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Structure of volcanoes

The structure of a volcano depends largely on the engineering properties of the rocks have been erupted, the central conduit (feeder channel. The central feeder channel may be a central vent or a fissure, but it is the viscosity of the magma will determine the type of eruption. Acid magmas are more viscous than basic magmas because they are richer in silica. Gases can not escape as efficiently from acid magmas and so these tend to be associated with more violent, explosive events and the volcanoes they give rise to tend to be made more of pyroclastic deposits. By comparison, the more fluid basic lavas give generate volcanoes that are constructed of layers of lavas that represent successive eruptions. Volcanoes that are built from pyroclastic deposits tend to grown much higher than those consisting predominantly of lavas. Single vent eruptions (monogenetic) are therefore smaller and have a less complex form compared to multiple vent (polygenetic) eruptions.

A lava dome is a mass of rock that has been formed when viscous lava is extruded from a volcanic vent. The lava is too viscous to flow more than a few tend to hundreds of metres form the vent before it solidifies. Lava domes may occur along fissures, on the summit of volcanoes or on their flanks. As the lava domes grow, their sides become unstable and they are subject to failure by landsliding or collapsing. These may sometimes associated with explosive events caused by the sudden release of gas. When these process occur a lava domes builds up, usually no more than a few hundreds of metres high, although these may be generated within a relatively small time frame of less than one year.

When viscous magma is explosively erupted this forms pyroclastic volcanoes. These tend to be much smaller than shield and composite volcanoes, reaching several hundreds of metres high. These tend to be located on the flanks of larger volcanoes, as parasitic cones and often they may consist entirely of cinder cones and pumice cones.

Some volcanoes only emit gas, often during the initial stages of an eruption. The explosive forces of the rapidly escaping gas may be sufficient to generate an explosion vent. These types of volcano are small in size and are composed of angular pyroclastic rocks, with a high proportion of country rock, which surrounds the volcano. The vents may be brecciated and filled with mixed, angular rocks.

Composite and strato-volcanoes are produced from multiple eruptions volcanoes are the most common types and consist of alternating layers of lava flows and pyroclasts. The basic, classic type of strato-volcano is cone shaped, with concave slopes and a summit crater from where the eruptions take place. As the volcano grows in height and evolves, radial fissure and parasitic eruptions may occur on the flanks and cinder cones may be scattered on the volcano slopes. The eruptive vents may migrate and therefore some volcanoes have two or more central craters. Violent explosions may also bring about changes to the structure and form of a volcano by blowing away part of the volcano.

When the uppermost part of the magma chamber has been depleted, following a major eruption, the volcanic structure may collapse generating a huge summit crater.  In extreme cases, the collapse of the superstructure of a volcano into the magma chamber generates a caldera that may reach several kilometres in diameter. Volcanic landforms may also be influenced by other geological and geomorphological processes such as landslides and mudflows.

Flood basalts describe the extensive eruptions of basaltic lava flows that cover areas extending hundreds of thousands of square kilometres (Tyrell 1937). For example, the lava flows which form the Antrim Plateau in Northern Ireland are at least 130,000km². The Deccan and plateau in India extends over 640,000 km² and at Bombay reaches a thickness of approximately 3000m. The individual flows are relatively thin, between 1 and 13 m in thickness. There are relatively few pyroclastic deposits. Red-brown, oxidised weathered material on the upper part of the lavas suggests that they were erupted intermittently. They appear to have been erupted from both fissure and central vent type eruptions, and occur throughout the world.

Profiles of volcanic landforms The landforms shown at left and right are vertically exaggerated, and those shown at right are out of scale to those shown at left. In reality a cinder cone would be approximately one-tenth the size of a stratovolcano.(Image Source: Encyclopædia Britannica, Inc.http://www.britannica.com/EBchecked/topic-art/632078/3256/The-major-types-of-volcanic-eruptions ) 

Types of volcanoes

There are several different types of volcanoes, as follows:

The major types of volcanic eruptions. (Image Source: Encyclopædia Britannica, Inc.http://www.britannica.com/EBchecked/topic-art/632078/3256/The-major-types-of-volcanic-eruptions )

Hawaiian (Shield)

Hawaiian type eruptions are characterised by quiet emissions of lava, due to their low viscosity (silica content) which allows the escape of gas. This allows the relatively fast flow of lava which can reach velocities of at least 50 kmh^-1. Lavas may be erupted from a summit crater or from fissures of the flanks of the volcano. Mauna Loa on the island of Hawaii typifies this type of eruption and other examples can be found on Iceland.

