VOLCANOES Volcanic eruptions are characterized by a number of events posing serious threats to living organisms and environment: pyroclastic flows; ash and ballistic projectiles falls; lava, tephra and lahars falls; gases; lightning.
Volcanoes are solid structures made up mostly by solidified lava and solid rocky material extruded from the vent, a conduit joining the earth surface/crust with the underneath lithosphere and asthenosphere [lithosphere includes the crust - crust is 5-11 km thick beneath the oceans; 19-64 km beneath the continents; averaging ~35km thickness - and part of the upper mantle - mantle is ~2,898 km thick; under the mantle there is the earth core: outer core ~ 2,250 km, inner core ~1,280 km thick*- The relatively plastic asthenosphere is a layer of the upper mantle beginning at 30 - 50 km under surface; it is ~ 300 km thick; it is the zone in which magma is generated. Asthenosphere is responsible for the movements of the crustal plates and continents.]
Areas where tectonic movements take place and continental plates converge, are sites of high energy release [earthquakes and/or volcanic activity]. Volcanoes usually occur on plate boundaries, where the plate forced deeper into the mantle is subjected to high temperature [earth temperature increase of ~10-20°C per km of dept. The likely source of heat in the earth interior is decay of radioactive uranium and thorium, concentrated in siliceous rocks as the granite. At the core mantle boundary temperature is about 3,000°C; at the center of the earth is about 5,000°C] and extraordinary high pressure [pressure increases steadily with increasing depth because of the weight of the overlaying rocks; at the boundary mantle core , 2,898 km deep, pressure is ~1.4 million atmospheres; at the earth center it reaches 3.5 million atmospheres], causing rocks to melt and forming magma.VOLCANIC ERUPTIONS
Eruption is usually preceded by some magma movements beneath the volcano; the movements of magma produce from some seismicity to very small earthquakes.
In explosive eruptions, gases escape violently from the vent together with magma fragments, in a gigantic column of 'ash'.
Huge quantities of lava, ash and tephra debris flows over the vent and other cracked rocks, with release of suffocating gases.In the first 15 minutes of a medium- to large-scale eruption an area with several kilometers radius from the volcano could be destroyed [Dobran and others, 1994].
According to recent analyses, about 274,000 fatalities were due to volcanic eruptions since AD 1; documented fatal volcanic eruptions were more than 400 [2 - 4 per year]; seven eruption produced more than 10,000 victims each; fatalities were due mostly to tephra, pyroclastic flows, collapse of ash covered roofs and to projectile impact [Simkin et al., 2001, Science 291(5502),255];
lava and gas give a significant contribution.
'Nearly two third of the fatalities and half the fatal events have taken place more than one month after the eruption start.'
[Simkin et al., 2001, Science 291(5502),255].
Most eruptions affect global climate for long periods and destroy the stratospheric ozone layer
[Nasa's Earth Observing Project Science Web page: Volcanoes and global climate change May 2000].ASH
Volcanic ash are very fine pieces of rocks, from less than 2 mm in diameter down to 0.001 mm formed during explosive eruptions. Suspended in vapor and gases, ash rises into the air as a gigantic cloudy column up to 30 kilometers high in about 10 minutes. The cloud may travel thousands kilometers away driven by blowing winds, posing serious hazard to aviation, compromising radio & telephone communications, and resulting in falling down ash over enormous area, adversely affecting vegetation and agricultural practices [health effects/poisoning of farm animals, grazing difficulties, loss of crops], reducing visibility, causing roads become slippery, clogging engines, air ventilation filters, accumulating over buildings up to the roof collapse [dry ash may weight up to 100 kilograms per square meter; double if wetted by rain] and injuring/killing peoples. Following Mt. St. Helen first eruption [1980] more than 500 million tons of ash fallen over Washington, Idaho and Montana; the estimated damage was of more than $ 1 billion in property and economic losses.
GASES
Gases are dissolved in magma, because of the high pressure present kilometers under the earth's surface. By sudden decrease of pressure occurring as magma rises the volcano conic vent, gases expand with explosive violence. Water vapor constitute about 75-95% of all eruption gases; 5-25% of erupted gases are carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, chlorine, nitrogen, etc.
