Volcanic ash consists of fragments of pulverized rock and volcanic glass, created during volcanic eruptions and measuring less than 2 mm in diameter. The term volcanic ash is often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere; the force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away. Due to its wide dispersal, ash can have a number of impacts on society, including human and animal health, disruption to aviation, disruption to critical infrastructure, primary industries and structures.
Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents. Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles to exsolve into gas bubbles; as more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs; when fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a efficient process of ash formation and is capable of generating fine ash without the addition of water. Volcanic ash is produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water groundwater, snow or ice; as the magma, hotter than the boiling point of water, comes into contact with water an insulating vapor film forms. This vapor film will collapse leading to direct coupling of the cold water and hot magma.
This increases the heat transfer which leads to the rapid expansion of water and fragmentation of the magma into small particles which are subsequently ejected from the volcanic vent. Fragmentation causes an increase in contact area between magma and water creating a feedback mechanism, leading to further fragmentation and production of fine ash particles. Pyroclastic density currents can produce ash particles; these are produced by lava dome collapse or collapse of the eruption column. Within pyroclastic density currents particle abrasion occurs as particles interact with each other resulting in a reduction in grain size and production of fine grained ash particles. In addition, ash can be produced during secondary fragmentation of pumice fragments, due to the conservation of heat within the flow; these processes produce large quantities of fine grained ash, removed from pyroclastic density currents in co-ignimbrite ash plumes. Physical and chemical characteristics of volcanic ash are controlled by the style of volcanic eruption.
Volcanoes display a range of eruption styles which are controlled by magma chemistry, crystal content and dissolved gases of the erupting magma and can be classified using the volcanic explosivity index. Effusive eruptions of basaltic composition produce <105 m3 of ejecta, whereas explosive eruptions of rhyolitic and dacitic composition can inject large quantities of ejecta into the atmosphere. Another parameter controlling the amount of ash produced is the duration of the eruption: the longer the eruption is sustained, the more ash will be produced. For example, the second phase of the 2010 eruptions of Eyjafjallajökull was classified as VEI 4 despite a modest 8 km high eruption column, but the eruption continued for a month, which allowed a large volume of ash to be ejected into the atmosphere; the types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it erupted. Considering that the most abundant elements found in silicate magma are silicon and oxygen, the various types of magma produced during volcanic eruptions are most explained in terms of their silica content.
Low energy eruptions of basalt produce a characteristically dark coloured ash containing ~45 - 55% silica, rich in iron and magnesium. The most explosive rhyolite eruptions produce a felsic ash, high in silica while other types of ash with an intermediate composition have a silica content between 55-69%; the principal gases released during volcanic activity are water, carbon dioxide, sulfur dioxide, hydrogen sulfide, carbon monoxide and hydrogen chloride. These sulfur and halogen gases and metals are removed from the atmosphere by processes of chemical reaction and wet deposition, by adsorption onto the surface of volcanic ash, it has long been recognised that a range of sulfate and halide compounds are mobilised from fresh volcanic ash.. While some 55 ionic species have been reported in fresh ash leachates the most
Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U. S. state of Hawaiʻi in the Pacific Ocean. The largest subaerial volcano in both mass and volume, Mauna Loa has been considered the largest volcano on Earth, dwarfed only by Tamu Massif, it is an active shield volcano with gentle slopes, with a volume estimated at 18,000 cubic miles, although its peak is about 125 feet lower than that of its neighbor, Mauna Kea. Lava eruptions from Mauna Loa are silica-poor and fluid, they tend to be non-explosive. Mauna Loa has been erupting for at least 700,000 years, may have emerged above sea level about 400,000 years ago; the oldest-known dated rocks are not older than 200,000 years. The volcano's magma comes from the Hawaii hotspot, responsible for the creation of the Hawaiian island chain over tens of millions of years; the slow drift of the Pacific Plate will carry Mauna Loa away from the hotspot within 500,000 to one million years from now, at which point it will become extinct.
Mauna Loa's most recent eruption occurred from March 24 to April 15, 1984. No recent eruptions of the volcano have caused fatalities, but eruptions in 1926 and 1950 destroyed villages, the city of Hilo is built on lava flows from the late 19th century; because of the potential hazards it poses to population centers, Mauna Loa is part of the Decade Volcanoes program, which encourages studies of the world's most dangerous volcanoes. Mauna Loa has been monitored intensively by the Hawaiian Volcano Observatory since 1912. Observations of the atmosphere are undertaken at the Mauna Loa Observatory, of the Sun at the Mauna Loa Solar Observatory, both located near the mountain's summit. Hawaii Volcanoes National Park covers the summit and the southeastern flank of the volcano, incorporates Kīlauea, a separate volcano. Like all Hawaiian volcanoes, Mauna Loa was created as the Pacific tectonic plate moved over the Hawaii hotspot in the Earth's underlying mantle; the Hawaii island volcanoes are the most recent evidence of this process that, over 70 million years, has created the 3,700 mi -long Hawaiian–Emperor seamount chain.
