Cerro Porquesa (

Aymara Aymara may refer to: Languages and people * Aymaran languages, the second most widespread Andean language ** Aymara language, the main language within that family ** Central Aymara, the other surviving branch of the Aymara(n) family, which today ...

) is an approximately high ( above base)

rhyodacite Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from r ...

lava dome In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruption ...

in the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

. It has a oval outline, and is high. The lava dome is accompanied by block-and-ash flows and

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

s and the presence of two volcanic cones has been reported. The lava dome is of

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Pleistocene The Pleistocene ( , often referred to as the '' Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed ...

age with little glacial features on the younger domes indicating young ages. The Quebrada de Piga, which flows to Salar de Huasco, originates at Cerro Porquesa. The dome was formed in at least three different eruption stages, with each stage contributing about two or three different lobes. Further, a

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surro ...

with 69.5% SiO₂ may be linked to the domes. It fills a valley in the south of the complex about thick. This ignimbrite is dated 0.73±0.16 and 0.63 +0.92/-0.63 mya by potassium-argon dating in

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more alum ...

, although with low precision. Another 0.28 mya old ignimbrite was originally attributed to Porquesa but instead comes from a wide

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber ...

in Bolivia. Three dates have been obtained from the domes, 0.63±0.63 mya, less than 1 mya and 1.1±0.4 mya. The volcanic complex was probably active after 280,000 years ago but not in the

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

. Some of the ignimbrites have been offset by faults. This lava dome is located 20° in a volcanic gap named

Pica gap Pica gap is a segment in the Central Volcanic Zone of Chile where volcanic activity is absent. It is named after the Altos de Pica region. This segment is long and extends between the volcanoes Isluga in the north and Irruputuncu in the south. ...

. In this gap volcanic activity younger than 2 mya isn't found and where

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

(Pb) isotope ratios in rocks change with the radiogenicity of the isotope ratio decreasing northward. Porquesa has intermediary isotope ratios. The lower ratio was principally imparted by the low-radiogenic Pb upper crust. Samples and the appearance of the domes in aerial photographs indicate a homogeneous composition with about 68% SiO₂. Their main component is

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained ( aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhy ...

, with

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic ro ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

phenocrysts.

References

External links

{{DEFAULTSORT:Porquesa, Cerro Pleistocene lava domes Pliocene lava domes Volcanoes of Tarapacá Region