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Cerro Azul (Chile volcano)


Cerro Azul ("blue hill" in Spanish) is an active stratovolcano in central Chile's Maule Region, immediately south of the Descabezado Grande volcano and part of the South Volcanic Zone. Capped by a summit crater that is wide and opens to the north, the lower slopes have numerous scoria cones and flank vents.

Azul has produced the largest eruptions ever in South America, once in 1846 and again in 1932. In 1846, an effusive eruption formed the vent at the site of present-day Quizapu Crater and sent lava flowing down the sides of the volcano, creating a lava field 89 square kilometers (33.5 square miles) in area. Phreatic and strombolian volcanism between 1907 and 1932 excavated Quizapu Crater. In 1932, of dacitic tephra erupted from Quizapu Crater on the northern flank of Cerro Azul in one of the largest explosive eruptions of the 20th century. The volcano's most recent eruption was in 1967.

Chile has almost 100 volcanoes, of which about 36 are active. The South Volcanic Zone has a long history of eruptions and poses a threat to the surrounding region. Any volcanic hazardranging from minor ashfalls to pyroclastic flowscould pose a significant risk to humans and wildlife.

Geography and geology

Regional setting

Volcanic activity in Chile varies widely, and includes explosive eruptions and both subaerial and submarine basalt flows. Volcanism in the Andes is caused by subduction of the Nazca and Antarctic tectonic plates under the South American Plate. Volcanoes in Chile (including Cerro Azul) occur in the Central (CVZ), South (SVZ), and Austral Volcanic Zones (AVZ). The gap that separates the Central and South Volcanic Zones is caused by shallow-angle subduction in the Pampean flat-slab segment where the more buoyant Juan Fernandez Ridge subducts under the South American continent.Stern et al., p. 147. This buoyant region prevents the slab (subducting tectonic plate) from diving deep into the mantle, where the heat and pressure would destabilize the mineral chlorite, releasing water that would in turn cause melting and volcanism. The Patagonian Volcanic Gap, which separates the South and Austral Volcanic Zones, is caused by the subduction of the Chile Ridge, though it is less clear whether this gap also is due to flat-slab subduction or is because melting of the subducting slab there produced felsic igneous rocks instead of volcanoes.

Offshore volcanism also occurs in Chile. Intraplate volcanism generated from the Easter and Juan Fernandez hotspots has formed many Chilean islands including Isla Salas y Gomez, Easter Island, and the Juan Fernandez Islands. Underwater volcanism occurs due to seafloor spreading along the Chile Ridge.

Nearly 100 Quaternary (Pleistocene- or Holocene-age) volcanoes exist in the country, as well as 60 complexes and caldera systems. Of the 200 historically active volcanoes in the Andean Range, 36 are found in Chile.

Local setting

Cerro Azul, just south of Descabezado Grande volcano, is part of the Andes' South Volcanic Zone, which runs through central and western Chile. The volcano is part of the Descabezado GrandeCerro Azul eruptive system, a volcanic field which comprises its two large namesake volcanic edifices and several smaller vents, including 12 Holocene calderas. Both volcanoes lie on top of the Casitas Shield, a plateau built of over 100 lava flows that erupted in at least 12 volcanic episodes during the Quaternary periodthe upper lava layers are dated at 340,000 years.

The South Volcanic Zone, of which Cerro Azul is a part, extends south to Argentina. This range includes at least nine caldera complexes, more than 70 of Chile's stratovolcanoes and volcanic fields that have been active in the Quaternary, and hundreds of minor eruptive centres. The South Volcanic Zone is the most volcanically active region in Chile, and produces around one eruption per year. Its largest historical eruption was at Quizapu crater, and its most active volcanoes are Llaima and Villarrica.Stern et al., pp. 154156.

As with the majority of the Andean volcanoes, Cerro Azul is a stratovolcano, meaning that it consists of layers, or strata, of volcanic ash and lava flows. The cone of Cerro Azul has a total volume of about 11 km3, and is a young feature, formed in the Holocene. It is made of agglutinated pyroclasts and some daciteandesine lavas. The cone has a few volcanic craters (calderas), with the majority of its eruptions in recorded history originating from Quizapu Crater on the northern flank of the Azul's cone. Two separate calderas lie within Quizapu: Cerro del Medio and Volcan Nuevo. Four other craters make up the volcano: Carasol, Crater los Quillayes, Crater la Resolana, and Crater sin Nombre. All of the craters lie between in elevation except Quizapu, which is up the volcano. The summit of Cerro Azul is crowned by an asymmetric crater about 500 m in diameter. Pleistocene glacial activity is evident in the form of deep struts in the volcanoes' sides. These deep cuts have revealed strata of older rock.

