Hot molten rock erupted onto the Earth's surface from a volcano. On solidification it forms volcanic igneous rocks such as rhyolite, andesite, or basalt. Lava temperature and viscosity depends on its chemical composition, with the more silica-rich lava being cooler (around 900°C) and more viscous than basic lavas (temperature up to 1100°C), which flow more freely, forming volcanoes with gentle slopes.
Although lava is quite viscous, about 100,000 times the viscosity of water, it can flow great distances before cooling and solidifying.Lava solidifies to form igneous rock. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain.
Lava composition
In general, a lava's composition determines its behavior more than the temperature of its eruption. Igneous rocks, which form lava flows when erupted, can be classified into three chemical types;
Felsic lavas such as rhyolite and dacite are often associated with strombolian eruptions, typically form lava domes and sheeted flows, and are associated with pyroclastic surge deposits and tuffs. Intermediate lavas form andesite domes and sheeted flows, are usually associated with strombolian eruptions, and form composite volcanoes.
Mafic or basaltic lavas are typified by their high ferromagnesian content, and generally erupt at temperatures in excess of 950 degrees Celsius.
Ultramafic lavas such as komatiite and high-magnesian magmas which form boninite take the composition and temperatures of eruptions to the extreme.
Lava behavior
The viscosity of lava is important because it determines how the lava will behave. Lavas with high viscosity are rhyolite, dacite, andesite and trachyte, with cooled basaltic lava also quite viscous;
Highly viscous lava shows the following behaviors:
It tends to flow slowly, clog, and form semi-solid blocks which resist flow It tends to entrap gasses, which form bubbles within the rock as they rise to the surface It correlates with explosive or phreatic eruptions and is associated with tuff and pyroclastic flowsHighly viscous lavas do not usually flow as liquid, and usually form explosive fragmental ash and tephra deposits. However, a degassed viscous lava or one which erupts somewhat hotter than usual may form a lava flow.
Lava with low viscosity shows the following behaviors:
It tends to flow easily, forming puddles, channels, and rivers of molten rock It tends to easily release bubbling gases as they are formed Eruptions are rarely pyroclastic and are usually quiescent Volcanoes tend to form as rifts, not steep conesThere are three forms of low-viscosity lava flows: ʻaʻā, pāhoehoe, and pillow lava.
Lavas also may contain many other components, sometimes including solid crystals of various minerals, fragments of exotic rocks known as xenoliths and parts of its own solidified lava products.
Volcanic Morphologies
The physical behaviour of a lava creates the physical forms of a lava flow or volcano. More fluid basaltic lava flows tend to form flat sheets and lobes of lava, whereas viscous rhyolite forms knobbly, rubbly masses of rock.
General features of volcanology can be used to classify volcanic edifices and provide information on the eruptions which formed the lava flow, even if the sequence of lavas have been buried or metamorphosed.
The ideal lava flow will have a brecciated top, either as pillow lava development, autobreccia and rubble typical of ʻaʻā and viscous flows, or a vesicular or frothy carapace such as scoria or pumice.
The centre of the lava flow will ideally be massive and crystalline, though usually the crystals will be microscopic. The more viscous lava forms tend to show sheeted flow features, and blocks or breccia entrained within the sticky lava.
The flow base tends to show evidence of hydrothermal activity, generally because the lava is erupted onto moist or wet substrates.
Discriminating between a sill and a lava flow in ancient rock sequences can be difficult. However, sills do not usually have brecciated margins, they show greater propensity to form a chilled margin, and may show a weak metamorphic aureole on both the upper and lower surface whereas a lava flow will only metamorphose the lower surface.
Lava domes
Cooling viscous lava often clogs a volcanic vent, allowing pressure behind the blockage to build; trapped gasses within the lava also add to the pressure, eventually producing cataclysmic explosions, ejecting great clouds of volcanic ash and gas, and producing pyroclastic flows.
Sometimes as a volcano extrudes silicic lava, it forms an inflation dome, gradually building up a large, pillow-like structure which cracks, fissures, and may release cooled chunks of rock and rubble.
Examples of lava dome eruptions include the Novarupta dome, and successive lava domes of Mount St Helens.
Sheeted flows
Sheeted flows are an uncommon form of eruptive phenomena of felsic and intermediate volcanoes.