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Strombolian

If the lavas are more viscous and gases can not readily escape, moderate explosions can occur. Lava is ejected into the air to form cinders and bombs, and therefore pyroclasts accompany the extrusion of lava. The ejected lavas and molten rocks fall close to the event and congeal in, or around the vent. During the next eruption, gases may escape through these deposits and throw blocks into the air. Steam and gases emitted often form clouds that may linger above the summit crater. The volcanic activity of the island of Stromboli, which is located on the Ionian Sea, north of Sicily and west of Italy, typifies this type of eruption. This type of eruption occurs consistently and at regular intervals throughout recent history.

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Vulcanian

The island of Vulcan, which also located in the Ionian Sea, gives its name to ‘volcanoes’ and the vulcanian type of central eruption. The lava is more viscous than the Strombolian type and therefore after an eruption the magma in the conduit quickly solidifies. Gases subsequently accumulate beneath the solidified lava that obstructs the vent until they reach a pressure that exceeds the strength of the obstructing lava. This results in an explosion, with fragments of lava and gas blown into the air. The lavas erupted breach the sides of the central vent, and flow for some distance before they congeal, and the process continues again.

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Vesuvian

The Vesuvian type of eruption is more violent than the Strombolian and Vulcanian type. The central conduit becomes sealed with a solidified plug of viscous lava allowing the build of considerable gas pressures beneath it. Lava may then begin to escape through routes of less resistance within the rock mass that forms the flanks of the volcano. As a result, lava erupts on the flanks of the volcano through fissures or parasitic vents on the middle and upper slopes. During these types of events the conduit may be emptied to a considerable depth. The release of pressure allows dissolved gases held in solution in the remaining magma to escape. This occurs with enormous force, therefore clearing the conduit of any solidified, obstructions of lava. This subsequently generates an explosion plume that travel high into the atmosphere. Vesuvian type eruptions are followed by periods of long quiescence. For instance, since the eruption of Vesuvius, Italy, in AD 79.

Plinian

A Vesuvian type eruption may increase in violence into a Plinian type. The eruption of Vesuvius and the destruction of the Pompeii and Herculaneum, near Naples, Italy, in AD 79, was a Plinian type of eruption. The last Plinian type eruption on Vesuvius occurred in 1944. The Plinian eruption itself lasted for only a few hours, but it was preceded by nine days of Vesuvian type activity. This type of activity is characterised by lava fountains that reach heights of 1 km and the violent explosion of gas sending an eruption cloud to heights of at least 12 km, where they disperse, level out and form a canopy (Carey and Sigurdsson 1989).

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Peléan

The Peléan type of central eruption is associated with extremely viscous magma which results in catastrophic explosions. Once again, the central conduit becomes clocked with viscous lava, allowing the build up of magma with highly charged gas pressures. When the gas pressure exceeds the strength of the volcano’s flanks the lava is violently extruded, by the expanding gas, through fissures. A cloud of superheated hot gas and ash is formed, called pyroclastic flows (known also as nuee ardentes). These travel along the sides of the volcano with tremendous velocity, destroying everything in its path. Before the eruption of Mount Pele, on the island on Martinique, West Indies, in 1902 this type of volcanic activity had been unknown.

Volcanic products and hazards

Volcanic eruptions generate a variety of products that influence the terrestrial and marine environments and affect people and infrastrcure in different ways. The affects will depend on, for example, the type, magnitude, location and duration of the eruption. The most common products and hazards are outlined below:

Lava flows

Lava flows are streams of molten rock that pour from an eruption vent at temperatures only just above their freezing point (MacDonald 1972). Lava may accompany explosive eruptions, forming lava fountains or non-explosive activity. Lava, once erupted, flows within laminar streams as it travels away from the eruptive vent or fissures, or along the flanks of a volcano. Lava continues to flow until it solidifies at a temperature of between 600 and 900^oC, although this will depend on the chemical composition of the lava, its viscosity and gas content. The velocity of a lava flow is highly variable and is controlled by the gradient and topography of the slope down which it is moving, its consistency (composition, temperature, viscosity and volatile content). Lava therefore flows faster nearer to its eruptive centre and its velocities progressively decreases until it solidifies. Lava with higher silica content will flow slower than basic lavas; the former may travel at speeds of between 30 and 100 km h^-1. Basaltic lavas may travel at least 50 km from the eruptive centre before they congeal and solidify, whereas andesitic flows rarely travel more than 8 km from their vents. Dacite and rhyolite lavas rarely form flows, but instead generate lava domes.

