Eruptive history Cerro Blanco (volcano)

1 eruptive history

1.1 pre-holocene
1.2 holocene
1.3 recent unrest , threats

eruptive history

human research in san buenaventura area goes stelzner 1885, mineral-focused research in 1895 , penck 1920. la hoyada formation identified 1963 , reinterpreted la hoyada volcanic complex in 2000. general geological research in region occurred 1988, 2000 , 2006, , research focused on cerro blanco done arnosio et al. 2005 , 2008.

activity of cerro blanco goes 8 mya, when maricunga belt in west active. volcano responsible large holocene eruption volcano explosivity index 7, largest in andean central volcanic zone 5000 years. comparable santorini , tambora s large eruptions.

pre-holocene

the aguada alumbrera ignimbrite white-rosa ignimbrite composed 2 units, 7.14 mya unidad blanquecina , 13.57 mya unidad rojiza. blanquecina unit of white white-rosa colour , contains amphibole, biotite , quartz crystals. contains lithic fragments of dacitic composition. groundmass of ignimbrite grey in colour , contains glass , phenocrysts. next laguna aguada alumbrera 100 metres (330 ft) thick. ignimbrite unconformably overlaid ignimbrites la hoyada volcano , overlies basement. there fluvial deposits contained within blanquecina ignimbrite, suggesting sedimentation occurred between eruptions. other subunit rojiza of white colour , rich in biotite , quartz. buried other deposits; south of aguada alumbrera principal deposit 40 metres (130 ft) thick. ignimbrite crops out south of cerro blanco.

the rosada ignimbrite located next cueros de purulla , has rosa colour, due fiamme formed welded pumice. pumice contains biotite, hornblende , quartz. contains lithic fragments basement , vulcanites. 2 dates of 6.3 , 7.3 mya have been obtained; younger age result argon loss , less accurate. , aguada alumbrera ignimbrites coeval different compositions indicate derive separate eruptions. rosada ignimbrite s main minerals biotite, quartz, plagioclase , sanidine. has volume of 50 cubic kilometres (12 cu mi).

a lava dome (26°40.26′s 67°45.90′w / 26.67100°s 67.76500°w / -26.67100; -67.76500) formed 7.3 ± 0.5 mya , red ignimbrite 8.1 ± 0.5 mya. 2.3–1.3 mya east-northeast chain of andesitic lava domes formed, youngest dome (26°40.63′s 67°45.1′w / 26.67717°s 67.7517°w / -26.67717; -67.7517) being 1.3 ± 0.4 mya old , located next cerro blanco. 0.8 mya mafic andesite erupted south , west of carachipampa (26°28.251′s 67°24.94′w / 26.470850°s 67.41567°w / -26.470850; -67.41567) , laguna de purulla (26°39.3′s 67°52′w / 26.6550°s 67.867°w / -26.6550; -67.867). cueros de purulla obsidian lava dome (26°34.06′s 67°44.97′w / 26.56767°s 67.74950°w / -26.56767; -67.74950) formed 0.4 ± 0.1 mya.

the barranca blanca , carachi ignimbrites covered lavas volcán carachipampa , older 0.75 mya. contain pumices of various colours , lithic fragments, including mylonite , schist. barranca blanca ignimbrite white in colour , contains rhyodacitic white pumice subordinate amounts of grey trachyandesitic pumice. carachi ignimbrite overlies barranca blanca ignimbrite. weakly welded , contains white pumice , crystals fiamme. both of these pyroclastic density currents still hot @ time of deposition.

the campo de la piedra pómez ignimbrites (26°39′s 67°43′w / 26.65°s 67.71°w / -26.65; -67.71) have surface area of 261.83 square kilometres (101.09 sq mi). have volume of 17 cubic kilometres (4.1 cu mi) , separated in 2 layers form 1 cooling unit despite different colour , mineral composition. both contain pumice , lithic fragments , poor in crystals. both have pyroclastic texture pumices bearing porphyritic texture. dating techniques yield different ages; reliable 1 (argon–argon dating on sanidine) indicates these ignimbrites erupted 73,000 ± 23,200 years ago. other proposed dates go far 0.555 ± 0.11 mya. not cross topographic surfaces , generated tranquil release ground rather through collapse of eruption column. surface has been exposed wind erosion, forming yardangs , ripple marks. source of eruption unknown; robledo caldera may origin of eruption controversial. ignimbrites of piedra pómez covered desert varnish similar coating, bearing signs of endolithic lifeforms.

