Monday, November 14, 2011

The Nature of Volcanoes

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The Nature of Volcanoes

Volcanoes are built by the accumulation of their own eruptive products—lava, bombs (crusted over ash flows), and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of Earth. The term volcano also refers to the opening or vent through which molten rock and gases are expelled.
Driven by buoyancy and gas pressure, the molten rock, which is lighter than the surrounding solid rock, forces its way upward and may ultimately break though zones of weaknesses in Earth's crust. If so, an eruption begins, and the molten rock may pour from the vent as nonexplosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fall-back fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind and fall to the ground many miles away. The finest ash particles may be injected miles into the atmosphere and carried many times around the world by stratospheric winds before settling out.
Molten rock below the surface of Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Upon cooling, the liquid magma solidifies to form an igneous or magmatic rock.
Lava is red-hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey, or in other cases, like pasty, blocky masses.
All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tremendous pressure and ultimately escape with explosive violence, throwing out great masses of solid rock as well as lava, dust, and ashes.
The violent separation of gas from lava may produce rock froth called pumice. Some of this froth is so light—because of the many gas bubbles—that it floats on water.


Recent Volcanic Activity

(Bold indicates activity in 2008, 2009, and 2010)

The following table shows the recent and ongoing volcanic history around the world by year of activity and volcano name.
VolcanoYear of last
eruption
or activity
Adatara, Honshu, Japan1997
Akan, Hokkaido, Japan2006
Akutan, Alaska1996
Ambrym Island, Vanuatu2007
Amukta, Alaska1996
Anatahan, Mariana Islands2007
Aoba, Ambae Island, Vanuatu2006
Arenal, Costa Ricaongoing
Asama, Honshu, Japan2003
Aso, Kyushu, Japan2004
Atka, Aleutian Islands, United States2006
Augustine, Cook Inlet, Alaska, United States2006
Axial Seamount1998
Bamus, New Britain Island, Papua New Guinea2006
Bandai, Honshu, Japan2000
Bagana, Bougainville, Papua New Guineaongoing
Barren Island, Indian Ocean2008
Batu Tara, Lesser Sunda Islands, Indonesia2007
Bezymianny, Kamchatka, Russia2009
Bromo, Java, Indonesia2000
Bulusan, Philippines2007
Mount Cameroon, Cameroon2000
Canlaon, Philippines2006
Cerro Azul, Galápagos Islands, Ecuador1998
Cerro Negro, Nicaragua1999
Chaitén, Chile2008
Chikurachki, Kurile Islands, Russia2005
Chiginagak, Alaska1997
Chuginadak, Alaska2005
Cleveland, Chuginadak Island, Alaska2008
Colima, Mexicoongoing
Conception, Island of Ometepe, Nicaragua2010
Copahue, Argentina and Chile2000
Dempo, Sumatra Indonesia2006
Dukono, Indonesiaongoing
East Epi, Vanuatu2004
Eastern Gemini Seamount, Vanuatu1996
Ebeko, Kuril Islands, Russia2007
Egoni, Indonesia2004
Erta Ale, Ethiopia2003
Etna, Sicily, Italy2007
Eyjafjallajokull, Iceland2010
Fernandina, Galápagos1995
Fogo, Cape Verde1995
Piton de la Fournaise, France2010
Fourpeaked, Alaska Peninsula, United States2006
Fuego, Guatemalaongoing
Fukutoku-Okanoba, Japan2010
Galeras, Colombia2008