Lava flows consist of numerous individual lava units, each representing a single eruption event. Hummocky, fragmented, splintery surfaces develop on the upper parts of recently deposited lava flows, where they have congealed in contact with the cooler atmosphere. It is this surface texture that allows basaltic lava to be categorised into ‘aa’ or ‘pahoehoe’ types. By comparison the central part may be characterised by polygonal columnar jointing.

Lava flows are the main hazards at basaltic volcanoes. Lava flows are rarely life threatening and deaths caused by lava flows are rare. This is because their directions of flow may be pre-determined and their relative low velocities, ranging from a few kilometres to several tens of kilometres per hour, mean that they can be avoided by people. Deaths caused by lava flows have been attributed to spectators and onlookers approaching an advancing flow, explosions when lava interacts with water, the collapse of a lava delta, asphyxiation due to associated toxic gases, lahars from glacier melt water and pyroclastic flows from lava domes. Infrastructure, houses and buildings will be almost always destroyed by an advancing lava flow. Land affected by lava will, in most cases, not be possible to rehabilitate.

Larger lava flows may be fed by complex lava streams and these may continue to flow beneath a solidified lava crust. However, when the lava supply has been exhausted the stream of lava may drain out of the tunnel through which it has been flowing to leave a lava tube. As these are susceptible to collapse they pose a subsidence hazard.

Cross-section showing phenomena characteristic of the toe of a lava flow. (Image Source: The Oxford Companion to the Earth www.oxfordreference.com/ )

Pyroclastic flows and surges

Pyroclastic flows are high-density, fast moving, mixtures of hot, dry rock fragments, violently expanding hot gases that rapidly move away from the vent (or lava dome) from which they were generated. They are usually associated with acidic, andesitic to rhyolitic volcanoes. Most pyroclastic flows consist of two parts; a basal flow of coarse rock fragments that move along the ground surface and a turbulent cloud of ash (the surge) that rises above the basal flow. Ash may subsequently fall from this cloud, to be deposited by the prevailing wind on the down wind (lee) side of the volcano. Pyroclastic flows travel at high velocities, tens to hundreds to metres per second and high temperatures, between 300 to 800^oC. They may be generated by range of mechanisms. These include an explosive eruption and the subsequent collapse of an eruption column or the gravitational collapse of a lava dome. The latter are known as ‘block and ash flows’ and tend to be smaller is size than the voluminous, pumice-rich ‘ignimbrites’ that accompany eruption column collapses.

Rock fragments in pyroclastic flows vary from ash to boulders in size and typically travel at speeds of at least 80 km h^-1. They also may be heavily loaded with debris which they have obliterated form their paths, such as tree stumps. The direction of pyroclastic flows tend to be controlled largely by the topography and in particular deeply incised stream and river channels that may have been eroded into the flanks of the volcano. The deposits of loose ash and volcanic rock debris vary from 1 to more than 200 m in thickness.

Pyroclastic flows and surges are one of the most life-threatening of all volcanogenic hazards. They are capable of complete total structural damage and destruction to life and land. Substantial pyroclastic flows may also give way to secondary, explosively driven secondary flows, or they may become remobilised to form destructive debris flows. On the margins of pyroclastic flows animals and people may be killed or serious injured as a result of burns and inhalations that accompanies the hot ash and gases forming the surge cloud.

Development of a pyroclastic flow from a lava dome collapse. (Image Source: volcanoes.usgs.gov )

Lahars

The term lahar is of Indonesian origin used to describe flows involving mixtures of water and debris, or sediment-laden flow, in and around volcanoes (Verstappen 1992; Hungr et al. 1987; Macedonio & Pareschi 1992; Caruso & Pareschi 1993). This is a generic term, referring to an event, rather than a deposit. Lahars have also referred to as; hyperconcentrated flows, volcanogenic debris flows, mudflows, debris flows, cohesive and non-cohesive flows; they may be hot or cold (United Nations 1996). This however, implies specific flow behaviour, characteristics and composition (Barclay et al. 2004; Mathews et al. 2002). Lahars occur during and after volcanic eruptions and their properties depend largely on whether they are hot or cold. Cold lahars, often called debris flows, may be triggered by rainstorms, earthquakes, melting snow or landslides. Hot lahars usually also release large volumes of gases and are generally more destructive than the cold lahars. Some lahars are generated by the secondary failure of fresh pyroclastic flow deposits, which may be still incandescent. These may flow as boiling rivers of mud and pyroclastic debris, at temperatures of several hundreds of degrees and for distances of over 100 km. Lahars vary considerably in size and speed. Small lahars may be less than a few metres wide and several tens of centimetres deep and may flow for a few metres per second. By comparison, larger lahars can be hundred of metres wide, tens of metres deep and can flow several tens of metres per second. Lahars, when they are flowing, appear to have the consistency of wet concrete that transports rock debris ranging in size from clay to boulders at least 10 m in diameter. Following a volcanic eruption, the erosion of large quantities of loose debris can led to flooding and extremely high rates of sedimentation on the middle and lower flanks of the volcano. Post eruption lahars, triggered by intense, prolonged rainfall and hurricanes, are able to bury towns, infrastructure and agricultural land. Upstream areas of rivers choked with volcanic debris and lahar deposits often become flooded. If the large becomes large enough it may eventually breach the blockage or break through causing a sudden release of huge volumes of water and sediments and threatening towns and villages located down-stream.

