Preface

Europe, despite its many changes through history, has been home to volcanoes for many millions of years. These volcanoes pay no attention to human foibles such as historical periods, political boundaries (and how they change) and scientific definitions. Thus, the title Volcanoes of Europe disguises several kinds of arbitrary choices. We have included, for instance, the Canary Islands and the mid-Atlantic islands of Jan Mayen, Iceland, Svalbard, and the Azores within the European umbrella, although two of the Azores and half of Iceland belong to the North American plate, and the Canary Islands belong to the African plate. On the other hand, we do not describe the volcanoes of Turkey and the Caucasus, which many would, no doubt, call European.

It is altogether more difficult to define those volcanoes that are active, dormant, or extinct. Volcanoes do not always display the secrets of their past, nor do they always reveal their future intentions. Several times, even in the course of the twentieth century, expert volcanologists have been puzzled – not to say surprised – when certain volcanoes have suddenly burst into life after a long period of calm. Volcanoes are generally considered active if they have erupted in the last 10,000 years, though such a value may include volcanoes that are effectively extinct. It is also valuable to consider the older records of many of the volcanic areas as they help reveal how these spectacular landscapes develop and grow.

The notion of historical time is also extremely flexible, and historical records count for little within the defined span of 10,000 years. Even within the limited European context, the period during which eruptions could actually be recorded has varied greatly from place to place. Probably no volcano on Earth has a longer recorded history than Etna, where eye-witness accounts have recorded its eruptions, with admittedly varying degrees of fantasy, for thousands of years. However, the Italian volcanoes were in an exceptionally favoured position in the classical world. On the other hand, records in Iceland extend back only to the early centuries after the settlement in AD874, and no human being even settled in the Azores until 1439.

Beyond the historical context, accurate dates of eruptions are only just becoming available in many areas. The traditional methods of geological dating by fossils and stratigraphy are very hard to apply to volcanic edifices. The timespan is too short; the volcanic products preserve few animal or vegetal remains; and the erosion of valleys and their subsequent occupation by further lavas make large and active volcanoes a stratigrapher’s nightmare. In many cases, too, the most recent eruptions have masked the products of their predecessors to such an extent that the story of the volcano can be difficult to read.

In recent decades, new techniques of absolute dating have done much to overcome these handicaps. Radiocarbon dates have been calibrated with greater precision, and volcanic rocks can be dated by thermoluminescence, argon dates (K–AR, Ar–Ar), and palaeomagnetic and archaeomagnetic studies, among other techniques. A whole range of these techniques is now being applied, especially to those more dangerous volcanoes whose tempestuous past must be discovered before their future furies can be predicted with accuracy. Nevertheless, the absolute dates of many European eruptions have yet to be established.

The availability of information is a further element that imposes its own limitations on any treatment of European volcanoes. In spite of the boom in volcanological research during the past few decades, some volcanoes are still imperfectly known. Thus, several Italian volcanoes are in intensive care, whereas those in the Azores, for instance, have undeservedly progressed little beyond the waiting list. Consequently, the balance and the treatment of active European volcanoes is, in part at least, influenced by the amount of the scientific literature that is available. The most recent of eruptions are being recorded at great length with a whole array of digital technologies from the ground, the air and up into space with satellites. Yet the older eruptions require the diligent studies of many scientists, and for the information to be made into the public domain. In this context, this contribution has used the wealth of information made available by the scientific and public communities that work on the volcanoes of Europe, as well as those gained by the authors in their volcanic careers.

Volcanoes hold a fascination that we have from a young age, and it is something that we would like to foster. The chief aim of this study is to stimulate a wide range of readers to encourage them to take an active, informed interest in some of the most sublime and fascinating volcanic features in the natural world and, especially, to go and see them. You can, in Europe, go and see an active volcano such as Stromboli, or explore ancient remains of these natural wonders. Aesthetic rewards will also enhance any scientific or knowledge-gaining pilgrimages, because the European volcanoes embellish landscapes beyond compare.