Currently, Volcanoes erupt more than 100 million tons of carbon dioxide/year.
During periods of eruptive activity Mt. St. Helen, Mammoth Mt, Kilauea volcanoes emitted more than 1,100 tons of carbon dioxide per day.Sulfur dioxide react with stratospheric water vapor, interacts with dust and sunlight, producing volcanic smog
[vog: a suspension of sulfuric acid aerosols, sulfates and trace amounts of selenium, arsenic, mercury, iridium, etc.]
remaining suspended many years at 15-25 km up into the air.Sulfuric acid aerosols affect global temperature by reflecting sun's rays [causing some cooling of earth surface] and forming acid rain.
Kilauea emitted more than 2,000 tons of sulfur dioxide per day during eruptive phases; during the 1991 cataclysmic eruption the Pinatubo emitted in the stratosphere more than 20 million tons of sulfur dioxide [The cataclysmic 1991 eruption of Pinatubo ]
PYROCLASTIC FLOWS
The term pyroclastic derives from the greek pyro [pur] meaning fire, and clastic [klastos] meaning broken, fragmented.
Pyroclastic flows represent
the highest threats to life.
Pyroclastic flows are mixtures of ash and tephra's [tephra is a general term used by volcanologists for airborne volcanic ejecta of any size] fine particles suspended in very hot - up to 1,000°C - gases, ejected from vents and falling down from volcano's sides over the land at high speed - from 1 to 30 meters/sec = 3.6 - ~ 100 km/h - burning everything in their path; they are extremely hazardous because most people are not aware of the coupled danger of speed and temperature.The density and viscosity of pyroclastic flow change as they move, owing to sedimentation of larger particles from the upper/more turbulent flow toward the base of the progressively more laminar flow.
[Fisher, R.V., 1966, 'Mechanism of deposition from pyroclastic flow' Amer. J. Sci., 264, 350; Sparks, R.S.J. & Wilson, L., 1976, J.Geol. Soc. London 132, 441]
Particles with greater settling velocity deposit over land along the path, forming dense layers of pyroclastic particles able to maintain local temperature at very high level as 100-300°C, even at several kilometers from the originating volcano.
Five years after Pinatubo eruption the debris accumulated from pyroclastic flows, thermally insulated by themselves, were still at about 500°C .
Energy for transport of pyroclastic fallout is supplied firstly by the eruptive explosion and later by winds.Lava
Magmas erupted from the volcano is called lava. Magma is formed many tens of kilometers beneath the earth surface. It is primarily a fluid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, manganese and trace amounts of many other elements; it contains crystals and fragments of unmelted rocks.
Lava flows are streams of molten rock
that move down the slope of the volcanoes.
The temperature of lava may be higher than 1,200°C [> 2,000°F].
Although changing somewhat following the magma composition, the color of flowing lava is reddish at about 500°C; it is dark/bright red at 600°C and orange/yellow at temperature higher than about 900°C.Lava is red hot when it pours out of vent but it soon changes to dark red, gray, black as it cools and solidifies.
Lava flows cause extensive damage and total destruction by burning, crushing, and burying everything in their paths.
Usually lava flows do not threaten human life, however, because of very slow rate of movement, ranging from a few meters to some kilometers per hour.
The distance traveled by lava flows depends on circumstances linked to composition of molten rocks and physical structure of the environment. Basalt flows are characterized by relatively low viscosity and may reach 50 -150 km from their sources.
Lahars
'Lahars are mixtures of water, rocks, sand and mud that rush down ...from the volcano..can travel over 50 miles...reaching speeds between 20 and 40 miles per hour...Close to the volcano they have the strength to rip huge boulders, trees and houses from the ground and carry them down valley ... they entomb everything in mud..Lahars have been one of the most deadly volcanic hazards.'
[ USGS]Volcanic Lightning
Electrical discharges occur during eruptions, between clouds and from the eruption column to the crater, probably caused by potential differences deriving from fractions between particles and gases. They can be of some hazard to life.
