From the Caribbean coast of Columbia to the frigid fjords of Patagonia, the Andean cordillera extends in an unbroken range; in places it is a single, narrow spine; in others, a crumpled web of several different ranges. Extending for about 7500 km, the cordillera forms one of the longest continuous mountain chains on Earth. Dozens of peaks along its length exceed 6000 m altitude. Many of them are potentially active volcanoes.

The Andean plate margin provides arguably the best developed example of continental margin magmatism in the world. Subduction of the oceanic Nazca plate beneath the western margin of the continental SouthAmerican plate has resulted in three principal zones of active volcanism: the Northern Zone (in Ecuador and Columbia), the Central Zone (southern Peru, northern Chile, southwestern Bolivia, and northwestern Argentina), and the Southern Zone (southern Chile and southern Argentina). Because they are often densely populated, the northern and southern zones are both relatively well known. Although much remains to be learned, at least the identities and locations of the major volcanoes have been documented. By contrast, the Central Volcanic Zone of the Andes remains one of the largest but least known areas of active volcanism in the world.

The Central Volcanic Zone of the Andes is located between latitudes 14ºand 28ºS of the Andean cordillera (Figure 2). An elevated region, much of it over 4,000m in altitude, constituting the altiplano of Bolivia and puna of north Chile and Argentina, dominates much of this zone (Figure 2). This high altitude plateau is second in size only to the great Tibetan plateau of Central Asia, and like the latter, is built on thickened continental crust which attains a maximum thickness of almost 70 km (James, 1971). Volcanism is largely
restricted to the margins of this remarkable physiographic province in theCordillera Occidental or Western Cordillera, with a few isolated examples in the Cordillera Oriental or Eastern Cordillera in Bolivia. So little is known about this area that the majority of the volcanoes have never been described, and in a few cases, have not even been named. 16 of them were considered to be "active" when the existing Catalog of Active Volcanoes of the World was published (1963-1966).

Eruptive: Eruptive volcanic activity.
Active: Records of magmatic or phreatomagmatic eruptions within the last 10 years
Fumarolic: Persistent fumarolic activity at present day.
Latent: Morphological evidence of very recent activity or records of historic activity.
Dormant: No historic records of activity or morpholgical evidence of recent activity, but with some evidence of Holocene activity( last 10000 years).
Unknown: No records or studies done.

At 6885 m, this is the world's highest active volcano, although the edifice itself is not large -- its basement is at about 4500 m, roughly the level of the camera. Nevado Ojos del Salado is a compound or multiple volcano, consisting of a massif covering some 70 square kilometers formed of at least a dozen andesitic and dacitic ones intermingled with lava domes and craters. The volcano has never been studied in detail. It has not erupted in historic times, though it exhibits continuing fumarolic acitivity. White material in gullies is snow; the greyer material is pumice from a large but undated plinian eruption.

This 6150 m high andesite-dacite composite volcano is an exceptionally well-exposed example of numerous similar volcanic constructs around the circum-Pacific "Ring of Fire." It is a large volcano, rising from a base at just over 3000 m. It consists of two essentially different units. At left, the reddish brown, snow-covered peak forms part of the "Old Cone," constructed mostly of basaltic andesite lavas, thin flows of which can be seen near the summit. At some unknown time, this cone failed, producing a massive débris avalanche deposit, not seen in the photo. Subsequently, a large dome of hornblende dacite lavas accumulated, forming the greyish massif on the right side. This dome became very steep, and many flow fronts of the lavas failed, yielding the visible scars, and forming an apron of hot avalanche debris, the upper parts of which are visible (pink toned) immediately below the lava scarps. Fumarolic activity continues to the present day on the summit of the dome. The foreground and middle distance is mantled with a plinian pumice fall deposit, creating a very smooth surface.

Volcan Láscar 5,641 m, is the most active volcano in the central Andes. Since 1988 a silicic lava dome has been growing in the summit crater, and there have been several recent eruptions, notably on April 19 and 20, 1993, when pyroclastic flows were erupted on both southern and northern flanks, and ash fell out as far distant as Buenos Aires,Argentina. The volcano consists of elongate series of six overlapping craters, trending roughly northeast, with the active, fuming crater located near the center of this cluster. It is about 800 m in diameter and 300 m deep. Grey pumice deposits from the 1993 eruption are visible in this photo, and the margins of some pyroclastic flows can be seen at right center. Most impressive, however, are the older, massive andesitic lava flows, exhibiting flow margins tens of meters high, well developed flow levées and transverse ogive ridges.

Remarks

Volcan Lascar is the only active volcano that had a part in the selection of sites, it is located on the Southern edge of the Chajnantor plain and to the East of the Salar de Atacama. From previous eruptions and known wind conditions, it seems that all manner of ash goes to the East over towards the Argentinian border. There are numerous other volcanos in the the first, second and third regions but these have been inactive and were usually located at high altitudes (5500m -6000m) and were covered with old lava fields, since one of the criteria in the search was that no mountains could have lava on them, this made these areas unsuitable for any sites. Very few of the proposed sites are located near a volcano.