When a lahar is first generated it is accompanied by a characteristic sound that resembles ‘leaves rustling in a wind storm’. The first pulse often has destructive properties that erode the rock mass and vegetation from the floor and side walls of the channel in which they are flowing. This addition debris incorporated into the lahar may result in the lahar growing considerably in magnitude. With increasing distance form the volcano, the velocity of the lahar becomes reduced, the ability to carry larger rock debris is therefore constrained and the heavy loaded material is deposited first. Eventually lahars cease flowing completely resulting in the deposition of mud across flood plains, large tracts of land, or on the floor of the sea or a lake.

One lahar deposit may lie above another, separated by periods of time varying from hours to several hundreds of thousands of years. Lahars are often generated as a series of discrete pulses, or waves. When they reach a break in slope, or a standing body of water (like a lake or the sea) their velocity dramatically reduces and they may spread out to form fans. Lahar deposist can impede drainage channels, block rivers, silt up estuaries and extends the land mass in a seawards direction by the generation of a delta.

The discharge of huge volumes of water from crater lakes may also generate lahars.

For example, in 1919 approximately 3.85 x10⁷m³ of water was expelled from the crater of Mont Kelut, in Indonesia. This subsequent mixed with pyroclastic material to form lahars that killed over 5100 people and destroyed 9000 homes (Suryo and Clarke 1985).

The generation and mechanisms of lahars are complex and appear to be generated in a series of discrete individual pulses, or surges, controlled by sediment and water availability, gradient and channel confinement and morphology (Hodgson & Manville 1999; Lavigne & Thouret 2002; Lavigne & Suwa 2004; (Donnelly 2007a, 2007b, Donnelly et al. 2006). Primary lahars are those generated as a direct consequence of volcanic activity, otherwise, when they are associated with other causes, such as heavy prolonged rainstorm or hurricanes, they may be known as secondary lahars.

Unlike lava flows, lahars can not be out run by people. Lahars that travel several tens of kilometres along river valleys and spread across flood plains often result loss of lives, the partial or complete burial of villages and towns that may become buried beneath a blanket of mud. The aftermath of a lahar may cause serious economic and environmental damage. The direct impact of a the turbulent flows that accompany the front of a lahar may shear off buildings at their foundation level, destroy bridges, choke rivers, block roads and railways, trap and drowned people and cause forests to be felled. The affects on people are magnified if the lahars deposist are deep, soft and hot.

Volcanic gases (volatiles)

When magma is erupted it separates into incandescent lava and a gaseous phase. Steam accounts for 90% of the gases emitted during a volcanic eruption. In fact, volcanic gases are thought to be responsible for the origin of water on Earth, in the atmosphere, on land and in the oceans. The main other gasses which are erupted include; carbon dioxide, carbon monoxide, sulphur dioxide, sulphur trioxide, hydrogen sulphide, hydrogen chloride, hydrogen fluoride. Secondary gases emitted may include; methane, ammonia, nitrogen, hydrogen thiocyanate, carbonyl sulphide, silicon tetra-fluoride, ferric chloride, aluminium chloride, ammonium chloride and argon. The composition of gases emitted from a volcano depends on the initial composition of the magma.

When a magma chamber is situated at a relatively shallow depth, say 7 to 14km, and the temperatures and pressures are relatively low, water tends to move to the top of the magma. This may help to explain the often violent, explosive start to the onset of a volcanic eruption. The resultant gas column may carry pumice and steam several thousands of metres into the air. At high pressures (when the magma is much deeper in the Earth’s crust), the gas is held in solution and the explosiveness of the eruptions will depend upon the rate which the gas is released from solution as the magma ascends to shallower crustal depths. Lavas may frequent contain vesicles, representing gas that has remained in the lava. The rapid escape of this gas may lead to surface frothing of lava and splattering.

