Yesterday, April 22^nd, 2015 (ironically the Earth Day) the Calbuco Volcano in Chile erupted. Let’s take a look to what volcanoes are and what are then main ones in the Patagonia region.

HOW VOLCANOS ARE FORMED

The inner structure of the Earth is made of three concentric layers: core, mantle and crust. The outermost part of the crust is called lithosphere, and there is where the geological processes occur. The core is an alloy of iron, solid in the centre and partially molten on the exterior. The mantle is made of molten rocks, with different temperatures and density; that creates a convective material circulation, where the hot material down deep the mantle upwells, while the cooler and heavier material sinks downward. This upward moving of hot rock material is called mantle plume and the molten material is the magma.

When the magma reaches the lithosphere, which is rigid, it flows below it, accumulating in big deposits called “magma chambers”. Typically the plume acts over the lithosphere like a blowtorch, so the latter cracks and tears apart, letting the magma reach the exterior; that process is called and eruption, the main volcanic phenomenon.

The shape of a volcano is determined by the kind of eruption occurred. When we think of eruptions two kind of images spring to our mind:

- Lava flowing over the Earth’s surface: that is an effusive eruption.

- Explosions with a huge cloud of column of ashes: that is an explosive eruption.

On effusive processes, when the lave reaches the surface it cools down, hardens in flows and scoria forming layers and shaping the volcano. On explosive processes the materials ejected on the explosions accumulate around the pipe, building the volcanic edifices. The greater volcanoes are made by the combination of these two types of eruption.

On each of these kinds of eruptions (effusive and explosive) there are different categories, going from 0 to 8: Hawaiian (0), Strombolian (1 and 2), Vulcanian (3 and 4) and then the mega-eruptions: Plinian (5 and 6) and Ultraplinian (7 and 8), where the column of ashes can reach 20 to 25 Km high and the lightest material is transported hundreds and even thousands miles away.

When we talk about volcanoes we always think about a cone, but there are many different forms, that are directly related with the kind of magma and lava material and the type of eruption suffered.

On shield volcanoes the edifice has soft gradients, resulting from abundant highly flowing lava that cover long distances while being not very thick. As time goes by the flows overlap. These volcanoes, resulting of and effusive eruption have a long diameter and low altitude. One great example, although not strictly in Patagonia, is Payún Matru, in Mendoza, a big shield volcano.

Stratovolcanoes have a conical shape, and are the result of the combination of effusive and explosive eruptions. They reach high altitudes, with flank vents proportionally short. They are very common in subductions areas, where two tectonic plates collide and one move under the other. A clear example is our emblematic Lanín Volcano.

Other shape that we can find is the volcanic calderas, which are circular depressions of great dimensions (5 to 24 Km) on the volcano summit. These are formed after a large explosive eruption, when the underlying chamber, partially empty, cannot support the weight of the upper part of the volcanic edifice, so it collapses leaving a huge crater.

On the Patagonian region we can find that type of structure on Caldera del Agrio, Caldera de Caviahue-Copahue, Caldera de Pino Hachado, Domuyo and Tromen Volcanos (all in Neuquén province), the Chubut River Volcanic Complex (Piedra Parada, Chubut) and the Chon Aike Formation on the north of Santa Cruz, on the Patagonian Atlantic coast.

There are also domes, which are bulbous masses of lava. These slowly extrude and, almost solid due to its high viscosity, cannot flow and accumulates over the vent blocking it. These can be found in different forms: inside the crater, linked to a different volcanic structure, forming domes fields or isolated.

Lastly, the piroclatic cones are small conical edifices, formed by the accumulation of lava fragments ejected to the atmosphere (pyro clasts) from one sole crater (monogenic); these fragments accumulate around a volcanic vent. As well as the domes, they can be associated with a different volcanic structure, forming fields or isolated.

In Patagonia, we can find them in almost every volcano in the Neuquén province, in the Basaltic Crater Volcanic Field in Chubut and on the Palei Aike Volcanic Field in Santa Cruz, near Rio Gallegos.

VOLCANES IN PATAGONIA

Volcanoes in Patagonia, both Argentinian and Chilean, are part of the so called “Ring of Fire”. This includes 452 volcanoes, the 75% of the volcanoes, both active and inactive of the Earth, over and horseshoe-shaped extension of approximately 40,000 Km over the Pacific Ocean. The Ring of Fire us the result of the movement of the tectonic plates over the crust of the Earth.

In Patagonia, on the Eastern side of the Ring, is the consequence of the subduction of the Nazca plate which is entirely oceanic) and the South American plate, that is part on the continent and part in the Atlantic Ocean. Both plates collide and the South American one moves under the Nazca one. This is how the Andes were formed.

