greek stem meaning ‘fire-broken’. The fragments or particles themselves are then ‘pyroclasts’. The terms ‘volcanic ash’ and ‘cinders’ still retain wide currency, although the idea of volcanoes producing ash or cinders dates back more than 200 years to a time when it was believed that volcanoes resulted from underground fires. Nonetheless, since the word ‘ash’ is still used so commonly with respect to volcanoes, I shall retain it rather than use any of the less familiar technical terms.

Where magma reaches the surface, piles of lava and pyroclasts may build up, sometimes forming volcanic mountains rising to heights of between six and seven kilometres above sea-level. If the magma conduit was cylindrical, a conical heap of products will result. Figure 1.1 depicts a simple central-type cone topped by a crater.

Fig. 1.1 Sketch of a simple central-type volcano with a crater and downward-narrowing conduit. (After F. Press and R. Siever, 1983.) Inset: Paracutin volcano, Mexico. (Photo, J.A.Cortés.)

If there are several feeders, a composite multi-vent volcano will be formed. If, as is common, large volumes of magma are stored at shallow levels beneath or within the volcanic edifice, their sudden emptying in a major eruption or by any other mechanism may result in collapse of the unsupported overlying pile, truncating the volcano and producing a large pit called a caldera. A caldera should be distinguished from a crater, which is a smaller hole marking the exit of gases, lavas and/ or ashes that have been blasted out by the materials erupted from it. The diameter of the calderas is believed to roughly equate with that of the magma body (reservoir or chamber) that preceded them; diameters of up to 10km are commonplace although terrestrial calderas several times larger are known.

If, rather than having a more or less identifiable focal point of eruption as in a central-type volcano, the conduit is an elongate split, a fissure volcano can result (Fig.1.2).

Few subaerial fissure eruptions have occurred in historic times, almost exclusively in Iceland. Some of the most recent fissure eruptions occurred at Krafla Volcano, northern Iceland, between 1975 and 1983, when opening of linear fissures, up to a kilometre long, allowed very mobile, incandescent basalt magma to fountain up as ‘curtains of fire’ attaining heights of several hundred metres (Fig. 1.3). The largest historically recorded fissure eruption occurred in 1783 when a fissure, along which a crater chain formed some 30km long, developed at Laki in SE Iceland (Fig. 1.4). Magma fountaining during those months when the fissure was active reached heights of at least 1400m and the plume of gases may have ascended to about 13km. Apart from these eruptions on the oceanic island of Iceland, the only continental fissure eruption in historical times is a small one in the horn of Africa in Djibouti. However, the geological record provides incontrovertible evidence that great fissure eruptions occurred at various times in the past, generally heralding the break-up of continents and the birth of new oceans. In the Scottish record we shall meet these first in chapter 5. Some of these involved fissure systems tens or even hundreds of kilometres long, and it is fair to assume that some of their eruptions will have exceeded that of Laki. The distinction between central-type and fissure-type volcanoes is useful but, as so often with natural phenomena, all intermediate varieties can be encountered and a single volcanic system may, during its active life, change from one to the other.

Fig. 1.2 Block diagram illustrating magma rising along tensional splits forming dykes. Magma reaching the surface gives rise to fissure eruptions of lava. Magma failing to reach the surface may intrude (typically parallel to strata) as near-horizontal sills. (After R. S. Fiske, 1971.)

Fig. 1.3 Fountains of incandescent lava from a fissure eruption, Krafla, northern Iceland, 1980. (Courtesy of Halldor Olafsson.)

Fig. 1.4 Map showing extent of lavas emitted from a NE-SW fissure system at Laki, 1783 and those from Eldgja, nearly 1000 years earlier, SE Iceland. The Laki flows extended c.60km, advancing at up to 6km/day.(Courtesy of Thor Thordarson.)

