WHAT IS A VALCANO

Unlike mountains formed by folding, a volcano is a mountain formed by the accumulation of its own products such as lava, ash flows and airborne ash and dust. Volcano also refers to the opening through which molten rock and gases are expelled.

Where did the word “volcano” originate? The word “volcano” comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, people living in this area believed that Vulcano was the chimney of the forge of Vulcan – the blacksmith of the Roman Gods. They thought that the hot lava fragments and ash clouds erupting from Vulcano came from Vulcan’s forge as he beat out thunderbolts for Jupiter, King of the gods.

What is an eruption? This occurs when magma rises from a storage reservoir and finally reaches the surface of the Earth. As it rises, the magma fractures overlying rocks, which causes earthquakes, and deformation also occurs as magma approaching the surface makes room for itself.

How much of planet Earth is volcanic? More than 80% (including above and below sea level) of the earth’s surface is volcanic. Countless volcanic eruptions have produced mountains, which subsequent erosion and weathering have shaped into majestic landscapes and formed fertile soils.

How many active volcanoes are there in the world? About 500, excluding those that are below sea level.

What is the “Ring of Fire?” Volcanoes are not randomly distributed over the Earth’s surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific “Ring of Fire.”

How many persons worldwide have been killed so far by volcanic eruptions? Scientists have estimated that at least 200,000 persons have lost their lives as a result of volcanic eruptions during the last 500 years.

What are some of the advantages of volcanic eruptions? The Earth’s crust, on which we live and depend, is in large part the product of millions of once-active volcanoes and tremendous volumes of magma that did not erupt but instead cooled below the Earth’s surface. Volcanic ash increases soil fertility for forests and agriculture by adding nutrients and acting as mulch. Groundwater heated by hot magma can be tapped for geo-thermal energy. This hot groundwater also has concentrated valuable minerals, including copper, tin, gold and silver, which are mined throughout the world.

What was the largest volcanic eruption of the 20th century? This eruption occurred in 1912 at Novarupta in Alaska. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6th. However, owing to the isolation and sparse population of the region affected, there were no human deaths and little property damage.

What is the world’s largest active volcano? Mauna Loa in Hawaii is the world’s largest active volcano, projecting 13,677 feet above sea level, its top being over 28,000 feet above the ocean floor. From its base below sea level to its summit, Mauna Loa is taller than Mount Everest, the earth’s highest peak.

What causes earthquakes? An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault. Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years.

What is a pyroclastic flow and where did the term originate? The term “pyroclastic” – derived from the Greek words pyro (fire) and klastos (broken) – describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Pyroclastic flows, sometimes called “nuees ardents” (French for “glowing clouds”) are hot, often incandescent mixtures of volcanic fragments and gases that sweep along close to the ground. Depending on the volume of material, proportion of solids to gas, temperature and slope gradient, the flows can travel at velocities as great as 450 mph. Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility. During the 1902 eruption of Mount Pelee in Martinique (West Indies), a nuee ardente demolished the coastal city of St Pierre, killing nearly 30,000 inhabitants.

What is the difference between magma and lava? Scientists use the term “magma” for molten rock underground and “lava” for molten rock that breaks through the Earth’s surface. In other words, lava is magma above the ground.

