San Pedro is a
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 ...
composite volcano
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and peri ...
in northern
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
and one of the tallest active volcanoes in the world. Part of the Chilean
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 ...
' volcanic segment, it is part of the
Central Volcanic Zone of the Andes, one of the four tracts of the
Andean Volcanic Belt
The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South American ...
. This region of volcanism includes the world's two highest volcanoes
Ojos del Salado
Nevado Ojos del Salado is a dormant complex volcano in the Andes on the Argentina–Chile border. It is the highest volcano on Earth and the highest peak in Chile. The upper reaches of Ojos del Salado consist of several overlapping lava domes, ...
and
Llullaillaco
Llullaillaco () is a dormant stratovolcano at the border of Argentina (Salta Province) and Chile (Antofagasta Region). It lies in the Puna de Atacama, a region of tall volcanic peaks on a high plateau close to the Atacama Desert, one of the dri ...
. San Pedro, like other Andean volcanoes, was formed by the
subduction of the
Nazca Plate beneath the
South America Plate
The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...
. It has a neighbouring volcano,
San Pablo, and is itself formed by two separate edifices usually known as the Old Cone and the Young Cone. These edifices are formed by rocks ranging from
basaltic andesite
Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Amer ...
over
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...
to
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 rhyolite ...
and are emplaced on a
basement formed by
Miocene
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
volcanic rocks.
The Old Cone was active over one hundred thousand years ago and was eventually truncated by a giant
landslide that removed its northwestern side. Within the landslide scar
lava flows and
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s constructed the Young Cone as well as the lateral centre La Poruña. This volcano was
glaciated
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
during the
Pleistocene
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 in ...
and a large
Plinian eruption occurred at the beginning of the Holocene. Some eruptions reportedly occurred during historical time; presently the volcano is
fumarolically active.
Geography and geomorphology
San Pedro is located in northern Chile, in the
Ollagüe
Ollagüe () or Ullawi () is a massive andesite stratovolcano in the Andes on the border between Bolivia and Chile, within the Antofagasta Region of Chile and the Potosi Department of Bolivia. Part of the Central Volcanic Zone of the Andes, it ...
commune
A commune is an alternative term for an intentional community. Commune or comună or comune or other derivations may also refer to:
Administrative-territorial entities
* Commune (administrative division), a municipality or township
** Communes of ...
,
El Loa Province,
Antofagasta Region
The Antofagasta Region ( es, Región de Antofagasta, ) is one of Chile's sixteen first-order administrative divisions. The second-largest region of Chile in area, it comprises three provinces, Antofagasta, El Loa and Tocopilla. It is bordered t ...
;
the border to Bolivia is away. The whole region is remote and thinly populated; towns in the area include
Ascotán,
Cupo,
Inacaliri and
Paniri;
and the San Pedro
railway station
Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a prep ...
lies southwest of the volcano. Tracks left by
sulfur miners lead up to San Pedro's summit, and roads run around the volcano; passes around the northern, northwestern and western flanks of the volcano.
San Pedro is part of the long
Central Volcanic Zone, one of the two belts of volcanoes in Chile, which contains a total of about 44 active volcanoes plus
silicic
Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks.
Silicic is the grou ...
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 is ...
and
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 ...
systems and
monogenetic volcano
A monogenetic volcanic field is a type of volcanic field consisting of a group of small monogenetic volcanoes, each of which erupts only once, as opposed to polygenetic volcanoes, which erupt repeatedly over a period of time. The small monogeneti ...
es.
Many of these volcanoes are of
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Pleistocene
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 in ...](_blank)
age and approach heights of about above sea level at
Llullaillaco
Llullaillaco () is a dormant stratovolcano at the border of Argentina (Salta Province) and Chile (Antofagasta Region). It lies in the Puna de Atacama, a region of tall volcanic peaks on a high plateau close to the Atacama Desert, one of the dri ...
and
Ojos del Salado
Nevado Ojos del Salado is a dormant complex volcano in the Andes on the Argentina–Chile border. It is the highest volcano on Earth and the highest peak in Chile. The upper reaches of Ojos del Salado consist of several overlapping lava domes, ...
; such sizes of the volcanoes occur because the edifices rise from a relatively high basement. Volcanism in this region is relatively poorly studied and there is not much present-day activity; with the exception of
Láscar, most present-day manifestations are limited to
fumarolic
A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volc ...
activity.
