The term volcano can either mean the vent from which magma erupts to the surface, or it can refer to the landform created by the solidified lava and fragmental volcanic debris that accumulate near the vent. One could say, for example, that large lava flows are erupted from Kilauea volcano in Hawaii, the world volcano here signifies a vent. Volcanoes are not the realm of any single scientific discipline.
Rather they require study from many scientists from several specialties: Geophysicist and Geochemist to probe the deep roots of volcano; Geologist to decipher prehistoric volcanic activity; Biologist to learn how life became established and evolve in barren volcanic islands; and meteorologist to determine the effects of volcanic dust and gases on the atmosphere, weather and climate. Volcanoes affect humankind in many ways. Their destructiveness is awesome, but risk involved can be reduced by assessing volcanic hazards and forecasting volcanic eruptions.
Body Volcanoes Risks and benefits Definition First of all, we should know what a volcano is. Volcano is an opening in the earth’s surface. Through this opening has come rock so hot that it is in a liquid or gaseous state. This melted rock deep in the earth is called magma. Philosophers once thought that volcanic eruptions came from the burning of natural fuels. Sir Charles Lyell and his associates later showed the volcanic mountains were piled up from the products of their own eruptions For hundreds of years, volcanoes have struck terror and wonder into the heart of man. In ancient time, they even moved man into worship.
The word volcano comes from Volcanus, the name of the Roman god of fire. The name was first used for volcano, one of the Lipari Islands in the Mediterranean Sea where the god was thought to live. Kinds of Volcano Volcanoes are commonly classified as active, dormant and extinct. The distinction between the categories is not very clear and consequently any classification based on this criterion and is highly arbitrary. The separation of dormant and extinct volcanoes is particularly difficult. A volcano may lie quiet many hundreds of years and then awaken, often violently. Some volcanoes are constantly active .
Izalco in El Salvador, and Stromboli in the Mediterranean Sea, erupt so regularly that they have been compared to light houses. Those that are quiet, but have not been dead for us to know when they will break out again are called dormant volcanoes. Volcanoes that have been remained quiet since the beginning of recorded history and probably will not erupt are called extinct volcanoes. Other volcanoes can be called intermittent because, they erupt fairly at regular periods. Many of these erupt in cycles, with the length of cycle being fixed by the amount of time needed to make enough heat to produce eruption.
Types of eruption In classification schemes based on character of eruption, volcanic activity and volcanic areas are commonly divided into six major order of increasing degree of explosiveness: (1) Icelandic, (2) Hawaiian, (3) Strombolian, (4) Vucanian, (5) Pelean and (6) Plinian. The Icelandic type of eruption is characterized by effusion of basaltic lave that flow from long parallel fissures. Such outpouring build lave patterns. The least violent type of eruption is termed Hawaiian and is characterized by extensive lava flows from central vents or fissures and occasionally accompanied by lava ountains.
Strombolian eruption is characterized by moderately fluid lava flows, usually accompanied by violent lava-fountaining that produces and abundance of volcanic bombs and cinders. Vulcanian eruptions are characterized by viscous lava that form short, thick lava flows around vents; very viscous or solid fragment of lava are violently ejected from these vents. Pelean eruptions are similar to vulcanian eruptions but have even more viscous lava; domes from over the vents, and ash flows commonly accompany the dome fountais.
Plinian eruptions, also known as ‘Vesuvian eruptions’, are volcanic eruptions marked by their similarity to the eruption of Mount Vesuvius in AD 79 (as described in a letter written by Pliny the Younger, and which killed his uncle Pliny the Elder). Plinian eruptions are marked by columns of gas and volcanic ash extending high into the stratosphere, a high layer of the atmosphere. The key characteristics are ejection of large amount of pumice and very powerful continuous gas blast eruptions. Risks Volcanoes release volcanic hazards that may cause the life of human kind to be in danger.