Every year about 60 volcanoes eruptions occur.
Currently erupting volcanoes
Shishaldin [Alaska], Villarica [Chile], Arena [Costa Rica], Mt. Cameroon [Cameroon], Nyamuragira [Congo], Guagua, Tungurahua [Ecuador], Fuego, Pacaya [Guatemala], Kilauea [Hawaii], Heckla [Iceland], Bromo, Kaba, Semeru, Krakatoa, Merapi, Mt. Karangetang [Indonesia], Etna [Italy], Komagatake, Sakura-Jima, Mt. Oyama, Bandai, Usu [Japan], Popocatepetl, Colima [Mexico], Ulawun, Tavurvur [New Guinea], White Island [New Zealand], San Cristobal [Nicaragua], Mayon [Philippines], Piton de la Fournaise [Reunion Ils], Sheveluch, Bezymianny, Karymsky [Russia],
Kavachi [Solomon Ils]. [ Reference]
Historic eruptions
The most powerful eruption in history was the cataclysmic one of Tambora, Indonesia [1815]. The '...volcanic cloud lowered global temperature by as much as 3 degrees C. Even a year after the eruption, most of the Northern hemisphere experienced sharply cooler temperature during the summer months. In parts of Europe and in North America, 1816 was known as "the year without a summer" '
[ Reference]The most Relevant / Deadliest Eruptions
79 AD. Vesuvius, Italy: ~ 3,400 victims
1586. Kelut, Indonesia: ~10,000 victims
1783. Lakagigar, Iceland: ~ 9,000 victims
1792. Unzen, Japan: ~15,000 victims
1815. Tambora, Indonesia: ~ 92,000 victims
1883. Krakatau, Indonesia: ~ 36,000 victims
1902.Mt. Pelee, Martinique: ~ 30,000 victims
1912. Novarupta, Alaska: erupted ~ 20 cubic kmeters of material
1980. Mt. St. Helen, Washington: devastated ~ 600 square km
1983. Kilauea, Hawaii: lava covered ~ 80 square kilometers
1984. Nevado del Ruiz, Columbia: more than 20,000 victims
1991. Mt. Pinatubo, Philippines: more than 300 victims.[Blong, R.J., 1984, 'Volcanic Hazard: A Soucebook on the Effects of Eruptions',Academic Press, Orlando, FL.] USGS
___________________________________________________ One of the largest volcanic eruption of 1900 occurred' at Mount Pinatubo in the Philippines on June 15, 1991: the volcano exploded ejecting more than 5 cubic kilometers of melted rock risen from about 32 kilometers beneath; the ash cloud rose 35 kilometers into the air; 'Nearly 20 million tons of sulfur dioxide were injected into the stratosphere..and the dispersal of this gas cloud around the world caused global temperature to drop temporarily by about 0.5°C.'
[The cataclysmic 1991 eruption of Mt. Pinatubo ]
VESUVIUS and SANTORINI ERUPTIONS. The eruptions of Vesuvius and of Santorini were among the largest ones in the last 10,000 years.
Herculanum was buried under about 23 m of ash deposited by a pyroclastic flow during the 79 A.D. eruption of Vesuvius. The column of ash cloud produced by eruption
was estimated to be more than 30 km high.
About 4 cubic kilometers of ash were erupted in less than 24 hours.
More than 3,300 people were killed by ash/pyroclastic flows and falls.
Vesuvius erupted several times since 79 A.D.
During the eruption of 1631
the lava/pyroclastic flows killed about 3,500 people.During the eruption of Santorini [Greece] in 1,650 B.C. about 30 cubic km of rhyodacite magma were erupted. The ash column of the eruption in the early phase was about 36 km high.
The eruption probably caused the end of the Minoan civilization on the island of Crete.
*The average diameter of Earth is 12,740 Km [7,910 miles]; the surface is ~ 510 million square km [197 million square miles]; circumference: 40,009 - 40,076 km; the mass ~ 5.976*1024 kg.