Fumaroles are commonly found near dormant volcanoes (for example the Salfatars to the north of Naples, Italy) and near to the summit of active volcanoes. Superheated steam may be observed to violently issue from fissures. These are composed mainly of steam with smaller proportions of carbon dioxide and hydrogen sulphide. Oxygen in the atmosphere may react with the hydrogen sulphide to form free water and sulphuric acid. Where steam escapes through saturated ash boiling mud pits may be formed.

Hot springs are common where active and extinct volcanoes occur. They originate when hot volatiles are driven off during the late stages of a magma chamber as it crystallises and solidifies. Hot springs contain carbon dioxide, hydrogen sulphide, dissolved salts, and when hot enough, dissolved silica. On cooling and where groundwater flows occur they may deposit sinter terraces or travertine. Geysers are hot springs that violent emit water explosively, often at regular intervals, up to about 100m in the air. The time between each eruption is minutes or days, and this varies over time.

Volcanic gases are harmful to life. The gases that present the greatest hazard to life are sulphur dioxide, carbon dioxide and hydrogen fluoride. Volcanic gases which contain fluorine, chlorine and sulphur (sulphur dioxide and hydrogen sulphide) can be toxic, which can also be emitted from hot springs and fumaroles. Fluorine can also poison live stock grazing on ash-coated vegetation and may also contaminate domestic water supplies.

Carbon dioxide is a colourless, odourless gas and does not pose a direct hazard to life as it normally occurs in low concentrations. However, carbon dioxide may become concentrated to levels that are hazardous to people and animals. Since carbon dioxide is denser than air it can sink down the flanks of the volcano, flow into confined spaces, voids and low lying areas, suffocating animals and humans where concentrations of carbon dioxide reach 30% in air.

Sulphur dioxide is a colourless gas with a pungent odour that irritates skin, tissues, membranes of the eyes, nose and throat. It affects the upper respiratory tract and bronchi. According to the World Health Organisation a person should not be exposed to no greater than 0.5 ppm in a 24 hour period. At concentrations of 6-12 ppm this will cause irritation of the throat and nose and at 20 ppm can cause eye irritation.

Chlorine is emitted from volcanoes in the form of hydrochloric acid. Exposure to this gas can irritate the membranes of the eyes and respiratory tract. Concentrations over 35 ppm cause irritation of the throat and >100 ppm result in pulmonary edema and laryngeal spasm. Hydrochloric acid also may generate acid rain on the down-wind flank of volcanoes. It is extremely soluble in condensing water to produce strong ‘acid’.

Fluorine, a pale yellow gas, adheres ash particles, pollutes streams and surface water bodies and covers vegetation. Exposure to this gas can cause conjunctivitis, skin irritations, bone degeneration, mottling of teeth and can cause fluorosis and death following prolonged exposure. Animals that eat vegetation covered with fluorine-ash are poisoned. Like hydrochloric acid it also caused acid rain on the down-wind side of volcanoes.

The cataclysmic eruption of Mount Pinatubo, Phillipines, on 15 June 1991 injected 17 million tonnes of sulphur dioxide gas into the stratosphere, where it subsequently combined with water vapour to form an aerosol (fine mist) of sulphuric acid. Such aerosols can reflect solar radiation and lower the Earth’s average surface temperature for extended periods of time, by several degrees centigrade. In the case of Mount Pinatubo, this resulted in a 0.6^oC cooling of the Earth’s surface in the northern hemisphere. Such aerosols can also contribute to the destruction of ozone in the upper atmosphere. Approximately 130-230 million tonnes of carbon dioxide is released annually from volcanoes into the atmosphere (Gerlach  1999, 1991). By comparison, the emissions of carbon dioxide by human activity, caused for example by the burning of fossil fuels, gas flaring and cement production, is about 30 billion tonnes per year (Marland et al. 2006, Gerlach et al. 2002).

Emissions of Carbon dioxide may occur in the absence of volcanic eruptions. Such events around crater lakes in Manoum in 1984 and Nyos in 1986, Cameroon, resulted in at least 1700 deaths. Carbon dioxide emissions were caused at Lake Nyos by the periodic convectional stirring of water layers in the lake which had different densities (Kling et al. 1987; Baxter and Kapila 1989; Walker et al. 1992).

The long term degassing of some volcanoes can cause damage to agricultural land and respiratory problems in the populations, as is occurring at Poas volcano in Costa Rica. Volcanic gases are also likely to be contributory factors that have a cooling effect on global climate, caused by gases such as sulphur dioxide in the stratosphere absorbing incoming solar radiation.

