116-528: Hekla ( Icelandic pronunciation: [ˈhɛhkla] ), or Hecla , is an active stratovolcano in the south of Iceland with a height of 1,491 m (4,892 ft). Hekla is one of Iceland's most active volcanoes ; over 20 eruptions have occurred in and around the volcano since the year 1210. During the Middle Ages , the Icelandic Norse called the volcano the "Gateway to Hell " and
232-432: A conical volcano built up by many alternating layers ( strata ) of hardened lava and tephra . Unlike shield volcanoes , stratovolcanoes are characterized by a steep profile with a summit crater and explosive eruptions. Some have collapsed summit craters called calderas . The lava flowing from stratovolcanoes typically cools and solidifies before spreading far, due to high viscosity . The magma forming this lava
348-462: A fissure vent and stratovolcano (built from mixed lava and tephra eruptions) sited at a rift- transform junction in the area where the south Iceland seismic zone and eastern volcanic zone meet. The unusual form of Hekla is found on very few volcanoes around the world, notably Callaqui in Chile . The 5.5 km (3.4 mi) Heklugjá fissure opens along its entire length during major eruptions and
464-436: A flow rate of around 1500 m³/s. For the first two hours, tephra was produced at the rate of 10000 m³/s. The cloud from the eruption, which had reached 53,000 feet (16,154 m) by 10:10 pm, caused a lightning storm . The tephra was transported northwards by the wind, causing the sky to turn black in places – 190 km (120 mi) away at Blönduós tephra fell from midnight until 2 am, and ash fell on
580-458: A trawler 330 km (210 mi) away at 2 am. Icelanders sampled the tephra fall in their locality by putting a plate outside to capture everything that fell onto it. This, and other measurements, showed the area covered was long and narrow with the 1 mm contour (an equivalent of 8 tonnes per hectare) extending to the north coast. By 5:30 on 6 May, the lava flow measured 4 km (2.5 mi) long. Many lava bombs were found near
696-747: A 4-inch thick ash layer can weigh 120-200 pounds and can get twice as heavy when wet. Wet ash also poses a risk to electronics due to its conductive nature. Dense clouds of hot volcanic ash can be expelled due to the collapse of an eruptive column , or laterally due to the partial collapse of a volcanic edifice or lava dome during explosive eruptions . These clouds are known as pyroclastic surges and in addition to ash , they contain hot lava , pumice , rock , and volcanic gas . Pyroclastic surges flow at speeds over 50 mph and are at temperatures between 200 °C – 700 °C. These surges can cause major damage to property and people in their path. Lava flows from stratovolcanoes are generally not
812-461: A 900 m (3,000 ft) long fissure opened 1 m (11 sq ft)1 km north of the main Hlídargígar crater. That night it contained 17 lava fountains, each 20–50 m (66–164 ft) in height. By the evening of the next day, 10–12 craters had formed, each throwing pieces of lava 50–100 m (160–330 ft) in the air. This row of craters was named Öldugígar. Gradually
928-458: A VEI of 3 and produced 0.2 km (0.048 cu mi) of lava covering an area of 18.5 km (7.1 sq mi) and 6.6×10 m³ of tephra, deposited over an area of 40,000 km (15,000 sq mi), mainly to the northwest of the volcano. The main Hekla fissure only erupted at its far southwest end, most of the eruption was from other fissures nearby. The eruption stopped in
1044-454: A century after the last eruption of Hekla proper, the longest dormant period since 1104. Before the eruption the volcano had been visible from the surrounding area but nothing remarkable was noticed. The eruption occurred at 6:41 am ± 3 min with a loud roar; later eruptions could be heard throughout Iceland. An earthquake at 6:50 am measured 6 on the Mercalli intensity scale and increased
1160-468: A couple of hours or less. Some are very short (a week to ten days) whereas others can stretch into months and years (the 1947 eruption started 29 March 1947 and ended April 1948). But there is a general correlation: the longer Hekla goes dormant, the larger and more catastrophic its opening eruption will be. The most recent eruption was on 26 February 2000. One of the largest Holocene eruptions in Iceland
1276-509: A crater at 780 m (2,560 ft) was producing a lava column which reached an altitude of around 1 km (0.62 mi). During the night a 700 m (2,300 ft) high lava fountain was thrown up from the main crater. A 500 m (1,600 ft) long fissure starting below the Lava Crater opened, and lava fountains and other lava flows emanated from it. One hour into the eruption, a new 400 m (1,300 ft) fissure opened to
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#17327727066481392-452: A decade. The dates were recently recalibrated of the major eruptions and a table is given below as the difference in dates could cause confusion. Unless otherwise stated eruption dates in Year (2019 values) column are from Global Volcanism Program and Catalogue of Icelandic Volcanoes, As other sources can disagree, there has been a recent literature update. The values and range given in
1508-479: A fast moving mudflow . Lahars are typically about 60% sediment and 40% water. Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete. Lahars have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour. In the 1985 eruption of Nevado del Ruiz in Colombia , Pyroclastic surges melted snow and ice atop
1624-427: A fearful underground groaning, which continued till mid-day on the 2nd. Then, with a tremendous crash, there were formed in the sides of the cone two large openings, whence there gushed torrents of lava, which flowed down two gorges on the flanks of the mountain. The whole summit was enveloped in clouds of vapour and volcanic dust. The neighbouring rivers became so hot as to kill the fish, and the sheep fled in terror from
1740-515: A final intermediate composition . When the magma nears the top surface, it pools in a magma chamber within the crust below the stratovolcano. The processes that trigger the final eruption remain a question for further research. Possible mechanisms include: These internal triggers may be modified by external triggers such as sector collapse , earthquakes , or interactions with groundwater . Some of these triggers operate only under limited conditions. For example, sector collapse (where part of