Authorship and acknowledgements

This updated colour version of Volcanoes of Europe has been put together by Dougal Jerram, expanding on the original content and scope of the first book, and with some new colour images provided by Jean-Claude. Updated colour images are used throughout, and new information is also included on the latest eruptions that have occurred. As with any of these ventures, debt is owed to many individuals and groups who have offered direct help, informative discussions over the years, and who have shared in a passion for earth science and particularly volcanoes. I will not be able to give credit to all here, but you know who you are. I must thank those who took part as Meet the Scientists: Matteo Lupi, Dave Pyle, Val Troll, Helen Robinson, Steffi Burchardt, Sigurður Gíslason, and Ben van Wyk de Vries, who also supplied materials. Specific help with parts and materials was also given by: Don Kaiser, John Howell, Angelo Cristaudo, Paraskevi Nomikou, Vikki Martin, Brian McMorrow, Josef Schalch, Sarah Gordee, Juan Carlos Carracedo, Alfredo Lainez Concepción, Breno Waichel, Paul Cole, Carlos Miguel, João Luís Gaspar, Abigail Barker, Frances Deegan, Morgan Jones, Sverre Planke, John Millett, Henrick Svensen, Reidar Trønnes, Torgeir Andersen, Trond Trosvik, Bæring Steinþórsson, Thórdís Högnadóttir, Jón Viðar Sigurðsson, Allan Treiman, Rolf Pedersen, Pierre Boivin, Cecile Olive-Garcia, Jörg Busch, Walter Müller, Universal Postal Union, TodoTenerife Team, Instituto Geográfico Nacional, Kappest/Vulkanpark, National Geopark Laacher See, US Geological Survey, NASA. Also, those people who made materials available to use through creative commons licences (credited along with the specific pictures) and from the Shutterstock archive. In the final construction of the book I give specific thanks to Helen Robinson for redrafting some original images, Francis Abbott for his efforts with Spain images, and Ben van Wyk de Vries for many efforts (both text and figures) within the chapter on France, as well as Anne Morton, David McLeod and Sue Butterworth for their great work in copy-editing, drafting and design and indexing. Final thanks to Anthony Kinahan at Dunedin for his continued support for the Volcanoes of Europe project. Dougal Jerram (August 2016).

In the original B/W edition Jean-Claude Tanguy was primarily responsible for the sections on Etna, Vesuvius, Pantelleria and parts of the Aeolian Islands. Alwyn Scarth bore the responsibility for the remaining chapters; he gratefully acknowledges the invaluable assistance of Juan-Carlos Carracedo, Victor Hugo Forjaz, Harry Hine, Maxime Le Goff, and especially Anthony Newton.

The photographs without explicit credits were taken by the authors.

Chapter 1 An introduction to volcanology, igneous rocks and the volcanoes of Europe

There are a magnitude of varying processes and ways in which igneous rocks are formed and emplaced both as intrusions and as volcanic units. As we explore the volcanoes of Europe we will be exposed to the whole variety of rock types and volcano types that we have on our planet, and in many ways some of the most important and well-studied examples that we have are found in Europe (Fig. 1.1). Volcanism and its plumbing can be explored in a number of ways. The first way follows the ‘present is the key to the past’ ethos, in that we can observe volcanism in the present day, where it occurs, and how the resultant material is defined (e.g. Fig. 1.2A). Just like sedimentology and geomorphology, understanding the modern systems can help us a great deal in deciphering the past rock record and palaeo-environments. In Europe there is also a rich history of Man that goes back thousands of years, where volcanoes and their eruptions have been observed and recorded in script and in art (e.g. Fig. 1.2B). This anthropological volcanology becomes more important in Europe’s recent past, when volcanoes such as Vesuvius are described erupting in great detail.

**Figure 1.1** The volcanoes of Europe provide a great opportunity to explore all the different types and styles of volcanic eruption and their plumbing systems (photo of Eyjafjallajökull courtesy of Jón Viðar Sigurðsson).

**Figure 1.2** How we look at and record the volcanoes of Europe. A) Sampling at the lava front, Holuhraun eruption Iceland 2014. B) Artistic impression of Vesuvius (Vesuvius from Portici by Joseph Wright of Derby: 18th century painting). C) Inspecting volcanic deposits from Santorini.

Finally, in outcrop we can use the normal range of field skills to measure and record ancient volcanic deposits, and to decipher the longer-term history of the volcanoes, where no other record of the eruptions is available (e.g. Fig. 1.2C). With the vast range of igneous rock types and the styles of deposits, we may be faced with everything from volcaniclastic sediments, lava flows or intrusions, to combinations of them all. In this case a basic understanding of the igneous rock types and styles of volcano will be useful. Additionally we can learn a great deal about the European volcanoes by understanding how rocks melt and where volcanoes occur on the planet. This in turn will help us understand the distribution of volcanoes in Europe, and why there is such a wide and rich variation.

Igneous rock types

The first thing to consider when looking at volcanic systems and their deposits is the variety of igneous rocks possible. Igneous rocks are classified according different relative amounts of key minerals such as olivine, feldspar and quartz. Each mineral is made up of its constituent elements, and varying the quantity of each mineral type will produce a different rock composition. The volcanoes of Europe cover all the possible magmatic/volcanic types, so a familiarization with some of the terminology and classification is helpful when reading this book. In general, igneous rocks range in composition from those relatively rich in iron and magnesium (known as mafic) to rocks that are rich in silica and aluminium (known as acidic). Rocks can be classified by their grain size as well as by their chemistry (see Fig. 1.3). This is used to help distinguish plutonic rocks (which have cooled slowly at depth), from shallow intrusions and volcanic rocks, which erupt at the Earth’s surface and cool very quickly. With plutonic rocks, the slow crystallization leads to large crystals, whereas a rapid cooling results in very fine-grained crystals or even glass.