San Pedro is one of the highest volcanoes in the world and is visible as far as away in
Calama. Its height has been variously given as ,
or . It is a
composite volcano
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and peri ...
, which forms a twin volcano with
San Pablo farther east.
Between the two lies the so-called "Central Crater" at an elevation of . San Pedro itself is formed from the easterly "Old Cone" whose most prominent expression is the eastern summit, and the westerly "Young Cone" which is only about lower. The "Young Cone" has two
craters, the eastern of which is the younger one.
Lava flows emerge from a breach in this crater and have filled an older wide older crater.
Hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
ly altered rocks and
sulfur deposits are found on San Pedro's summit and were mined into the 1930s.
A number of
lava flows of volumes between make up part of the edifice. The upper part of the Younger Cone is covered with such lava flows, which have steep fronts
and are up to thick. Downslope the lava flows are found mainly over the northwestern flank where they radiate away from San Pedro's crater.
Scars at elevations of on the edifice have been the origin of
mudflow
A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...
s; the largest is known as Estación flow and covers much of the southern and western foot of San Pedro to distances of ; the debris apron reaches the
Loa River
The Loa River (Spanish: Río Loa) is a U-shaped river in Chile's northern Antofagasta Region. At long, it is the country's longest river and the main watercourse in the Atacama Desert.
Course
The Loa's sources are located on Andean mountain sl ...
and
San Pedro River. These mudflows occurred between 110,000 – 36,000 years ago and they filled the river bed of the Loa, while the impact on the San Pedro River was much more minor.
Hot avalanche deposits or
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s are also found on the volcano; the largest of which covers large parts of the edifice and has a total volume of approximately . These deposits contain prismatic jointed blocks and poorly sorted pumice, with their thickness ranging from a thick layer to scattered rocks. Their formation was probably due to the steep incline of San Pedro's slopes which tended to destabilize lava bodies high on the edifice, similar to regular
landslides. At least one
Plinian fall deposit is known from San Pedro; the size of the deposit indicates that it was as large as the
AD 79 eruption of
Vesuvius
Mount Vesuvius ( ; it, Vesuvio ; nap, 'O Vesuvio , also or ; la, Vesuvius , also , or ) is a somma- stratovolcano located on the Gulf of Naples in Campania, Italy, about east of Naples and a short distance from the shore. It is one of ...
in
Italy
Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...
. This Plinian eruption was accompanied by the formation of an
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 ...
that covers much of the southern, southwestern and western slopes of San Pedro and reaches thicknesses of . It has a volume of and is called the El Encanto ignimbrite.
There is evidence of a large
sector collapse
A sector collapse is the collapse of a portion of a volcano due to a phreatic eruption, an earthquake, or the intervention of new magma. Occurring on many volcanoes, sector collapses are generally one of the most hazardous volcanic events, and will ...
on San Pedro, which removed the northwestern flank of the volcano and formed a large debris avalanche deposit. This avalanche deposit covers a surface area of and its front is high in the Pampa de la Avestruz. The debris flow formed by the sector collapse does not have many of the typical hummock-like hills of volcanic sector collapse deposits and instead features grooves and radial ridges. Such flank collapse occurred notably on
Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
during
its eruption in 1980. A high scarp west of the eastern summit was left by the collapse of San Pedro; otherwise much of the evidence was buried by later volcanic activity. As with the mudflows, the steep slopes of San Pedro probably facilitated the onset of the sector collapse, which descended over an elevation difference of about .
A
parasitic vent
A parasitic cone (also adventive cone or satellite cone) is the cone-shaped accumulation of volcanic material not part of the central vent of a volcano. It forms from eruptions from fractures on the flank of the volcano. These fractures occur ...
named La Poruña lies on the western foot of San Pedro, its formation was probably influenced by a
normal fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecton ...
that runs in that area. It is a wide and high
scoria cone on top of a field of
lava flows which extend as far as west of San Pedro and from the cone. The cone's western side slumped and
lava flows emanate from that area, reaching a considerable distance from the cone. These flows are subdivided into a small proximal unit and two distant ones, and feature structures such as ogives and levees. In addition, a
lava dome at an elevation of lies on the southwestern flank of San Pedro and also seems to be a parasitic vent. Andesite lava flows emanate from this area as well. They are the largest found on San Pedro, with volumes of and a surface area of ; they reach a distance of from the vent.
