Composite Volcanoes

II. Morphology of Composite Volcanoes

F. Changes in Vent Locations through Time

The classic conical shape owes its form to the constancy of vent location over a protracted period of time. This ensures that volcanic products, and therefore mass, are always added from the summit area and decrease radially away from it. However, many volcanoes show evidence that activity has migrated with time (Table II, Fig. 6). Several interesting questions arise. For instance, it is not clear whether activity migrates in the same direction with time throughout the region, in response to migration of the volcanic front as a whole, or whether it is segment specific or even random.

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FIGURE 6 More complex or compound composite cones, illustrating effects of local vent migration through time. (a) Lascar from the north, an extensive composite cone that is elongate east to west due to migration of the vent region. (b) Aerial photograph of Lascar clearly showing the nested craters resulting from vent migration (north to top of page). (c) Irruputuncu (Chile/Bolivia), another typical nonconical, ridgelike composite cone (view from the west). The oldest part of the cone is the left. (Photograph courtesy of Gerhard Worner; see also color insert.) (d) Coropuna, Peru, a compound volcano made up of at least five different overlapping cones. (e) Nevados de Payachata, central Andes, comprising twin volcanoes (view from southwest). The older glaciated Pomerape cone is to the north (left), while Parinacota is the young, more symmetrical cone. The hummocky terrane in the foreground is a debris avalanche deposit. (See also color insert.) (f) Klyuchevskoy group volcanoes, Kamchatka, view from the southeast. The symmetrical Klyuchevskoy cone in the foreground and the vapor-shrouded Bezymianny volcano (which collapsed in 1956 and now consists of a large active dome in the amphitheater) in the distance are both active, while the central peak (Kamen), which shows the scarp of a collapse event, is inactive.

Some volcanoes are twin systems comprising two quite distinct but neighboring composite volcanoes. Examples include San Pedro-San Pablo and the Nevados de Payachata in the central Andes (Fig. 6e). Typically, twin volcanoes reflect a vent shift such that one of the two is active, leaving its twin extinct. Examples are found, however, such as Bezymi anny and Klyuchevskoy volcanoes in Russia (Fig. 6f), where neighboring volcanoes are concurrently active. Others are elongate amalgamated edifices with a migrating summit complex. Examples of these include the following: in the central Andes, Aucanquilcha, Lascar (Figs. 6a and 6b), Irruputuncu (Fig. 6c), Coropuna (Fig. 6d), and Ojos del Salado, the world's highest active volcano; in New Zealand, Ruapehu (Fig. 2); in Turkey, Ararat; in Kamchatka, Zhupinovsky. Tongariro (Fig. 2) is an extreme example of this as it is a cluster of as many as 30 relatively small edifices. These compound volcanoes offer even greater challenges of interpretation than on "simple" cones. The complex geometries of nested and overlapping cones make determination of individual cone volumes difficult as we commonly do not see basement exposed and the preexisting topography is unknown. Volcanic stratigraphy is similarly problematic to unravel, particularly in light of the close proximity (in both space and time) of potential source vents.

Subsidiary vents, commonly called satellite, parasitic, or flank vents, are common features at many composite volcanoes. The general use of the term parasitic should, however, be discouraged as it implies an often-false dependence on the main volcano. These subsidiary vents are typically in the form of small monogenetic volcanoes like cinder cones or domes (Figs. 1 and 7). Such features are commonly arranged along lineaments and are interpreted as reflecting the influence of faults along which magma can migrate laterally away from the main magma chamber or, more commonly, vertically from a separate magma batch. In some cases, these satellite vents occur as part of an areally extensive field spatially associated with the main volcano as at Mount Adams in Washington State or Payun Matru in Argentina. Geochemically, the erupted products of flank eruptions are commonly found to be significantly different from those of the main vent. Typically, they are more primitive but they can show a wide range in compositions—those of the main vent tend to be more restricted in their range. These observations suggest that in these systems the main vent is supplied by a steady-state magmatic system, whereas flank eruptions might be supplied by separate small magma batches that do not intercept the main magma chamber. In the case of the volcanic fields peripheral to the main cone (e.g., Mount Adams, Washington, U.S.A.; Payun Matru, Argentina; Mount Ararat, Turkey), the more mafic magmas that typically erupt from the satellite vents are probably prevented from ascending beneath the main cone by its subjacent magma storage system.

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FIGURE 7 Examples of satellite centers and their relation to composite volcanoes: (a) San Pedro composite volcano (central Andes) and La Poruna cinder cone from west (arrowed), together with (b) thematic Mapper image of San Pedro and La Poruna (arrowed). Note the very large lava flow that erupted during the formation of La Poruna. North is to the top, and the image is 15 km x 15 km. (c) Ground view (from the north) of monogenetic dacite flank domes (arrowed) on Ollague volcano and (d) Landsat image of Ollague volcano, central Andes, with dacite flank domes of (c) arrowed.


GlossaryIntroductionDistribution of Composite VolcanoesMorphology of Composite VolcanoesEvolution of MorphologyFactors Controlling MorphologyDegradationChanges in Vent Locations through TimeLifetimes of Composite VolcanoesCharacteristics and Distribution of Volcanogenic Products at Composite VolcanoesConcluding Remarks and Future Research Directions

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