Composite Volcanoes

IV. Characteristics and Distribution of Volcanogenic Products at Composite Volcanoes

A. Introduction

Composite volcanoes are characterized by a greater diversity of volcanogenic products than any other volcanic landform (Fig. 8). The controls on the nature and distribution of erupted products are twofold--magmatic differentiation processes and syneruptive modulation by interaction with the surface environment. In the former category are included the melting and differentiation processes discussed earlier in this volume (e.g., "Composition of Magmas," "Origin of Magmas," "Volatiles in Magmas," and "Magma Chambers") that determine the physical properties—density and viscosity—of the magma. In this respect, the role of volatiles is critical ("Volatiles in Magmas"). High volatile concentrat ions increase the propensity for eruption as fragmented magma (pyroclastic) relative to lava, although the influences of effective magma viscosity and ascent rate will ultimately determine the occurrence and extent of explosivity. In the latter category, the surface morphology controls the distribution of lava flows. As gravity-driven phenomena, both pyroclastic and lava flows are typically channeled into valleys, producing a complex interplay of erosion and construction that leads to frequent topographic inversions on volcano flanks.


FIGURE 8    Broad classification of volcanogenic products (lavas vs volcaniclastic rock types, distinguished by shading of boxes) at composite volcanoes.

The morphology of the summit region can have a significant effect in directing volcanogenic flows, particularly if there are distinct crater breaches. Lava domes, a common feature of evolved composition volcanoes, may dominate vent region morphologies over long periods of time—occupying either craters (e.g., Soufrière and Montserrat) or sector-collapse scar amphitheaters (e.g., Mount St. Helens). The domes are commonly active, growing by slow addition of magma to the interior and periodically gravitational collapse—which may or may not be accompanied by explosions. Dome talus landslides or avalanches may on occasion supply more extensive block-and-ash flows. Lava flows in steep summit regions or on upper flanks may also sustain gravitational instability collapse of their flow fronts, producing hot block avalanches. Sustained dome growth can result in a significant fraction of the upper part of a composite cone being formed entirely of one or more domes (e.g., Bezymianny).

Finally, the effects of surface conditions (climate/weather), discussed in the previous section, will have a profound effect on the redistribution of volcanogenic material. In fact, perhaps the single most important element in defining volcano morphology is the balance between construction (a function of magmatic flux to the surface) and erosion/weathering (a function of local climate). The control on the distribution of volcanogenic products is largely a result of this competition and is illustrated schematically in Fig. 9a.

While Fig. 9a usefully summarizes the distribution of volcanogenic products around a composite cone, perhaps the most helpful way to discuss the materials themselves and how they vary in nature and distribution relative to vents and topography is by use of the facies concept traditionally used to describe sedimentary environments. The specific associations that characterize a composite cone can be defined largely on the basis of distance from the vent, which accounts for both the relative predominance of immediate volcanic products near the vent, versus recycled or mixed (with water) volcaniclastic material, and topographic effects with slope steepness typically decreasing away from vents (Fig. 9b). A summary is provided in Table V.

B. Main Vent Association

This association is defined as lithologies related to long-term vents at which volcanic products are erupted to the surface. Typically, for a simple cone this would be a central/summit vent, but in cases where compound cones have been developed, this may be an inaccurate description. The most important defining characteristic is that it is a long-term feature, and for a given volcano, it may change location several times during the history of activity. At the cone surface, the lithofacies found associated with the main vent are of two main types. On the one hand, if the vent takes the form of a crater, it is commonly surrounded by (1) vent-filling breccia comprising disaggregated wall rock material from the vent and (2) coarse primary tephra, commonly welded due to proximity to the vent. The distribution of this material is concentric to the vent, thinning rapidly away from the vent region, where it may simply pinch out (interfingering with proximal cone-building facies association) or grade into well-defined tephra layers or even lava flows of the upper cone. In cases where the vent crater is filled by a crater lake, additional deposits such as lacustrine sediment, intracrater lahars, and phreatic/phreatomagmatic deposits may be an important element of the main vent lithofacies. On the other hand, the vent at the cone surface can be occupied by a lava plug or dome that for ms a steep-sided morphological feature occupying or completely overfilling and obscuring a preexisting crater depression. Main vent domes are vulnerable to collapse if they overfill the summit crater [e.g., Mount Unzen (Japan), Soufrière (Montserrat)]. They are also vulnerable to destruction during later eruptions when located over the volcano's main vent. The lava dome itself is commonly surrounded by carapace breccia and breccia aprons—which again may be continuous with upper cone slope facies. In either case, the subsurface vent association is defined by a concentration of brecciated material and by an intensity of hydrothermal alteration. The latter reflects both the intense supply of heat and volcanic gases that define the vent and the permeability of the vent lithologies to vapors and fluids over long periods of time--commonly extending beyond the period of eruptive activity for the cone.