1856-401: A fluorine content of 0.2%, and two days after the eruption contaminated grass had a dry weight content of up to 0.4% fluorine. 450 farms and 95,000 sheep were affected by the eruption. Some sheep were kept inside and fed on hay or moved, but other farmers were forced to graze their flocks outside. Stratovolcano A stratovolcano , also known as a composite volcano , is
1972-407: A glass with no true melting point, can be used as a glass fibre for fibreglass. Silicon dioxide is mostly obtained by mining, including sand mining and purification of quartz . Quartz is suitable for many purposes, while chemical processing is required to make a purer or otherwise more suitable (e.g. more reactive or fine-grained) product. Precipitated silica or amorphous silica is produced by
2088-494: A heat capacity minimum. Its density decreases from 2.08 g/cm at 1950 °C to 2.03 g/cm at 2200 °C. The molecular SiO 2 has a linear structure like CO 2 . It has been produced by combining silicon monoxide (SiO) with oxygen in an argon matrix. The dimeric silicon dioxide, (SiO 2 ) 2 has been obtained by reacting O 2 with matrix isolated dimeric silicon monoxide, (Si 2 O 2 ). In dimeric silicon dioxide there are two oxygen atoms bridging between
2204-450: A low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz, the Si–O–Si angle is 144°. Alpha quartz is the most stable form of solid SiO 2 at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since
2320-769: A result of intraplate volcanism on oceanic islands far from plate boundaries. Examples are Teide in the Canary Islands , and Pico do Fogo in Cape Verde . Stratovolcanoes in the East African Rift include Ol Doinyo Lengai in Tanzania, and Longonot in Kenya. Subduction zone volcanoes form when hydrous minerals are pulled down into the mantle on the slab. These hydrous minerals, such as chlorite and serpentine , release their water into
2436-461: A significant threat to humans or animals because the highly viscous lava moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such as explosions and asphyxiation from toxic gas . Lava flows can bury homes and farms in thick volcanic rock which greatly reduces property value. However, not all stratovolcanoes erupt viscous and sticky lava . Nyiragongo , near Lake Kivu in central Africa ,
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#17327727066482552-410: A silicon wafer enables it to overcome the surface states that otherwise prevent electricity from reaching the semiconducting layer. The process of silicon surface passivation by thermal oxidation (silicon dioxide) is critical to the semiconductor industry . It is commonly used to manufacture metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon integrated circuit chips (with
2668-477: A very shallow layer of about 1 nm or 10 Å of so-called native oxide. Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry oxidation with O 2 or wet oxidation with H 2 O. The native oxide layer is beneficial in microelectronics , where it acts as electric insulator with high chemical stability. It can protect
2784-483: A violent storm on the night of the 2nd of September in that year, the surface of the ground in the Orkney Islands was found strown with volcanic dust. There was thus conveyed to the inhabitants of Great Britain an intimation that Hecla had been again at work. Accordingly, tidings soon after arrived of a great eruption of the mountain. On the night of 1 September, the dwellers in its neighbourhood were terrified by
2900-524: A white powder with extremely low bulk density (0.03-0.15 g/cm ) and thus high surface area. The particles act as a thixotropic thickening agent, or as an anti-caking agent, and can be treated to make them hydrophilic or hydrophobic for either water or organic liquid applications. Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production. It consists of amorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without
3016-531: Is a passive release of gas during periods of dormancy. As per the above examples, while eruptions like Mount Unzen have caused deaths and local damage, the impact of the June 1991 eruption of Mount Pinatubo was seen globally. The eruptive columns reached heights of 40 km and dumped 17 megatons of SO 2 into the lower stratosphere . The aerosols that formed from the sulfur dioxide (SO 2 ), carbon dioxide (CO 2 ), and other gases dispersed around
3132-439: Is also the primary component of rice husk ash , which is used, for example, in filtration and as supplementary cementitious material (SCM) in cement and concrete manufacturing. Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates). Prominent examples include: About 95% of the commercial use of silicon dioxide (sand)
3248-399: Is an oxide of silicon with the chemical formula SiO 2 , commonly found in nature as quartz . In many parts of the world, silica is the major constituent of sand . Silica is one of the most complex and abundant families of materials , existing as a compound of several minerals and as a synthetic product. Examples include fused quartz , fumed silica , opal , and aerogels . It
3364-568: Is closely studied today for parameters such as strain , tilt, deformation and other movement and seismic activity. Earthquakes in the volcano's vicinity are generally below magnitude 2 while it is dormant and magnitude 3 when erupting. The earliest recorded eruption of Hekla took place in 1104. Since then there have been between twenty and thirty considerable eruptions, with the mountain sometimes remaining active for periods of six years with little pause. Eruptions in Hekla are varied and difficult to predict. Precursor seismic activity may only be for
3480-604: Is covalently bonded in a tetrahedral manner to 4 oxygen atoms. In contrast, CO 2 is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule . Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In crystalline form, this substance can be found naturally occurring as quartz , tridymite (high-temperature form), cristobalite (high-temperature form), stishovite (high-pressure form), and coesite (high-pressure form). On
