|
Lava is molten rock expelled by a volcano during an eruption. Magma is molten rock below the earth's surface. Lava, when first exuded from a volcanic vent, is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F). 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. The term "lava flow" refers to the hardened formation, whereas the one still having molten rock associated, is called an "active lava flow". The word 'lava' comes from Italian, and is probably derived from the Latin word labes which means a fall, slide, or sinking in. The first use in connection with extruded magma was apparently in a short account written by Francesco Serao * on the eruption of Vesuvius between May 14 and June 4, 1737. 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, intermediate, and mafic. These classes are primarily chemical; however, the chemistry of a lava also tends to correlate with the magma temperature, its viscosity and its mode of eruption. 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. Felsic lavas are extremely viscous. This is caused primarily by the chemistry of the magma, which is high in silica, aluminium, potassium, sodium, and calcium, forming a polymerized liquid rich in feldspar and quartz, which is thus much more sticky than other magma types. Felsic magmas can erupt at temperatures as low as 650 to 750 degrees Celsius, although they can be hotter. Intermediate lavas are lower in aluminium and silica, and usually somewhat richer in magnesium and iron. Intermediate lavas form andesite domes and sheeted flows, are usually associated with strombolian eruptions, and form composite volcanoes. Poorer in aluminium and silica than felsic lavas, and also hotter (in the range of 750 to 950 degrees Celsius), they tend to be less viscous. Greater temperatures tend to destroy polymerized bonds within the magma, promoting more fluid behaviour and also a greater tendency to form phenocrysts. Higher iron and magnesium tends to manifest as a darker groundmass, and also occasionally amphibole or pyroxene phenocrysts. Mafic or basaltic lavas are typified by their high ferromagnesian content, and generally erupt at temperatures in excess of 950 degrees Celsius. Basaltic magma is high in iron and magnesium, and has relatively lower aluminium and silica, which taken together reduces the degree of polymerization within the melt. Due to the higher temperatures, viscosities can be relatively low, although still thousands of times more viscous than water. The low degree of polymerization and high temperature favors chemical diffusion, so it is common to see large, well-formed phenocrysts within mafic lavas. Basalt volcanoes tend to form shield volcanoes, as the fluid magma tends to form thin, widely distributed flows. Ultramafic lavas such as komatiite and high-magnesian magmas which form boninite take the composition and temperatures of eruptions to the extreme. Komatiites contain over 18% magnesium oxide, and are thought to have erupted at temperatures of 1600 °C. At this temperature there is no polymerization of the mineral compounds, creating a highly mobile liquid with viscosity as low as that of water. Most if not all ultramafic lavas are no younger than the Proterozoic, with a few ultramafic magmas known from the Phanerozoic. No modern komatiite lavas are known, as the Earth's mantle has cooled too much to produce highly magnesian magmas. 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; those with low viscosities are freshly erupted basalt, carbonatite and the unusual sulphide lavas, and occasionally andesite. Highly viscous lava shows the following behaviors: Highly 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. Viscous lavas have two forms of non-pyroclastic eruptions, lava domes and sheeted flows. Lava with low viscosity shows the following behaviors: There are three forms of low-viscosity lava flows: aā, pāhoehoe, and pillow lava. They are described in relation to basaltic flows from Hawaii, shown in the following sections. 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 aā and viscous flows, or a vesicular or frothy carapace such as scoria or pumice. The flow top will tend to be glassy, having been flash frozen in contact with the air or water. 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 crystal size at the centre of a lava will in general be greater than at the margins, as the crystals have more time to grow. The flow base tends to show evidence of hydrothermal activity, generally because the lava is erupted onto moist or wet substrates. The flow base may have vesicles, perhaps filled with minerals (amygdules). The substrate upon which the lava has flowed may show signs of scouring, it may be broken or disturbed due to the boiling of trapped water, and in the case of soil profiles, may be baked into a brick-red clay. 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. However, it is often difficult in practise to identify these metamorphic phenomenon because they are usually weak and restricted in size. Lava domes
Sheeted flows Sheeted flows are an uncommon form of eruptive phenomena of felsic and intermediate volcanoes. Internal pressure of gases tend to promote pyroclastic and explosive eruptions. However, a viscous magma will flow, though very slowly, across the surface of the Earth. 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. The hard skin forms a chaotic igneous breccia called autobreccia, as the flow creeps along, churning the outer margins apart. This is similar to an aā flow except that the inner lava will show evidence of stretching, plastic deformation and even foliation of the highly viscous lava. Examples of laminar or sheeted flows include the Tertiary aged volcanic edifices of the Glasshouse mountains, and the cliffs of Kangaroo Point in Brisbane, Australia. {{okina}}A{{okina}}ā Aā (Hawaiian English, from Hawaiian meaning "stony with rough lava", but also to "burn" or "blaze") is one of three basic types of flow lava. Aā is characterized by a rough or rubbly surface composed of broken lava blocks called clinker. The loose, broken, and sharp, spiny surface of a solidified aā flow makes walking difficult and slow. (Naturally, walking upon a nonsolidified aā flow is not advised.) The clinkery surface actually covers a massive dense core, which was the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an aā flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an aā flow. Accretionary lava balls as large as 3 m (10 ft) are common on aā flows. Aā is usually of higher viscosity than pāhoehoe (often spelled just pahoehoe). Pāhoehoe can turn into aā if it becomes turbulent due to meeting impediments or steep slopes. The sharp, angled texture makes aā a strong radar reflector, and can easily be seen from an orbiting satellite (bright on Magellan pictures). Pāhoehoe
Pillow lava
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
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. Cinder cones are formed from tephra or ash and tuff which is thrown from an explosive vent. Spatter cones are formed by accumulation of molten volcanic slag and cinders ejected in a more liquid form. Lava domes Lava domes are formed by the extrusion of viscous felsic magma. They can form prominent rounded protuberances, such as at Valle Calderas. 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
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. There are only a few sites in the world where permanent lakes of lava exist. These include: Composition 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). The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. Unusual lavas Three types of unusual volcanic rocks have been recognised as erupting onto the surface of the Earth; 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. Flowing down the steep slopes of the volcano at up to 60 miles per hour (100 km per hour), the lava swiftly overwhelmed several villages whilst their residents were asleep. As a result of this disaster, the mountain was designated a Decade Volcano in 1991*. 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*. Towns destroyed by lava flows
Towns partially destroyed by lava flows | |||||||||||||||||||
|
| ||||||||||||||||||||
 |
|
| |