These volcanic hazards are Pyroclastic Density Currents (pyroclastic flows and surges), Lahars, Structural Collapse: Debris flow-Avalanches, Dome Collapse and the formation of pyroclastic flows and surges, Lava flows, Tephra fall and ballistic projectiles, volcanic gas, Tsunamis and Volcanic Lightning Pyroclastic density currents are are gravity-driven, rapidly moving, ground-hugging mixtures of rock fragments and hot gases. This mixture forms a dense fluid that moves along the ground with an upper part that is less dense as particles fall toward the ground.
The behavior of the fluid depends upon the solids concentration relative to the amount of hot gases. High concentration density flows are called “pyroclastic flows” and are essentially nonturbulent and confined to valleys. Low concentration density flows are called “pyroclastic surges” which can expand over hill and valley like hurricanes. Temperatures may be as hot as 900 degrees Celsius, or as cold as steam. Pyroclastic flows and surges are potentially highly destructive owing to their mass, high temperature, high velocity and great mobility. Deadly effects include asphyxiation, burial, incineration and crushing from impacts.
Many people and the cities of Pompeii and Herculaneum were destroyed in 79 AD from an erupion of Mount Vesuvius; 29,000 people were destroyed by pyroclastic surges at St. Pierre, Martinique in 1902; 2000 died at Chichonal Volcano in southern Mexico in 1982 from pyroclastic surges. The only effective method of risk mitigation is evacuation prior to such eruptions from areas likely to be affected by pyroclastic density currents. Lahars are part of the family of debris flows that are fluids composed of mixtures of water and particles of all sizes from clay-size to gigantic boulders.
The abundance of solid matter carries the water, unlike watery floods where water carries the fragments. Debris flows have the viscous consistency of wet concrete, and there is a complete transition to watery floods. Lahars are composed of volcanic particles and originate directly or indirectly from volcanic action. Lahars can form by hot pyroclastic surges or flows entering watershed systems or flowing over snow and ice, by eruptions through crater lakes, by heavy rains on loose volcanic debris that is, any process by which volcanic particles can become saturated by water and move downs lopes.
They can move with velocities as low as 1. m/s to as great as 40 m/s on steep slopes (1 m/s = 2. 55 miles per hour). They are known to have travelled as far as 300 km (1 km = 0. 63 miles). Lahars have destroyed many villages and lives living on Indonesian volcanoes because most people live in valleys where lahars flow. The 21,000 lives lost at Armero, Colombia, were from a lahar that formed during the eruption of Nevado Del Ruiz in 1985. It was generated by melt water from the interaction of pyroclastic surges with snow and ice, from a very small eruption. Lahars can transform into regular floods as they become increasingly diluted with water downstream.
This phenomenon was first discovered at Mount St. Helens where hot pyroclastic surges transformed to lahars, which further transformed to hyper concentrated stream flow and then to normal stream-flow turbulence. The eruption of Mount St. Helens on May 18, 1980 started with a relatively small volcanic earthquake that caused collapse of the north side of the volcano because it was over steepened and therefore unstable. When the landslide occurred, it decreased the pressure on the pressurized interior of the volcano which expanded explosively to form a lateral blast that devastated the countryside north of the volcano.
Most of the debris flow avalanche was diverted down the North Fork Toutle River, but some moved directly northward over a 300 meter ridge and down into the next valley. Since the 1980 Mount St. Helens eruption, dozens of volcanoes that have given rise to avalanches have been discovered. For example, 40 avalanches exceeding 1 Km3 in volume, and 22 with a volume of less than 1 km3, are now known from the Quaternary alone, and 17 historic volcanic avalanches have been identified. The hilly topography north of Mount Shasta in northern California is now known to be the result of a have debris-flow avalanche.
