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Volcanoes have been of concern to mankind since prehistory. For example, the fertile soils that form from volcanic material have encouraged men to farm on their slopes since agriculture began. Some volcanic products such as obsidian were central to the Middle and Near-Eastern neolithic trading industry from 12 000 BC. Vesuvius, a volcano in Italy, was planted with vines right up to the crater rim when the volcano erupted in AD 79, to drown the nearby city of Pompeii in ash.
Volcanoes are named after the Roman god of fire and metalworking, Vulcan. Vulcano itself is a small volcanic island in the Aeolian Islands in the Tyrhennian Sea. Volcanoes are areas where magma (molten rock) erupts (breaks through the surface of the Earth). Strabo (b. c. 60 BC), a geographer who travelled extensively in Asia Minor, interpreted volcanoes as burns where the ground had been scorched and blistered by escaping tongues of subterranean fire. In the nineteenth century, observations of lava flows in the Massif Central of France led an early school of geologists, the plutonists, to propose that all rocks were formed by volcanic activity. However, a German group, the neptunists, whose area of study was the sediments of Germany, were strongly in support of the hypothesis that all rocks were formed under the sea by sedimentation. It was not until a few mutual field trips had been undertaken that the two schools were able to come to an agreement whereby both methods of rock formation were admissible. The study of volcanoes and volcanic rocks is called volcanology or vulcanology.
The behaviour of magmas, and hence the type of volcanic edifice that they form, is dependent upon their composition and the volume of lava erupted. Gas-rich lavas, particularly those that are of high viscosity, form large quantities of cinders and ash which collect in a cinder cone. Where gas-free, or low-viscosity, lava erupts, the absence or ease of escape of gas means that the eruption is quiet and the volcanic landscape is dominated by lava flows. This type of eruption forms a shield volcano. The volcanic terrain of Iceland is dominated by shield volcanoes. The composition and volume of the volcanic rocks is dependent upon the magma-generating processes, which are the result of plate-tectonic activity. Specific tectonic environments are thus associated with particular types of volcanic edifice. The most abundant volcanoes on land are found round the margins of the Pacific Ocean where plates are being destroyed by subduction, a process commonly associated with volcanic activity. These chains of volcanoes are sometimes called the circum-Pacific Ring of Fire.
Volcanoes
Most magmas erupt at about 1200 °C, at which temperature they glow red; hence the popular idea that volcanic material is aflame. Gases that are dissolved in the magma until the time of eruption bubble out (exsolve) as the magma approaches the surface of the Earth. The material that erupts out of the ground is a mixture of molten rock, solid rock, and gas. Figure 1 is a diagram of a volcano, showing the main features of an eruption. The volcanic liquid is called a magma while it is still inside the ground and a lava after it has been erupted. Magma is fed to the eruption from a magma chamber. The pressure of the rock overlying the magma chamber forces the liquid out of the ground. The hole from which lava erupts is known as a vent. While the melt is resident in the magma chamber it cools, crystals form in it and bubbles rise out of it. Many of the crystals remain suspended in the melt while the remainder sink to the bottom of the magma chamber to form a cumulate rock. When irregularities on the roof of the chamber overflow with bubbles there is a rush of gas through the vent. Experimental work on bubbles collecting in magma-chamber-shaped tanks of water or syrup show that bubbles forming, collecting, and escaping in this way can lead to periodicity in volcanic eruptions.
Fig. 1. Principal attributes of a volcano during an eruption. If a strong wind is blowing, the ash cloud will be elongated in the direction of the wind.
If the hot magma reaches the water table before it reaches the surface of the Earth through a vent, the water boils and expands violently, causing an explosion. This type of eruption, called a phreatic eruption, is characterized by a volcanic crater that goes beneath the level of the surrounding landscape and is encircled by a wide, low ring of volcanic debris. During a phreatic eruption the early magma is chilled to form a glass, which is broken by the explosion to form angular clasts of pelagonite. Where there is no groundwater and the magma is gassy, it travels to the surface and sprays out of the vent in a fire-fountain, the bubbles in it expand, and small vesicular stones called scoria (or lapillae) are formed. The bubbles in lava are called vesicles. If, after the lava has cooled and set, the vesicles are filled with minerals, they are known as amygdales.
Small vesicular pieces of volcanic material, scoria, collect around the vent to form a cinder cone. As cinders build up near the vent the pile becomes unstable and eventually cinders roll down the slope so that the cone, although it grows larger, maintains its profile. Smaller vents on the flanks of a volcanic cone also form small cones of their own, called parasitic cones. Large masses of volcanic material falling down the sides of the volcano are called debris flows. The debris flows in the 1980 eruption of Mount St Helens (Washington State) contained enormous volumes of material. Detritus incorporated into these flows included trees and large logging machines that were being used in the vicinity of the volcano at the time of eruption.