Tephra, pyroclasts and ash

Tephra is a general term for fragments of volcanic rocks and lava, regardless of size, that are blasted into the air by explosions or carried upwards by hot gasses in an eruption columns or lava fountains. These may range from less than 2mm to over 1 m in diameter.

‘Pyroclastic’ is a collective term applied to material that has been fragmented by explosive volcanic activity. Pyroclasts may consist of lava fragments exploded during an eruption and fragments of country rock that have been blown from the central neck of a volcano. The size of pyroclasts is highly variable depending on the amount of gas coming out of solution, the violence of the volcanic activity and the height of the eruption column. Blocks over 1 tonne in with may be ejected into the air and ash so fine may take years to fall back to the Earth’s surface (or into the sea).

Volcanic bombs are larger pyroclasts consisting of molten lava or wall rock fragments. These tend to fall within a few kilometres radius of the eruption. They may destroy structures on which they fall, ignite crops, forests and homes. Lapilli is pyroclastic material that has a diameter ranging from 10 to 50 mm. Scoria or cinder is irregular shaped material of lapilli size.  These are vesicular, glassy and can be ejected within a radius of several kilometres from a volcano.

Volcanic ash is the finest pyroclastic material. Ash consists of tiny fragments of lava, less than 2 millimetres in diameter, or rock blasted into the air by volcanic explosions. Ash is hard, abrasive, corrosive, conducts electricity when wet and is insoluble in water. More ash is produced from acidic rather than basaltic volcanoes because the former is more viscous which prevents the gas from escaping. Larger diameter, heavier ash particles fall back closer to the eruptive centre whereas finer particles are blown away and may travel hundreds to thousands of kilometres (for example during the eruption on Mount St. Helens in 1980).

Ash deposits show vertical and horizontal variations and their thicknesses generally decrease exponentially with greater distances from a volcano. The resulting ash deposist will depend on the particle size of the ejecta, height of eruption column, rate and duration of eruption and prevailing wind conditions.

As the ash particles becomes compacted, by subsequent ash falls, after they have been deposited on the ground, their bulk density increases by as much as 50% within a few weeks of an eruption and the thickness of the ash deposits decrease over time. The density of ash particles varies from approximately 700-1200 kg m^-3 for pumice, 2700-3300 kg m^-3 for glass shards, 2600-3200 kg m^-3  for lithic (crystals and rock fragments) particles (Shipley and Sarna-Wojcicki 1982). However, actual bulk densities will vary depending on the degree of saturation of the ash, grain size, composition and particle shape (more spherical, well rounded grains will result in a lower porosity and higher bulk density). For example, 10mm of ash may add 19 kg weight to each square metre of roof.

Ash is abrasive due to the hardness of the mineral forming particles and their shape being made mainly of sharp, broken high silica minerals. The minerals within volcanic ash are derived from magma, where they crystallised before the eruption.  Volcanic glass shards represent fragments of the molten magma that cooled and solidified during an eruption. They are the remnants of small gas bubbles that grew in size during the ascent of the magma towards the Earth’s surface. They were subsequently erupted and the rapidly expanding gas disintegrated the bubbles and surrounding glass into shards of various shapes and sizes. Violent eruptions that accompany phreatomagmatic eruptions generate particularly angular shaped shards caused by the violent interaction between water and magma.

Volcanic ash may be spread over large areas by winds and is can turn daylight into complete darkness. Accompanied by rain and lightening, it can cause power outages, hinder communications and disorient people caught up in the ash plume. Ash falls can cover and bury houses, farmland and infrastructure, cause roofs to collapse, destroy crops, block streams and rivers.

Long term exposure to volcanic ash can cause respiratory aliments in people and can lead to silicosis. Ash deposits also may destroy vegetation, which in turn can promote increased rates of surface run-off following rainstorms, increase the availability of loose debris to aid erosion, result in the persistence of air borne ash, increase sediment and debris supply into rivers. In some cases the increased sediment supply combined with running water may severely damage infrastructure, cause the impounding of rivers and flood hazards.

When ash falls in areas which are paved, or there is little vegetation, human activities, moving vehicles and agricultural activities may cause the ash to become mobile. Wet ash has more cohesive properties and will reduce the reworking and disturbance of the fallen ash deposits. The upper surface layers of the ash may also be impermeable caused by the precipitation of insoluble salts by evaporation.

Extremely fine ash particles may be ejected into the stratosphere and circle the Earth for months or years. This can scatter, absorb and reflect solar radiation and may obscure the sun for several hours or day. This may potentially also lead to small reductions in average temperatures.