The prevailing view states that the hotspot has been stationary within the planet's mantle for much, if not all of the Cenozoic Era. However, while the Hawaiian mantle plume is well understood and extensively studied, the nature of hotspots themselves remains enigmatic. Mauna Loa is one of five subaerial volcanoes; the oldest volcano on the island, Kohala, is more than a million years old, Kīlauea, the youngest, is believed to be between 300,000 and 600,000 years of age. Lōʻihi Seamount on the island's flank is younger, but has yet to breach the surface of the Pacific Ocean. At 1 million to 700,000 years of age, Mauna Loa is the second youngest of the five volcanoes on the island, making it the third youngest volcano in the Hawaiian – Emperor seamount chain, a chain of shield volcanoes and seamounts extending from Hawaii to the Kuril–Kamchatka Trench in Russia. Following the pattern of Hawaiian volcano formation, Mauna Loa would have started as a submarine volcano building itself up through underwater eruptions of alkali basalt before emerging from the sea through a series of surtseyan eruptions about 400,000 years ago.
Since the volcano has remained active, with a history of effusive and explosive eruptions, including 33 eruptions since the first well-documented eruption in 1843. Although Mauna Loa's activity has been overshadowed in recent years by that of its neighbor Kīlauea, it remains active. Mauna Loa is the largest subaerial and second largest overall volcano in the world, covering a land area of 5,271 km2 and spans a maximum width of 120 km. Consisting of 65,000 to 80,000 km3 of solid rock, it makes up more than half of the surface area of the island of Hawaiʻi. Combining the volcano's extensive submarine flanks and 4,170 m subaerial height, Mauna Loa rises 9,170 m from base to summit, greater than the 8,848 m or 29,029 ft elevation of Mount Everest from sea level to its summit. In addition, much of the mountain is invisible underwater: its mass depresses the crust beneath it by another 8 km, in the shape of an inverse mountain, meaning the total height of Mauna Loa from the start of its eruptive history is about 17,170 m.
Mauna Loa is a typical shield volcano in form, taking the shape of a long, broad dome extending down to the ocean floor whose slopes are about 12° at their steepest, a consequence of its fluid lava. The shield-stage lavas that built the enormous main mass of the mountain are tholeiitic basalts, like those of Mauna Kea, created through the mixing of primary magma and subducted oceanic crust. Mauna Loa's summit hosts three overlapping pit craters arranged northeast-southwest, the first and last 1 km in diameter and the second an oblong 4.2 km × 2.5 km feature. Mokuʻāweoweo's caldera floor lies between 170 and 50 m beneath its rim and it is only the latest of several calderas that have formed and reformed over the volcano's life, it was created between 1,000 and 1,500 years ago by a large eruption from Mauna Loa's northeast rift zone, which emptied out a shallow magma chamber beneath the summit and collapsed it into its pres
Mount Merapi, Gunung Merapi, is an active stratovolcano located on the border between Central Java and Yogyakarta provinces, Indonesia. It is the most active volcano in Indonesia and has erupted since 1548, it is located 28 kilometres north of Yogyakarta city which has a population of 2.4 million, thousands of people live on the flanks of the volcano, with villages as high as 1,700 metres above sea level. Smoke can be seen emerging from the mountaintop, several eruptions have caused fatalities. Pyroclastic flow from a large explosion killed 27 people on 22 November 1994 in the town of Muntilan, west of the volcano. Another large eruption occurred in 2006, shortly before the Yogyakarta earthquake. In light of the hazards that Merapi poses to populated areas, it has been designated as one of the Decade Volcanoes. On 25 October 2010 the Indonesian government raised the alert for Mount Merapi to its highest level and warned villagers in threatened areas to move to safer ground. People living within the range of a 20 km zone were told to evacuate.