San Pedro has been glaciated in the past. Evidence of such glaciation is found especially on the southern side of the Old Cone and it includes
moraines at elevations of over as well as other glacially modified surfaces such as
rock pavements and
striated boulders. The chronology of glaciations in the Central Andes is poorly known but stratigraphic relations indicate that San Pedro was glaciated during the late Pleistocene. Some
cirques on the southern flank contain evidence of past
rock glaciers, but there are no glaciers presently on the mountain.
The
San Pedro River flows at the foot of San Pedro volcano, and joins the
Loa River
The Loa River (Spanish: Río Loa) is a U-shaped river in Chile's northern Antofagasta Region. At long, it is the country's longest river and the main watercourse in the Atacama Desert.
Course
The Loa's sources are located on Andean mountain sl ...
southwest of the volcano;
Pleistocene
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 in ...
lava flows from San Pedro created a
lava dam
A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those creat ...
on the San Pedro River, forming a lake which no longer exists. As reported in 1926, the slopes of San Pedro were used to cultivate
llareta, a plant that was grown as a fuel source.
Geology
Beneath northern Chile, the
Nazca Plate subducts beneath the
South America Plate
The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...
. This subduction process is responsible for volcanic activity in the
Western Cordillera, as well as elsewhere in the Andes.
The volcanoes of Chile are part of the
Pacific Ring of Fire
The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a region around much of the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur. The Ring o ...
, which in Chile contains about 2000 volcanoes over a length of . In Chile, it is subdivided into two volcanic zones, the
Central Volcanic Zone which also spans Peru, Bolivia and Argentina, and the
Southern Volcanic Zone which spans Chile and Argentina. In addition, Colombia and Ecuador feature the
Northern Volcanic Zone, while the subduction of the
Antarctic Plate
The Antarctic Plate is a tectonic plate containing the continent of Antarctica, the Kerguelen Plateau, and some remote islands in the Southern Ocean and other surrounding oceans. After breakup from Gondwana (the southern part of the superconti ...
beneath the
South America Plate
The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...
at the southernmost tip of South America forms the
Austral Volcanic Zone south of the Southern Volcanic Zone.
There were two cycles of volcanic activity in Chile, the first during the
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...
-
Triassic
The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
and a second starting from the
Tertiary
Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago.
The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
. In the San Pedro region, this volcanic activity migrated east from its point of inception but recently has moved back westward. West of San Pedro lies the
Pampa del Tamarugal
Pampa del Tamarugal ("Plateau of the Tamarugal") is a vast plain encompassing a significant portion of the Norte Grande, Chile, and originally named for the '' Prosopis tamarugo'' trees that used to cover its surface. It is located between the par ...
and the
Coastal Cordillera, neither of which show evidence of recent volcanic activity.
Regional
Volcanoes in this region of Chile often form lineaments perpendicular to the
volcanic arc
A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate,
with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...
, with northwest–southeast and north–south lineaments common. One such lineament is the long San Pedro-Linzor lineament, which includes San Pedro,
Paniri,
Cerro Chao
Cerro Chao is a lava flow complex associated with the Cerro del León volcano in the Andes. It is the largest known Quaternary silicic volcano body and part of the most recent phase of activity in the Altiplano–Puna volcanic complex.
Cerro Chao ...
,
Cerro del León
Cerro del León is a stratovolcano located in El Loa province, Antofagasta Region, Chile. It is part of the Chilean Central Volcanic Zone and forms a volcanic lineament with neighbouring Paniri and Toconce that was active into the Holocene. Cer ...
and
Toconce
Toconce is a small Chilean village located on the south rim of the Toconce River Canyon at 3,350 m above sea level. To the north, the landscape is dominated by the volcanoes Cerro Paniri, Cerro del León and Toconce.
See also
*Ayquina
Ayquina (a ...
. Other volcanoes with such trends are
Carasilla-
Polapi-
Cerro Cebollar-
Cerro Ascotan
Cerro Ascotan (also known as ) is a volcano on the border between Chile and Bolivia. It is high, above the terrain and a maximum slope in the summit area of 26°. A breach in the edifice is wide and long, with an azimuth of 252°. The curren ...
-
Palpana Palpana (from , ram) is a volcano in the Andes of Chile. It has a summit elevation of .
It is part of the dividing range between Upper Loa River basin and Salar de Ascotán basin. Together with Inacaliri and Cerro del Azufre, Azufre, it forms a lo ...
and the
Aucanquilcha complex; the youngest edifice is the westernmost one.
