Composite Volcanoes II. Morphology of Composite Volcanoes C. Factors Controlling Morphology Notwithstanding the complexities discussed above, the primary shape of a composite volcano is conical. Many of the controls on this shape can be understood by considering the geometry of a cone. The volume v of a simple cone is given by (1)  , where r is the radius of the base and h is the height of the cone. For a typical steady-state volcano with the concave-upward profile, a more complex exponential form is needed: (2)  . B and M are constants and the volume has to be found by integration. Below we address the main factors that control the morphology of composite volcanoes. D. Aggradation While there is a strong dependency of edifice height on the volume of a composite cone, there also appears to be a limit to the size of composite cones. Edifice heights (base to summit ) of composite volcanoes rarely exceed 3000m and the vast majority of arc-related composite volcanoes are between 2000 and 2500m. Volumes are similarly restricted; notwithstanding the difficulty in measuring accurate volumes, maximum volumes for arc-related composite volcanoes are around 200 km³ (e.g., Mount Adams and Mount Shasta in the Cascades of the United States). Intraplate composite cones may attain larger volumes. One explanation for this is that the conical form exerts a fundamental control on the growth of a volcano. The relationships in Eqs.(1) and (2) suggest that if the conical form is to be maintained, every additional increment in height requires a huge additional increase in volume—the additional volume has to be distributed all over the cone. At some point, this becomes prohibitive to further growth. However, closer examination of volcanoes reveals that the height increment does not necessarily always involve the large volume increment suggested by the relationships described in Eq. (2), because in many cases the increase in height is achieved by adding material to the uppermost parts of t he edifice only. In many cases the uppermost parts of composite cones are characterized by small and stubby lava flows or domes reflecting their evolved nature and attendant high viscosity and yield strength (Fig. 4c, part iv). Effusion of these lavas builds up the edifice without much new volume being added and results in the steep slopes that characterize the uppermost slopes. The consistency of edifice heights and volumes in comparable tectonic settings around the world suggests that some simple geophysical relation must exist to limit the size of composite volcanoes. Several factors can affect the height to which a composite volcano can grow. Among these are the nature of the volcanic products, the duration of magma supply, differentiation of the magmas, and the crustal density profile. Magma supply is certainly an important control because the largest composite volcanoes are intraplate volcanoes such as Mount Ararat, Turkey; Mount Damavand, Iran; and Mount Kilimanjaro, Kenya. Here magma production and supply rates are higher than those associated with arcs and the resulting edifices are much larger. However, even though magma supply rates and other factors are likely to be different between regions or arcs, they may be close to constant for a given region, arc, or portion of an arc. Mature composite volcanoes in comparable tectonic settings are still similar in size the world over. Most eruptions from composite volcanoes are driven by the hydrostatic "head" or, more appropriately, the overpressure (that in excess of lithostatic (Fig. 5) in the magma reservoir. Geophysical studies of active volcanoes commonly reveal the presence of shallow (5-10 km) magma storage zones beneath active volcanoes. Petrologic studies confirm shallow reservoir depths as most composite volcanoes are characterized by eruptions of porphyritic magmas dominated by plagioclase. Furthermore, most volcanic products also show evidence of extensive magma mixing—a phenomenon that is best understood in the context of shallow magmatic systems. While it is debatable whether a shallow magma reservoir exists throughout the entire lifetime of the volcano, it is clear that one must exist during the actively erupting stages. Therefore it is reasonable to assume that most eruptions from composite volcanoes occur from shallow magma reservoirs and therefore that the overpressure required to erupt material from the chamber is unlikely to vary much. The consistency of the ballistics of volcanic bombs from composite volcano eruptions supports this contention. As a volcano grows, two factors may conspire to decrease the eruption rate. First, the growing mass of the volcano increases the lithostatic load on the shallow magma chamber and eventually overcomes the hydrostatic head. Inflation and deflation of volcanic edifices are often modeled as the result of a shallow source of deformation (a Mogi source), suggesting strong feedback between the edifice and the magma reservoir. Second, the distance the magma has to ascend to the surface increases. It should be clear from Fig. 5 that if h and the load are increased, there will be a limit (observational evidence suggests this is about 3000 m of edifice height) above which it will be physically unlikely that further lava can be erupted from the summit. Two general observations support this reasoning. First, early lavas tend to be more voluminous and extensive than later lavas. Second, early lavas are commonly less evolved than later lavas. The first directly relates growth to changing effusion rate, and the second implies that the critical overpressure (Pex > Plith ) can only be maintained if the reservoir magma becomes more evolved and consequently less dense, but more viscous as the volcano grows. Only the higher viscosity of the lower density, evolved lavas permits the condition Pex > Plith to be maintained—the more evolved lavas typically have lower magma densities due to removal of dense crystalline phases and can sustain greater overpressures on vesiculation since the higher viscosities impede volatile outgassing. This control is well illustrated in Fig. 4b, parts iii and iv. Here, the youthful cone of Licancabur is built from shorter younger flows radially disposed around the vent, overlying older more extensive flows. The true equilibrium profile has not yet developed because there has been insufficient time to significantly degrade the flanks and develop a talus apron. A dramatic decrease in density (with consequent increase in overpressure) can also be achieved by vesiculation of the magma. This condition will depend on magma composition (volatile species, solubility, and content) and on its capacity to achieve supersaturation through depressurization, which is ultimately also controlled by the parameters illustrated in Fig. 5. FIGURE 5 Schematic diagram showing the relation between lithostatic pressure P and eruption driving overpressure Pex in the magmatic system of an active volcano. The lithostatic pressure P changes with the mass of the growing volcano and the depth to the magma chamber h. Eruption can take place as long as Pex remains greater than P. At h2, the edifice has grown to the critical height beyond which Pex is less than lithostatic and therefore eruption from the summit is no longer viable. Back to Contents