Glossary of Volcanological Terms

Andesite	The name given to volcanic rocks that have chemical compositions intermediate between basalt and rhyolite.
Ash	This is defined as volcanic particles less than 4 mm in diameter.
Ballistics	These are large fragments forcibly ejected by volcanic explosions which are typically about 0.5 to 2.0 m in diameter.
Caldera	A large depression generated by the collapse of a volcanic edifice into its magma chamber. Transfer of magma from the chamber to the surface causes the edifice to founder, either as a coherent piston or chaotically as a jumble of blocks. Some calderas form due to prolonged eruption of basaltic lava, like on Hawaii. The world’s largest calderas form during explosive eruptions and can be several tens of kilometres wide and up to two or more km deep. For example, the great caldera of Lake Toba in Sumatra is 100x30km. It formed by collapse associated with more than one ignimbrite eruption, the last of which was about 75,000 years ago. Some small calderas are only a few km across. The composite caldera of Santorini is 6x8km and extends to 400m below sea level. It formed as a result of at least four explosive eruptions during the last 200,000 years.
Conduits	These are paths along which magma rises to the earth’s surface from the magma chamber.
COSPEC	This means Correlation Spectrometer and is the instrument that measures the amount of sulphur dioxide in the atmosphere from the absorption of ultraviolet radiation from the sun.
Cristobalite	This is a particular kind of crystalline silica and it forms by precipitation of tiny crystals in the pores of the Soufriere Hills andesite and by devitrification of the volcanic glass.
Deformation	This is one of the principal phenomena monitored during an eruption. The surface of the volcano responds to changes within the interior of the volcano or deeper in the magma chamber. The volcano can swell (inflation) or subside (deflation) to allow inferences to be made about the magma pressure.
DOAS	DIfferential Optical Absorption Spectroscopy. A method of processing the UV spectral data used at MVO, whereby the "blue" or background atmospheric spectrum is subtracted from the spectra of the emissions plume - allowing  calculation of SO2 emission rates from the volcano.
Gases	Volcanic gases are dissolved in the magma at depth in the chamber and are released in the low-pressure environment of the earth’s surface. The main volcanic gas is usually water with minor amounts of sulphur dioxide, carbon dioxide and halogen gases such as chlorine and fluorine.
GPS	This stands for Global Positioning System. It is a navigation system which allows the location of a point on the earth’s surface to be determined precisely. This is done when a receiver obtains signals from satellites which allow the location to be fixed.
Hornblende	A complex hydrous silicate of calcium, magnesium, and iron which occurs as crystals in the lavas of Montserrat.
Hybrid	This term is used with earthquakes. A hybrid earthquake is characterised by seismic signals containing long and short frequencies.This type of earthquake occurs at shallow depth (usually less than 2 km) and is interpreted as fractures forming under high gas pressures. Hybrid earthquakes occur when magma is rising.
Inflation	This describes the swelling of a volcano due to an increase in internal pressure.
Lava Domes	These are morphologies formed by eruption of extremely viscous or semi-solid magma which piles up around and above the vent. The lava is too viscous or stiff to flow away from the vent and a thick pile forms.
LP Quake	This means long-period earthquake. They have low frequencies and are though to be formed by the flow of pressurised gases along fractures which are caused to resonate.
Magma	This is molten or semi-molten rock that has sufficiently melted to be able to flow to the earth’s surface and erupt. The ‘magma chamber’ is the region which supplies magma to the volcano.
Magma Chamber	Reservoir of magma underneath the volcano.
Magnetite	Natural magnetic iron oxide, Fe3O4.
Mineral	A naturally occurring crystalline substance. Magmas crystallise different minerals as they cool from high temperature. Common minerals in rocks include olivine, pyroxene, plagioclase, magnetite,.and hornblende. Many lavas consist of millimetre-sized phenocrysts of different minerals set in a fine groundmass, the crystals of which are only visible under a microscope.
Mudflows	These are called lahars. They are concentrated slurries of volcanic debris and water. They form during and after heavy amounts of rainfall.
Phenocrysts	Crystals in a lava, pumice, or scoria which form by slow cooling of the magma in the magma chamber. When the magma rises to the surface it cools more rapidly and the remaining melt either chills to a glass (as in pumice or scoria) or crystallises to a fine mesh of microscopic crystal called the groundmass (as in many lavas). Phenocrysts in Montserrat rocks may reach a millimetre or more in size, and may sometimes occur in clusters.