Officials said about 500 volcanic earthquakes had been recorded on the mountain over the weekend of 23–24 October, that the magma had risen to about 1 kilometre below the surface due to the seismic activity. On the afternoon of 25 October 2010, Mount Merapi erupted lava from its southern and southeastern slopes; the mountain was still erupting on 30 November 2010, but due to lowered eruptive activity on 3 December 2010 the official alert status was reduced to level 3. The volcano is now 38 metres lower than before the 2010 eruptions. After a large eruption in 2010 the characteristic of Mount Merapi was changed. On 18 November 2013 Mount Merapi burst smoke up to 2,000 meters high, one of its first major phreatic eruptions after the 2010 eruption. Researchers said that this eruption occurred due to combined effect of hot volcanic gases and abundant rainfall; the last eruption was so far May 11, 2018 The name Merapi could be loosely translated as the Mountain of Fire. The etymology of the name came from Meru-Api.
Merapi is the youngest in a group of volcanoes in southern Java. It is situated at a subduction zone, where the Indo-Australian Plate is subducting under the Sunda Plate, it is one of at least 129 active volcanoes in Indonesia, part of the volcano is located in the Southeastern part of the Pacific Ring of Fire–a section of fault lines stretching from the Western Hemisphere through Japan and South East Asia. Stratigraphic analysis reveals that eruptions in the Merapi area began about 400,000 years ago, from until about 10,000 years ago, eruptions were effusive, the out flowing lava emitted was basaltic. Since eruptions have become more explosive, with viscous andesitic lavas generating lava domes. Dome collapse has generated pyroclastic flows, larger explosions, which have resulted in eruption columns, have generated pyroclastic flows through column collapse. Small eruptions occur every two to three years, larger ones every 10–15 years or so. Notable eruptions causing many deaths, have occurred in 1006, 1786, 1822, 1872, 1930.
Thirteen villages were destroyed in the latter one, 1400 people killed by pyroclastic flows. The large eruption in 1006 is claimed to have covered all of central Java with ash; the volcanic devastation is claimed to have led to the collapse of the Hindu Kingdom of Mataram. In April 2006, increased seismicity at more regular intervals and a detected bulge in the volcano's cone indicated that fresh eruptions were imminent. Authorities put the volcano's neighboring villages on high alert and local residents prepared for a evacuation. On 19 April smoke from the crater reached a height of 400 metres, compared to 75 metres the previous day. On 23 April, after nine surface tremors and some 156 multifaced quakes signalled movements of magma, some 600 elderly and infant residents of the slopes were evacuated. By early May, active lava flows had begun. On 11 May, with lava flow beginning to be constant, some 17,000 people were ordered to be evacuated from the area and on 13 May, Indonesian authorities raised the alert status to the highest level, ordering the immediate evacuation of all residents on the mountain.
Many villagers defied the dangers posed by the volcano and returned to their villages, fearing that their livestock and crops would be vulnerable to theft. Activity calmed by the middle of May. On 27 May, a 6.3 magnitude earthquake struck 50 km southwest of Merapi, killing at least 5,000 and leaving at least 200,000 people homeless in the Yogyakarta region, heightening fears that Merapi would "blow". The quake did not appear to be a long-period oscillation, a seismic disturbance class, associated with major volcanic eruptions. A further 11,000 villagers were evacuated on 6 June as lava and superheated clouds of gas poured down its upper slopes towards Kaliadem, a location, located southeast of Mt. Merapi; the pyroclastic flows are known locally as "wedhus gembel". There were two fatalities as the result of the eruption. In late October 2010 the Center for Volcanology and Geological Hazard Mitigation, Geological Agency, reported that a pattern of increasing seismicity from Merapi had begun to emerge in early September.
Observers at Babadan 7 kilometres west and Kaliura
Guadalajara is the capital and largest city of the Mexican state of Jalisco, the seat of the municipality of Guadalajara. The city is in the central region of Jalisco in the Western-Pacific area of Mexico. With a population of 1,460,148 inhabitants, it is Mexico's second most populous municipality; the Guadalajara metropolitan area has a reported population of 5,002,466 inhabitants, making it the second most populous metropolitan area in Mexico, behind Mexico City. The municipality is the second most densely populated in Mexico, the first being Ciudad Nezahualcóyotl in the State of Mexico, it is a strong business and economic center in the Bajío region. Guadalajara is the 10th largest Latin American city in population, urban area and gross domestic product; the city is named after the Spanish city of Guadalajara, its economy is based on services and industry information technology, with a large number of international firms having regional offices and manufacturing facilities in the Guadalajara Metropolitan Area, several domestic IT companies headquartered in the city.
