Composite Volcanoes II. Morphology of Composite Volcanoes B. Evolution of Morphology Most composite volcanoes around the world have very similar characteristics. These volcanoes are characterized by a concave-upward profile, which should be considered as the equilibrium or steady-state profile of an active composite cone. Classic textbook volcanoes such as Mount Mayon in the Philippines, Mount Fuji in Japan, and Mount Taranaki (formerly Egmont, New Zealand) exemplify this (Fig. 4). While this is a useful general observation, it is instructive to note that more youthful or nonequilibrium volcanoes have a more pure conical shape with a more restricted base (Fig. 4b). At most composite volcanoes, it is clear that this shape is produced by early lavas being more voluminous and extensive than later lavas (Fig. 4b, parts iii and iv). Certainly, there is a slight concavity to the flanks, but these volcanoes lack the extensive aprons of talus that accentuate the concavity of the slopes. This suggests that mass transfer through destructive processes modifies the shape of a volcano with time from a cone with simple slopes and a restricted base to one with a more concave-upward profile and a wider base (Fig. 4c). Most active volcanoes are equilibrium or steady state. A B     i   ii   iii   iv FIGURE 4 (A) Idealized evolution of the shape of a composite volcano. Initially, a pristine volcano will form a pure conical form with constant slopes and a volume given by Eq. (1). Mass wasting results in transfer of mass from the upper parts of the edifice to the lower flanks, building out a talus apron. The edifice evolves to a steady-state profile with concave-upward slopes and a volume given by Eq. (2). Compare with examples of profile evolution given in (B) and (C), respectively. (B) Young nonequilibrium conical profiles: (i) Klyuchevskoy volcano, Kamchatka, one of the world's most active subduction zone volcanoes and <10,000 years old (view from north); (ii) Nguaruhoe cone (ca. 2500 years old), New Zealand (cf. Fig. 2); (iii) Licancabur volcano, view from west together with (iv) Landsat TM image of Licancabur. Photographs iii and iv show the early more extensive lava flows around the base and a more restricted cone formed from later more restricted lavas, outlined in a white dashed line. C   i   ii   iii   iv FIGURE 4 (continued)   (C) Equilibrium profiles: (i) Taranaki volcano, New Zealand (two views from the north emphasizing classic extensive equilibrium profile of concave-up flanks); (ii) Volcan Misti (Peru) from southeast showing extensive ring plain or talus apron and a developing equilibrium profile; (iii) lahar deposits of the ring plain or talus apron (foreground) of Mount St. Helens. Low-angle aprons of this type of material extend out to several kilometers from the base of the cone. View is from the southeast. (iv) Aguas Calientes from Lascar (central Andes), showing the short, stubby lava flows that have built up the steep upper portions of the cone. Back to Contents