The
basement of the region is formed by various
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
and
sedimentary
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
rocks intruded by
plutons ranging in their age from the
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
to the
Tertiary
Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago.
The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
; one of these is the 65 million years old Cerro Colorado pluton. However, in the area of San Pedro this basement is entirely buried beneath volcanites of Miocene age, including
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 ...
s, remnants of
composite volcano
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and peri ...
es and volcanic debris. At least three individual ignimbrites have been found; one of the ignimbrites, the San Pedro Ignimbrite, may have originated from a vent now covered by San Pedro volcano although the neighbouring Caracanal volcano is also a candidate source. The older composite volcano centres include Cerro Carcanal and Cerro Huiche south-southeast of San Pedro and Cerro del Diablo due north. This surface slopes down to the Loa River in the west.
Composition
Old Cone lavas range from
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...
to
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
-containing
basaltic andesite
Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Amer ...
. Young Cone lavas include both
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...
and
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 rock ...
-containing
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 rhyolite ...
. Volcanic rocks erupted during both stages of San Pedro belong to the
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
-rich
calc-alkaline
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic m ...
suite.
San Pedro volcanic rocks are usually glassy with only tiny
phenocryst
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
s.
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 ...
and
pyroxene are the dominant minerals, with
amphibole and
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
being secondary components. The
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
feeding the volcano probably formed from the mixing of magmas of distinct temperatures, as indicated by various clues indicating significant thermal disequilibrium between various components. Ultimately they originate in the
Altiplano-Puna Magma Body, but undergo storage in shallower crustal
magma chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...
s. The eruption of La Poruña appears to have been accompanied by a change in the magma system of the general volcano, a more recent hypothesis regards this cone as a separate volcanic system from San Pedro.
Climate
The region has an
arid
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...
climate with infrequent precipitation.
Moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
ultimately originates on the tropical
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
and much of it rains out as it crosses the
Eastern Cordillera and the
Altiplano
The Altiplano (Spanish for "high plain"), Collao (Quechua and Aymara: Qullaw, meaning "place of the Qulla") or Andean Plateau, in west-central South America, is the most extensive high plateau on Earth outside Tibet. The plateau is located at the ...
, so that little reaches the western Cordillera, with precipitation in the San Pedro area less than or about annually. This dryness may go back to the late
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
, but was interrupted during the late ice age between 17,000 and 11,000 years
before present
Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becaus ...
by a wetter period during which
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
s expanded. Temperatures strongly fluctuate between day and night; they can go as low as and as high as .
Eruptive history
San Pedro formed in two stages, which are known as the Old Cone and the Young Cone. The Old Cone was formed by
lava
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 ...
and
scoria
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...
and later largely buried by
glacial
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
,
mudflow
A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...
s and volcanic deposits of the Young Cone; it forms about 80% of the volume of the present-day San Pedro edifice. One date has been obtained on Old Cone lavas by
argon-argon dating: 160,000 years ago. After activity of the Old Cone ceased, glacial and fluvial erosion dissected San Pedro until the large
sector collapse
A sector collapse is the collapse of a portion of a volcano due to a phreatic eruption, an earthquake, or the intervention of new magma. Occurring on many volcanoes, sector collapses are generally one of the most hazardous volcanic events, and will ...
occurred. The so-called "white airfall" with a volume of as well as
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s on the western summit may have been erupted during the collapse but this is uncertain.
Activity of the Young Cone began after the collapse within the scar left by the failure of the edifice. This activity involved the extrusion of four groups of lavas of both andesitic and dacitic composition as well as
lava domes and one
pumice
Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...
flow. Apparently, the emission of each lava unit was preceded by
explosive eruption
In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such ...
s which formed
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s.
Argon-argon dating on one of these units has yielded an age of 100,000 ± 35,000 years ago.
Radiometric
Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which cha ...
and
surface-exposure dating has yielded ages of between 110,000 and 56,000 years ago for La Poruña; it was previously proposed that this cone formed in the late 19th century. The La Poruña cone formed in two separate eruptions, each of which yielded a long lava flow; the second eruption remains undated. The southwest dome is of comparable age to La Poruña, 107,000 ± 12 years ago. In general, dates obtained by
argon-argon dating range from 168,000 to 68,000 years ago. The entry of
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
magma at depth about 100,000 years ago may have stimulated the occurrence of eruptions not only at San Pedro but also at neighbouring volcanoes.
