Organization of Volcanoes of the Central Andes

VCA contains descriptions of 44 major volcanoes and several examples of monogenetic centers and large silicic systems. It will be apparent that the descriptions of individual centers are highly variable. Several centers are sufficiently well known that extensive entries were possible, and in some cases selection of the available material to comply with space requirements was a major challenge. Many centers, however, are so poorly known thatVolcanoes of the Central Andes contains their first published description, which, regrettably, is often meagre.

A standard format is adopted for the 44 major volcano entries. Each entry contains a TM image and a statistical block of information on volcano location and physical features which form the essential core of information. Other additional information is presented where appropriate in the text. A summary of the statistical data is given in Table 1. The following information is included in the statistics block and the main text:

Volcano Name and Geographic locality:

Volcanoes of the Central Andes are described beginning in the north, starting in Peru, and progressing southwards through northern Chile, southwestern Bolivia and northwestern Argentina.

As with much of the data in this poorly studied area, even the names of volcanos have been a problem. Because so many volcanoes are located on the frontiers between different countries, different names are sometimes applied to the same volcano by the cartographic agencies of the respective countries. In general, we have used names employed in previous work, or taken what appears to be the most appropriate name from the best available topographic map. In a few cases entirely new names had to be made up, based on nearby volcanic or geographic features, simply to provide a means to refer to previously unstudied or even unknown volcanic features. Many volcanoes have the prefix Volcan or Cerro in their Spanish formulation, but these prefixes have not been included in the statistics block or in the index. In cases where the same name refers to two entries where the prefix is the only means of differentiating (e.g.Volcan andCordon Punta Negra VCA nos. 29 & 31), use of the prefix is unavoidable.

Below each volcano name is the country or countries containing the volcano. For the most part, the modern cordillera (volcanic front) defines the frontier between neighbouring countries so many of the volcanoes straddle the boundary. Relatively few are located wholly within one country.

The sections on monogenetic centers and large silicic systems do not follow the general format of the Volcanoes of the Central Andes entries. In section 3, the monogenetics centers are described in groups which are considered to be similar, i.e. monogenetic fields, scoria cones, maars, and silicic domes etc. The best examples from each of these groups are described, where possible from different regions of the Central Andes. In section 4, entries for the large silicic systems each cover an individual system and follow a general north-south progression. Some overlap between sections 3 and 4 is inevitable due to their close spatial, and sometimes genetic inter-relationships.

Synonyms: In many cases, the dual nationality of many volcanoes, and the use of Quechua and Aymara names by some workers and of Spanish by others, has resulted in the same volcano being called different names. This has led to some difficulties in deciding on a suitable name. Volcanoes of the Central Andes therefore contains a list of synonyms where appropriate.

Location: This is presented in the form of the latitude and longitude for the center of the volcano or complex, or the center of the major volcano of the complex. Locations are taken from available 1:250,000 or 1:50,000 maps and the values usually are given to minutes of a degree.

Type: It soon became apparent in attempting to classify the various volcanoes that a plethora of terms would be needed to account for the various volcanic landforms encountered. However, rather than have a long list of descriptive terms to encompass each nuance of each volcano, we have elected to use the following four categories, based on morphology, evolutionary history and spatial relations of vents, to classify them:

       Simple - Volcanoes with a simple (usually conical) morphology, radial symmetry, and a single summit vent (e.g. Misti, Licancabur).

       Composite - Volcanoes with more than one stage in their evolutionary history represented by different vents, but which still retain their conical shape and an overall radial symmetry. Some show evidence of cone collapse and subsequent healing (e.g. Parinacota, Ollague, San Pedro, Socompa) .

       Compound - A single volcanic massif with a well defined basal region but whose edificeis a coalesence of several individual cones. The term multiple is used by some authors for such volcanoes (i.e. Cotton, 1944).

       Complex - An areally extensive collection of spatially, temporally, and genetically related major and minor centers and associated lava flows and pyroclastic deposits. In most cases the number of discernible vents is given in parentheses.

The last two categories often include several vents aligned in well defined structural lineaments.

Clearly, as in any classification scheme, there is some room for overlap and confusion, but we stress that these terms are only meant to provide a general description on the basis of our definition here. The details given in the body of the text should be used to supplement and refine this classification scheme.

The terms used in the sections of monogenetic centers (section 3) and large silicic systems (section 4) should be familiar and self explanatory.

Elevation: The summit altitude of the volcano, or tallest volcano in a complex, is given in meters above sea level. Heights quoted are taken from published maps. Although quoted to 1 m, there are differences of more than 100 m between heights of the same volcano printed on different map editions, and on maps prepared by the cartographic agencies of different countries. In some cases, heights printed on maps do not appear to relate to the highest summit, but to points identifed photogrammetrically.

Edifice Height: This is the height (or relief) of the volcanic landform above its base level. Substantial errors arise as the base level of a volcano is highly variable due to the regional slope and local topography. Furthermore, the large-scale of the available maps (100 m contours) makes the choice of the base level of an individual volcano subjective. The data presented are our best estimates of edifice height, with errors of more than 100 m in many cases.

Category: Although each of the volcanoes is thought to be potentially active, several levels or states of activity are represented. Four categories are used:

       1 - Active: Records of magmatic or phreatomagmatic eruptions within the last 10 years.

       2 - Fumarolic :Persistent fumarolic activity at present day

       3 - Latent: Morphological evidence of very recent activity or records of historic activity (i.e Llullaillaco).

       4 -Dormant: No historic records of activity or morphological evidence of very recent activity, but with some evidence of Holocene activity.

Once again there is room for overlap between two categories. The general absence of historic records means that the divisions are largely based on our subjective analysis of morphological features, and field observations by ourselves and others.

Structure and Evolution: A basic description of the morphology and structure of each volcano is given and where possible this is used to outline an evolutionary history for the center. Volcanoes of the Central Andes is mostly concerned with the Holocene activity of the volcanoes (activity since the 10,000 yr BP glacial regression), but in a few cases some aspects of important pre-Holocene activity (i.e. cone collapse events) are also presented. Given the general absence of radiocarbon dates from Central Andean volcanoes, a stringent chronology of activity is not possible.

Where records exist, dates of historic activity of a volcano are presented.Usually however, in the absence of historic records for much of the region, we simply provide our assessment of the most recent (morphologically youthful) activity from the center.

Current State: A statement of current (1990) activity of the volcano.

Hazard: Many of the volcanoes in the region are situated in remote, unpopulated areas, where volcanic hazard assessment would be an academic excercise with little societal relevance. However, a few volcanoes, particularly those in the northern part of the region, offer considerable hazard to local communities. A statement of likely hazard is presented where relevant.

Geochemistry: Many of the recent studies done on the volcanoes of the Central Andes have been largely petrological in nature. Most of these have been reconnaissance studies, but a few detailed studies now exist. While a wealth of petrographic and geochemical data is available for some volcanoes, the quality and quantity of data for individual centers are highly variable. The data presented in Volcanoes of the Central Andes are a summary of what is available, which is often little or nothing.

References: We have attempted to cite all references to individual volcanoes which are meaningful from a volcanological viewpoint, and which we have been able to locate. Others of which we are not aware doubtless exist in the Spanish literature. We have cited some peripheral literature, such as botanical studies made on some volcanoes.