3596-436: Is fed by a magma reservoir estimated to have a top 4 km (2.5 mi) below the surface with centroid 2.5 km (1.6 mi) lower. The chamber extends to an unusual depth of more than 10 km (6.2 mi), and the more silicic lavas have matured at more than 9 km (5.6 mi). Many of the eruptions commence with thicker more explosive rhyolite , dacite or andesite eruptives which create tephra and have
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3712-465: Is in the construction industry, e.g. in the production of concrete ( Portland cement concrete ). Certain deposits of silica sand, with desirable particle size and shape and desirable clay and other mineral content, were important for sand casting of metallic products. The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons. Crystalline silica
3828-519: Is likely that this was both the second greatest lava eruption of Hekla whilst Iceland was inhabited and the second greatest lava eruption in the world in the period 1900–1970. A total lava volume 0.8 km (0.19 cu mi) was produced with 0.21 km (0.050 cu mi) of tephra. The height of Hekla was 1,447 m (4,747 ft) before the eruption, increasing to a maximum of 1,503 m (4,931 ft), before dropping to 1,491 m (4,892 ft) subsequently. The eruption occurred over
3944-615: Is likely to be the source of the Efrahvolshraun lava on Hekla's west. The VEI-3 eruption began on 4 December. The VEI-2 eruption and the 1206 eruption distributed around 0.24 km (0.058 cu mi) of tephra mainly to the northeast. This VEI-4 eruption, which started on 11 July and lasted for a year, was the second largest tephra eruption of Hekla since Iceland was settled, covering 30,000 km (12,000 sq mi) of land with 0.31 km (0.074 cu mi) of tephra. Over 0.5 km (0.12 cu mi) of lava
4060-526: Is not classed as an eruption of Hekla based on the SiO 2 content of the lava. Details of the 1510 eruption were not recorded until a century later. It started on 25 July and was particularly violent (VEI 4), firing volcanic bombs as far as Vördufell , 40 km (25 mi) west. Tephra was deposited over Rangárvellir , Holt and Landeyjar , 0.2 km (0.048 cu mi) in total. A man in Landsveit
4176-651: Is obtained by the dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment. The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m /g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid . Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water : negative temperature expansion, density maximum at temperatures ~5000 °C, and
4292-476: Is often felsic , having high to intermediate levels of silica (as in rhyolite , dacite , or andesite ), with lesser amounts of less viscous mafic magma . Extensive felsic lava flows are uncommon, but can travel as far as 8 km (5 mi). The term composite volcano is used because the strata are usually mixed and uneven instead of neat layers. They are among the most common types of volcanoes; more than 700 stratovolcanoes have erupted lava during
4408-410: Is part of a volcanic ridge, 40 km (25 mi) long. The most active part of this ridge, a fissure about 5.5 km (3.4 mi) long named Heklugjá [ˈhɛhklʏˌcauː] , is considered to be within Hekla proper. Hekla looks rather like an overturned boat, with its keel being a series of craters, two of which are generally the most active. Hekla has a morphological type between that of
4524-421: Is rapidly cooled, it does not crystallize, but solidifies as a glass. Because of this, most ceramic glazes have silica as the main ingredient. The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. The difference between the glass and crystalline forms arises from
4640-426: Is the major constituent of sand . Even though it is poorly soluble, silica occurs in many plants such as rice . Plant materials with high silica phytolith content appear to be of importance to grazing animals, from chewing insects to ungulates . Silica accelerates tooth wear, and high levels of silica in plants frequently eaten by herbivores may have developed as a defense mechanism against predation. Silica
4756-451: Is the most famous example of a hazardous stratovolcano eruption. It completely smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of pyroclastic surges and pumice ranging from 6–7 meters deep. Pompeii had 10,000-20,000 inhabitants at the time of eruption. Mount Vesuvius is recognized as one of the most dangerous of the world's volcanoes, due to its capacity for powerful explosive eruptions coupled with
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4872-428: Is the only Icelandic volcano to produce calc-alkaline lavas. Phenocrysts in Hekla's lava can contain plagioclase , pyroxene , titanomagnetite , olivine , and apatite . When not erupting Hekla is often covered with snow and small glaciers ; it is also unusually aseismic with activity only starting 30–80 minutes before an eruption. Hekla is located on the mid-ocean ridge, a diverging plate boundary . Hekla
4988-457: Is the only polymorph of silica stable at the Earth's surface. Metastable occurrences of the high-pressure forms coesite and stishovite have been found around impact structures and associated with eclogites formed during ultra-high-pressure metamorphism . The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks . In many parts of the world, silica
5104-504: Is the process by which a semiconductor surface is rendered inert, and does not change semiconductor properties as a result of interaction with air or other materials in contact with the surface or edge of the crystal. The formation of a thermally grown silicon dioxide layer greatly reduces the concentration of electronic states at the silicon surface . SiO 2 films preserve the electrical characteristics of p–n junctions and prevent these electrical characteristics from deteriorating by
5220-450: Is typically between 700 and 1,200 °C (1,300-2,200 °F). Volcanic bombs are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything below 64mm is classified as a volcanic block . When erupted Bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with