Airborne ash can obscure sunlight and cause temporary darkness or reduced visibility. Wet ash is hazardous when wet, especially to roads, highways and airport runways that may become impassable. Car and jet engines may stall from ash-clogged air filters and moving engine parts (including breaks, transmission and bearings) may be damaged by the abrasive properties of ash. Since the early 1980s at least 80 civil aircraft have been damaged by flying into ash laden eruption clouds, at least two of these events in Alaska (1989-90) and Indonesia (1982) almost resulted in crashes. The 14 km high ash cloud generated by the 17 September 1986 eruption of the Soufriere Hills Volcano in Montserrat, caused engine and windscreen damage to an Air Canada jet.

The Tambora eruption in 1815 caused the failure of crops due to widespread ash deposits, resulting in 90,000 deaths due to famine. Widespread crop failure and starvation caused by ash falls also accompanied the 1793 Laki eruption, in Iceland, which resulted in the deaths of 25% of the population and 200,000 livestock (Thordson and Self 1993). Most of the 'recent’ volcanic eruptions have also resulted in significant ash hazards including  for example Mount St Helens, USA, 1980; El Chicon, Mexico, 1982; Pinatubo, Phillipines, 1991; Cerro Hidson, Chile 1991; Rabaul, Paupa New Ginea, 1994, Souffiere Hills, Volcano, Montserrat, West Indies 1996-ongoing.

Some comparative volumes of ejecta from volcanic eruptions. (Image Source: Prof. Bill McGuire, University College London)

Explosions

Explosive volcanoes rather than effusive volcanoes have the potential to create widespread structural damage and deaths. Explosions vary enormously in scale, form localised explosions that affect only local communities who live on the flanks of a volcano, mainly by the generation of pyroclasts and bombs, to entire regions. Explosive eruptions are characterised by the Volcano Explosivity Index (VEI) (Newhall and Self 1982).

The VEI is a logarithmic scale from 0 to 8 and is based on the amount of material ejected during an eruption and the size of the eruption column. VEI events which measure 1 to 3 are localised, while the effects of a 4 to 5 eruption can disrupt life on a regional scale. VEI 6 and greater events can affect the population of the Earth through their impact on climatic change. For example, the May 1980 eruption of Mount St Helens in the USA had a VEI of 5 and the 1991 eruption of Pinatubo in the Phillipines had a VEI of 6. Only once every thousand years does a VEI 7 event occur, the last being the 1815 eruption of Tambora in Indonesia. There is no record of a VEI 8 event during the Holocene. The last being the ‘super-eruption’ of Toba, in Sumatra, Indonesia, a cataclysmic event that occurred approximately 74, 000 years BP (Rampino and Self 1992). This event may have triggered global climatic changes and accelerated an already cooling planet into an ice-age. Modern civilisations has yet to face a VEI 8 eruption, however, interestingly, the estimated return period for a VEI 8 is 50,000 years (Simkin and Siebert 1994). This potentially may reduce the average temperature of the Earth by 3-5^oC (Rampino and Self).

Large explosive eruptions may be preceded and accompanied by earthquakes and huge emissions of noxious gases, mainly sulphur dioxide and carbon dioxide, which can cause structural damage and loss of life, respectively.

The Volcano Explosivity Index (VEI) scale.

Volcanogenic landslides (volcanogenic debris avalanches)

Landslide is the relatively rapid movement of a mass of rock, earth (soil) or debris (a mixture of rock and earth) down a slope, under the influence of gravity. Landslides may be classified in several ways depending upon; mode of failure, initial rupture surface, dominant form of displacement, behaviour of the rock and/or soil once movement has commenced and the subsequent deformation of the material (Varnes 1978, 1984; UNESCO 1990, 1993; Dikau et al. 1996; Hutchinson 1988). The type of ‘materials’ displaced during landsliding is recognised by the further division based on the following:

Soil: fine grained engineering soil (silt, clay and organic material)

Rock: insitu bedrock

Debris: coarse grained engineering soil, of sand size or greater and/or an admixture of gravel or boulders, (rock and soil)

Volcanogenic landslides (sometimes called volcanogenic debris avalanches) occur when a sizeable part of a volcanic edifice becomes unstable and slides under gravity catastrophically. These may be associated with an eruption or occur during a period of reduced activity. Such landslides may be generated rapidly, over periods from weeks to months or occur progressively over period of thousands of years (McGuire 1995, 1996; McGuire and Saunders 1993; Siebert 1984, 1992; Donnelly 2007a, Donnelly 2007b, Donnelly et al. 2006). On siliceous volcanoes the gravitational failure of an over-steepened lava dome can lead to the generation of a volcanogenic landslide that may lead to the generation of a pyroclastic flow.