Shallow intrusions comprising various geometries, including plugs, dikes, and sills, reflect the focusing of magma into the vent conduit and its movement upward through the edifice. The main vent association at shallow subvent depths (referred to as "hypabyssal") may also host concentrations of metal ores (cf. "Mineral Deposits Associated with Volcanism"), which can be exposed by modest erosion in the deeper portions of the edifice and in the subedifice basement between the surface and the original magma chamber.

(a) 

(b)


FIGURE 9    (a) Schematic illustration of the distribution of deposits relative to vent for a composite volcano. "Completeness of volcanic record" refers to the number of events recorded in a succession at a given location. The most complete record is preserved in tephra that are deposited over wider areas. However, on the slopes of the volcano, tephra are rapidly reworked and may be washed away. Thus a more complete, albeit condensed, section is typically found in the region immediately surrounding the volcano—the ring plain. (b) Schematic illustration of lithofacies associations for a typical composite cone, showing types of deposits-compare with distribution of deposits in (a).

C. Cone-Building Association

The lithofacies of this association are those that build and define the edifice itself. They include a diverse array of volcanic products ranging from lava flows to pyroclastic flows to the products of flow transformations or reworking, involving mixing of water with volcaniclastic material. The relative distribution of lithofacies is largely dependent on (1) the style of activity—which we have seen is determined mainly by the composition of the magma supplied to the volcano—and (2) the balance between the supply of volcanogenic material (eruption frequency) and the rate of erosion. If erosion is able to produce a well-developed drainage pattern between eruptions, then many of the erupted products will be channelized and form ribbon-like bodies radial to the main vent(s). As a result, there may be common stratigraphic inversions—younger flows occupy valley floors while older flows form the canyon walls at higher elevations. Topographic inversions may also occur—if valleys are effectively filled by lava flows that are resistant to erosion, then subsequent incision may take place on former ridges.

TABLE V    Lithofacies Associations for Typical Composite Volcanoes


If erosion is less effective and the edifice is a relatively smooth-surfaced cone, then, at least for low-viscosity erupted products (basaltic lavas, pyroclastic flows, and hyperconcentrated flows), more widely distributed wedge-shaped veneers will form, again focused at the vent(s) of origination. The distribution of fall tephra is less influenced by the cone topography, but rather will form a veneer with a thickness that is greatest near the vent and strongly influenced by wind directions during eruption.

Lava flows, and their associated autobreccias, originate principally from the main vent, where, in the case of basaltic flows, they are typically the result of quiet effusion from a summit crater (overspill of a lava lake) or rapid accumulation and congelation of fire fountain spatter (clastogenic flows). More silicic andesite and dacite flows for m more prominent radial ridges by virtue of their greater viscosities and tend to be topography-formers rather than topography-fillers, as in the case of their more fluid basaltic counterparts. They may be short, stubby coulees, connected to main vent domes, or may be rootless if the steep uppermost portions of the cone cause detachment of the main mass of the flow from its source.

Pyroclastic flows are also dispersed radially from the main vent. Given a well-established topography—that is, deep radial valleys—they will tend to become valley fills; otherwise they may form veneers over quite large angular segments of the flanks. Such flows may originate as column collapse from vulcanian or plinian eruptions or as dome/plug collapses in the vent region. Mixing with stream water in channels or snow and ice on the cone surface may lead to flow transformations, so that the ratio of primary pyroclastic flow deposits to hyperconcentrated flows (lahars and debris flows) typically decreases away from the vent (Fig. 9a). Unmodified pyroclastic flow deposits, as with pyroclastic fall tephra, are also rapidly reworked and removed from the upper parts of the cone to be redeposited low on the cone or on the ring plain.