**Figure 1.3** How we classify igneous rocks. A) Classification based on abundance of key minerals and crystal size. B) The Total Alkali-Silica plot used to classify igneous rocks from their geochemistry.

A simple classification scheme for igneous rocks is presented in Figure 1.3A, which combines the relative compositions of the rock types with the grain size to give a number of rock classification names. In some instances there may be some chemical analysis of the rocks, and where this is available the rock units can be classified according to the silica and alkali contents (e.g. Fig. 1.3B). Basic rocks rich in iron and magnesium range from coarse-grained gabbro through medium-grained dolerite to fine-grained basalt. Acidic rocks range from coarse-grained granites, through micro-granites to finegrained rhyolite. Intermediate rocks include andesites and dacites, etc. (see Fig. 1.3). You may also come across rocks being termed ‘basaltic’ in composition, which can sometimes be used in a non-grain size sense, as well as the term ‘granitic’ to indicate silicic compositions.

Why rocks melt and where volcanoes occur

In order to understand at what temperature different rocks melt, and at what temperatures different crystals will start to solidify from molten rocks, a number of simple, yet highly revealing, melting experiments were used. A classic study in the early 1900s by Norman L. Bowen used melt experiments to show that minerals such as olivine, pyroxene and (calcium Ca-rich) plagioclase crystallize from hotter, more mafic compositions (e.g. basalt), and minerals such as quartz, potassium feldspar, and micas from cooler acidic compositions (e.g. granite). In the Earth the rate at which the temperature increases with depth is known as the geothermal gradient, and commonly ranges from 25–30°C per kilometre of depth in most parts of the world. Exceptions to this are areas of thin crust/hot spots with very high geothermal gradients (30–50°C/km) and thickened crust in subduction settings (5–10°C). If we were to consider the geothermal gradient alone, we would soon reach the temperature to melt rocks. This is where the other factor of pressure comes in. If you try to melt rocks at high pressures, you need even higher temperatures than normal to do so. This simple relationship means that, for most circumstances on Earth, the pressure/temperature relationships are such that the rocks are below their melting point, and not hot enough to melt.

So how do we get rocks to melt? To help us understand this, we can consider a simple plot of an average geothermal gradient as shown in Figure 1.4. Also contained in this plot is a line known as the solidus. This marks the temperature/pressure relationship with depth at which melting will occur. At normal conditions the geotherm does not cross the solidus line, and so no melting will occur. However, if water is added to rocks (as happens at subduction zones) their melting temperature drops. This has the effect of moving the solidus line towards the geotherm, and if they cross, the rocks will start to melt. The other way of getting rocks to melt is by removing pressure from them. If we can quickly remove the pressure from a rock faster than it is allowed to cool, we will have the effect of raising the geotherm, as the rock will be hotter than expected at a shallower depth (see Fig. 1.4C). It can be shown that temperatures of ~700°C or greater are needed to melt rocks (this is at the low end for granitic compositions and in excess of 1100°C for basalts). Melting occurs at depths of 50–200km on average, and we need either to add fluids to the rocks or to release the pressure in order to get them to melt.

So where and how do we get volcanoes on earth? Well, this boils down mainly to plate tectonics and the convection of the Earth as it cools. The surface of the Earth is made of a number of rigid plates that are moved around on a more ductile, deeper mantle. The plates are made of the crust and the lithosphere, and can be continental or oceanic. As the Earth cools and convects, the plate boundaries are either pushed/rubbed against each other, or are pulled apart. The Earth cross-section on Figure 1.4 displays a schematic picture of various plate-tectonic scenarios involving both oceanic and continental plates.

Where plates pull apart, at constructive/divergent plate boundaries, we get upwelling of mantle and decompression resulting in melting, and rift volcanoes. Where plates subduct beneath one another (destructive/convergent plate boundaries) we introduce water and fluids into the hot mantle and cause melting where we lower the melting point, also resulting in volcanoes. Finally, where hotspots occur, marking where hot plumes of mantle material rise independently of the plates, we get decompression melting and volcanoes (see Fig. 1.4). Thus at various settings on the Earth we can generate melt, and this explains why we have volcanoes on Earth. As we explore the volcanoes of Europe, we shall see how different plate-tectonic settings and hotspots are present, helping us to elucidate the origins of our European volcanoes.