Quizapu Crater

Quizapu, which formed during the 1846 eruption, is the most prominent caldera. The volcanic vent formed during an effusive eruption involving hornblende-dacite flows accompanied by tephra, and crater was excavated by phreatic and strombolian eruptions between 1907 and 1932. Pent up pressure within the volcano spawned an enormous Plinian eruption in 1932. The volume of lava ejected during this single event is roughly equal to that erupted during the rest of the eruptive history at Quizapu, since its formation in 1846. In spite of the fact that of material was ejected, no subsidence was detected in response to the removal of the magma. Due to aerodynamic drag, a Plinian eruption excavates a circular caldera. Because the earlier eruptions had already formed a nearly-circular caldera, the Plinian eruption was able to proceed efficiently, with minimal drag and reshaping of the crater.

The Quizapu Crater is nearly perfectly circular and rises to a prominence of above the surrounding portions of the volcano. At between elevation, Quizapu is one the highest known Plinian calderas. The radius of the crater floor, which is the current inner vent, is ~, and the radius of its rim is . The crater floor lies at , and the rim lies above that, giving the walls a near-angle of repose average slope of 3435 degrees. It is cut by two long, dacitic lava flows which are probably the remnants of a dome or an eruption. It is surrounded by debris from its 1932 eruption and topped by thick layers of mafic scoria and ash.

Climate and vegetation

Cerro Azul is situated in the Mediterranean climate zone. It is characterized by hot and dry summers, but mild and wet winters. The temperatures and precipitation are strongly dependent on topography. In the Andes the annual average maximum temperatures lie in the range of 2025 C, while minimum temperatures are below 0 C. Annual precipitation is up to 800 mm.

Vegetation in Andes varies with elevation. Above 1600 m the slopes of mountains are covered by Alpine like steppe, while below there are zones of Nothofagus forest, Hygrophilous forest, Sclerophylous forest and matorral. The number of plant species is likely to exceed 2,000, although no comprehensive study of the flora of Central Chile has been undertaken.

Eruptive history

Cerro Azul has a history of eruptions, dating back to at least 1846. The known events include effusive eruptions (lava flows), which created the Quizapu vent, explosive eruptions, and phreatic eruptions. Pyroclastic flows have also been observed as a result of some of these explosive eruptions. The earliest recorded eruption began on November 26, 1846, while the volcano's last eruption began on August 9, 1967. The volcano has produced the two largest eruptions in South America in recorded history, in 1846 and 1932. Both released of the dacitic magma.

First documented activity, 1846

On November 26, 1846, Cerro Azul erupted. This was the first documented activity at the volcano, and there is no trace of fumaroles, adjacent vents, or pre-eruptive activity. Most descriptions of the eruption come from the backcountry herdsman who saw it. One was camped in a valley approximately east of Quizapu when in the late afternoon, "a great noise and a cloud of ash" emanated from the mountain. No precursor activity was reported, and the herdsman claimed that there were no earthquakes during the late afternoon eruption.

That night, the two herdsmen near the site heard a continuous roar punctuated by loud bangs and crackling sounds "like that of great rockslides". Lightning and thunder accompanied the spectacle. They saw many blue flames, and were choked by sulfurous gas. Observers in Talca away heard the eruption noises, and the sulfurous odors reached them the day after the eruption. None of the reports mention earthquakes or ash fall, though the crackling and banging sounds could be from block lavas (aa).

This first recorded eruption of Cerro Azul was effusive, and formed the volcanic vent at Quizapu. Hornblende-dacite lava erupted with small masses of tephra, which had been degassed shortly before the eruption. Lava flows flowed over the Estero Barroso Valley and westward into the Rio Blanquillo Valley. By November 28, the volcano appeared at rest, and the arrieros returned to the place of first observation. There, they found a blocky lava field. The lava was still hot, fuming and crackling with gas and flame. Fascinated by the volcano, Ignacy Domeyko traveled to Chile to study the field and measured its its width to be . Today, the field is twice that size.

Early twentieth century

Cerro Azul was quiet from 1846 to the beginning of the 20th century. Beginning in 1907, though with a possible precursor explosive event in 1903, Cerro Azul once again erupted. Between 1907 and 1914, plumes and clouds of ash frequently were seen rising out of the caldera, and at least a few of these events were explosive. On September 8, 1914, an explosive eruption sent a plume 6 or 7 kilometers (about 4 mi) into the air over a span of 8 minutes. By 1916, these eruptions had produced a caldera that is nearly identical to the one in existence today.