Some volcanogenic landslides frequently begin as a rockslide, which disintegrates during failure, into fragments ranging in size from small particles to large blocks hundreds of metres in size. Volcano landslides have been observed to occur ranging in volume form less than 1 km³ to at least 100 km³. Volcano landslides are major destructive events that have destroy structures and land, cause loss of lives, trigger volcanic eruptions, lahars and tsunamis. Landslides may travel down-slope and up-slope for several hundreds of metres. Volcano landslides are triggered by a number of inter-related factors that may include the following:

Size (height) of the volcano slopes

Deposits of lava and pyroclastic materials

Magmatic intrusions

Hydrothermal activity

Magmatic and phreatic intrusions

Earthquakes

Intense prolonged rainfall

The displacement of a significant portion of a volcano, caused by a landslide, can reduce the confining pressure on shallow magmatic and hydrothermal systems. This can generate explosions and volcanic eruptions that range from small phreatic (steam) explosions, to magma driven blasts. Associated affects may include the damming of stream and rivers, flooding, burying landscapes and the transformation of landslides into lahars.

Historically, the most deadliest volcano landslides occurred at Mount Rainer volcano, Washington, USA and from Mt Mayuyama (in 1782), which is located near Unzen Volcano, Japan. The landslide entered the Ariaka Sea and generated a tsunami that killed 15,000 people (Tsuji and Hino 1993).

On 18 May 1980 a landslide on Mount St. Helens, located in the Cascades Range in the Pacific Northwest, USA, had a volume of 2.5 km³, travelled at a speed of 180-288kmh^-1 (50-80ms^-1) and surged up, then over a 400 m high ridge, some 5 km from the volcano. The landslide triggered the most destructive and extensive volcano in the USA in recent times (Lipman & Mullineaux 1981, Foxworthy and Hill 1982, Tilling 1987). Other notable volcano landslides have occurred at; Otake Volcano, Japan (1984), Huila Volcano, Colombia (1994) and Casita Volcano, Nicaragua (1999).

Volcanogenic earthquakes

Volcanogenic earthquakes are a consequence the movement of magma within the volcanic complex, geothermal and hydrothermal activity and from surface processes such as the collapse of an andesitic dome, explosions, rock falls and eruptions. Volcano-seismicity can result in damage to civil engineered structures and loss of life. These may accompany volcanic eruptive activity or be induced by the movement of magma beneath a volcano without eruptive activity.

Seismic monitoring of volcanoes provides information on earthquakes that accompany the movements of magma, migration of volcanic gases, movement of hydrothermal groundwater systems, high-frequency earthquakes that are associated with rock fracturing, low frequency tremors, explosions and landslides. These earthquakes occur in swarms, less than 10 km beneath a volcano and are normally less than a magnitude 2 or 3. Volcanogenic earthquakes generally have lower magnitude than those caused by tectonic activity, but they may persist for longer, from hours to weeks. Structural damage is often limited to areas in the vicinity of the volcano, especially where houses and buildings have been poorly constructed, and rarely affect the wider regions. For example, volcanogenic earthquakes at Campi Flegrei caldera, Napoli, Italy from 1982 to 1984 caused severe structural damage and resident living in the town of Pozzuoli, located near to the epicentre, were evacuated (Barberi et al 1984).

Volcanogenic fault reactivation and ground fissuring

Aseismic creep may occur along faults located on the flanks of volcanoes. At Mount Etna, Sicily, these ground movements in the vicinity of faults that are situated near potentially unstable slopes are monitored to investigate the landslide and tsunami risks (Lo Guidice and Rasa 1992, Rasa et al. 1996).

Volcanogenic tsunami

Tsunamis may be associated with major explosive volcanic eruptions. Such events occurred during the eruption of Krakatoa, in Sumatra, in 1883, which killed 36,000 people who lived along the coasts of neighbouring islands (Simkin and Fiske 1983). Other notable tsunami events generated by volcanic eruptions occurred in Hawaii (Moore and Moore 1984) and Australia (Young and Bryant 1992). Volcano induced tsunamis may potentially affect and devastate large parts of the Earth from an explosive event of major volcanogenic landslide entering the ocean. In the latter case, such an event has been postulated to occur (by some volcanologists) from the partial collapse of western flank of Cumbre Vieja volcano, on the island of La Palma, the Canary Islands, which may generate tsunamis that could catastrophically affect the eastern sea board of the USA.

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