Volcanic eruptions in inhabited regions result in loss of life and destruction of habitat and agricultural land. Destruction is caused by the ejection of molten lava (either as flows or as fountains thrown hundreds of feet into the air), angular blocks, mud, ash (possibly incandescent), dust and gas. The speed of the lava flows (up to 160 km/hour) and the ash may mean a community in the neighbourhood is obliterated in just a few seconds. An eruption may also be prolonged for several years or even centuries. The most terrible of all volcanic manifestations are glowing avalanches or nuÃ©es ardentes which give little forewarning, travel at hurricane speeds and are hot enough both to kill instantly by cadaveric spasm and to carbonize the corpse by destructive distillation.
Volcanic eruptions can be placed into two general categories: those that are explosive, such as at Mount St. Helens, and those that are effusive, such as in Hawai'i. The most active volcano in the world, Kilauea Volcano on the big island of Hawai'i, is generally a nonexplosive volcano (though there have been occasions when it erupted explosively). Eruptions from it normally result in gently flowing lava flows, spatter cones, and lava fountains. Another type of nonexplosive volcanism is flood basalts. Lava flows from this type of eruption are extruded from fissures and cover vast areas. These nonexplosive eruptions are the least dangerous type of volcanic eruption since people rarely get killed by them. However, they are devastating and may have global consequences.
Many eruptions are explosive in nature. They produce fragmental rocks from erupting lava and surrounding country rock. Some eruptions are highly explosive and produce fine volcanic ash that rises many kilometers into the atmosphere in enormous eruption columns. Explosive activity also causes widespread ash fall, pyroclastic flows, debris avalanches, landslides, pyroclastic surges, and lahars. Explosivity is usually the result of gases expanding within a viscous lava. Another mechanism for explosions at volcanoes occurs when surface water or ground water enters a magma chamber. These eruptions are likely when a volcano occurs in a wet area or in the sea.
It is probable that every part of the earth's surface has been the site of volcanic activity at some stage. The active and most recently active volcanoes are however at present concentrated largely in a great belt encircling the Pacific Ocean and in a shorter belt extending from the Solomon Islands through New Guinea and Indonesia; the active volcanoes of the Mediterranean region are regarded as lying on an extension of the latter belt. There are also several active volcanoes in the Atlantic. Of the world's 543 volcanoes with known recorded activity, 90 erupted between 1966 and 1970. Of the 760 catalogued active volcanoes of the world, only 95 are known to have caused casualties during historic times. In the casualty league Japan tops the list with 14 disastrous eruptions; Java comes second with 12; the Philippines and Central America are equal third with 6 each. However, it was Mt Etna in Sicily, which caused the greatest loss of life during an eruption, when in 1669 some 100,000 people died in a particularly violent outburst. A similar event today could result in as many as two million deaths. As many as 20,000 people died in central Columbia when the Nevado del Riuz volcano erupted in November 1986. It was one of the worst volcano eruptions of this century.
A volcanic hazard refers to the probability that a given area will be affected by a potentially destructive volcanic process. A volcanic risk is a loss (this loss can be life, property, jobs, etc.) in the hazardous area. Any area near an active volcano is potentially hazardous. Some areas have higher probabilities of being affected by a destructive volcanic process than do others. A home built near the summit of a volcano may have the same level of risk as one built in a valley 60 km from the volcano.
It is estimated that 500 million people will be at risk from volcanic hazards by the year 2000. In the past 500 years, over 200,000 people have lost their lives due to volcanic eruptions. An average of 845 people died each year between 1900 and 1986 from volcanic hazards. The number of deaths for these years is far greater than the number of deaths for previous centuries. The reason behind this increase is not due to increased volcanism, but due, instead, to an increase in the amount of people populating the flanks of active volcanoes and valley areas near those volcanoes.
In June 1991, Mt. Pinatubo in the Philippines erupted with terrifying force, killing 800 people and forcing thousands to flee. It thrust 30 million tonnes of dust into the stratosphere, forming a cloud around the world which many scientists believe slowed down global warming.
When Toba, in Sumatra, exploded into life 73,500 years ago, it put out enough material to accelerate the onset of the last ice age. In 1815, Tambora, in Indonesia, erupted, the second biggest (after Toba) in the past million years. It is reckoned to have thrown out five times more dust than Pinatubo in 1991. Krakatoa, perhaps the most famous and apocalyptic volcano, was followed by cold, wet summers through Europe for years - but also by beautiful sunsets, caused by high-level dust scattering light from the setting sun to create spectacular violet and purple afterglows. The biggest eruption in the 20th century was Mt. Katmai in Alaska in 1912.