Some of the steepest-sided volcanoes are those whose products are predominantly fragmental. The stable slopes (or angles of rest) of these are commonly around 35° to the horizontal, much as in ash-piles or slag-heaps. If the fragments become cemented into coherent material they form pyroclastic rocks. For those volcanoes where lavas predominate over pyroclastic materials, the angle of slope is commonly defined by the fluidity of the lavas. However, fragmental deposits of volcanic origin commonly originate through debris flows and avalanching as unstable flanks collapse. Extremely viscous lavas, which may have flow properties more akin to pitch or toothpaste, flow thickly and slowly, and consequently do not travel far from their eruptive vent. Steep-sided domes or spines of such tacky lava will result.

Basalt lavas are far and away the most abundant variety. With temperatures well over 1000°C they are very mobile and flow and spread readily. Huge basaltic eruptions in ancient times appear to have flowed for distances of over 100km from their vents with such low angles of slope as to be virtually horizontal. Such mobile lavas are depicted in Figure 1.3. In brief, the subject of volcano-morphology is complex and volcanoes can present many forms, often differing strikingly from the commonly held image of a simple cone surmounted by a relatively small vent. Furthermore, active volcanoes should be thought of as dynamic entities whose shape may change greatly during their evolution. Of the various factors controlling their geometry, two of the most important are the relative ratio of pyroclastic deposits to lavas, and the viscosity of the lavas.

The constitution of the Earth and the nature of tectonic plates

Before attempting any account of Scotland’s old volcanoes, an overview of the Earth’s structure and the subject of plate tectonics is called for, to gain some understanding of how volcanoes come about and why they occur where they do. The distance from the surface of the Earth to its centre is about 6300km (Figure 1.5). Getting on for halfway down, at around 2900km, we know from a whole series of indirect observations (mainly on the behaviour of earthquake waves) that there is a dramatic change in the physical and chemical nature of the materials. This defines the break between the enveloping mantle and the central core. The core is inferred to be predominantly composed of an iron-nickel metallic alloy, whereas the surrounding mantle is mainly composed of silicate rock. Although the composition of the mantle does not change dramatically from the core boundary outwards, it does show some notable changes in its physical properties, of which the most important are in its density, which decreases in several steps as it approaches the surface. In its outermost density shell (about 650km thick) the rock is mainly composed of the mineral olivine and is known as peridotite, derived from the French word peridot, meaning olivine.

Fig. 1.5 Cut-away block drawing of Earth.

Although rocks appear hard and brittle, they have a Jekyll and Hyde nature; given sufficient time, in conjunction with the application of heat and pressure, they are capable of flow and exhibit some of the properties of a fluid. In contrast, in response to short-term stress (for example, a blow with a hammer!) they will exhibit their familiar brittle behaviour and will fracture. A well-known illustration of this dual behaviour can be found in ice. An icicle is hard and brittle and will snap like a twig. But, given time enough, ice can also flow, as we know from the existence of glaciers. A more dramatic demonstration of a material which, according to the rate at which stresses are applied, changes its character is the silicone material known as ‘bouncing putty’. Thrown on the floor it lives up to its name by bouncing like an elastic solid, whereas a lump left for a while in a saucer will collapse and spread out like melted butter. All of this is relevant because there is now ample evidence that the rocks composing the mantle are not static but, despite staying solid, are in continuous slow motion. The concept that ‘solid’ rocks are capable of flow may take some time to digest!

Small quantities of radioactive elements (for example, uranium) are present in the mantle but are not uniformly distributed. The spontaneous decay of such unstable radioactive atoms generates heat and the irregular distribution of these heatproducing elements produces, over long periods of time, localized ‘hot-spots’ in the mantle. The consequent thermal expansion reduces the density; i.e., it increases the buoyancy, giving rise to the slow ascent of the hot rock through solid-state flow. At the same time, colder and denser portions of rock in the outer zones of the Earth sink deep into the mantle to participate in an age-old pattern of convective overturn, similar to a pan of simmering porridge. Whereas rocks of the deep mantle are confined under such high pressures that melting does not occur, those of the outer part (from a few tens to a hundred kilometres down), and held under lower pressures, are at temperatures only slightly below (and sometimes above) those at which melting commences. The arrival of hot ascending mantle rock, in the fo...