Types of volcano

We can consider volcanoes firstly in terms of their morphology, and then in terms of the types and size of eruptive events (next section). This first step is important as volcanoes can have very similar morphologies but on quite different scales, for example from the smallest basalt lava flows to some of the largest basalt flows, known as flood basalts. The basic categories of volcano are presented in Figure 1.5, and we shall briefly consider the main morphological types below.

Fissure vents form as linear features on the Earth’s surface and essentially represent lava erupting out of faults or cracks in the Earth’s crust. Fissure vents can be found at constructive plate margins where the crust is being ripped apart under extension, e.g. Iceland and the Afar rift in Ethiopia, but are also common in hotspot volcanoes like Hawaii. In settings where the main lava type is predominantly basaltic, shield volcanoes are formed. These are large volcanoes that have a very broad relief, due mainly to the low viscosity of the lavas that make them up. Although they have a relatively low profile, some examples can reach great heights due to their immense size (e.g. Mauna Loa, Hawaii 4169m (13,679ft). Cinder/scoria cones develop from small to moderate scale eruptions where pyroclastic scoria are erupted in fire-fountain and Strombolian type eruptions (see next section). These cones are very common and can be found as isolated or clustered vents in volcanic fields or along fissures.

**Figure 1.4** How and where rocks melt. Three-dimensional plate boundaries model (USGS) and simple graphical sketches of how to modify solidus or geotherm to promote melting. The numbers highlight the location of melting styles depicted in the graphs.

**Figure 1.5** Types of volcano with examples (modified from USGS).

Composite volcanoes (stratovolcanoes), rise up from shallow slopes at the base with steep-sided tops to the volcano, resulting in a cone-shaped mountain. They are a common form of volcano, often occurring in areas where more explosive silicic eruptions dominate. The sides of the volcanoes are littered with a mixture of both lava flows and pyroclastic debris, and commonly a number of valleys run off from the sides, which channel pyroclastic flows and debris flows from the volcano. Stratovolcanoes are mainly acidic (silicic) in composition, with rhyolites and dacites, and hence their association with explosive eruptions, but they can show the full compositional spectrum including andesites and basalts. Lava domes are bulges of new lava that form in the craters of volcanoes. These often occur after large explosive eruptions and can be termed resurgent domes. Calderas/caldera volcanoes are a type of volcanic landform where the profile of the volcano, instead of being that of a classic cone or conical shape, reflects a circular/semi-circular depression on the surface. Such depressions or calderas are formed by the collapse of the land after the evacuation of magma from a shallow chamber that was present before the eruption.

Some volcanoes that are intricately involved with water are also significant. Submarine volcanoes occur when a volcano starts to erupt on the sea floor, and build up initially as underwater volcanoes. In some instances the build-up and growth of the volcanic edifice fills the water space and the volcano breaks through the sea surface with dramatic effects, known as an emergent volcano. Other water-influenced volcanoes include subglacial volcanoes, where an eruption occurs under ice. These are very complex in that they involve interactions between magma, water and ice, and are also constrained by the ice that covers them. Having introduced the types of volcano, we shall now look at the different styles of eruption and their relationship to the size of the eruptive event.

Types and scales of eruption

Types of eruption range from those that are not very explosive and have little or no eruption column to the largest type of explosive activity. An additional factor that can affect the style of eruption is the interaction with water or ice. If you have predominantly lava flows, explosive products in your volcanic basin will depend on the prominent type of eruption, and it should be noted that the style of eruption can change with time and also switch back and forth. A general classification of eruptions has developed, based on, and often named after, key volcanic events (Fig. 1.6). This captures the general scales of eruption based on observations of the eruption column and the explosiveness of the event. Below we shall briefly go through the different types of volcanic eruption; examples of some of these are also presented in Figure 1.7.

Surtseyan eruptions occur when a submarine volcano erupts through the waves and starts to produce a new volcanic island, also known as an emergent volcano. This style of eruption is named after the example that occurred off the coast of Iceland in 1963, leading to the formation of the Island of Surtsey (e.g. Fig. 1.7A). More generally they are known as hydrovolcanic (also phreatomagmatic) eruptions, signifying the interaction of lava with water.

Effusive eruptions are the very simplest of volcanic eruption on land, where lava effuses out of a vent forming a lava flow, and little or no explosive activity is involved. Such eruptions can occur commonly at lava lakes when they fill up and spill over, but it can often be difficult to develop a predominantly effusive eruption without some sort of more explosive event at its start. Hawaiian eruptions are more commonly seen in basaltic-dominated volcanoes. They are intimately associated with effusive eruptions, and this style of eruption can often feed much larger lava flows through time. Lava fountains that reach a few hundred metres into the air are common, and examples of this style of eruption along fissures can sometimes result in a wall of fire, which collapses to the ground to feed lava flows. A classic example of this type of eruption is the recent Icelandic eruptions of 2014 and 2015 (e.g. Fig. 1.7B).

