Pyroclastic flow
A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that moves away from a volcano about 100 km/h on average but is capable of reaching speeds up to 700 km/h. The gases can reach temperatures of about 1,000 °C. Pyroclastic flows are a devastating result of certain explosive eruptions, their speed depends upon the density of the current, the volcanic output rate, the gradient of the slope. The word pyroclast is derived from the Greek πῦρ, meaning "fire", κλαστός, meaning "broken in pieces". A name for pyroclastic flows which glow red in the dark is nuée ardente. Pyroclastic flows that contain a much higher proportion of gas to rock are known as "fully dilute pyroclastic density currents" or pyroclastic surges; the lower density sometimes allows them to flow over higher topographic features or water such as ridges, hills and seas. They may contain steam and rock at less than 250 °C. Cold pyroclastic surges can occur when the eruption is from a vent under the sea. Fronts of some pyroclastic density currents are dilute.
A pyroclastic flow is a type of gravity current. There are several mechanisms that can produce a pyroclastic flow: Fountain collapse of an eruption column from a Plinian eruption. In such an eruption, the material forcefully ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometers. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano. Fountain collapse of an eruption column associated with a Vulcanian eruption; the gas and projectiles create a cloud, denser than the surrounding air and becomes a pyroclastic flow. Frothing at the mouth of the vent during degassing of the erupted lava; this can lead to the production of a rock called ignimbrite. This occurred during the eruption of Novarupta in 1912. Gravitational collapse of a lava dome or spine, with subsequent avalanches and flows down a steep slope.
The directional blast when part of a volcano explodes. As distance from the volcano increases, this transforms into a gravity-driven current; the volumes range from a few hundred cubic meters to more than 1,000 cubic kilometres. The larger ones can travel for hundreds of kilometres, although none on that scale have occurred for several hundred thousand years. Most pyroclastic flows are around travel for several kilometres. Flows consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an hot ash plume lofts above it because of the turbulence between the flow and the overlying air and heating cold atmospheric air causing expansion and convection; the kinetic energy of the moving cloud will flatten buildings in its path. The hot gases and high speed make them lethal, as they will incinerate living organisms instantaneously: The cities of Pompeii and Herculaneum, for example, were engulfed by pyroclastic surges on August 24, 79 AD with many lives lost.
The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. Despite signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys between it and the volcano, but the pyroclastic flow charred the entirety of the city, killing all but two of its 30,000 residents. A pyroclastic surge killed volcanologists Harry Glicken and Katia and Maurice Krafft and 40 other people on Mount Unzen, in Japan, on June 3, 1991; the surge started as a pyroclastic flow and the more energised surge climbed a spur on which the Kraffts and the others were standing. On 25 June, 1997 a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large energized pyroclastic surge developed; this flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area. Several others in the area suffered severe burns. Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence, shows that pyroclastic flows can cross significant bodies of water.
However, that might be a pyroclastic surge, not flow, because the density of a gravity current means it cannot move across the surface of water. One flow reached the Sumatran coast as much as 48 km away. A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes, demonstrated tests by a research team at Kiel University, Germany, of pyroclastic flows moving over water; when the reconstructed pyroclastic flow hit the water, two things happened
Country rock (geology)
Country rock is a geological term meaning the rock native to an area, in which there is an intrusion of viscous geologic material magma, or rock salt or unconsolidated sediments. Magma is less dense than the rock it intrudes and filling existing cracks, sometimes melting the already-existing country rock; the term "country rock" is similar to, in many cases interchangeable with, the terms basement and wall rocks. Country rock can denote the widespread lithology of a region in relation to the rock, being discussed or observed. Settings in geology when the term country rock is used include: When describing a pluton or dike, the igneous rock can be described as intruding the surrounding country rock, the rock into which the pluton has intruded; when country rock is intruded by dyke, perpendicular to the bedding plane, it is called discordant intrusion, while a parallel intrusion by a sill indicates a sub-parallel or concordant intrusion. Most intrusions into country rock are via magma. Country rock is intruded by an igneous body of rock which formed when magma forced upward through fractures, or melted through overlying rock.