The volcano also erupted phreatically several times, as recorded by Vogel in 1913 and 1920, with its activity increasing from 1916 to 1926. During these years, the eruptions grew more frequent and more violent. A major outburst on November 2, 1927, started a period of nearly continuous violent eruptions that lasted until 1929. During this period, Cerro Azul sometimes erupted daily, sending columns of ash as far as 6 or 7 kilometers (about 4 mi) into the air. Quizapu Crater grew slightly during this eruptive period.

Pre-1932 volcanism was largely phreatic or fumarolic, as evidenced by the lack of tephra generated by these eruptions. Photographs from 1912 show vapor plumes containing little ash rising 12 kilometers above the crater.

Major eruption, 1932

By 1932, Quizapu had produced many phreatic events and one effusive eruption, but had produced no large Plinian eruptions. The frequency of activity proved to be a precursor for a major eruption. Ash clouds were ejected, not frequently, and became larger than the other activity. On 25 January 1932, a large black cloud was observed in Malargue over the summit. By 9 April, the volcano emitted green gas and started to "bellow like a bull".

On April 10, Cerro Azul erupted, releasing a towering column of white gas. After 10 AM, the plume turned black with ash and began to form an umbrella shape. The ash was carried by wind into Puesto El Tristan in Argentina, , where beginning at 1 pm, it rained down for hours. At 4 pm, coarser sandy material and some pumice lapilli began to fall.

Cerro Azul's April 1932 was one of the largest of the 20th century. Releasing of lava, the volcano ejected primarily dacitic tephra, accompanied by rhyodacite, andesite, and minuscule amounts of andesitic and basaltic scoria. At least one eruptive period lasted for 18  hours, creating an "exceptionally uniform" deposit. Eruption columns, extending into the air, were sighted. Phenocrysts compared similar to the effusive eruption in 1846. Soon after, both the Tinguiririca and Descabezado Grande volcanoes began erupting, sending clouds of ash into Argentina. The eruption had a Volcanic Explosivity Index (VEI) of at least 5.

Threats and preparedness

Chile remains at risk from a volcanic eruption. Several volcanoes, such as Mount Hudson and Villarrica, are still active. According to John Ewert and Ed Miller in a 1995 publication, "a great majority of the world's potentially active volcanoes are unmonitored". Of the historically active volcanoes in the world, less than one fourth are monitored. Only twenty-four volcanoes in the entire world are thoroughly monitored for activity. They also state that "seventy-five percent of the largest explosive eruptions since 1800 occurred at volcanoes that had no previous historical eruptions".

Many South Volcanic Zone volcanoes pose a threat to human life. Every known type of eruption, including Hawaiian eruptions, Strombolian, Plinian, sub-Plinian, phreatomagmatic, and Vulcanian activity, has occurred at some point in the range. The type of eruption tends to correspond with lava type. Volcanoes such as Llaima have produced Strombolian activity, while more silicic and rhyolitic lavas have been linked to Plinian eruptions . Because of this versatility, any type of volcanic hazard could threaten life. Ashfall, for example, could interfere with air traffic. Lava flows and lahars could wipe out entire cities or towns; both have been present during eruptions in recent time. Most threatening of all is the risk of pyroclastic flows or avalanches, which have in the region historically traversed as far as .

Past eruptions of Quizapu Crater ejected enormous amounts of ash, which traveled as far as Brazil. After the 1932 eruption, the local vegetation was devastated, and the area remained barren until the 1990s. However, human life remained normal. Also indicative of a possible deadly eruption are the remnants of a lahar evident at Descebezado Grande. Historically, lahars have killed thousands, especially in the Andes.

Despite the extent of its eruptions, due to the remoteness of Cerro Azul, the threat from Quizapu is small. However, the size of past eruptions is enough for scientists to be worried. If a volcano were to erupt, relief efforts could be orchestrated. The Volcanic Disaster Assistance Program (VDAP), which formed in response to Nevado del Ruiz's famous eruption, has provided relief efforts to victims of volcanic disasters in the country before. After Mount Hudson erupted in 1991, the VDAP saved lives by evacuating the area. The team's stated aim is to "reduce eruption-caused fatalities and economic losses in developing countries". Made up of various USGS offices, such as the Cascades Volcano Observatory (CVO), responsible for monitoring Mount St. Helens, the team is outfitted with advanced equipment which can monitor any volcano.

Bibliography

Isopach mapping of the volcanic deposits, between , contradict 1930 estimates by about half.

Further reading

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Cerro Azul (Chile volcano)


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