**Figure 1.6** Scales of volcanic eruption. A) Eruption column height vs. explosivity used to define types of volcanic eruption (note the addition of water helps promote explosivity). B) The Volcanic Explosivity Index in graphical form with some European eruptions listed (adapted from USGS).

**Figure 1.7** Examples of different eruption types and volcanoes with European examples. (3D images by Ivan d’Hostingue (aka Sémhur), inspired by the document about volcanism from the web portal on the prevention of major risks from the French Minister of Ecology, Environment and Sustainable Development) (photo A, NOAA; photo D by Brisk_g; photo E courtesy of Jacques Cordier).

Strombolian eruptions, some would say, are the most picturesque of volcanic styles, named after the volcano forming the Island of Stromboli in the Mediterranean. A Strombolian eruption is driven by the bursting of gas bubbles within the magma. Strombolian activity is characterized by scoria and bombs of hot material being ejected as the bubble bursts. In extreme cases, columns can measure from hundreds of metres to a few kilometres in height, but more commonly a Strombolian eruption looks somewhat like an energetic Roman candle firework when viewed at night (e.g. Fig. 1.7B).

Vulcanian eruptions, named after the 1888–1890 eruptions on Vulcano Island (Fig. 1.7C) in the Mediterranean, are more explosive still. These eruptions result from more viscous magma (andesite/dacite), from which it is difficult for gases to escape. The pressure builds up and eventually the blocked top of the volcano gives way, with an initial set of cannonlike explosions sending bombs and blocks through the air. The deposits contain a much larger amount of ash than their Strombolian counterparts, with eruptive columns reaching up to 10km. Some of the high gas contents in Vulcanian eruptions have been attributed to the interaction of the magma with groundwater, known as hydrovolcanic eruptions. In August 1888 to 1890, the island of Vulcano erupted with a number of explosions sounding like cannons going off at irregular intervals – these were the iconic eruptions that gave their name to this type. Peléan eruptions (also termed nuée ardente) are named after the 1902 explosive eruption of Mount Pelée in Martinique, and are similar in scale to vulcanian eruptions, but characterized by hot, glowing clouds of pyroclastic flows. These are driven by the collapse of domes, formed in the volcano’s crater, which over-steepen and collapse.

Plinian eruptions are the largest types of eruption seen on Earth. They are named from the famous AD79 eruption of Mt. Vesuvius, observed by Pliny the Elder and Pliny the Younger. These eruptions are very powerful and result in a large column of ash and ejecta and a cloud of ash that travels high up into the atmosphere (up to 55km) and stretches out horizontally where it reaches the stratosphere (e.g. Fig. 1.7D).

In order to place some measure of the relative sizes of explosive volcanic eruptions, a classification scheme called the Volcanic Explosivity Index (VEI) has been developed. This uses a number of criteria to estimate the overall size and force of a volcanic event. The VEI runs from 0 where eruption is not explosive through Small (1), moderate (2), moderate-large (3), large (4) and very large (5–8). These different sizes in turn are related to a type or style of eruption (see table 1.1, Fig. 1.6B).As you can see from this table, the different types of eruption discussed above can be placed into the VEI with relative descriptions of explosivity and the height of the eruption column (as in Fig. 1.6).

The largest eruptions – Large Igneous Provinces

The largest eruptions that have occurred on earth are not necessarily the most explosive ones, but are made up, commonly, of basalt rocks similar to those that are erupted on Iceland and Hawaii today. These volcanic events are known as flood basalts and have occurred at key points in the Earth’s history. Some of the largest individual eruptions measure thousands to tens of thousands in cubic kilometres, and can cover vast areas of land. Through geological time the Earth has experienced a number of large outpourings of lavas known as Large Igneous Provinces (LIPs), which have been mapped out to show their distribution (e.g. Fig 1.8). These LIPs or flood basalt provinces (a term used when they occur on land) have been associated with significant events in Earth’s history, such as the break-up of supercontinents like Gondwanaland, and have been linked with extinction events. In the context of the volcanoes of Europe, the North Atlantic Igneous Province represents one of these large events that formed part of the break-up of Europe from North America. In this case the flood basalts are found outcropping in the UK and the Faroes Islands and are found extensively offshore UK and Norwegian margins. The ancient volcanoes that helped feed this massive province are exposed in some of the most classic examples of fossil volcanoes in the British Tertiary, and the volcanic activity in Iceland today is a legacy of this process, which started some 60 million years ago.

