Cerro Blanco (caldera)

Cerro Blanco (, "White Hill") is a caldera in the Andes of the Catamarca Province in Argentina. Part of the Central Volcanic Zone of the Andes, it is a volcano collapse structure located at an altitude of in a depression. The caldera is associated with a less well-defined caldera to the south and several lava domes.

The caldera has been active for the last eight million years, and eruptions have created several ignimbrites. An eruption occurred 73,000 years ago and formed the Campo de la Piedra Pómez ignimbrite layer. About 2,300 ± 160 BCE, the largest known volcanic eruption of the Central Andes, with a VEI-7, occurred at Cerro Blanco, forming the most recent caldera as well as thick ignimbrite layers. About of tephra were erupted then. The volcano has been dormant since then with some deformation and geothermal activity. A major future eruption would put nearby communities to the south at risk.

The volcano is also known for giant ripple marks that have formed on its ignimbrite fields. Persistent wind action on the ground has shifted gravel and sand, forming wave-like structures. These ripple marks have heights up to and are separated by distances up to . These ripple marks are among the largest on Earth and have been compared to Martian ripple marks by geologists.

Geography and geomorphology

The volcano lies at the southern margin of the Argentine Puna, on the border between the Antofagasta de la Sierra Department and the Tinogasta Department in the Catamarca Province of Argentina. Trails run through the area, and there are abandoned mining operations. Provincial Route 34 (Catamarca) between Fiambalá and Antofagasta de la Sierra runs past Cerro Blanco. The volcano is sometimes known as Cerro Blanco, meaning "white hill" in Spanish, and sometimes as Robledo; the Smithsonian Institution uses the latter name.

Calderas and lava domes

Cerro Blanco lies at an elevation of and consists of four nested calderas with discontinuous borders, fallout deposits, lava domes and pyroclastic deposits. The two inconspicuous El Niño and Pie de San Buenaventura calderas are nested in the northern part of the complex and form a wide depression; El Niño is sometimes referred to as a scarp. Only their northern margins are recognisable in satellite images; their southern parts are filled with block-and-ash flows from the southern calderas. The southern calderas are the Robledo and Cerro Blanco calderas, which form a southeast-northwest trending pair. Alternative interpretations consider the Pie de San Buenaventura, Robledo and Cerro Blanco calderas as one caldera, that the Robledo and Cerro Blanco calderas are one system or envisage the existence of only three calderas.

The Cerro Blanco caldera is about wide and its walls are up to high. They are formed by ignimbrite breccia, ignimbrites and lava domes cut by the caldera margins. The caldera floor is almost entirely covered by block-and-ash flows, apart from an area where hydrothermal activity has left white sinter deposits. A slight circular uplift on the caldera floor may be a cryptodome.

The caldera has an almost perfectly circular outline with the exception of the southwestern margin which is cut by a wide lava dome. This dome is also known as Cerro Blanco or Cerro Blanco del Robledo and reaches a height of above sea level. Three additional lava domes surround this dome, and an explosion crater lies to its southwest. West of this crater there are three pinkish lava domes lined up in west-southwest direction away from the main dome; these are surrounded by pyroclastic cones and depressions.

Owing to erosion, the Robledo caldera is less well defined than the Cerro Blanco caldera. A site southeast of the Robledo caldera is known as Robledo. South of the Robledo caldera lies the Portezuelo de Robledo mountain pass, the south-eastward trending El Médano plain and the Robledo valley.

About northeast of Cerro Blanco lies a wide and deep vent known as El Escondido or El Oculto. It does not have a strong topographic expression but is conspicuous on satellite images as a semi-circular patch of darker material. Gravimetric analysis has found a number of gravity anomalies around the caldera.

Surrounding terrain

The terrain northeast-east from Cerro Blanco is covered by its ignimbrites and by Plinian fallout deposits which radiate away from the calderas. Cerro Blanco lies at the southwestern end of the Carachipampa valley, a volcano-tectonic depression flanked by normal faults which extends to Carachipampa. This depression appears to have formed in response to north-south tectonic extension of the Puna and is covered by volcanic deposits from Cerro Blanco. These volcanic deposits form the "Campo de Pedra Pomez" and extend away from the volcano. To the north, the El Niño scarp of the El Niño caldera separates the Cerro Blanco caldera from the Purulla valley.

Other valleys are the Purulla valley northwest from Cerro Blanco and Incahuasi due north; all three contain both volcanic deposits from Cerro Blanco and salt flats or lakes. In the Incahuasi valley an ignimbrite also known as the "white ignimbrite" reaches a distance of over . Wind has carved deep channels into the ignimbrites.

Aeolian landscapes

One of the most spectacular aeolian landscapes is found at Cerro Blanco, where large wind-formed ripple marks occur. These ripples cover Cerro Blanco ignimbrites and reach heights of and wavelengths of , making them the largest ripples known on Earth and comparable to similar ripple fields on Mars. Wind-driven erosion of ignimbrites has generated the ripples, which consist of gravel, pebbles and sand and are covered with gravel. Smaller gravelly ripples lie atop the larger ripples and troughs and there are intermediate sized forms ( high); they may be precursors to the large ripples and make up most of the ripples in the fields. Their wind-driven movement is fast enough that trails abandoned four years before are already partly covered with them.