Other, more traditional industries, such as shoes and food processing are important contributing factors. Guadalajara is a cultural centre of Mexico, considered by most to be the home of mariachi music and host to a number of large-scale cultural events such as the Guadalajara International Film Festival, the Guadalajara International Book Fair, globally renowned cultural events which draw international crowds, it is home to the C. D. Guadalajara, one of the most popular football clubs in Mexico; this city was named the American Capital of Culture for 2005. Guadalajara hosted the 2011 Pan American Games; the city was established in five other places before moving to its current location. The first settlement in 1532 was in Mesa del Cerro, now known as Zacatecas; this site was settled by Cristóbal de Oñate as commissioned by Nuño de Guzmán, with the purpose of securing recent conquests and defending them against the still-hostile natives. The settlement did not last long at this spot due to the lack of water.
Four years Guzmán ordered that the village be moved to Tlacotán. While the settlement was in Tlacotán, the Spanish king Charles I granted the coat of arms that the city still has today; this settlement was ferociously attacked during the Mixtón War in 1543 by Caxcan and Zacateco peoples under the command of Tenamaxtli. The war was initiated by the natives due to the cruel treatment of Indians by Nuño de Guzmán, in particular the enslavement of captured natives. Viceroy Antonio de Mendoza had to take control of the campaign to suppress the revolt after the Spanish were defeated in several engagements; the conflict ended after Mendoza made some concessions to the Indians such as freeing the Indian slaves and granting amnesty. The village of Guadalajara survived the war, the villagers attributed their survival to the Archangel Michael, who remains the patron of the city, it was decided to move the city once again, this time to Atemajac. The city has remained there to this day. In 1542, records indicate that 126 people were living in Guadalajara and, in the same year, the status of city was granted by the king of Spain.
Guadalajara was founded on February 14, 1542 in the Valley of Atemajac. The settlement's name came from the Spanish hometown of Nuño de Guzmán. In 1559, royal offices for the province of Nueva Galicia were moved from Compostela to Guadalajara, as well as the bishopric. Construction of the cathedral began in 1563. In 1575, religious orders such as the Augustinians and Dominicans arrived, which would make the city a center for evangelization efforts; the historic city center encompasses what was four centers of population, as the villages of Mezquitán, Analco and Mexicaltzingo were annexed to the Atemajac site in 1669. In 1791, the University of Guadalajara was established in the city, the capital of Nueva Galicia; the inauguration was held in 1792 at the site of the old Santo Tomas College. While the institution was founded during the 18th century, it would not be developed until the 20th century, starting in 1925. In 1794, the Hospital Real de San Miguel de Belén, or the Hospital de Belén, was opened.
Guadalajara's economy during the 18th century was based on agriculture and the production of non-durable goods such as textiles and food products. Guadalajara remained the capital of Nueva Galicia with some modifications until the Mexican War of Independence. After Miguel Hidalgo y Costilla decided not to attack Mexico City, despite early successes, he retreated to Guadalajara in late 1810, he and his army were welcome in the city, as living conditions had become difficult for workers and Hidalgo promised to lower taxes and put an end to slavery. However, violence by the rebel army to city residents royalists, soured the welcome. Hidalgo did sign a proclamation ending slavery, honored in the country since after the war. During this time, he founded the newspaper El Despertador Americano, dedicated to the insurgent cause. Royalist forces marched to Guadalajara. Insurgents Ignacio Allende and Mariano Abasolo wanted to concentrate their forces in the city and plan an escape route should they be defeated, but Hidalgo rejected this.
Their second choice was to make a stand at the Puente de Calderon just outside the city. Hidalgo had between 80,000 and 100,000 men and 95 cannons, but the better-trained royalists won, decimating the insurgent army, forcing Hidalgo to flee toward Aguascalientes. Guadalajara remained in royalist hands until nearly the end of the war. After the state of Jalisco was erect
Jalisco the Free and Sovereign State of Jalisco, is one of the 31 states which, with Mexico City, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and is bordered by six states which are Nayarit, Aguascalientes, Michoacán and Colima. Jalisco is divided into 125 municipalities, its capital city is Guadalajara. Jalisco is one of the most important states in Mexico because of its natural resources as well as its history. Many of the characteristic traits of Mexican culture outside Mexico City, are from Jalisco, such as mariachi, ranchera music, tequila, etc. hence the state's motto: "Jalisco es México." Economically, it is ranked third in the country, with industries centered in the Guadalajara metropolitan area, the second largest metropolitan area in Mexico. The state is home to two significant indigenous the Huichols and the Nahuas. There is a significant foreign population retirees from the United States and Canada, living in the Lake Chapala and Puerto Vallarta areas.