5336-436: Is used as a fining agent for wine, beer, and juice, with the E number reference E551 . In cosmetics, silica is useful for its light-diffusing properties and natural absorbency. Diatomaceous earth , a mined product, has been used in food and cosmetics for centuries. It consists of the silica shells of microscopic diatoms ; in a less processed form it was sold as "tooth powder". Manufactured or mined hydrated silica
5452-489: Is used as the hard abrasive in toothpaste . Silicon dioxide is widely used in the semiconductor technology: Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors like gallium arsenide or indium phosphide . Silicon dioxide could be grown on a silicon semiconductor surface. Silicon oxide layers could protect silicon surfaces during diffusion processes , and could be used for diffusion masking. Surface passivation
5568-418: Is used in hydraulic fracturing of formations which contain tight oil and shale gas . Silica is the primary ingredient in the production of most glass . As other minerals are melted with silica, the principle of freezing point depression lowers the melting point of the mixture and increases fluidity. The glass transition temperature of pure SiO 2 is about 1475 K. When molten silicon dioxide SiO 2
5684-498: Is used in structural materials , microelectronics , and as components in the food and pharmaceutical industries. All forms are white or colorless, although impure samples can be colored. Silicon dioxide is a common fundamental constituent of glass . In the majority of silicon dioxides, the silicon atom shows tetrahedral coordination , with four oxygen atoms surrounding a central Si atom ( see 3-D Unit Cell ). Thus, SiO 2 forms 3-dimensional network solids in which each silicon atom
5800-538: Is used to produce elemental silicon . The process involves carbothermic reduction in an electric arc furnace : Fumed silica , also known as pyrogenic silica, is prepared by burning SiCl 4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO 2 . It can also be produced by vaporizing quartz sand in a 3000 °C electric arc. Both processes result in microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles,
5916-403: Is very dangerous because its magma has an unusually low silica content , making it much less viscous than other stratovolcanoes. Low viscosity lava can generate massive lava fountains , while lava of thicker viscosity can solidify within the vent, creating a volcanic plug . Volcanic plugs can trap gas and create pressure in the magma chamber, resulting in violent eruptions. Lava
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#17327727066486032-560: The Flatey Book Annal it was recorded that during the 1341 eruption, people saw large and small birds flying in the mountain's fire which were taken to be souls. In the 16th century Caspar Peucer wrote that the Gates of Hell could be found in "the bottomless abyss of Hekla Fell". The belief that Hekla was the gate to Hell persisted until the 19th century. There is still a legend that witches gather on Hekla during Easter . Hekla
6148-824: The Holocene Epoch (the last 11,700 years), and many older, now extinct, stratovolcanoes erupted lava as far back as Archean times. Stratovolcanoes are typically found in subduction zones but they also occur in other geological settings. Two examples of stratovolcanoes famous for catastrophic eruptions are Krakatoa in Indonesia (which erupted in 1883 claiming 36,000 lives) and Mount Vesuvius in Italy (which erupted in 79 A.D killing an estimated 2,000 people). In modern times, Mount St. Helens (1980) in Washington State , US, and Mount Pinatubo (1991) in
6264-579: The Philippines have erupted catastrophically, but with fewer deaths. Stratovolcanoes are common at subduction zones , forming chains and clusters along plate tectonic boundaries where an oceanic crust plate is drawn under a continental crust plate (continental arc volcanism, e.g. Cascade Range , Andes , Campania ) or another oceanic crust plate ( island arc volcanism, e.g. Japan , Philippines , Aleutian Islands ). Stratovolcanoes also occur in some other geological settings, for example as
6380-547: The last ice age . During the last 7,000 years, one third of the volcanic ash deposited in Scandinavia, Germany, Ireland, and the United Kingdom originated from Hekla. Hekla had been dormant for at least 250 years when it erupted explosively in 1104 (probably in the autumn), covering 55,000 km (21,000 sq mi) which is over half of Iceland with 1.2 km / 2.5 km of rhyodacitic tephra. This
6496-891: The magma is too viscous to allow easy escape of volcanic gases . As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma . Following the breaching of the vent and the opening of the crater, the magma degasses explosively. The magma and gases blast out with high speed and full force. Since 1600 CE , nearly 300,000 people have been killed by volcanic eruptions . Most deaths were caused by pyroclastic flows and lahars , deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash , fragmented lava , and superheated gases that can travel at speeds over 150 km/h (90 mph). Around 30,000 people were killed by pyroclastic flows during
6612-443: The mantle which decreases its melting point by 60 to 100 °C. The release of water from hydrated minerals is termed " dewatering ", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. This allows the mantle to partially melt and generate magma . This is called flux melting . The magma then rises through the crust , incorporating silica-rich crustal rock, leading to
6728-555: The planar process ). Hydrophobic silica is used as a defoamer component . In its capacity as a refractory , it is useful in fiber form as a high-temperature thermal protection fabric. Silica is used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes . Silica aerogel was used in the Stardust spacecraft to collect extraterrestrial particles. Pure silica (silicon dioxide), when cooled as fused quartz into
6844-547: The ʻAʻā lava type with Pāhoehoe and lava a budella (lava tubes) areas. In April and May 1948 CO 2 emitted from cracks in the ground pooled in hollows near to Hekla, killing 15 sheep and some wild animals and birds. In total 24,000 tonnes (26,000 tons) of CO 2 was emitted. Ditches were dug by farmers to drain these hollows, and the CO 2 emission had stopped by the end of the year. The 1970 eruption of Hekla started at 9:23 pm on 5 May 1970 and lasted until 5 July. It had