The ripple marks cover areas of about or in the Carachipampa and or in the Purulla valley. A field of large ripples covers an area of in the Purulla valley and is accompanied by yardangs; this field is also the place where the largest ripples occur.

Various wind-dependent mechanisms have been proposed to explain their large size, including the presence of roll vortexes, Helmholtz instability-like phenomena, atmospheric gravity waves or creep-like movement when pumice fragments and sand are lifted from the ground by wind and fall back. The latter view envisages that undulating terrain triggers the development of ripples through the accumulation of gravel and sand at such undulations. Their formation appears to be influenced by whether the rock material available can be moved by wind while a role of the bedrock structure or the size of the material is controversial.

Wind has also formed demoiselles and yardangs in the ignimbrites. These are particularly well expressed in the Campo de Piedra Pomez area southeast of the Carachipampa valley, a area where yardangs, hoodoos and wind-exposed cliffs create a majestic landscape. The structures reach widths of and heights of and form an array-like assembly. They have fluted surfaces. The yardangs appear to form beginning from either a pre-existing topographic elevation or a fumarolic vent where the rock has been hardened, and eventually develop through a series of early, intermediate and late yardang forms as wind and wind-transported particles erode the rocks. Their layout may be influenced by regional tectonics, pre-existent topography and the patterns formed by the ignimbrite deposits. Exposed rocks are often covered with brown, orange or beige desert varnish and sometimes are oversteepened and collapse.

Bedrock ridges are cut into ignimbrites of the Incahuasi valley. This terrain gradually leads over into the megaripple-covered surface through an increased gravel cover. The development of these megaripples appears to have been influenced by the underlying bedrock ridges which move along with the overlying ripples. These bedrock ridges are formed through erosion by wind and by wind-transported particles, it is not clear how they are then exposed from the ripples. Additional aeolian landforms in the region are known and include ventifacts and so-called "aeolian rat tails"; these are small structures which form when erosion-resistant rock fragments slow wind erosion in their lee, thus leaving a tail-like area where less rock is eroded. Wind streaks occur in groups.

The Campo de Piedra Pómez makes up the , a protected area of Catamarca Province. It was among the finalists in the "Seven Wonders of Argentina" contest but was not selected when the results were announced in 2019.

Regional

Cerro Blanco is located south of the southern end of the Filo Colorado/Los Colorados mountain range and at the eastern end of the . The Cordillera de San Buenaventura marks the southern margin of the Puna and extends west-southwestwards from Cerro Blanco to the volcanoes San Francisco and Falso Azufre and the Paso de San Francisco. It marks the boundary between the steep subduction to the north from the shallower subduction to the south.

A series of andesitic to dacitic stratovolcanoes ranging in age from 1 to 6 million years old make up the Cordillera de San Buenaventura, and Quaternary basaltic volcanoes are dispersed over the wider region. In the surroundings of Cerro Blanco lies the Cueros de Purulla volcano north and the Nevado Tres Cruces-El Solo- Ojos del Salado complex farther west.

Geology

Subduction of the Nazca Plate beneath the South America Plate occurs in the Peru-Chile Trench at a rate of . It is responsible for the volcanism in the Andes, which is localised in three volcanic zones known as the Northern Volcanic Zone, Central Volcanic Zone and Southern Volcanic Zone. Cerro Blanco is part of the Andean Central Volcanic Zone (CVZ), and one of its southernmost volcanoes. The CVZ is sparsely inhabited and recent volcanic activity is only poorly recorded; Lascar is the only regularly active volcano there.

The CVZ extends over the Altiplano-Puna where calc-alkaline volcanism has been ongoing since the Miocene. Characteristic for the CVZ are the large fields of ignimbritic volcanism and associated calderas, chiefly in the Altiplano-Puna volcanic complex. In the southern part of the CVZ such volcanic systems are usually small and are poorly studied. During the Neogene, volcanism commenced in the Maricunga belt and eventually shifted to its present-day location in the Western Cordillera. Tectonic processes also took place, such as two phases of east-west compression; the first was in the middle Miocene and the second began 7 million years ago.

Volcanism in the southern Puna region initiated about 8 million years ago and took place in several stages, which were characterised by the emplacement of lava domes and of ignimbrites such as the 4.0–3.7 million year old Laguna Amarga-Laguna Verde ignimbrites. Some of the domes are located close to the border with Chile in the Ojos del Salado and Nevado Tres Cruces area. Later there also were mafic eruptions, which generated lava flows in the Carachipampa and Laguna de Purulla area. The late mafic eruption products and the Cerro Blanco volcanics are geologically classified as making up the "Purulla Supersynthem". From the Miocene to the Pliocene

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