Introduction

A volcano is a vent through which molten rock escapes to the earth's surface. Unlike other mountains, which are pushed up from below, volcanoes are built by surface accumulation of their eruptive products - layers of lava, ashflows, and ash. When pressure from gases within the molten rock becomes too great, an eruption occurs. Volcanic hazards include gases, lava and pyroclastic flows, landslides, earthquakes, and explosive eruptions.

Let's first take a look at the earth. The earth is made up of 4 main layers-the inner core, the outer core, the mantle and the crust. The crust is where we live, on the surface of the earth. Below the crust lies the mantle, which is made up of solids, liquids and gases.

The lithosphere is made up of the crust and the upper most layer of the mantle, and is divided up into 16 major plates. As the plates of the lithosphere shift, they disrupt portions and pockets of the mantle below it. This disruption causes steam and gasses within the mantle to into liquid magna-these pockets of liquid magma are called "hot spots". The liquid magma then makes its way up through the lithosphere and erupts from onto the earth as lava. The places where they erupt are called volcanoes.

Where these plates meet are where most of the earth's volcanoes are located. Plates can shift towards each other, away from each other, or they can shift side by side. The way in which these plates move in relation to each other cause the different types of physical conditions that create volcanoes and earthquakes.

Parts of a Volcano

This cross-section shows the parts of a volcano. A conduit feeds magma to the surface. Near the surface, the gas expands and fragments the lava into ash. Some magma passes through dikes to feed vents. Some magma intrudes parallel to layers to make sills.

Image source: http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/parts.html

Main Vent
The Main vent is the path taken by the liquid rock from the Magma Chamber to the surface. It is like a pipe up which the lava can flow. Sometimes the main vent has branches which, if they reach the surface may form Secondary Cones or Fumeroles.
When a volcano erupts, lava, gasses and fragments of rock travel right up the main vent and are thrown out through the crater. When the eruption finishes the lava may drop back down the pipe or form a lava lake in the crater.

Lava Flow
When magma reaches the surface it is called lava. ( pronounced 'larver'). If the level of lava in the crater overflows the rim, it will begin to flow down the sides of the volcano. Different types of lava are more runny than others and the most runny types can flow down the slopes at over 30 miles per hour.
Although the rock is liquid, is is very dense. If you stood on it, you wouldn't sink, but you would burn. The rock surface may seem to have cooled and set, but below the very thin crust the lava can remain liquid for a long time, at temperatures of 500 ° or more.

Ash and Lava Strata
The volcano in the diagram is made of layers, coloured light and dark grey. These are the layers of ash and lava which have been thrown out of the volcano during its life. Ash consists of small fragments of rock, some as fine as tiny dust particles, other chunks being bigger than your fist. The layers of lava are old lava flows which cooled and set as they flowed down the volcano. The volcanic ash is usually thrown out of the volcano before the lava. It settles to the ground and forms a steep sided pile. You can see the same effect by pouring salt onto a flat surface. It will form a pyramid, the steepness of which is determined by the size and shape of the individual salt grains. The ash is soft and is easily worn away by wind and rain. The layers of lava which flow over the ash protect it from erosion and thus allow the volcano to keep growing upwards.

Secondary Cone
When the Main Vent develops branches, the volcano may begin to grow secondary cones. The rock and ash layers which make up the volcano are often cracked and weakened by the explosions that occur during eruptions. If these cracks form a path from the main vent to the surface, magma is able to move up the new pipe and reach the surface. As it erupts the ash and lava is sprayed into the air like a fountain. Splashes of lava mix with the ash to form a new cone. Given time, a secondary cone may divert so much of the magma that it becomes the main vent and the original cone becomes less important.

Magma Chamber
Deep below the Earth's surface, between 100 and 200km down, the rocks are semi liquid. In certain parts of the world there are 'hot spots', areas where the rocks are hotter than elsewhere. These areas are believed to be the sources of the magma which rises to the surface via volcanoes. Although diagrams often show the Magma Chamber as having well defined sides, it is very unlikely that they are like that. It is much more likely that the rocks slowly cool as distance from the hot spot increases, so the rock turns from liquid, to semi-molten, to thick sticky stuff and finally becomes solid.