6960-616: The 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean . During March and April 1982, El Chichón in the State of Chiapas in southeastern Mexico , erupted 3 times, causing the worst volcanic disaster in that country's history and killied more than 2,000 people in pyroclastic flows . Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On 15 June, Mount Pinatubo erupted and caused an ash cloud to shoot 40 km (25 mi) into
7076-485: The 1982 eruption of Galunggung in Java , British Airways Flight 9 flew into the ash cloud, causing it to sustain temporary engine failure and structural damage. Although no crashes have happened due to ash, more than 60, mostly commercial aircraft , have been damaged. Some of these incidents resulted in emergency landings. Ashfalls are a threat to health when inhaled and are also a threat to property. A square yard of
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#17327727066487192-585: The 5,321 m (17,457 ft) high Andean volcano. The ensuing lahar killed 25,000 people and flooded the city of Armero and nearby settlements. As a volcano forms, several different gases mix with magma in the volcanic chamber. During an eruption the gases are then released into the atmosphere which can lead to toxic human exposure. The most abundant of these gases is H 2 O ( water ) followed by CO 2 ( carbon dioxide ), SO 2 ( sulfur dioxide ), H 2 S ( hydrogen sulfide ), and HF ( hydrogen fluoride ). If at concentrations of more than 3% in
7308-477: The Lava Crater. The longest lava stream produced was 8 km (5.0 mi) long and stopped in Stóraskógsbotnar. A scientist filming one of the lava streams on 2 November was hit by a block of lava and was killed. The lava flow stopped after 13 months on 21 April, having covered 40 km (15 sq mi) and with a maximum depth of 100 m (330 ft). The lava beds produced were mainly
7424-671: The Shoulder Crater had a 960 m (3,150 ft) circumference at its top and the Summit Crater a 700 m (2,300 ft)circumference at its highest point, 90 m (300 ft) above the ridge. Sandy tephra and ash fell over Iceland in May and June, sometimes making it dark in the daytime near Hekla. The tephra caused fluorine poisoning of grazing sheep, making them unable to walk. That winter more craters formed, building up cones. Explosive activity had ceased six months after
7540-411: The Year (2024 values) column used IntCal20 for H-5 and H-3 and ice core data for H-4 which were not available in 2019. Hekla 3, 4, and 5 produced huge amounts of rhyolitic ash and tephra, covering 80% of Iceland and providing useful date markers in soil profiles in other parts of Europe such as Orkney , Scandinavia, and elsewhere. H 3 and H 4 produced the largest layers of tephra in Iceland since
7656-496: The acidification of solutions of sodium silicate . The gelatinous precipitate or silica gel , is first washed and then dehydrated to produce colorless microporous silica. The idealized equation involving a trisilicate and sulfuric acid is: Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles. Thin films of silica grow spontaneously on silicon wafers via thermal oxidation , producing
7772-405: The adjoining heaths, some being burnt before they could escape. On the night of 15 September, two new openings were formed — one on the eastern, and the other on the southern slope — from both of which lava was discharged for twenty-two hours. It flowed to a distance of upwards of twenty miles, killing many cattle and destroying a large tract of pasturage. Twelve miles from the crater, the lava-stream
7888-819: The air, when breathed in CO 2 can cause dizziness and difficulty breathing. At more than 15% concentration CO 2 causes death. CO 2 can settle into depressions in the land, leading to deadly, odorless pockets of gas. SO 2 classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption. H 2 S has an even stronger odor than SO 2 as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death. HF and similar species can coat ash particles and once deposited can poison soil and water. Gases are also emitted during volcanic degassing, which
8004-748: The air. It produced large pyroclastic surges and lahar floods that caused a lot of damage to the surrounding area. Pinatubo , located in Central Luzon just 90 km (56 mi) west-northwest of Manila , had been dormant for six centuries before the 1991 eruption. This eruption was one of the 2nd largest in the 20th century. It produced a large volcanic ash cloud that affected global temperatures, lowering them in areas as much as .5 °C. The volcanic ash cloud consisted of 22 million tons of SO 2 which combined with water droplets to create sulfuric acid . In 1991 Japan's Unzen Volcano also erupted, after 200 years of inactivity. It's located on
8120-675: The areas west and southwest of Hekla, leading to many cattle deaths, probably mainly from fluorosis . In late 1389 Hekla erupted again (VEI-3), starting with a large ejection of tephra to the southeast. Later "the eruption fissure moved itself out of the mountain proper and into the woods a little above Skard". Skard and another nearby farm were destroyed by a large lava flow that now forms the 12.5 km (4.8 sq mi) Nordurhraun . In total around 0.3 km (0.072 cu mi) of lava and 5 × 10 m (1.8 × 10 cu ft) of tephra were produced. An eruption may have occurred around 1440 at Raudölder ; despite being close to Hekla this
8236-552: The branching of the pyrogenic product. The main use is as pozzolanic material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders. Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti- caking agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets. It can adsorb water in hygroscopic applications. Colloidal silica
8352-509: The connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. One example of this ordering is the preference to form rings of 6-tetrahedra. The majority of optical fibers for telecommunications are also made from silica. It is a primary raw material for many ceramics such as earthenware , stoneware , and porcelain . Silicon dioxide