Typically the lava flow forms a sheeted flow or laminar flow, with the upper and lower margins of the flowing lava forming a hard, brittle shell inside of which the sticky, viscous lava will be flowing. This is similar to an ʻaʻā flow except that the inner lava will show evidence of stretching, plastic deformation and even foliation of the highly viscous lava.
ʻAʻā
ʻAʻā (also spelled aa, aʻa, ʻaʻa and aa-aa, IPA: /ˈɑːɑː/, Hawaiian English, from Hawaiian meaning "stony with rough lava", but also to "burn" or "blaze") is one of three basic types of flow lava.
Accretionary lava balls as large as 3 m (10 ft) are common on ʻaʻā flows.
Pāhoehoe
Pāhoehoe (also spelled pahoehoe, IPA: /pəˈhəʊɪhəʊi/, Hawaiian English, from Hawaiian, meaning "smooth, unbroken lava") is basaltic lava that has a smooth, billowy, undulating, or ropy surface.
Pillow lava
Pillow lava is the rock type typically formed when lava emerges from an underwater volcanic vent or a lava flow enters the ocean. The viscous lava gains a solid crust immediately upon contact with the water, and this crust cracks and oozes additional large blobs or "pillows" as more lava emerges from the advancing flow.
Lava landforms
Due to being formed from viscous molten rock, lava flows and eruptions create distinctive formations, landforms and topographical features from the macroscopic to the microscopic.
Volcanoes
Volcanoes are the primary landform created by lava eruption and consist of flattish, shallow shield volcanes formed from basalt to steeply-sided ash and lava composite volcanic cones typical of andesite and rhyolite lavas.
Volcanoes can form calderas if they are obliterated by large pyroclastic or phreatic eruptions, and such features typically include volcanic crater lakes and lava domes after the event.
Cinder and spatter cones
Cinder cones and spatter cones are small-scale features formed by lava accumulation around a small vent on a volcanic edifice.
Lava domes
Lava domes are formed by the extrusion of viscous felsic magma.
Lava tubes
Lava tubes are formed when a flow of relatively fluid lava cools on the upper surface sufficiently to form a crust. Beneath this crust, which by dint of being made of rock is an excellent insulator, the lava can continue to flow as a liquid. When this flow occurs over a prolonged period of time the lava conduit can form a tunnel-like aperture or lava tube, which can conduct molten rock many kilometres from the vent without cooling appreciably. Often these lava tubes drain out once the supply of fresh lava has stopped, leaving a considerable length of open tunnel within the lava flow.
Lava tubes are known from the modern day eruptions of Kīlauea, and significant, extensive and open lava tubes of Tertiary age are known from North Queensland, Australia, some extending for 15 kilometres.
Lava cascades and fountains
The eruptions of lava are sometimes attended by peculiarities which impart to them much additional grandeur.
Lava lakes
Rarely, a volcanic cone may fill with lava but not erupt. Lava which pools within the caldera is known as a lava lake. Lava lakes do not usually persist for long, either draining back into the magma chamber once pressure is relieved (usually by venting of gases through the caldera), or by draining via eruption of lava flows or pyroclastic explosion. These include:
Mount Erebus, Antarctica Kīlauea Volcano, Hawaiʻi Erta Ale, Ethiopia Nyiragongo, Democratic Republic of CongoComposition of volcanic rocks
The sub-family of rocks which form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks which form from magma, below the surface of the earth, called igneous plutonic rocks).
Unusual lavas
Three types of unusual volcanic rocks have been recognised as erupting onto the surface of the Earth; Sulfide lavas have been recognised from Chile and Peru Iron oxide lavas are thought to be the source of the iron ore at Kiruna, Sweden, erupted in the Proterozoic, and in Chile associated with highly alkaline igneous rocks
Hazards
Lava flows are enormously destructive to property in their path but generally move slowly enough for people to get out of their way, so casualties caused directly by active lava flows are rare. Nevertheless injuries and deaths have occurred, either because people had their escape route cut off, because they get too close to the flow or, more rarely, if the lava flow front travels too quickly.
This notably happened during the eruption of Nyiragongo in Zaire (now Democratic Republic of Congo) on 10 January 1977 when the crater wall was breached during the night and the fluid lava lake in it drained out in less than an hour.
Deaths attributed to volcanoes frequently have a different cause, for example volcanic ejecta, pyroclastic flow from a collapsing lava dome, lahars, or explosions caused when the flow comes into contact with water.
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