Fumerole
A fumerole is a crack in the surface through which steam and gas can escape. The magma below the surface heats water to the point where it turns to steam and is able to dissolve minerals from the surrounding rock. As the gas reaches the surface it is both hot and under pressure. It cools and expands, depositing the dissolved minerals around the vent. In some parts of the world local industries have built up collecting the sulphur deposited around fumerole vents.

Crater
A volcanic crater is a funnel shaped hollow at the top of the vent. It is formed as lava, gas and ash are blasted upwards from the main vent. Material falls back down to earth around the vent and slowly piles up forming a rim around it. The inside of the crater is kept clear by the force of upward moving material constantly removing any debris which falls there.

Ash and Gas Clouds
Gas escapes all the time from active volcanoes. It may be just steam, ( 90% of all volcanic gas is water and carbon dioxide ) but more often is water vapour containing dissolved minerals such as sulphur. During an eruption the volume of gas released increases considerably, tonnes of material being thrown into the atmosphere, forming a mixture of gas, ash and rock fragments. Along with the smaller particles there are often larger chunks of liquid rock, thrown high into the air by the force of the eruption. These pieces of rock cool as they spin through the air, forming rod shaped chunks which are called 'volcanic bombs'. Dangerous though the bombs are, they are not usually the cause of most casualties. The hot ash and poisonous gasses kill many more people.

Types of Volcanoes

Volcanologists have classified volcanoes into groups based on the shape of the volcano, the materials they are built of, and the way the volcano erupts.

The groups are:

• Composite Volcanoes (also called strato volcanoes)
• Shield Volcanoes (also called shields)
• Cinder Cones
• Fissure
• Caldera

	Composite Volcanoes These volcanoes are typically tens of miles across and ten thousand or more feet in height. As illustrated in the figure above, they have moderately steep sides and sometimes have small craters in their summits. Volcanologists call these "strato-" or composite volcanoes because they consist of layers of solid lava flows mixed with layers of sand- or gravel-like volcanic rock called cinders or volcanic ash.
	Shield Volcanoes This type of volcano can be hundreds of miles across and many tens of thousands of feet high. The individual islands of the state of Hawaii are simply large shield volcanoes. Mauna Loa, a shield volcano on the "big" island of Hawaii, is the largest single mountain in the world, rising over 30,000 feet above the ocean floor and reaching almost 100 miles across at its base. Shield volcanoes have low slopes and consist almost entirely of frozen lavas. They almost always have large craters at their summits.
	Cinder Cones As you might expect from the name, these volcanoes consist almost entirely of loose, grainy cinders and almost no lava. They are small volcanoes, usually only about a mile across and up to about a thousand feet high. They have very steep sides and usually have a small crater on top.
	Fissure In this type, there is no central crater at all. Instead, giant cracks open in the ground and expel vast quantities of lava that spread far and wide to form huge pools that can cover almost everything around. When these pools of lava cool and solidify, the surface remains mostly flat. Since the source cracks are usually buried, there is often nothing "volcano-like" to see--only a flat plain.
Images reproduced from: Wheeling Jesuit University/Center for Educational Technologies	Caldera Calderas, which are simply circular depressions, are found on the summits of many volcanoes. "Giant" calderas are the largest of these: huge craters up to many tens of miles across. Giant Calderas form by collapse (see animation) in gigantic eruptions that spew volcanic rocks out hundreds or even a thousand miles in all directions. Sometimes the calderas are so filled with lava and volcanic ash that there is no recognizable depression at all. These can only be found by carefully locating the big fractures or "faults" in the ground that mark the edges of the caldera.

When is a volcano considers active, dorman or extinct?

There are over 1,000 volcanoes in our world. Classifying a volcano as active, dormant, or extinct is a subjective and inexact exercise. A volcano is generally considered active if it has erupted in historic time. This definition, however, is rather ambiguous, because recorded history varies from thousands of years in Europe and the Middle East, to only a few hundred years in other regions of the world, like the Pacific Northwest of the United States. Scientists generally consider a volcano active if it is currently erupting, or exhibiting unrest through earthquakes, uplift, and/or new gas emissions. The Smithsonian Institution's catalog of active volcanoes, recognizes 539 volcanoes with historic eruptions. In addition, there are 529 volcanoes that have not erupted in historic times, but which exhibit clear evidence of eruption in the past 10,000 years. These latter volcanoes are probably best considered "dormat," since they have the potential to erupt again.