Magma cooled into solid rock, different from the surrounding country rock. Sometimes, a fragment of country rock will break off and become incorporated into the intrusion, is called a xenolith, from Greek, ξένος, xenos, "strange,", λίθος, the ancient Greek word for "stone." The heat of the intrusions changes the country rock to contact metamorphic rock. Hornfels is produced, or skarn; when describing recent alluvium, the material that has arrived through volcanic, glacial or fluvial action can be described as a veneer on the country rock
Lapilli
Lapilli is a size classification term for tephra, material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli is Latin for "little stones". By definition lapilli range from 2 to 64 mm in diameter. A pyroclastic particle greater than 64 mm in diameter is known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash. Lapilli are spheroid-, teardrop-, dumbbell- or button-shaped droplets of molten or semi-molten lava ejected from a volcanic eruption that fall to earth while still at least molten; these granules are not accretionary, but instead the direct result of liquid rock cooling as it travels through the air. Lapilli tuffs are a common form of volcanic rock typical of rhyolite and dacite pyroclastic eruptions, where thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff.
The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out and become welded. Welded tuff textures are distinctive, with flattened lapilli, fiamme and bombs forming oblate to discus-shaped forms within layers; these rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs, which are unconsolidated and eroded. Rounded balls of tephra are called accretionary lapilli if they consist of layered volcanic ash particles. Accretionary lapilli are formed by a process of wet ash aggregation due to moisture in volcanic clouds that sticks the particles together, with the volcanic ash nucleating on some object and accreting to it in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus; this texture can be confused with axiolitic texture. These lapilli are a variety of accretionary lapilli, though they contain lithic or crystal cores coated by rinds of coarse to fine ash.
Armoured lapilli only form in hydroclastic eruptions. The vapour column contains cohesive ash. Tuff Tephra Cinder Lava Rock microstructure of volcanic rocks Volcanic Materials Identification Tephra fall from Mt St. Helens
Agglomerate
Agglomerate is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records fluidal surfaces: they may, for example, have ropy, scoriaceous, or folded, chilled margins and spindle, ribbon, ragged, or amoeboid shapes. Globular masses of lava may have been shot from the crater at a time when molten lava was exposed, was shattered by sudden outbursts of steam; these bombs were viscous by rotation in the air acquired their shape. They are 1 to 2 feet in diameter, but specimens as large as 12 feet have been observed. There is less variety in their composition at any one volcanic centre than in the case of the lithic blocks, their composition indicates the type of magma being erupted. Agglomerates are found near volcanic vents and within volcanic conduits, where they may be associated with pyroclastic or intrusive volcanic breccias. Older publications in Scotland, referred to any coarse-grained volcaniclastic rock as'agglomerate', which led to debris flow deposits, talus deposits and other types of breccia being mistaken for vents.
Agglomerates are poorly sorted, may contain a fine ash or tuff matrix and vary from matrix to clast support. They may be monolithologic or heterolithic, may contain some blocks of various igneous rocks. There are various differences between ordinary ash beds or tuffs. Agglomerates are coarser and less well-bedded. Agglomerates can be non-welded or welded, such as coarse basaltic'spatter', they form proximally during Strombolian eruptions, are common at peralkaline volcanoes. Some large agglomerate deposits are deposited from pyroclastic density currents during explosive caldera-forming eruptions, such as at Santorini and Campi Flegrei, they may be massive to crudely bedded, can attain great thicknesses. Crystalline masses of a different kind occur in some numbers in certain agglomerates, they consist of volcanic minerals much the same as those formed in the lava, but exhibiting certain peculiarities which indicate that they have formed under pressure at considerable depths. They bear a resemblance to plutonic igneous rocks, but are more to be regarded as agglomerations of crystals formed within the liquid lava as it rose towards the surface, at a subsequent period cast out by violent steam explosions.
The sanidinites of the Eifel belong to this group. At Vesuvius, Ascension, St Vincent and many other volcanoes, they form a considerable part of the coarser ash-beds, their commonest minerals are olivine, hornblende, augite and leucite. This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed.. "Agglomerate". Encyclopædia Britannica. 1. Cambridge University Press. Pp. 374–375
Peperite
A peperite is a type of volcaniclastic rock consisting of sedimentary rock that contains fragments of younger igneous material and is formed when magma comes into contact with wet sediments. The term was used to describe rocks from the Limagne region of France, from the similarity in appearance of the granules of dark basalt in the light-coloured limestone to black pepper; the igneous fragments are glassy and show chilled-margins to the sedimentary matrix, distinguishing them from clasts with a sedimentary origin. The term has been used to describe a wide variety of rocks that are interpreted to have formed by the interaction between magma and sediments; this usage has led to overlap with other terms such as hyaloclastite. In the most recent edition of'Igneous rocks' by Le Maître et al. the definition is given as "A local term for a tuff or breccia, formed by the intrusion of magma into wet sediments. Consists of glassy fragments of igneous rock and some sedimentary rock", while White defines peperite as "a genetic term applied to a rock formed in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated wet sediments.