holocene

2,300 ± 160 bce large plinian eruption occurred @ cerro blanco. rhyolites volume of 110 cubic kilometres (26 cu mi) generated ashfalls , ignimbrites, forming largest known eruption in central andes during holocene. ash eruption covered area of 440,000 square kilometres (170,000 sq mi). these purulla/el médano ignimbrites (26°51′s 67°43′w / 26.85°s 67.72°w / -26.85; -67.72) cover area of 51.61 square kilometres (19.93 sq mi) , dated have occurred less 12,200 years ago on basis of radiocarbon analysis of organic material contained between various ignimbrite layers. 1 layer in la hoyada dated between 8,830 , 5,480 bp. other data attribute purulla ignimbrites 22,000 years ago. date of major holocene eruption constrained between 5,500 , 4,000 years ago. based on stratigraphic relationships, cerro blanco caldera formed during eruption. location of vent did not change during course of eruption, indicating annular fractures around caldera possibly uninvolved in process. low population density in fiambalá valley south of volcano during 10,000–3,000 b.p interval ascribed eruption.

the ignimbrites caldera-forming eruption have volume of 17 cubic kilometres (4.1 cu mi) , cover surface area of 527 square kilometres (203 sq mi) south, north , east of caldera. partly welded , contain fiamme , lithic fragments. material vitreous , may have been fluid @ time of deposition. breccia derived column collapse found. deposits microcristalline glass. few kilometres calderas abruptly turn medium-sized pumice deposits. ignimbrites stage derived lava domes placed within small depressions, largest of 1.3 kilometres (0.81 mi) wide. aeolian erosion has reworked ashes generated eruption, generating dunes.

the purulla/el médano ignimbrites have precaldera lower layer various methods have dated 20,000 5,000 years ago. stage effusive , accompanied emplacement of lava domes. postcaldera stage represented lava domes on southeast edge of caldera, block , ash flows inside caldera. these rhyolitic in composition. age unknown, postdates caldera formation.

recent unrest , threats

there no reports of historical activity. small geothermal field associated volcanic complex, consisting of fumaroles, mud volcanoes , thermal anomalies.

the caldera floor has been subsiding in 1990s according satellite images. diameter of deformation region less 30 kilometres (19 mi) , deformation occurs @ depths of less 4.8 kilometres (3.0 mi). deflation speed has been decreasing more 2.5 centimetres per year (0.98 in/year) between 1992 , 1997 on 1.8 centimetres per year (0.71 in/year) between 1996 , 2000 , 1.2 centimetres per year (0.47 in/year) between 2003 , 2007 0.87 centimetres per year (0.34 in/year) between 2005 , 2010. deformation centered between robledo , cerro blanco calderas , accompanied uplift northwest. deflation has been interpreted depend on magma chamber diameter of 17 kilometres (11 mi) unlikely caused magma withdrawal or tectonic expansion. may linked hydrothermal effects however. seismic swarm noted @ 15 kilometres (9.3 mi) depth in 2007 , 2009.

evidence indicates such calderas can fill in within timespan of hundreds thousands of years. 2 possible scenarios of eruption formation of plinian eruption columns or more tranquil subhorizontal eruption of pyroclastic flows vents. given low population density of 0.06 people per 1 square kilometre (0.39 sq mi), impact of eruption on local population small. westerly winds may cause larger impact west of centre, air travel particularly affected. community of bolsón de fiambalá 50 kilometres (31 mi) south of caldera beneath level of cerro blanco , connected valleys. in future eruption, pyroclastic currents , less dense clouds associated them cerro blanco might reach community.