Gamalama, Indonesia2003
Gaua, Vanuatu2009
Grimsvotn, Iceland1998
Guagua Pichincha, Ecuador2006
Hachijo-Jima, Izu Islands, Japan2002
Hakkoda, Japan1997
Heard Island, Southern Indian Ocean, Australia2006
Hekla, Iceland2000
Mount Hili Aludo, Indonesia1997
Hokkaido, Japan2006
Home Reef, Tonga Islands, Tonga2006
Hosho, Kyushu, Japan1995
Ijen, Java, Indonesia2001
Iwate-san, Honshu, Japan1998
Jackson Segment, N. Gorda Ridge (nr. Oregon)2001
Kaba, Sumatra, Indonesia2000
Mount Karangetang, Indonesia2007
Karthala, Comoros Islands, Indian Ocean2007
Karymsky, Kamchatka, Russiaongoing
Kavachi Seamount, Solomon Islands2004
Kelut, Java, Indonesia2006
Kick-'em-Jenny (nr. Grenada)2003
Kikai, Japan2004
Kilauea, Hawaiiongoing
Kirishima, Japan2010
Kliuchevskoi, Kamchatka, Russia2009
Komagatake, Hokkaido, Japan2000
Korovin, Alaska1998
Krakatau, Indonesia2001
Langila, New Britain, P.N.G.2007
Lamongan, Indonesia2003
Lascar, Chile2007
Leroboleng, Indonesia2003
Mount Lewotobi, Indonesia2003
Llaima, Chile2008
Loihi Seamount, Hawaii1996
Lokon, Sulawesi, Indonesia2003
Long Valley caldera, California1996
Lopevi, Central Islands, Vanuatu2007
La Madera, Nicaragua1996
Manam, Papua New Guineaongoing
Maroa, New Zealand2001
Masaya, Nicaraguaongoing
Mauna Loa, Hawaii2003
Mayon, Philippines2009
McDonald Island, Australia1996
Merapi, Indonesia2007
Metis Shoal, Tonga1995
Momotombo, Nicaragua1996
Monowai Seamount, Kermadec Islands1997
Nevado Del Huila, Colombia2007
Nyamuragira, Congo (Dem. Rep.)2010
Nyiragongo, Congo (Dem. Rep.)2003
Okmok, Alaska1997
Ol Doinya Lengai, Tanzania, Africa2006
Oyama, Miyake-jima, Izu Islands, Japan2006
Pacaya, Guatemala2007
Pagan, Mariana Islands, central Pacific Ocean2006
Pago, Papua New Guinea2004
Papandayan, Java, Indonesia2002
Pavlof, Alaska1997
Peuet Sague, Indonesia1998
Piparo, Trinidad1997
Piton de la Fournaise, RĂ©union2007
Poas, Costa Rica2006
Popocatepetl, Mexico2007
Rabaul, Papua New Guinea2008
Raoul Island, New Zealand2006
Redoubt, Alaska2009
Reventador, Ecuador2007
Rincon de la Vieja, Costa Rica1998
Ritter Island, Papua New Guinea2006
Rotorua, New Zealand2001
Ruapehu, New Zealand2006
Ruby Seamount, Mariana Islands1995
Mount St. Helens, Washingtonongoing
Sakura-Jima, Japanongoing
San Cristobal, Nicaragua2006
San Miguel, El Salvador2006
Sangay, Ecuadorongoing
Santa Ana, El Salvador2006
Santa Maria, Guatemalaongoing
Semeru, Java, Indonesiaongoing
Shiveluch, Kamchatka, Russiaongoing
Shin-dake, Kuchinoerabujima Island, Japan1999
Shishaldin, Unimak Island, Alaska2000
Simbo, Solomon Islands2006
Slamet, Java Indonesia2007
Soputan, Indonesia2006
Soufriere Hills, Montserrat, West Indiesongoing
South Sister, Oregon2001
Stromboli, Italyongoing
Sulu Range, New Britain Island, Papua New Guinea2006
Suwanose-Jima Ryukyu Islands, Japanongoing
Tavurvur, Papua New Guinea2000
Taal, Philippines2006
Talang, Indonesia2007
Telica, Nicaragua2007
Tengger Caldera, Java, Indonesia2006
Terceira, Azores1999
Tonga (unnamed volcano)1999
Tongariro Volcanic Centre, New Zealand2006
Tungurahua, Ecuadorongoing
Turrialba, Costa Rica2010
Ubinas, Peru2007
Ulawun, Papua New Guinea2007
Usu, Japan2000
Veniaminof, Alaska2006
Villarrica, Chile2009
White Island, New Zealand2007
Yasur, Tanna Island, Vanuatu2004
Yellowstone, Wyoming1998
Zacatecas, Mexico1997



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