Volcanoes

Volcanoes : Volcanoes


volcano : volcano A volcano is a vent at the Earth's surface through which magma (molten rock) and associated gases erupt, and also the cone built by effusive and explosive eruptions The lifespan of a volcano can vary from months to several million years


volcano : volcano


volcano : volcano


volcano : volcano How they form: Divergent boundaries (sea floor spreading)


volcano : volcano Convergent boundaries


volcano : volcano Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest Many scientists also consider a volcano active if it has erupted in historic time Recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawai`i, little more than 200 years.


volcano : volcano Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again Extinct volcanoes are those that scientists consider unlikely to erupt again


Types of volcanic cones or structures : Types of volcanic cones or structures caldera cinder cone hornito maar mud volcano shield volcano spatter cone stratovolcano volcanic dome


Shield volcano : Shield volcano Volcanoes with broad, gentle slopes and built by the eruption of fluid basalt lava The largest volcanoes on Earth are shield volcanoes Basalt lava tends to build enormous, low-angle cones because it flows across the ground easily and can form lava tubes that enable lava to flow tens of kilometers from an erupting vent with very little cooling


Shield volcano : Shield volcano


Stratovolcano : Stratovolcano Sometimes called composite volcanoes Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions typically consists of many separate vents, some of which may have erupted cinder cones and domes on the volcano's flanks


Stratovolcano : Stratovolcano


Cinder cone : Cinder cone a steep, conical hill of volcanic fragments that accumulate around and downwind from a vent range in size from tens to hundreds of meters tall usually erupt lava flows, either through a breach on one side of the crater or from a vent located on a flank


Cinder cone : Cinder cone


eruption types : eruption types Effusive eruption An eruption dominated by the outpouring of lava onto the ground ‘a’a pahoehoe obsidian pillow basalt


eruption types : eruption types `A`a - (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers


eruption types : eruption types Pahoehoe - Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface


eruption types : eruption types Obsidian - dense volcanic glass, usually rhyolite in composition and typically black in color often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow


eruption types : eruption types Pillow lava - when basalts erupt underwater; mounds of elongate lava "pillows" formed by repeated oozing and quenching of the hot basalt


eruption types : eruption types Geyser - hot springs that episodically erupt fountains of scalding water and steam. Such eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space (for example, a fracture or conduit)


eruption types : eruption types Phreatic eruptions - steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits).


eruption types : eruption types Phreatic eruptions


eruption types : eruption types Lava fountain - a jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock Lava fountains erupt from isolated vents, along fissures, within active lava lakes, and from a lava tube when water gains access to the tube in a confined space


eruption types : eruption types Lava fountain


eruption types : eruption types Lava lakes - large volumes of molten lava, usually basaltic, contained in a vent, crater, or broad depression can form (1) from one or more vents in a crater that erupts enough lava to partially fill the crater; (2) when lava pours into a crater or broad depression and partially fills the crater; and (3) atop a new vent that erupts lava continuously for a period of several weeks or more and slowly builds a crater higher and higher above the surrounding ground.


eruption types : eruption types Lava lakes


eruption types : eruption types pyroclastic flow - a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight of the overlying material.


eruption types : eruption types pyroclastic flow


eruption types : eruption types Strombolian - characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly lava fragments generally consist of partially molten volcanic bombs that become rounded as they fly through the air.


eruption types : eruption types Strombolian


eruption types : eruption types vulcanian - explosive eruption that ejects new lava fragments that do not take on a rounded shape during their flight through the air moderate-sized explosive eruptions commonly eject a large proportion of volcanic ash and also breadcrust bombs and blocks


eruption types : eruption types vulcanian


eruption types : eruption types Plinian - large explosive events that form enormous dark columns of tephra and gas high into the stratosphere Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D - 2,000 were killed


eruption types : eruption types Plinian


volcano : volcano fissure - a fracture or crack in rock along which there is a distinct separation; an elongate fracture or crack at the surface from which lava erupts


volcano : volcano caldera - a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir


volcano : volcano Vent - openings in the Earth's crust from which molten rock and volcanic gases escape onto the ground or into the atmosphere consist of a single circular-shaped structure, a large elongate fissure and fracture, or a tiny ground crack


volcano : volcano vent


volcano : volcano Volcanic ash - consists of rock, mineral, and volcanic glass fragments smaller than 2 mm


volcano : volcano fumaroles - vents from which volcanic gas escapes into the atmosphere may occur along tiny cracks or long fissures may persist for decades or centuries if they are above a persistent heat source or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools


volcano : volcano fumaroles a fumarole on Kilauea Volcano


volcano : volcano Basalt - hard, black volcanic rock with less than about 52 weight percent silica (SiO2) it has a low viscosity (resistance to flow) basaltic lava can flow quickly and easily move


volcano : volcano Magma - molten or partially molten rock beneath the Earth's surface When magma erupts onto the surface, it is called lava Magma typically consists of (1) a liquid portion (often referred to as the melt); (2) a solid portion made of minerals that crystallized directly from the melt; (3) solid rocks incorporated into the magma from along the conduit or reservoir, called xenoliths or inclusions; and (4) dissolved gases.


volcano : volcano Magma types: Dacite - light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2) erupts at temperatures between 800 and 1000°C. Rhyolite - light-colored rock with silica (SiO2) content greater than about 68 weight percent erupted at temperatures of 700 to 850° C Andesite - gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2) erupt at temperatures between 900 and 1100° C.


volcano : volcano Silica: A chemical combination of silicon and oxygen (SiO2).


Slide48 : Dacite Andesite Rhyolite


volcano : volcano Magma


volcano : volcano Tephra - a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains includes large dense blocks and bombs, and small light rock debris such as scoria, pumice, reticulite, and ash.


volcano : volcano Tephra Tephra erupted by Mount St. Helens on 18 May 1980 ranging in size from ash (left 2 piles) to lapilli (right 2 piles).


volcano : volcano bombs - lava fragments that were ejected while viscous (partially molten) and larger than 64 mm in diameter Many acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs


volcano : volcano block - solid rock fragment greater than 64 mm in diameter that was ejected from a volcano during an explosive eruption commonly consist of solidified pieces of old lava flows that were part of a volcano's cone


volcano : volcano block


volcano : volcano Bombs These basaltic lava bombs were erupted by Mauna Kea Volcano, Hawai`i.