With a total area of 78,599 square kilometers, Jalisco is the seventh-largest state in Mexico, accounting for 4.1% of the country's territory. The state is in the central western coast of the country, bordering the states of Nayarit, Aguascalientes, Guanajuato and Michoacán with 342 kilometers of coastline on the Pacific Ocean to the west. Jalisco is made up of a diverse terrain that includes forests, beaches and lakes. Altitudes in the state vary from 0 to 4,300 meters above sea level, from the coast to the top of the Nevado de Colima; the Jalisco area contains all five of Mexico's natural ecosystems: arid and semi arid scrublands, tropical evergreen forests, tropical deciduous and thorn forests and mesquite grasslands and temperate forests with oaks and firs. Over 52% of the bird species found in Mexico live in the state, with 525, 40% of Mexico's mammals with 173 and 18% of its reptile species. There are 7,500 species of veined plants. One reason for its biodiversity is, lies in the transition area between the temperate north and tropical south.
It lies at the northern edge of the Sierra Madre del Sur and is on the Trans-Mexican Volcanic Belt, which provides a wide variety of ecological conditions from tropical rainforest conditions to semi arid areas to areas apt for conifer forests. Its five natural regions are: Northwestern Plains and Sierras, Sierra Madre Occidental, Central Plateau, Trans-Mexican Volcanic Belt, which covers most of the state, the Sierra Madre del Sur, it has an average altitude of 1,550 meters MASL, but ranges from 0–4,300 m. Most of the territory is semi-flat between 600–2,050 m, followed by rugged terrain of between 900–4,300 m and a small percentage of flat lands between 0–1,750 m. Principle elevations include the Nevado de Colima, the Volcan de Colima, the Sierra El Madroño, the Tequila Volcano, the Sierra Tapalpa, Sierra Los Huicholes, Sierra San Isidro, Sierra Manantlán, Cerro El Tigre, Cerro García, Sierra Lalo, Sierra Cacoma, Cerro Gordo, Sierra Verde and the Sierra Los Guajolotes. Jalisco's rivers and streams empty into the Pacific Ocean and are divided into three groups: the Lerma/Santiago River and its tributaries, rivers that empty directly into the Pacific and rivers in the south of the state.
Jalisco has several river basins with the most notable being that of the Lerma/Santiago River, which drains the northern and northeastern parts of the state. The Lerma River enters extends from the State of Mexico and empties into Lake Chapala on the east side. On the west, water flows out in the Santiago River, which crosses the center of Jalisco on its way to the Pacific, carving deep canyons in the land. Tributaries to the Santiago River include the Zula, the Verde River, the Juchipila and the Bolaños. About three quarters of the state's population lives near this river system. In the southwest of the state, there are a number of small rivers that empty directly into the Pacific Ocean; the most important of these is the Ameca, with its one main tributary, the Mascota River. This river empties into the Ipala Bay; the Tomatlán, San Nicolás, Purificación, Marabasco-Minatitlán, Tuxcacuesco, Armería and Tuxpan rivers flow perpendicular to the Pacific Ocean and drain the coastal area. Another river of this group is the Cihuatlán River, which forms the boundary between Jalisco and Colima emptying into the Barra de Navidad Bay.
The southeastern corner belongs to the Balsas River basin. This includes the Tuxcacuesco, which join to form the Armería and the Tuxpan; the other main surface water is Lake Chapala, is the largest and most important freshwater lake in Mexico, accounting for about half of the country's lake surface. The lake acts as a regulator of the flow of both the Santiago Rivers. There are a number of seasonal and salty lakes linking to form the Zacoalco-Sayula land-locked system. There are other smaller lakes called Cajititlán, San Marcos, Atotonilco. Dams include Santa Rosa, La Vega, Tacotán and Las Piedras. Jalisco's surface water accounts for fifteen percent of the surface freshwater in Mexico. In 1987, four beaches in Jalisco were designated as federal marine turtle sanctuaries: El Tecuán, Cuitzmala and Playón de Mismaloya, with an extension of 8 km. Playa Majahuitas is 27 km southwest of Puerto Vallarta with a rugged coastline, numerous inlets and outcroppings; the Cañon Submarino underwater canyon is located offs
Mexico City, or the City of Mexico, is the capital of Mexico and the most populous city in North America. Mexico City is one of the most important financial centres in the Americas, it is located in the Valley of Mexico, a large valley in the high plateaus in the center of Mexico, at an altitude of 2,240 meters. The city has 16 boroughs; the 2009 population for the city proper was 8.84 million people, with a land area of 1,485 square kilometers. According to the most recent definition agreed upon by the federal and state governments, the population of Greater Mexico City is 21.3 million, which makes it the largest metropolitan area of the Western Hemisphere, the eleventh-largest agglomeration, the largest Spanish-speaking city in the world. Greater Mexico City has a GDP of $411 billion in 2011, making Greater Mexico City one of the most productive urban areas in the world; the city was responsible for generating 15.8% of Mexico's GDP, the metropolitan area accounted for about 22% of total national GDP.