Phreatic	This term is used with explosions. Phreatic explosions are caused by ground water being heated by rising magma to high temperatures. The phase change from superheated liquid to vapour close to the earth’s surface causes explosive activity.
Plagioclase	A mineral with the chemical composition calcium sodium silicate formed by the crystallization of all magmas across the range basalt to rhyodacite.
Plinian eruption	A sustained, explosive eruption which forms a high, jet-like column of pumice and ash in the atmosphere. As silica-rich magma such as dacite or rhyodacite rises from depth, dissolved gases come out of solution and form bubbles. When the percentage of bubbles reaches about 50%, the magma froth blows itself apart, forming a mixture of pumice, ash, and rock fragments dispersed in gas. This mixture then accelerates to the surface and erupts at supersonic speeds. Plinian eruptions commonly last several hours and lay down thick layers of pumice as pyroclastic fall deposit. The clouds of such eruptions may be 30 km or more high and reach well into the stratosphere. Named after the AD79 eruption of Mount Vesuvius, Italy, described by Pliny the Younger.
Pumice	A porous rock formed during explosive eruptions, generally of silica-rich magma. Gas dissolved in magma at high pressure comes out of solution as the magma ascends towards the earth’s surface. This forms a froth which then fragments violently, driving the explosive eruption. Pumice clasts are pieces of the magmatic froth chilled to glass in contact with the air. Pumice commonly contains phenocrysts of plagioclase, hornblende, and magnetite. About 80% of many pumices are void space, causing them to float on water.
Pyroclastic	This is a general term for fragmented volcanic material. A pyroclastic flow is a concentrated flow of hot pyroclastic fragments. These flows are formed by dome collapses and contain a mixture of blocks and ash.They are also formed when there is an explosive eruption to form an avalanche of pumice blocks and ash. Pyroclastic surges are dilute turbulent suspensions of pyroclastic particles in volcanic gas and air that form a density current.
Pyroclastic flow	A highly mobile avalanche of high-temperature volcanic debris. Pyroclastic flows are common products of eruptions at Montserrat and elsewhere. They form by the gravitational collapse of lava domes and also (on a much larger scale) during explosive eruptions such as those which generate calderas. In the latter case such flows are believed to form by the fountaining of high eruption columns and may have very high speeds. Velocities as high as 60 m/s have been measured on historic pyroclastic flows, and some are believed to travel as fast as 250 m/s. Owing to their high velocities and to fluidization by escaping gases, some pyroclastic flows may travel many tens of kilometres from the source vent, and may even travel large distances across the sea, as well documented at Krakatau in 1883. Pyroclastic flows which contain abundant pumice give rise to the deposits called ignimbrites.
Pyroxene	A family of minerals with the general chemical composition calcium, magnesium, iron silicate.
Rockfalls	These are small-scale rock avalanches and falls of individual rock from a growing dome.There is no definite boundary between a rockfall and a pyroclastic flow, although in general pyroclastic flows develop when avalanching rocks disintegrate to form a large amount of fine-grained ash.
Scoria	Porous, glassy rock formed by the rapid chilling of frothy, relatively silica-poor magma such as basalt or andesite. Scoria is typically dark-coloured; it is denser than pumice and does not float on water.
Spine	This is a protrusion of semi-solid lava that form at the surface of may lava flows. In some volcanoes the lava dome is in fact a single large spine.
Steam Vent	This is formed where superheated water reaches the earth’s surface and boils explosively to form a jet of stem from a vent. This is common when rising magma heats ground water.
Swarms	This term is used a lot with earthquakes. An earthquake swarm simply means a large number of volcanic earthquakes that occur over a definite period of time.
Tectonic	This term is used to describe the deformation and movements of the earth’s surface, to a large extent as a consequence of the movements of tectonic plates.Volcano-tectonic earthquakes or VT’s are generated by rock breaking as magma pushing its way to the surface.
Tiltmeters	These are instruments that measure changes of angle of the ground slope on the flanks of a volcano. Slope increases (inflation) as the pressure in a volcano increases and decreases (deflation) as pressure decreases.
Vents	These are surface openings through which volcanic materials (magma and gas) are emitted at the earth’s surface.