8468-413: The deaths of both fish and livestock. Rangárvellir, Land and Hreppar also suffered damage. During the eruption up to 0.5 m (1 ft 8 in) lava bombs were thrown 15–20 km (9.3–12.4 mi) away, and flooding was caused by the sudden melting of snow and ice on Hekla's slopes. Hekla was dormant for more than sixty years before 1845, when it suddenly burst forth on 2 September at 9 am: After
8584-488: The densest of the low-pressure forms, which has a density of 2.648 g/cm . The difference in density can be ascribed to the increase in coordination as the six shortest Si–O bond lengths in stishovite (four Si–O bond lengths of 176 pm and two others of 181 pm) are greater than the Si–O bond length (161 pm) in α-quartz. The change in the coordination increases the ionicity of the Si–O bond. Faujasite silica, another polymorph,
8700-525: The eruption had started, ash fell on Helsinki , Finland, having covered 2,860 km (1,780 mi) in this time. The initial tephra production rate in the first 30 minutes of the eruption was 75,000 m·s, dropping to 22,000 m·s for the next half-hour. The initial phase produced 0.18 km (0.043 cu mi) of tephra, equating to 4.5 × 10 km (1.1 × 10 cu mi) of Dense-rock equivalent , covering 3,130 km (1,210 sq mi) of land and sea. 98 farms were damaged by
8816-455: The eruption intensity until it covered a 4 km (2.5 mi) fissure on the ridge. The cloud from the eruption had ascended to a height of 30 km by 7:08 am, the wind then carried it southwards towards Eyjafjallajökull , turning it black. Pumice first landed on Fljótshlíð at around 7:10 am, and tephra and ash continued falling until it formed a 3–10 cm (1.2–3.9 in) layer. A lava bomb that landed 32 km (20 mi) from Hekla
8932-401: The eruption was one of Hekla's most destructive (VEI-4). Initially tephra was produced at 60,000 m·s, 0.18 km (0.043 cu mi) during the entire eruption, which also caused lahars and tsunami . The tephra was deposited to the northwest, destroying and damaging farms and woodland in Þjórsárdalur, Land, Hreppar and Biskupstungur . Fine ash from the eruption reached Norway. There
9048-472: The eruption, but only 2 were no longer farmed in 1970. A large volunteer effort was mobilized to clear the tephra – around 1000 man-days by the end of July. The eruption produced around 3 ML (110 × 10 ^ cu ft) of water (snowmelt and directly from the fissure) which caused flooding of the Ytri Rangá river. In the first 20 hours of the eruption approximately 3,500 m·s of lava
9164-488: The evening of the eruption; the largest had a magnitude of 4. The eruption started weakly at 9:23 pm IMT ± 2 min before increasing in power. The first pumice fell on Búrfell power station, 15 km (9.3 mi) away, at 9:35 pm causing people to evacuate. The eruption seems to have started in two locations at the same time – to the Shoulder crater's south-southwest and below the Lava Crater. At 10:30 pm
9280-546: The first eruption. Lava flowed from the Lava Crater continuously during the eruption, starting at a rate of over 100 m·s, dropping to 5–10 m·s in April and early May at a speed of around 20 cm·s before increasing, eventually reaching 150 m·s at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5 m·s. From there it gradually decreased to under 10 m·s in November. Initially
9396-540: The flank of a volcano collapses in a massive landslide) can only trigger the eruption of a very shallow magma chamber . Magma differentiation and thermal expansion also are ineffective as triggers for eruptions from deep magma chambers . In recorded history , explosive eruptions at subduction zone ( convergent-boundary ) volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, such as Mount St. Helens , Mount Etna and Mount Pinatubo , typically erupt with explosive force because
9512-411: The gaseous ambient environment. Silicon oxide layers could be used to electrically stabilize silicon surfaces. The surface passivation process is an important method of semiconductor device fabrication that involves coating a silicon wafer with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below. Growing a layer of silicon dioxide on top of
9628-478: The ground. Volcanic Bombs are associated with Strombolian and Vulcanian eruptions and basaltic lava . Ejection velocities ranging from 200 to 400 m/s have been recorded causing volcanic bombs to be destructive. Lahars (from a Javanese term for volcanic mudflows) are a mixture of volcanic debris and water. Lahars can result from heavy rainfall during or before the eruption or interaction with ice and snow. Meltwater mixes with volcanic debris causing
9744-458: The high population density of the surrounding Metropolitan Naples area (totaling about 3.6 million inhabitants). In addition to potentially affecting the climate, volcanic ash clouds from explosive eruptions pose a serious hazard to aviation . Volcanic ash clouds consist of ash which is made of silt or sand sized pieces of rock, mineral, volcanic glass . Ash grains are jagged, abrasive, and don't dissolve in water. For example, during
9860-419: The idea spread over much of Europe. The volcano's frequent large and often initially explosive eruptions have covered much of Iceland with tephra , and these layers can be used to date eruptions of Iceland's other volcanoes. Approximately 10% of the tephra created in Iceland in the last thousand years has come from Hekla, amounting to 5 km (1.2 cu mi). Cumulatively, the volcano has produced one of
9976-504: The island of Kyushu about 40 km (25 mi) east of Nagasaki . Beginning in June, a newly formed lava dome repeatedly collapsed. This generated a pyroclastic flow that flowed down the mountain's slopes at speeds as high as 200 km/h (120 mph). The 1991 eruption of Mount Unzen was one of the worst volcanic disasters in Japan's history, once killing more than 15,000 people in 1792. The eruption of Mount Vesuvius in 79 AD
10092-426: The largest volumes of lava of any in the world in the last millennium, around 8 km (1.9 cu mi). In Icelandic Hekla is the word for a short hooded cloak, which may relate to the frequent cloud cover on the summit. An early Latin source refers to the mountain as Mons Casule . After the eruption of 1104, stories, probably spread deliberately through Europe by Cistercian monks, told that Hekla