Types of Volcanic Eruptions

During an episode of activity, a volcano commonly displays a distinctive pattern of behavior. Some mild eruptions merely discharge steam and other gases, whereas other eruptions extrude quantities of lava. The most spectacular eruptions consist of violent explosions that blast great clouds of gas-laden debris into the atmosphere.

The type of volcanic eruption is often labeled with the name of a well-known volcano where characteristic behavior is similar—hence the use of such terms as “Strombolian,” “Vulcanian,” “Vesuvian,” “Pelean,” “Hawaiian,” and others. Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity—others may display an entire sequence of types.

In a “Strombolian”-type eruption observed during the 1965 activity of Irazu Volcano in Costa Rica, huge clots of molten lava burst from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

In contrast, the eruptive activity of Parícutin Volcano in 1947 demonstrated a “Vulcanian”-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.

In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash-laden gas are violently discharged to form cauliflower-shaped cloud high above the volcano.

In a “Peléan” or “Nuée Ardente” (glowing cloud) eruption, such as occurred on the Mayon Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move downslope at velocities as great as 100 miles per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mont Pelée on Martinique, Lesser Antilles.

The most powerful eruptions are called “Plinian” and involve the explosive ejection of relatively viscous lava. Large plinian eruptions—such as during 18 May 1980 at Mount St. Helens or, more recently, during 15 June 1991 at Pinatubo in the Philippines—can send ash and volcanic gas tens of miles into the air. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving deadly pyroclastic flows (“nuées ardentes”) are also commonly associated with plinian eruptions.

“Hawaiian” eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950; or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.

“Phreatic” (or “steam-blast”) eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak, but can be quite violent in some cases, such as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76 activity at La Soufrière, Guadeloupe (Lesser Antilles).

The eruptive products are highly variaible and largely dependent on the composition, viscosity, and gas content of the erupting magma. Lava flows, for example, are more common in relatively non-explosive basaltic eruptions associated with sheild volcanoes, scoria cones, and fissures. On the other hand, pyroclastic flows, lahars and voluminous tephra deposits are more common in explosive andesitic-to-rhyolitic eruptions associated with stratovolcanoes. Gaseous emissions are also examined, as are their harmful effects on both local and global scales. (Image courtesy of USGS)

Volcanoes in the Caribbean

The Caribbean region has its own tectonic plate, called the Caribbean plate. Though one of the smallest plates on the Earth's outer shell, and one of the youngest, a mere fifty million years old, the Caribbean plate has always been active. In the Lesser Antilles, the North American plate is pushed under the Caribbean plate. The westernmost islands form an arc that reaches from just south of the Anegada Passage (near the British Virgin Islands) almost to South America. This string of islands has been referred to as the Volcanic Caribbees. Since the arrival of Europeans, these islands have produced some 26 volcanic eruptions, ten submarine eruptions (all from a rising underwater volcano north of Grenada known as Kick'em Jenny), and 11 volcanic seismic crises (clusters of earthquakes around a volcano).

Montserrat

Montserrat is a typical volcanic island. It began as a volcano on the ocean floor and grew over the last, few million years into the small, mountainous island that it is today. Although, disturbed by volcanic eruptions from time to time, this "new" island was colonized by plants, birds, and animals typical of all the Caribbean islands.

The Amerinds, Montserrat's earliest inhabitants, arrived on the island about 2000 years ago. There are no records of volcanic activity until the Europeans arrive. Let's flash forward, then, to the early 1600s and see the island's volcano put on a display for Montserrat's new European settlers. They learned first-hand why the soil was so rich. Montserrat is truly and entirely a volcanic island. Although only some 11 miles long, it rapidly ascends to 3,000 feet at its highest point, and it has three volcanic formations, Silver Hill, Centre Hills, and Soufriere Hills. Soufriere Hills is the youngest.
For centuries, Soufriere Hills slept peacefully beneath nature's bounty. Deep-green rainforest grew along its slopes, its rivers descended to the sea, and indigenous animals roamed freely, finding ample food and shelter. For all of these reasons, Montserrat, located about one-third of the way down the Lesser Antillean chain, became known as the Emerald Isle of the Caribbean. At different times, island farmers relied on sugarcane, limes, and sea-island cotton for their livelihood. More recently, internationally known rock stars traveled there to record their music. Occasional small earthquakes would startle the islanders, but it wasn't until recently that scientists connected the earthquake rumblings with possible volcanic eruptions.