The term ‘Super Volcano’ has been used to describe a volcano that has the capability of erupting >1000km³ of material in a single event (100–1000 times larger than historic eruptions). Clearly the eruptions associated with LIPs fit into this category, with many eruption events occurring with volumes in excess of 1000km³, with the largest estimated at around 8000km³. In the modern context six current volcanoes have been identified as possible supervolcanoes based on their previous activity: Yellowstone, Long Valley, and Valles Caldera, US; Lake Toba, Indonesia; Taupo Volcano, New Zealand; and Aira Caldera, Japan. Within Europe there is also one of these sleeping giants, Campi Flegrei, Italy, which in the past has erupted large volumes ~500km³ of material (e.g. the Campanian Ignimbrite ~39ka).

Table 1.1 The Volcanic Explosivity Index with examples from European volcanoes (There is a discontinuity in the definition of the VEI between indices 1 and 2. The lower border of the volume of ejecta jumps by a factor of 100 from 10,000 to 1,000,000m3 while the factor is 10 between all higher indices).

Figure 1.8 Map of Large Igneous Provinces around the world with the main eruption phase in red and the trails of plumes/hotspots in blue. The North Atlantic Igneous Province straddles the European plate and North American plate with its current expression of the Iceland plume still erupting today (image courtesy of Mike Coffin).

The plumbing systems of volcanoes

With the most modern volcanic systems we are mainly dealing with the eruptive products, e.g. lava flows, tephra, etc. Yet even in the modern systems it is useful to have an insight into the possible pathways or plumbing systems beneath our volcanoes. In some instances these can be excavated in caldera-forming eruptions such as that at Santorini, or they are exposed over time through erosion, which can be either very fast or over hundreds of thousands, if not millions, of years. A variety of types and geometries of intrusion can be found that feed the volcanic system (e.g. Fig. 1.9). These are often simply split into sheet-like intrusions, forming sills and dykes (e.g. Fig 1.10), and those more irregular and often low-aspect-ratio (large width vs. height) laccoliths, plugs and volcanic centres. It is important to have a grasp of these volcanic feeding systems, as they can actually be monitored in real time beneath active volcanoes (for example, the recent activity in Iceland 2014–2015).

More commonly, we need some erosion to explore the magma plumbing at depth. In dynamic environments such as Iceland, relatively young volcanoes can be excavated by ice and floods and their insides exposed. In other areas the fragility of the volcanic edifice can cause collapse, exposing material. Some of the most classically studied examples of the insides of volcanic systems are somewhat older and carved by glaciers and deeper erosion. In this context we can consider the volcanic centres in the British Palaeogene Province (formally the British Tertiary), as these eroded volcanoes formed a vital linchpin in connecting observations made at the modern systems in Europe with what must have been happening at depth.

Figure 1.9 Schematic to show the types and styles of plumbing system beneath volcanoes.

Understanding the volcanoes of Europe

The distribution of the volcanoes of Europe is perhaps more diverse than on any other continent because of the complications caused by the collision between the Eurasian and the African plates. Most of the volcanoes occur on the margins of the European continent: in the Mid-Atlantic Ridge, the Canary Islands, southern Italy, and the Aegean Sea. The remaining areas of volcanic activity are broadly associated with old systems in France and Germany. So we have volcanoes on constructive margins related to the growth of the Eurasian plate along the Mid-Atlantic Ridge; others related to collision and subduction linked to the clash between Europe and Africa; and others that have no plate boundary connection and seem to relate to hotspots (e.g. Fig. 1.11). The old structural grain within Europe has a role to play in some of the locations of volcanoes, as well as the complex plate organizations and relationships in the Mediterranean, with small terrains and mini-plates complicating a simple plate-tectonic explanation.

The volcanoes on the Mid-Atlantic Ridge are clearly linked to the growing edge of the Eurasian plate. Eruptions are largely submarine and continuous along the whole length of the Ridge, which has been explored by remote geophysics and with submarine-based sampling. These eruptions occur chiefly from multitudes of fissures that produce the basalts that make the world’s oceanic crust. In Jan Mayen, Iceland and the Azores, the crest of the Mid-Atlantic Ridge and part of its flanks have been built up above the waves, so that this vital volcanic activity can be inspected and analysed at close range. The emissions on these islands are not wholly basaltic, and they have also included eruptions of more evolved magmas after reservoirs have developed, and their complex relationships with the mid-Atlantic ridge and hotspot activity have, in some sense or another, helped in their emergence from the ocean. The Canary Islands, which lie on the African plate, seem to have been generated by a complex hotspot system that does not interact directly with the plate boundaries. Practically all the volcanic islands in the North Atlantic Ocean represent considerable accumulations of volcanic material. Their bases often lie more than 2000m deep on the sea floor, and several volcanic peaks rise more than 2000m above the waves. Thus, Beerenberg in Jan Mayen, Öraefajökull in Iceland, Pico in the Azores, and Teide in the Canary Islands, form some of the most prominent mountains in the North Atlantic Ocean – and are, at least, on a par with any volcanic centres in the rest of Europe.