If it were an independent country, in 2013, Mexico City would be the fifth-largest economy in Latin America, five times as large as Costa Rica and about the same size as Peru. Mexico’s capital is both the oldest capital city in the Americas and one of two founded by Native Americans, the other being Quito, Ecuador; the city was built on an island of Lake Texcoco by the Aztecs in 1325 as Tenochtitlan, completely destroyed in the 1521 siege of Tenochtitlan and subsequently redesigned and rebuilt in accordance with the Spanish urban standards. In 1524, the municipality of Mexico City was established, known as México Tenochtitlán, as of 1585, it was known as Ciudad de México. Mexico City was the political and financial center of a major part of the Spanish colonial empire. After independence from Spain was achieved, the federal district was created in 1824. After years of demanding greater political autonomy, residents were given the right to elect both a Head of Government and the representatives of the unicameral Legislative Assembly by election in 1997.
Since, the left-wing Party of the Democratic Revolution has controlled both of them. The city has several progressive policies, such as abortion on request, a limited form of euthanasia, no-fault divorce, same-sex marriage. On January 29, 2016, it ceased to be the Federal District, is now known as Ciudad de México, with a greater degree of autonomy. A clause in the Constitution of Mexico, prevents it from becoming a state, as it is the seat of power in the country, unless the capital of the country were relocated elsewhere; the city of Mexico-Tenochtitlan was founded by the Mexica people in 1325. The old Mexica city, now referred to as Tenochtitlan was built on an island in the center of the inland lake system of the Valley of Mexico, which it shared with a smaller city-state called Tlatelolco. According to legend, the Mexicas' principal god, indicated the site where they were to build their home by presenting a golden eagle perched on a prickly pear devouring a rattlesnake. Between 1325 and 1521, Tenochtitlan grew in size and strength dominating the other city-states around Lake Texcoco and in the Valley of Mexico.
When the Spaniards arrived, the Aztec Empire had reached much of Mesoamerica, touching both the Gulf of Mexico and the Pacific Ocean. After landing in Veracruz, Spanish explorer Hernán Cortés advanced upon Tenochtitlan with the aid of many of the other native peoples, arriving there on November 8, 1519. Cortés and his men marched along the causeway leading into the city from Iztapalapa, the city's ruler, Moctezuma II, greeted the Spaniards. Cortés put Moctezuma under house arrest. Tensions increased until, on the night of June 30, 1520 – during a struggle known as "La Noche Triste" – the Aztecs rose up against the Spanish intrusion and managed to capture or drive out the Europeans and their Tlaxcalan allies. Cortés regrouped at Tlaxcala; the Aztecs thought the Spaniards were permanently gone, they elected a new king, Cuitláhuac, but he soon died. Cortés began a siege of Tenochtitlan in May 1521. For three months, the city suffered from the lack of food and water as well as the spread of smallpox brought by the Europeans.
Cortés and his allies landed their forces in the south of the island and fought their way through the city. Cuauhtémoc surrendered in August 1521; the Spaniards razed Tenochtitlan during the final siege of the conquest. Cortés first settled in Coyoacán, but decided to rebuild the Aztec site to erase all traces of the old order, he did not establish a territory under his own personal rule, but remained loyal to the Spanish crown. The first Spanish viceroy arrived in Mexico City fourteen years later. By that time, the city had again become a city-state, having power that extended far beyond its borders. Although the Spanish preserved Tenochtitlan's basic layout, they built Catholic churches over the old Aztec temples and claimed the imperial palaces for themselves. Tenochtitlan was renamed "Mexico"; the city had been the capital of the Aztec empire and in the colonial era, Mexico City became the capital of New Spain. The viceroy of Mexico or vice-king lived in the viceregal palace on Zócalo; the Mexico City Metropolitan Cathedral, the seat of the Archbishopric of New Spain, was const
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