Some are known to extend up to 85 km from their sources and to cover tens to more than 1000 km2 in area. Lava flows rarely threaten human life because lava usually moves slowly a few centimeters per hour for silicic flows to several km/hour for basaltic flows. An exceptionally fast flow at Mt. Nyiragongo, Zaire (30-100 km/hour) overwhelmed about 300 people. Major hazards of lava flows burying, crushing, covering, burning everything in their path. Sometimes lava melts ice and snow to cause floods and lahars. Lava flows can dam rivers to form lakes that might overflow and break their dams causing floods.
Methods for controlling paths of lava flows: (1) construct barriers and diversion channels, (2) cool advancing front with water, (3) disruption of source or advancing front of lava flow by explosives. Tephra consists of pyroclastic fragments of any size and origin. It is a synonym for “pyroclastic material. ” Tephra ranges in size from ash (2 mm) to lapilli (2-64 mm) to blocks and bombs (>64 mm). Densities vary greatly, from that of pumice (0. 5) to solid pieces of lava with density about 3. 0. Blocks from basement material may exceed 3. 0.
Material may be juvenile (formed of magma involved in the eruption) or accidental (derived from pre-existing rock). Tephra fall and ballistic projectiles endanger life and property by (1) the force of impact of falling fragments, but this occurs only close to an eruption, (2) loss of agricultural lands if burial is greater than 10 cm depth, (3) producing suspensions of fine-grained particles in air and water which clogs filters and vents of motors, human lungs, industrial machines, and nuclear power plants, and (4) carrying of noxious gases, acids, salts, and, close to the vent, heat.
Burial by tephra can collapse roofs of buildings, break power and communication lines and damage or kill vegetation. Even thin (2 cm) falls of ash can damage such critical facilities as hospitals, electric-generating plants, pumping stations, storm sewers and surface-drainage systems and sewage treatment plants, and short circuit electric-transmission facilities, telephone lines, radio and television transmitters. When dispersed widely over a drainage basin, tephra can change rainfall/runoff relationships. Low permeability of fine ash deposits leads to increased runoff, accelerated erosion, stream-channel changes and hazardous floods.
In contrast, thick, coarse-grained deposits closed to the source can increase infiltration capacity and essentially eliminate surface runoff. Many of the hazards of tephra falls can be mitigated with proper planning and preparation. This includes clearing tephra from roofs as it accumulates, designing roofs with steep slopes, strengthening roofs and walls, designing filters for machinery, wearing respirators or wet clothes over the mouth and nose because tephra can contain harmful gases adsorbed on the particles as acid aerosols and salt particles.
Magma is molten rock containing dissolved gases that are released to the atmosphere during an eruption and while the magma lies close to the surface from hydrothermal systems. The most abundant volcanic gas is water vapor; other important gases are carbon dioxide, carbon monoxide, sulfur oxides, hydrogen sulfide, chlorine, and fluorine. The gases are transported away from vents as acid aerosols, as compounds adsorbed on tephra and as microscopic salt particles. Sulfur compounds, chlorine and fluorine react with water to form poisonous acids damaging to the eyes, skin and respiratory systems of animals even in very small concentrations.
The acids can destroy vegetation, fabrics and metals. Atmospheric veils of dust or acid aerosols caused by large-volume explosive eruptions can affect regional or global climate. Most volcanic gases are noxious and smell bad, but they can cause mass fatalities. A rare case of mass deaths by volcanic gases in 1986 at Lake Nyos, in Cameroon, West Africa. Tons of carbon dioxide spilled out of Lake Nyos, and flowed silently down a canyon and through 3 villages occupied by 1700 people. They and 3000 cattle died instantly from lack of oxygen.
Carbon dioxide emissions are now being monitored at Mammoth Mountain, California. A tsunami is a long-period sea wave or wave train generated by a sudden displacement of water. Tsunamis travel at very high speeds through deep water as low broad waves and build to great heights as they approach the shallow bottom of shores. Most are caused by fault displacements on the sea floor, but many have been caused by volcanic action. The eruption of Krakatau in 1883 produced tsunamis that killed 36,000 people. The pyroclastic flow generated by this eruption displaced the water that initiated the tsunamis.