The term refers to similar mixtures generated by the same processes operating at the contacts of lavas and other hot volcaniclastic deposits with such sediments". When magma comes into contact with wet sediment several processes combine to produce the mixture of sedimentary and igneous clasts, characteristic of a peperite; these processes are required to produce both the disintegration or fragmentation of magma to form juvenile clasts and the mingling of these clasts within the sediment. Mechanisms proposed; the main mechanism suggested for mingling of the igneous clasts with the sediment is fluidisation, in the sense of particle support and transport by a fluid. Peperites are found world-wide in sediments with a significant water content at the time of formation associated with igneous rocks covering the compositional range from basalt to rhyolite
Lava
Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption at temperatures from 700 to 1,200 °C. The structures resulting from subsequent solidification and cooling are sometimes described as lava; the molten rock is formed in the interior of some planets, including Earth, some of their satellites, though such material located below the crust is referred to by other terms. A lava flow is a moving outpouring of lava created during a non-explosive effusive eruption; when it has stopped moving, lava solidifies to form igneous rock. The term lava flow is shortened to lava. Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic and shear thinning properties. Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows; the word lava comes from Italian, is derived from the Latin word labes which means a fall or slide.
The first use in connection with extruded magma was in a short account written by Francesco Serao on the eruption of Vesuvius in 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; the composition of all lava of the Earth's crust is dominated by silicate minerals feldspars, pyroxenes, amphiboles and quartz. Igneous rocks, which form lava flows when erupted, can be classified into three chemical types: felsic and mafic; these classes are chemical, the chemistry of lava tends to correlate with the magma temperature, its viscosity and its mode of eruption. Felsic or silicic lavas such as rhyolite and dacite form lava spines, lava domes or "coulees" and are associated with pyroclastic deposits. Most silicic lava flows are viscous, fragment as they extrude, producing blocky autobreccias; the high viscosity and strength are the result of their chemistry, high in silica, potassium and calcium, forming a polymerized liquid rich in feldspar and quartz, thus has a higher viscosity than other magma types.
Felsic magmas can erupt at temperatures as low as 650 to 750 °C. Unusually hot rhyolite lavas, may flow for distances of many tens of kilometres, such as in the Snake River Plain of the northwestern United States. Intermediate or andesitic lavas are lower in aluminium and silica, somewhat richer in magnesium and iron. Intermediate lavas form andesite domes and block lavas, may occur on steep composite volcanoes, such as in the Andes. Poorer in aluminium and silica than felsic lavas, commonly hotter, they tend to be less viscous. Greater temperatures tend to destroy polymerized bonds within the magma, promoting more fluid behaviour and a greater tendency to form phenocrysts. Higher iron and magnesium tends to manifest as a darker groundmass, occasionally amphibole or pyroxene phenocrysts. Mafic or basaltic lavas are typified by their high ferromagnesian content, erupt at temperatures in excess of 950 °C. Basaltic magma is high in iron and magnesium, has lower aluminium and silica, which taken together reduces the degree of polymerization within the melt.
Owing to the higher temperatures, viscosities can be low, although still thousands of times higher 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 lavas tend to produce low-profile shield volcanoes or "flood basalt fields", because the fluidal lava flows for long distances from the vent; the thickness of a basalt lava on a low slope, may be much greater than the thickness of the moving lava flow at any one time, because basalt lavas may "inflate" by supply of lava beneath a solidified crust. Most basalt lavas are of pāhoehoe types, rather than block lavas. Underwater, they can form pillow lavas, which are rather similar to entrail-type pahoehoe lavas on land. Ultramafic lavas such as komatiite and magnesian magmas that form boninite take the composition and temperatures of eruptions to the extreme. Komatiites contain over 18% magnesium oxide, are thought to have erupted at temperatures of 1,600 °C.