Figure 1.10 Sills and Dyke intrusions with examples. A) Santoríni, Greece. B) Kilt rock, Isle of Skye, Scotland. C) Streymooy Sill, Faroes.

The volcanoes in Italy and Greece are closely linked to the prolonged collision of the African and Eurasian plates, during which several microplates/terrains have detached themselves from their parent masses and pursued varying and independent courses. This adds to the complexity of volcanism in the Mediterranean. At the same time, the edges of the microplates and the Eurasian plate were smashed, fractured and crumpled as the African plate advanced broadly northwards. Thus, continental sediments carried on the plates were thrust up and contorted to form the Atlas Mountains, the Alps, the Apennines, and the chains of the Balkans, Greece, and Turkey; and subduction/collision-related magma made its way to the land surface up major faults that transect the Earth’s crust along these sutures. Etna and Vesuvius may have formed in this way, as complex volcanoes straddled between subduction and collision. Parts of the forward edges of the African plate were also subducted beneath the Eurasian plate and the adjacent microplates, in a simpler sense. This subduction caused the eruptions that formed the Aeolian Islands and the volcanic islands in the Hellenic Arc in the Aegean Sea. Subduction is thus probably responsible for the most violent European eruption during the past 4000 years, on the Greek island of Santoríni, and for the world’s most diligent volcanic performer in modern times, Stromboli. But these Mediterranean volcanoes are as varied as the tectonic conditions that have given them birth. Some, such as Etna and Stromboli, have long histories of moderate and mainly basaltic eruptions; others, such as the Fossa cone at Vulcano, have erupted tuffs in more vigorous outbursts; yet others, such as Santoríni and Vesuvius, have erupted huge volumes of fragments (pyroclasts) during episodes of great violence that buried whole cities. In the case of Campe Flegre we have a system associated with tectonics, but maybe also a more localized hotspot activity, which is capable of very large, >500km³ eruptions, as demonstrated with the Campanian ignimbrite (~39ka).

Figure 1.11 Summary map of the volcanoes of Europe. (Map based on NASA World Wind.)

The third main group of European volcanoes formed broadly in relation to the discontinuous rifts that traverse the continent from Oslo, in the north, to the Rhine Rift Valley and on to the Limagnes of central France. Eruptions have given rise to many cones and maars, in both the Eifel Massif in Germany and the Chain of Puys in central France, and the association of these with complex heat anomalies under these parts of Europe has been the subject of debate in recent years.

The Mediterranean volcanoes

The volcanoes and the major fold structures of the Mediterranean area have been caused fundamentally by the collision between the Eurasian and African plates. However, this generalization hides a great complexity of events that perhaps has no equal anywhere else on Earth. Although the Mediterranean area has been intensively studied, much work still needs to be done before the intricacies of the scenario of its development can be truly unravelled. It is thus difficult to explain the distribution and the causes of the Mediterranean volcanoes without making generalizations that may prove to be misleading, inadequate or even inaccurate as research progresses.

The collision takes place between two plates carrying continents that are themselves directly involved in the impact, which has shattered their edges, formed micro-plates that have moved in different directions, and crumpled and faulted the rocks for millions of years. Thus, collision has not only brought about subduction and deep faults that transect the whole crust, but also areas of crustal extension. As a result, individual Mediterranean volcanoes can have several different causes or, indeed, combinations of causes, which in some instances straddle our classic view of volcanic settings (e.g. Fig. 1.4). In broad terms, subduction seems to be responsible for the Greek volcanoes on the Hellenic arc in the Aegean Sea, and for the volcanoes in the Aeolian Islands, whereas Etna and Vesuvius perhaps owe their growth to eruptions at the intersection of deep major faults and complex subduction. But the relationships are far from simple, and the specialists rarely agree about the exact details of the course of events.

With this varied tectonic background, it is not surprising that the Mediterranean volcanoes have displayed virtually the complete range of eruptive styles. Thus, Vesuvius has been extremely violent, often erupting Plinian columns and pyroclastic density currents, but was largely effusive from 1631 to 1944; Etna has been chiefly effusive, but had some violent outbursts about 2000 years ago; Stromboli has been mildly explosive for many centuries; and Santoríni has produced only moderate eruptions since its great explosion in the Bronze Age. Thousands of smaller, yet not less fascinating, eruptions have formed cinder cones, lava flows and domes, and fumaroles and mudpots are commonly an indication of magma simmering close to the surface. Italy is the zone of greatest tectonic complexity, and it has also been the forum of the greatest, most varied and most closely studied volcanic activity in Europe.