From the subduction area to the west, a volcanic arch arises over the Andes mountain range. On the interior side of this arch there is a depressed area where there are also plenty of volcanoes, as the lithosphere stretches out and being thinner cracks so the magma passes through the surface. This area is called Bac-arc region, as is just behind the volcanic arc, and these volcanos are Back-Arc volcanos.

In the Argentinian Patagonia the major volcanos on the volcanic arc are the following, most of them are considered active, as there are records of eruptions on the last 10.000 years.

DOMUYO VOLCANO (36°38´ S, 70°26´ W - Neuquén - 4702 m)

The highest mountain in Patagonia, it is a stratovolcano associated with a large caldera of 15 Km of diameter and other eruptive centres. Chenque Mallín volcano, of 3,555 is the highest of this group.

TROMEN VOLCANO (37°08´ S, 70°03´ W - Neuquén - 4114 m.)

Another stratovolcano formed during the Holocene. The summit rises in between of two calderas. An eruption was reported in 1822, although it hasn’t been confirmed.

TROLON COPLEX (37°44′ S, 70°54′ W - Neuquén - 2500 m.)

A lava dome complex formed in the Plestocene-Holocene, NE of the Caviahue caldera. It has two craters and a pyroclastic cone.

COPAHUE VOLCANO (37° 51´ S, 71° 12´ W - Neuquén, Chilean border – 2,953 m.)

It is a compound cone, formed on the edge of the Caviahue caldera (20 x 15 Km.). Nine aligned craters have been identified, the easternmost being active and containing and acid and hot lake of 250 of diameter. It shows an intense fumarole activity. Several geothermal areas are located inside the caldera, 7 Km away from the active crater. More than 12 eruptions were reported on the last 250 years, the most recent ones in 1992, 1995 and 2000.

LANÍN VOLCANO (39° 38´S, 71° 30´ W - Neuquén, Chilean border, 3,776 m.)

A mainly effusive stratovolcano. It belongs to a group of volcanoes including Villarrica, in Chile. It was formed in 4 eruptive cicles that stated on the Pliocene-Pleistocene. The last 2 occurred during the Plesitocene-Holocene. A small dome on the summit fed lava flows approximately 2,200 years ago and some others 1,600 ago. Lanín was reported active after an earthquake in 1906, although there a not records to confirm this.

HUANQUIHUE COMPLEX(39° 53´S, 71° 35´ W - Neuquén - 2189 m.)

A chain of stratovolcanoes formed during the Pleistocene. An ash cone with 3 craters of 400 m. of diameter and a pyroclastic cone built on the Epulafquen Lake. A recent lava flood (200 years ago) coming from the base of the ash cone of Achen Ñiyeu flowed north falling into the Epulafquen Lake, forming a prominent lava delta. The eruption that produced this flow is still remembered by the local residents, from the stories told by their ancestors.

MOUNT TRONADOR (41° 10´S, 71° 53´ W - Río Negro, Chilean border - 3478 m.)

Its activity dates back to the Early Pleistocene. The well-preserved cone was the origin of a lava flow after the last glaciations, dated 70,000 to 14,000 years ago.

VIEDMA VOLCANO (49°21.5′ S, 73°17′ W - Santa Cruz - 1500 m.)

The eruptive centre of the Viedma volcano is sub-glacial, with an eruption confirmed in 1988. Just the oldest part of the volcano emerges over the ice-caped surface. There are 4 big craters or calderas, between 1,5 and 4 Km. wide.

The following volcanoes are located on the Back-Arc area:

BASALTIC CRATER VOLCANIC FIELD (42°1’S, 70°11′ W - Chubut – 1,359 m.)

Covering a 700 km^{2 area}, 9 ash and scoria cones produced lava spillage. One of its main features are the cone in Cerro San Fermin and the highest one, Cerro Negro.

PALEI AIKE VOLCANIC FIELD (52°0′ S, 70°0′ W - Santa Cruz - 282 m.)

This volcanic field shows 3,000 km² of lava, maars and scoria cones associated with recent lava spills (Pleistocene to Holocene). The earliest eruptions produced maars and lava flows with are now exposed on the river valleys. The most recent cones and lava flows are SE of the volcanic field and are also exposed.

SOURCES:

GESVA Website – Buenos Aires University)

“Volcanes, Nacimiento -Estructura- Dinámica”, Eduardo J. LLambías

(Vazquez Mazzini Editores, 2009)

National Geograpic Institute (www.ign.com.ar)

Grupo de Estudio y Seguimiento de Volcanes Activos Website (www.gesva.gl.fcen.uba.ar)