At this temperature there is no polymerization of the mineral compounds, creating a mobile liquid. 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 magnesian magmas; some lavas of unusual composition have erupted onto the surface of the Earth. These include: Carbonatite and natrocarbonatite lavas are known from Ol Doinyo Lengai volcano in Tanzania, the sole example of an active carbonatite volcano. Iron oxide lavas are thought to be the source of the iron ore at Kiruna, Sweden which formed during the Proterozoic. Iron oxide lavas of Pliocene age occur at the El Laco volcanic complex on the Chile-Argentina border. Iron oxide lavas are thought to be the result of immiscible separation of iron oxide magma from a parental magma of calc-alkaline or alkaline composition. Sulfur lava flows up to 250 metres 10 metres wide occur at Lastarria volcano, Chile.
They were formed by the melting of sulfur deposits at temperatures as low as 113 °C
Krakatoa
Krakatoa, or Krakatau, is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The name is used for the surrounding volcanic island group comprising four islands: two of which and Verlaten, are remnants of a previous volcanic edifice destroyed in eruptions long before the famous 1883 eruption. In 1927, a fourth island, Anak Krakatau, or "Child of Krakatoa", emerged from the caldera formed in 1883. There has been new eruptive activity since the late 20th century, with a large collapse causing a deadly tsunami in December 2018; the most notable eruptions of Krakatoa culminated in a series of massive explosions over 26–27 August 1883, which were among the most violent volcanic events in recorded history. With an estimated Volcanic Explosivity Index of 6, the eruption was equivalent to 200 megatons of TNT —about 13,000 times the nuclear yield of the Little Boy bomb that devastated Hiroshima, during World War II, four times the yield of Tsar Bomba, the most powerful nuclear device detonated at 50 Mt.
The 1883 eruption ejected 25 km3 of rock. The cataclysmic explosion was heard 3,600 km away in Alice Springs, on the island of Rodrigues near Mauritius, 4,780 km to the west. According to the official records of the Dutch East Indies colony, 165 villages and towns were destroyed near Krakatoa, 132 were damaged. At least 36,417 people died, many more thousands were injured from the tsunamis that followed the explosion; the eruption destroyed two-thirds of the island of Krakatoa. Eruptions in the area since 1927 have built a new island at the same location, named Anak Krakatau. Periodic eruptions have continued since, with recent eruptions in 2009, 2010, 2011, 2012, a major collapse in 2018. In late 2011, this island had a radius of 2 kilometres, a highest point of about 324 metres above sea level, growing five metres each year. In 2017, the height of Anak Krakatau was reported as over 400 m above sea level. Although there are earlier descriptions of an island in the Sunda Strait with a "pointed mountain," the earliest mention of Krakatoa by name in the western world was on a 1611 map by Lucas Janszoon Waghenaer, who labelled the island "Pulo Carcata".
About two dozen variants have been found, including Crackatouw and Krakatao. The first known appearance of the spelling Krakatau was by Wouter Schouten, who passed by "the high tree-covered island of Krakatau" in October 1658; the origin of the Indonesian name Krakatau is uncertain. The Smithsonian Institution's Global Volcanism Program cites the Indonesian name, Krakatau, as the correct name, but says that Krakatoa is employed. Indonesia has the most of any nation, they make up the axis of the Indonesian island arc system produced by northeastward subduction of the Indo-Australian Plate. A majority of these volcanoes lie along Indonesia's two largest islands and Sumatra; these two islands are separated by the Sunda Strait located at a bend in the axis of the island arc. Krakatau is directly above the subduction zone of the Eurasian Plate and the Indo-Australian Plate where the plate boundaries make a sharp change of direction resulting in an unusually weak crust in the region. At some point in prehistory, an earlier caldera-forming eruption had occurred, leaving as remnants Verlaten.
At least two more cones formed and joined with Rakata, forming the main island of Krakatoa. At the time of the 1883 eruption, the Krakatoa group comprised Lang and Krakatoa itself, an island 9 km long by 5 km wide. There were the tree-covered islet near Lang and several small rocky islets or banks between Krakatoa and Verlaten. There were three volcanic cones on Krakatoa island: Rakata, to the south; the Javanese Book of Kings records that in the year 338 Saka: A thundering sound was heard from the mountain Batuwara, answered by a similar noise from Kapi, lying westward of the modern Bantam. A great glowing fire, which reached the sky, came out of the last-named mountain; the water of the sea rose and inundated the land, the country to the east of the mountain Batuwara, to the mountain Rajabasa, was inundated by the sea. The water subsided but the land on which Kapi stood became sea, Java and Sumatra were divided into two parts. There is no geological evidence of a K