The Mid-Atlantic Ridge

The Mid-Atlantic Ridge is the most clearly defined, and probably the best known, of all the mid-ocean ridges. It forms a sinuous curve bisecting the Atlantic Ocean from the Arctic to Antarctica, in a continuous chain of volcanic accumulations rising 2km or more from the ocean floor. A longitudinal rift runs along its crest, which marks the site of continual volcanic eruptions, where oceanic crust is generated as the North American and Eurasian plates diverge (e.g. Fig. 1.11). The main source of these eruptions is the multitude of fissures and dykes that run parallel to the trend of the crest. They have provided the basaltic pillow lavas and the black smokers that are characteristic of this environment. Generally speaking, the youngest volcanic rocks occur at the central parts of the ridge, whereas increasingly older rocks are found in roughly parallel strips further and further from the crest.

The ridge is mostly submerged, and it is only in exceptional circumstances that volcanic eruptions are so frequent as to have built it above sea level. The most common explanation for these exceptional conditions is that a mantle hotspot/plume lies beneath the mid-ocean ridge, and this seems to be the most likely reason for the emergence of Jan Mayen, Iceland, and the Azores as some of the culminating points on the Mid-Atlantic Ridge. Jan Mayen lies on the Eurasian flank of the ridge; Iceland is transected by the ridge so that its western part belongs to the North American plate and the eastern part to the Eurasian plate; and the ridge divides the Azores. Conversely, the Canary Islands are apparently not related to the Mid-Atlantic Ridge, and seem to have developed chiefly in response to one or more hotspots beneath the African plate.

The study of the islands along the Mid-Atlantic Ridge also shows significant variations from the simplified convergent margin pattern. Major offsets develop in the trend of the Mid-Atlantic Ridge that are associated with notable transform faults and fracture zones more or less at right angles to the trend of its crest. One of their broad effects is to create further fissures up which magma can then rise. They also tend to facilitate activity on the flanks of the ridge, where eruptions can build up and widen the ridge itself. Thus, the Azores rise from a broad, submerged platform on the flanks of the ridge. The rocks on these flanks usually increase in age with their distance from its crest, but the materials emitted during the flank eruptions are much younger than the rocks upon which they lie, and they do not usually increase in age with their distance from the crest of the ridge.

The morphology of the Mid-Atlantic Ridge is further complicated near the Azores by the development of a triple junction. Rifting occurs not only between the Eurasian and North American plates but also between the Eurasian and African plates. A zone of secondary spreading seems to have developed, and may have formed a microplate supporting the central and eastern Azores.

The fourth rather abnormal feature of the ridge is the eruptions from central clusters of vents, which are often related to flank volcanism. In Iceland, for instance, they often form large basaltic shields and pahoehoe surfaces, but sometimes more explosive eruptions take place if the magma has undergone some evolution in a reservoir. In these conditions, the prevalent basalts are replaced by intermediate lavas such as andesites or even rhyolites. At the same time, lava flows become less numerous and fragments become increasingly important as a stratovolcano is constructed. In this way, for instance, Hekla and Oraefajökull have grown up in Iceland, and Beerenberg in Jan Mayen. In the Azores, most of the stratovolcanoes have also undergone a markedly explosive phase that led to the formation of large calderas on their summits. Iceland, too, has more than a dozen calderas, some of which, like Grímsvötn, are hidden beneath ice caps. Clearly the interaction of hotspots with the ridge leads to more complex crust and subsequently more complex volcanic systems.

Iceland is by far the largest emerged zone of any midocean ridge in the world, and it is the best studied of the three European components of the Mid-Atlantic Ridge. The growth of the much smaller and less complex island of Jan Mayen has also been broadly elucidated. In the Azores, where the mixture of eruptions is greater, several important detailed studies in recent years have begun to clarify the picture of their development.

Summary

With some of the most iconic volcanoes in the world and some of the most intriguing historical examples of the struggle between volcanoes and Man, Europe’s fiery past and its explosive present make for a fascinating subject. In the Volcanoes of Europe, we will explore the main active volcanic areas and some of the most recently active areas in Europe, as well as taking a look into the ancient volcanic centres that helped forge the study of volcanology and igneous geology. The structure of this book will look at the regions of interest from Italy (chapter 2), Greece, Spain, Portugal, Iceland, Norway, France, and finally Germany. Where possible we will touch on examples of Man’s interaction with, and recording of, some of the most infamous eruptions, and we will find out from modern scientists about some of the exciting work that is going on at the volcanoes of Europe in our ‘meet the scientist’ sections. A glossary is provided to help you navigate through some of the terminology, and a list of the most well-documented eruptions in Europe’s history is also provided.