D. Ring Plain Association

The lithofacies of the ring plain are dominated by fall tephra and by the tephra that has been rapidly reworked from the cone. The ring plain is defined as the area immediately surrounding the volcano, but not including the constructional edifice itself. In cases where the cone is isolated from neighboring volcanoes or other mountainous terrain (e.g., Taranaki and Ruapehu, Fig. 2), the definition is quite obvious. However, in many volcanic terranes, individual edifices merge with their neighbors, including ancestral degraded volcanoes, and the surrounding topography is far from flat so that classic "ring plain" deposits as discussed here will rather be concentrated into drainages, reworked and removed from the system quite rapidly. Furthermore, for many oceanic composite cones such as those of island arcs, the immediate edifice is surrounded by ocean and a true ring plain does not exist—although quite complete records of activity can be recovered from the tephra mantling the nearby ocean floor.

As emphasized in Fig. 9, the ring plain is potentially where the most complete record of explosive activity is preserved. It is sufficiently close to the volcano that the products of volcanism are not too widely dispersed, yet it is beyond the edifice, which, through its topography, has the effect of promoting rapid removal of volcanic products and concentrating them into narrow drainages. Lithofacies include numerous tephra layers—pyroclastic fall material that may be correlative with flow-forming events on the cone but that has been dispersed over much larger areas. The thicknesses of individual tephra layers reflect both the flux and duration of an eruption and the dispersal control by wind. Lateral variations in the thicknesses of individual layers (isopachs) and in the grain sizes of tephra within the layers can be used to accurately reconstruct eruption histories (cf. "Plinian Eruptions" and "Tephra Fall Deposits").

Interbedded with fall tephra are the fluvial and laharic deposits, which may form the distal edges of wedgelike fans of debris from the lower cone slopes or may be concentrated along well-established drainage channels through the ring plain. The distal edges of the most extensive of flows (lava and pyroclastic) from the cone may also reach as far as the ring plain (e.g., Fig. 4b, parts iii and iv).

Atypical, albeit occasional, product of composite volcano activity is the debris avalanche—the widely dispersed large-volume product of large-scale gravitational instability, or sector collapse (cf. "Debris Avalanches"). Debris avalanche deposits are a common ring plain lithofacies, distinguished by a hummocky topography, with the size of hummocks decreasing away from the edifice from large toreva blocks at the slope base to the more subtle topography of flow transition lahars and debris flows that drain the deposit far out on the ring plain (e.g., Fig. 6e). The hummocks consist of blocks mobilized from the cone slopes, which have suffered various degrees of disruption. At the base of the deposit, the lithologies are strongly disrupted and smeared, whereas in the interiors of many blocks disruption is minimal and a jigsaw texture is observed among the components reflecting intense fracturing but no turbulent techanical mixing.

E. Satellite Vent Association

While by no means ubiquitous at composite cones, satellite vents are nevertheless common. As pointed out above, the magma compositions erupted at such vents are commonly distinct from those of the main vent that characterize the bulk of the edifice. The eruptions are also typically monogenetic—a single eruptive episode rather than prolonged and repeated eruptions of the main vent. It should come as no surprise then that there may be distinct lithofacies associations at satellite vents. Scoria cones are defined by the steep piles of scoria that are accumulated at the angle of repose about the vent crater. A high flux of pyroclast accumulation around the vent can lead to the formation of spatter ramparts or even limited clastogenic lava flows. However, most lava flows associated with scoria cones (and some may be quite extensive; Fig. 7b) breach the cone and may raft large fragments of the cone walls along on the flow.

If a basaltic satellite vent interacts with near-surface water, then maars and tuff rings may form during phreatomagmatic eruptions. The water may occur as shallow groundwater reservoirs or in standing bodies of water such as lakes. In either case interception of shallow water by ascending magma is more likely to occur on the shallow slope lower reaches of the edifice, or on the ring plain, rather than on the cone itself. The common occurrence of accretionary lapilli in such phreatomagmatic deposits is also consistent with the important role for water.

Lava domes may also occur as monogenetic eruptions at satellite vents (e.g., Figs. 7c and 7d). These occurrences, by virtue of their location, have a higher preservation potential than those emplaced at the main vent. Silicic magma batches supplied to flank locations are typically small and degassed, resulting in slow and passive effusion of lava. The earliest phases of emplacement may be volatile-charged, resulting in emplacement within a pumice cone—an effect analogous to effusion of basalt lava from a cinder cone. As with basaltic equivalents, tuff cones of silicic tephra may also be found where silicic magma supplied to flank vents intercepts shallow near-surface water reservoirs.


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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