10208-474: The lava comprised 57–58% SiO 2 and 11% Fe 2 O 3 , from the time of peak flow onwards this changed to 54% SiO 2 and 13.5% Fe 2 O 3 . The lava river sometimes ran through lava tubes before emerging again. The lava front had a height of up to 15 m (49 ft). On 15 and 16 June, a branch of lava flow to the south of Melfell traveled over 1 km (0.62 mi) in 30 hours before slowing and stopping by 21 June, 7.8 km (4.8 mi) from
10324-551: The main crater, one had an area of 6 m (65 sq ft) and a likely weight of 12 tons. Xenoliths formed around 2% of the material produced by the craters. These were of rock types including basalt, andesite, ignimbrite and sedimentary rock . The eruption became stronger at Skjólkvíar on 12 May, with columns of steam attaining a height of 2,500 m (8,200 ft). The eruption intensity then gradually reduced until it stopped on 20 May. The lava field then had an area of 5.8 km (2.2 sq mi). Later that day
10440-510: The northeast, producing two main lava fountains, and shortly after another adjoining fissure opened producing lava fountains to a height of 500 m (1,600 ft). At around midnight, another fissure opened northwest of the Lava Crater, later hurling an over 300 m (980 ft) long lava fountain, 200–300 m (660–980 ft) into the air. By midnight lava had already covered over 1 km (0.39 sq mi) and this extended to 7.5 km (2.9 sq mi) by next morning implying
10556-421: The number of active craters decreased, the most active of these built a cone 100 m (330 ft) higher than the level of the ridge. Lava flowed from its base until mid-June when the lava cut through the north crater wall. The larger cones produced more tephra, occasionally with lightning within the tephra cloud. By 5 July, the eruption had stopped. During eruptions of Hekla, fluorine is produced and sticks to
10672-400: The other hand, amorphous silica can be found in nature as opal and diatomaceous earth . Quartz glass is a form of intermediate state between these structures. All of these distinct crystalline forms always have the same local structure around Si and O. In α-quartz the Si–O bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. The Si–O–Si angle also varies between
10788-428: The potential for pyroclastic flows . Other or the later part of eruptions come from thinner basalt tending magma which forms lava fields. The tephra produced by its eruptions is high in fluorine , which is poisonous to animals. Hekla's basaltic andesite lava generally has a SiO 2 content of over 54%, compared to the 45–50% of other nearby transitional alkaline basalt eruptions (see TAS classification ). It
10904-689: The same directions leading to many livestock deaths through fluorosis for the next two years. A small eruption (VEI-2) occurred between 27 February 1878 and April 1878, around 10 km (6.2 mi) east of Hekla, and produced 0.2 km (0.048 cu mi) of lava from two parallel fissures covering 15.5 km (6.0 sq mi). A small eruption (VEI-2) occurred between 25 April 1913 and 18 May 1913, around 10 km (6.2 mi) east of Hekla, and caused large fissures at Mundafell and Lambafit which produced 3.8 and 6.3 km (1.5 and 2.4 sq mi) of lava respectively. The VEI-4 eruption started on 29 March 1947 and ended on 21 April 1948. It
11020-401: The silicon atoms with an Si–O–Si angle of 94° and bond length of 164.6 pm and the terminal Si–O bond length is 150.2 pm. The Si–O bond length is 148.3 pm, which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol. SiO 2 is most commonly encountered in nature as quartz , which comprises more than 10% by mass of the Earth's crust. Quartz
11136-418: The silicon, store charge, block current, and even act as a controlled pathway to limit current flow. Many routes to silicon dioxide start with an organosilicon compound, e.g., HMDSO, TEOS. Synthesis of silica is illustrated below using tetraethyl orthosilicate (TEOS). Simply heating TEOS at 680–730 °C results in the oxide: Similarly TEOS combusts around 400 °C: TEOS undergoes hydrolysis via
11252-412: The size of Mount Pinatubo affected the weather for a few years; with warmer winters and cooler summers observed. A similar phenomenon occurred in the April 1815, the eruption of Mount Tambora on Sumbawa island in Indonesia . The Mount Tambora eruption is recognized as the most powerful eruption in recorded history. Its eruption cloud lowered global temperatures as much as 0.4 to 0.7 °C. In
11368-467: The smoke. By the fourth, fifth, and sixth days, the eruption was greatly diminished, and only the shoulder and summit craters were erupting explosively. The explosive eruption increased in strength from 9–12 April and then from 28 April it reduced again. On 3 May, the volcano stopped throwing out lava in sudden explosions from its craters and changed to continuously ejecting tephra and ash for long periods, until early June when this reduced. On 2 September,
11484-492: The so-called sol-gel process . The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is: Being highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value, combustion of silane gives silicon dioxide. This reaction is analogous to the combustion of methane: However the chemical vapor deposition of silicon dioxide onto crystal surface from silane had been used using nitrogen as
11600-402: The south-southwest on 10 May and in Hlídargígar on 20 May, but a new fissure opened on the same day and lava flowed from this until 5 July. The lava was andesite containing olivine, similar to the lava produced later in the eruption of 1947. Before the eruption, a greater than normal amount of snow melting had occurred, indicating the volcano was heating up. Earth tremors began at 8:48 pm on
11716-470: The surfaces of the grains of tephra. Fine grains can have a fluorine content of 350 ppm, and fluorine poisoning can start in sheep at a diet with fluorine content of 25 ppm. At 250 ppm, death can occur within a few days. In 1783, 79% of the Icelandic sheep stock were killed, probably as a result of fluorosis caused by the eruption of Lakagígar . Some of the ash produced in this eruption had