A large eruption covered parts of the upper edifice with
scoria
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...
about 15,000 years before present. The Plinian eruption was even more recent, occurring about 10,000 years ago (10,085 ± 45
radiocarbon
Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and coll ...
years ago). After this event four small
lava domes formed in the summit region, and partly underwent collapse forming hot avalanche deposits; it is possible that the 1901 eruption was one of these lava dome forming events.
Historical activity
Eruptions at San Pedro are reported from possibly 1885, when a newspaper from
Valparaiso reported "strong shaking of the earth", the emission of large quantities of white steam, and damage to the railway, houses and
water pipe
Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water delivery ...
s.
Other eruptions reported from San Pedro were in 1901 when an eruption caused damage, May–August 1910, 1911, February 1938 and a minor event in December 1960. Additional eruptions are mentioned in records and dated to 1870, 1916, 1917, 1923.
All these eruptions are uncertain and apparently of
phreatic
''Phreatic'' is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption.
Hydrology
The term phreatic (the word originates from the Greek , meaning "well" ...
nature, and no geological evidence of them has been found.
The timing of eruptions at San Pedro sometimes is unclear; one eruption reported from 1877 may instead have occurred in 1891 as there are two reports of an eruption before 1910 and they may either refer to one eruption with two candidate dates or two eruptions. This volcanic activity makes San Pedro one of the highest active volcanoes.
Presently,
fumarole
A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...
s in the eastern
summit crater
A volcanic crater is an approximately circular depression in the ground caused by volcanic activity. It is typically a bowl-shaped feature containing one or more vents. During volcanic eruptions, molten magma and volcanic gases rise from an unde ...
are the only ongoing activity at San Pedro; a steam plume is barely visible. The fumarole has formed
sulfur deposits. Based on measurements made in December 2013, the output of San Pedro is about . Fumaroles were also reported from the Ojos de San Pedro area south of San Pedro volcano, and a report published in 1894 mentions a smell of sulfur at La Poruña.
In addition,
seismic
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
activity has been observed at the volcano.
The history of volcanic activity at San Pedro indicates two principal hazards from future eruptions. The first is the formation of long debris avalanches or
pyroclastic flow
A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s that could reach the which is just away from the volcano. The second hazard is formed by a large explosive eruption resulting in ash fall and the emplacement of ignimbrites; the whole region could be affected if the
eruption column
An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air a ...
is over high. In light of the height of the edifice, partial collapses are also a possibility.
A few
seismic station
A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, Types of volcanic eruptions, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to f ...
s monitor
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
activity at San Pedro.
The Chilean
SERNAGEOMIN
250px, Sernageomin building in Providencia, Santiago.
The National Geology and Mining Service ( es, Servicio Nacional de Geología y Minería; SERNAGEOMIN) is a Chilean government agency. Its function is to provide geological information and adv ...
publishes a volcano hazard level for the volcano.
Climbing and First Ascent
San Pedro is climbed relatively frequently due to its proximity to the tourist town of
San Pedro de Atacama
San Pedro de Atacama is a Chilean town and commune in El Loa Province, Antofagasta Region. It is located east of Antofagasta, some 106 km (60 mi) southeast of Calama and the Chuquicamata copper mine, overlooking the Licancabur volcano. ...
. The easiest route is by the north slopes to the col then by the east slope to the summit.
It was first climbed by George Courty (France) and Filemón Morales (Chile) July 16, 1903.
File:Volcan_San_Pedro.jpg, San Pablo volcano as seen from just below the summit of San Pedro
See also
*
List of volcanoes in Chile
The Smithsonian Institution's Global Volcanism Program lists 105 volcanoes in Chile that have been active during the Holocene.List of Ultras of South America
This is a list of the 209 ultra prominent peaks, or Ultras in South America. An ''Ultra'' is a mountain summit with a topographic prominence of or more.
Guiana Highlands
Sierra Nevada de Santa Marta, Cordillera Oriental, Cordillera de Mér ...
References
Sources
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External links
{{Commons category, San Pedro Volcano (Antofagasta), San Pedro (volcano)
"Volcán San Pedro, Chile" on Peakbagger
Stratovolcanoes of Chile
Active volcanoes
Volcanoes of Antofagasta Region
Six-thousanders of the Andes
Mountains of Chile
Pleistocene stratovolcanoes
Holocene stratovolcanoes