If the lava is very viscous (sticky), and there is a great deal of gas present, pumice is formed. Pumice is a very pale rock of low density, composed of a foam of glass enclosing gas bubbles. Pumice is of such low density that it can float on water. Submarine volcanoes have been known to erupt so much floating pumice that ships have been stranded in mid-ocean. Sometimes there is so much gas in the magma and the siliceous magma is so viscous that the gas cannot form bubbles quickly enough and the pumice is exploded into Y-shaped sherds. These sherds are blown out of the volcano as ash. Occasionally, volcanic glass is spun out into fine threads like candy floss, called Pelée's hair.
Lava flows
When gas from the magma chamber has been exhausted, the lava erupts more quietly to form a broader and lower edifice, a shield volcano. If a suitable crater has been formed, a lava lake develops. The surface of the lava lake freezes while the liquid underneath continues to convect, disturbing it and creating patterns reminiscent of moving tectonic plates. Should the wall of the lava lake be breached, lava escapes and moves down the side of the volcano as a lava flow. When the lava flow reaches flat land it spreads out to form a lava field. A lava field is composed of large numbers of flows winding through the field. Wherever the side of a lava flow breaks, lava gushes out and starts to form a new flow. Figure 2 shows how a lava flow advances. The molten lava in contact with the air cools quickly to form a skin over the flow. As the lava flows onwards, this skin is left behind as a lava tunnel. Lava tunnels may be several kilometres long, but in many instances the roof of the tunnel collapses to form a trough in the lava field, along which a later lava flow may pass. At the downhill termination of a flow, lava flowing down the lava tunnel from the volcano accumulates behind a flow front which is made of solidified lava. The pressure of the lava behind builds up on the flow front until it breaks and molten lava pours out. When this lava solidifies, it forms a new flow front. Thus, lava flows advance in small steps. The new lava flows over the debris that has fallen down the old flow front to form a type of basal breccia called an agglomerate.
Fig. 2. Cross-section showing phenomena characteristic of the toe of a lava flow.
When a lava flow meets trees or other obstructions, it flows round them. The intense heat of the lava causes trees to catch fire. After the lava has solidified the trees are burnt away by the heat of the lava flow, leaving behind tree-shaped holes. The carbonized remains of trees and plants can be used to date recent lava flows by the carbon-14 radiometric dating method.
As lava flows along a tunnel, gas exsolving from the lava collects immediately beneath the roof of the tunnel. Where there is a hole in the roof, gas bursts out and forms a cinder cone in the middle of the lava flow, called a hornito. Eventually the supply of lava to a tunnel ceases and the flow stops advancing. The higher parts of the tunnel are evacuated and lava ponds at the downhill termination of the flow. As the ponded lava cools, it shrinks and cracks. The cracks form perpendicularly to the cooling surface (the top of the flow) unless there are elongated vesicles, which act like the perforations in postage stamps, bending the cracks round them. Many joints in lava flows are vertical cracks which divide the solid lava into polygonal columns. A well-known example of this phenomenon of columnar jointing is the 'Giant's Causeway' in Antrim, Northern Ireland.
The surface of a lava flow weathers, particularly in wet climates, to form a rich, reddish volcanic soil, called a bole. Although soil may form on the surface, lava flows are usually quite resistant to erosion. Where volcanic material has been extruded on to a soft substrate, the rate of erosion of the substrate can exceed that of the lava flow. Where this is the case, the lava, although having originally flowed down a valley, becomes the site of a ridge. Should more lava flow through the area, it, like its predecessor, will flow down the valleys, with the result that the older lava is topographically higher than the new lava. This is contrary to normal stratigraphical principles, which dictate that older rocks are usually preserved underneath younger rocks.
The propensity of lava to seek out topographic lows means that it often flows into river beds or the sea. Contact with water causes chilling and fracturing of the lava to form a hyaloclastite. Where there is an eruption underneath a glacier, hyaloclastites are particularly common. Lava erupting into deep water forms 'pillows'. Initially it is squeezed out like toothpaste from a tube into the water. Then the outside of the lava freezes to form a glassy, vesicle-free skin which inflates like a balloon until the surface is ruptured and a new pillow begins to form. Internally the pillows are characterized by concentric layers of vesicles. While the pillows are still partially molten they are compacted together so that they display convex upper surfaces and downwards-facing cusps at their contacts. Sediments that have settled on the lava are cooked by the heat of the new pillows. Where these sediments contain clay (as would be expected in an oceanic environment) jasper, a semi-precious red form of silica, may be formed.
