A volcano is an opening in the earth’s crust that spews out gaseous liquid or solid materials from the surface of the earth. Volcanoes are not always mountain shaped.
Below the earth’s crust lies an extremely hot region called the mantle. Although this area is almost entirely solid, it can form small pockets of liquid rock and hot gases. The molten rock, called magma, slowly forces its way upward through weaknesses in the earth’s crust to become volcanoes.
Some rocks deep within the earth’s surface would get so hot that they slowly melt and become magma. This is lighter than the rock around it and therefore it rises and collects in magma chambers. As time goes by, some of the magma pushes through vents and fissures in the earth’s surface causing an eruption.
The types of volcanic eruptions vary greatly based on the composition of the magma. Volcanic eruptions can be violent and highly explosive, or not as explosive and produce a large amount of lava.
Many volcanic eruptions also hurl molten magma and solids of various sizes into the air in the form of boulders, cinders, and ash.
There are five types of volcanoes – the cinder cone, the shield, the dome, the stratovolcano and the caldera.
The Cinder Cone – This is the simplest type of volcano. It is built from particles of semi-solid lava ejected from a single vent. As the lava is blown into the air, it breaks into fragments that solidify and fall as cinders around the vent to form a circular or oval cone.
The Shield Volcano – Shield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior’s shield. Some of the largest volcanoes in the world are shield volcanoes. The largest of all is Mauna Loa in Hawaii.
The Lava Dome – Volcanic or lava domes are formed by relatively small, bulging masses of lava too viscous to flow any great distance. As a result, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides.
The Stratovolcano or Composite – These are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth’s crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.
The Caldera – Caldera volcanoes are the most powerful and catastrophic types of volcanoes. This type of volcano is shaped more like an inverse volcano. An enormous magma chamber bulges up beneath the ground from the extremely high pressures of the trapped gases within. Caldera volcanoes are the largest on earth, with some calderas measuring from 15 to 100 kilometers wide.
There are three major types of volcanic eruptions:
1. Hawaiian Eruptions
Hawaiian eruptions are neither explosive nor destructive. The lava thrown out is low in gas content and it flows down slowly. Sometimes the volcanoes throw up a fire fountain where bright lava is sprayed into the air for several hours or few minutes. However, the most common form is the lava lake where the lava forms craters or depressions.
2. Strombolian Eruptions
Though these eruptions are not dangerous, they are more impressive than Hawaiian eruptions. There are regular eruptions where the small amounts of lava are exuded into the air followed by booming sounds. The Strombolian eruptions produce small quantities of ashy tephra.
3. Plinian Eruptions
This was the eruption that not only destroyed but also buried Pompeii and Herculaneum. These eruptions are caused by magma, which has high gas content and has high viscosity.
The lava is spewed as high as 50 km in the air. This eruption lasts for days. Plinian eruptions throw out large quantities of tephra which is heaped on one side, depending upon the direction of the wind. Here, the lava flows quickly and will destroy anything that it comes across.
To sum up, volcanic eruptions can cause destruction to all living things. They can also cause substantial changes in the climate. The particles that are thrown into the atmosphere hinder the sunlight from reaching the surface of the earth. This can lead to low global temperatures. Large quantities of gases such as sulfur dioxide, hydrogen chloride and hydrogen fluoride are sometimes thrown out. These react with atmospheric water to produce acid rain. Such acid rain will not only destroy the crops but also harm all living things.
There are 19 ‘live’ (likely to erupt again) volcanoes in the Eastern Caribbean. According to the University of the West Indies Seismic Research Centre, every island from Grenada to Saba is subject to the direct threat of volcanic eruptions. Islands such as Grenada, St. Vincent, St. Lucia, Martinique, Dominica, Guadeloupe, Montserrat, Nevis, St. Kitts, St. Eustatius and Saba have ‘live’ volcanic centres.
Other islands such as Anguilla, Antigua and Barbuda, Barbados, British Virgin Islands, most of the Grenadines and Trinidad & Tobago are not volcanic but are close to volcanic islands and are subject to volcanic hazards such as severe Ash Fall and volcanically-generated tsunamis.
Volcanoes in the Eastern Caribbean are mainly steep-sided and roughly conical in shape. They consist of alternating layers of solid lava (magma that has reached the Earth’s surface) and broken fragments of lava called pyroclastic rocks. They are called stratovolcanoes.
Blasted Projectiles: These are large projectiles that can damage buildings and start fires.
Mud flows : A mixture of mud, rocky debris and water. These are also called Lahars.
Pyroclastic flows: A mixture of hot gases, ash and rocks. Danger lies in the density and temperature of the ash and rock fragments. Pyroclastic flows can be as hot as 1,500°F and move at speeds of 100 to 150 miles per hour.
Gases: These can include carbon dioxide, sulphur dioxide or hydrogen. They may be asphyxiants which are concentrated near the volcanic crater or fissure or respiratory irritants and can be harmful at lower concentrations.
Lava flows: Flows of extremely hot molten rocks spewed out by the volcano. The viscosity and high temperature make these flows very dangerous and they are capable of destroying all in their path.
Landslide or debris avalanche: A rapid downhill movement of rocky material, snow, and or ice. Volcano landslides range in size from small movements of loose debris on the surface of a volcano to massive collapses of the sides of a volcano.
Local earthquakes: These can result in possible loss of human life and property.
Tsunamis: Tsunami is Japanese for “harbour wave”, the seismic wave that can move across oceans at up to 600 miles per hour. Tsunamis can destroy coastlines and are known to have killed many.
Ash Fall: Volcanic ash consists of tiny jagged particles of rock and natural glass blasted into the air by a volcano. Ash can threaten the health of people and livestock causing respiratory problems and pose a hazard to aircraft among others.
BEFORE THE ERUPTION
DURING THE ERUPTION
There are no volcanoes in Antigua/Barbuda however, due to the island’s proximity to Montserrat and other neighbouring volcanic islands such as Guadeloupe, Martinique, Dominica, St. Lucia, St. Vincent, Grenada, Trinidad, Montserrat, Nevis, St. Kitts, St. Eustatius, the occasional change in wind direction means that Antigua/Barbuda could be affected by the eruption of any of these volcanoes.
Volcanic events which have affected Antigua and Barbuda