11832-446: The transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit. The high-pressure minerals, seifertite , stishovite, and coesite, though, have higher densities and indices of refraction than quartz. Stishovite has a rutile -like structure where silicon is 6-coordinate. The density of stishovite is 4.287 g/cm , which compares to α-quartz,
11948-412: The world. The SO 2 in this cloud combined with water (both of volcanic and atmospheric origin) and formed sulfuric acid , blocking a portion of the sunlight from reaching the troposphere . This caused the global temperature to decrease by about 0.4 °C (0.72 °F) from 1992 to 1993. These aerosols caused the ozone layer to reach the lowest concentrations recorded at that time. An eruption
12064-476: The year following the eruption, most of the Northern Hemisphere experienced cooler temperatures during the summer. In the northern hemisphere , 1816 was known as the " Year Without a Summer ". The eruption caused crop failures, food shortages, and floods that killed over 100,000 people across Europe , Asia , and North America . Silicon dioxide Silicon dioxide , also known as silica ,
12180-429: Was 0.5 m (1 ft 8 in) across and weighed 20 kg (44 lb). Between Vatnafjöll and Hekla, a layer of tephra up to 1 m (3 ft 3 in) thick was deposited, and this included bombs with a diameter larger than 0.5 m (1 ft 8 in). Bombs with surface areas of 50 m (540 sq ft) were dropped onto the slopes of Hekla, for up to 1 km (0.62 mi). 51 hours after
12296-443: Was also expelled. The tephra caused significant damage to the settlements of Skagafjörður and Fljót, leading to over 500 deaths that winter. The material output from this eruption had SiO 2 levels of between 56% and 64%, and apart from a slight abundance of olivine the lava, was typical of Hekla eruptions. A small eruption (VEI-3) started on 19 May and deposited around 5 × 10 km (1.2 × 10 cu mi) of tephra over
12412-404: Was between forty and fifty feet deep and nearly a mile in width. On 12 October a fresh torrent of lava burst forth, and heaped up another similar mass. The mountain continued in a state of activity up to April 1846; then it rested for a while, and began again in the following month of October. Since then, however, it has enjoyed repose. The effects of these eruptions were disastrous. The whole island
12528-399: Was damage to wildlife with significant numbers of trout , salmon , ptarmigan and farm animals dying. A very small eruption, possibly only VEI-1, took place on 2 April 1725, producing flows of lava from locations around Hekla which have since been covered by later lava flows. These eruptions are not classed as of Hekla itself based on the SiO 2 content of the lava. The eruption of 1766
12644-463: Was killed. A VEI-4 eruption began on 3 January and lasted for over 6 months, with 0.15 km (0.036 cu mi) of tephra being deposited to the south-southeast, damaging Mýrdalur . A small (VEI-3) eruption began on 8 May 1636 and lasted for over a year. The 5 × 10 m (1.8 × 10 cu ft) of tephra from the eruption damaged pasture to the northeast causing death of livestock. Starting 13 January and lasting for over 7 months
12760-452: Was large (VEI-4) and produced the second largest lava flow, 1.3 km (0.31 cu mi) covering 65 km (25 sq mi), and third largest tephra volume, 0.24 km (0.058 cu mi), of any Icelandic volcano during the inhabited era. The eruption started at around 3:30 am on 5 April 1766 and ceased in May 1768. Initially a 2–4 cm layer of tephra was deposited over Austur-Húnavatnssýsla and Skagafjördur, resulting in
12876-557: Was produced at 20,000 m·s. The tephra deposition of a total amount of 0.17 km (0.041 cu mi) was mainly to the east-southeast; immediately to the east of Hekla the layer was 20–40 cm (7.9–15.7 in) deep. Fine ash was carried to the Faroes , Shetland and Orkney. Lava flows to the west and north-west covered an area of 25 km (9.7 sq mi) with a volume of 0.63 km (0.15 cu mi) of lava. Large quantities of dark ash were deposited over pasture in
12992-479: Was produced from the fissure, dividing into various branches and covering 12–15 km (4.6–5.8 sq mi). On the second day, 8 distinct eruption columns were discernible. A crater formed at 860 m (2,820 ft) called the Lava Crater ( Hraungígur ), producing a constant flow of lava. Another crater named the Shoulder Crater ( Axlargígur ) produced a column of smoke every 10 seconds together with loud explosions that created visible compression waves in
13108-494: Was strewn with volcanic ash, which, where it did not smother the grass outright, gave it a poisonous taint. The cattle that ate of it were attacked by a murrain , of which great numbers died. The ice and snow, which had gathered about the mountain for a long period of time, were wholly melted by the heat. Masses of pumice weighing nearly half a ton were thrown to a distance of between four and five miles. The eruption ceased around 5 April 1846. Initially in this VEI-4 eruption tephra
13224-645: Was the Hekla 3 (or H 3 ) eruption c. 1000 BC , which threw about 7.3 km (1.8 cu mi) of volcanic rock into the atmosphere, placing its Volcanic Explosivity Index (VEI) at 5. This would have cooled temperatures in the northern parts of the globe for a few years afterwards. Traces of this eruption have been identified in Scottish peat bogs , and in Ireland a study of tree rings dating from this period has shown negligible tree ring growth for
13340-423: Was the gateway to Hell. The Cistercian monk Herbert of Clairvaux wrote in his De Miraculis (without naming Hekla): The renowned fiery cauldron of Sicily , which men call Hell's chimney ... that cauldron is affirmed to be like a small furnace compared to this enormous inferno. A poem by the monk Benedeit from c. 1120 about the voyages of Saint Brendan mentions Hekla as the prison of Judas . In
13456-524: Was the second largest tephra eruption in the country in historical times with a VEI of 5. Farms upwind of the volcano 15 km (9.3 mi) in Þjórsárdalur valley, 50 km (31 mi) at Hrunamannaafréttur and 70 km (43 mi) at Lake Hvítárvatn were abandoned because of the damage. The eruption caused Hekla to become famous throughout Europe. A VEI-4 eruption began on 19 January 1158 producing over 0.15 km (0.036 cu mi) of lava and 0.2 km (0.048 cu mi) of tephra. It
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