Plinian eruptions
Plinian eruptions are those that are dominated by a plume of hot gas and ash. They are commonly associated with Pliny the Younger, who described them after the eruption of Vesuvius in AD 79. The eruption buried the town of Pompeii in ash, drowned nearby Heraculaneum in lava, and caused the demise of his uncle, the elder Pliny, a natural historian.
Hot gas and ash that are pushed out of a volcanic vent form a jet. The force with which the gas is pushed out diminishes quickly away from the vent, but where the ash and gas are hot they rise through the atmosphere as a plume. As the plume rises it mixes with, and heats, cold air to become more buoyant and rise still further. (If a strong wind is blowing, the plume will be asymmetric.) Under certain conditions (depending on the velocity of the volcanic jet and the size of the vent) the ash cloud may become unstable and collapse down the side of the volcano. The hot cloud of gas and ash ingests air as it rolls down the side of the volcano as a density current. The heat and speed of the cloud are such that the shearing forces within the cloud are large and fragments of pumice and ash are crushed and welded together. Streaks of crushed pumice are called fiammé and are diagnostic of an ash deposit called an ignimbrite. If the ash deposit has been welded but does not contain fiammé it is called a welded tuff. Ash deposits that have no fiammé and have not been welded are called simply tuffs. These tuffs may be compacted and turned to stone (lithified) during later geological events. The large quantities of gas (including water vapour) and fine ash particles that are emitted by volcanoes often result in violent rainstorms: water vapour from the volcano and from the atmosphere is nucleated by the fine ash particles to form rain clouds. The deluges of rain upon the volcano slopes, which may be augmented by melting ice, help to mobilize ash and debris flows (lahars).
In addition to scoria and ash, other volcanic ejecta include blocks, bombs, and xenoliths. Rocky material formed by the accumulation of large ejecta is classified as agglomerate. Blocks and xenoliths are pieces of material that are broken off the sides of the volcanic feeder as the magma rises from it source. Blocks are pieces of volcanic material that have been picked up from the volcanic edifice; xenoliths are pieces of non-volcanic country rock. If the magma has risen quickly from the source region of the volcano, the xenoliths may represent country rock from all levels of the crust through which it has travelled. Bombs are pieces of molten lava that are thrown out of the volcano. They are named according to their form. For example, bread-crust bombs are those whose outer surface has solidified while the inside is still evolving gas and expanding as a foam. As the inside expands it cracks the outer shell, giving it the appearance of the crust of a loaf of bread. Splatter bombs are those that do not have time to solidify before they hit the ground. They have a morphology reminiscent of a cow-pat.
Some volcanic vents emit only gases. Water, carbon dioxide, and sulphurous gases are common volcanic gases. Where the vent is in a hollow, the gas collects under certain weather conditions to form a pool of noxious gas. Pools of carbon dioxide have been reported as forming on the Icelandic volcanoes overnight; the casualties are usually livestock that have taken shelter in the hollow. If a gas vent opens into a pool of water, the gas, particularly carbon dioxide (which has a higher solubility at high pressure), dissolves in the water until it reaches a critical point. At this point the gas bubbles out of the water, causing the waters of the pool to overturn, which causes more evolution of gas. The most devastating example of a volcanic gas eruption in recent years was the outgassing of Lake Nyos in Cameroon on 21 August 1986. The waters of Lake Nyos overturned one cold night, releasing an enormous surge of cold carbon dioxide gas. The gas rushed down the valley in a deadly, invisible cloud, suffocating 1700 people in their beds.
After a magma chamber has been exhausted and eruption has ceased, the centre of a volcanic edifice may collapse into the void that represents the former site of the magma chamber. This process of volcanic collapse is called caldera collapse. A lake may collect in the caldera until such time as more magma arrives beneath the volcano to end the period of quiescence. Many caldera lakes have islands in the middle, formed where more magma has arrived and erupted in the centre of the lake. A lava dome of this type may be a prelude to further activity. Volcanoes that are prone to erupt but are not doing so at the time of inspection are designated dormant, while volcanoes that are not expected ever to erupt again are extinct.
Judith M. Bunbury
Fischer, R. V. and Schminke, H.-U. (1984) Pyroclastic rocks. Springer-Verlag, Berlin.
Francis, P. (1993) Volcanoes: a planetary perspective. Oxford University Press.
From The Oxford Companion to the Earth
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