![]() 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. At the cone surface, the lithofacies found associated with the main vent are of two main types. 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. 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. This association is defined as lithologies related to long-term vents at which volcanic products are erupted to the surface. 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. 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 control on the distribution of volcanogenic products is largely a result of this competition and is illustrated schematically in Fig. 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). 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).įinally, the effects of surface conditions (climate/weather), discussed in the previous section, will have a profound effect on the redistribution of volcanogenic material. Lava flows in steep summit regions or on upper flanks may also sustain gravitational instability collapse of their flow fronts, producing hot block avalanches. Dome talus landslides or avalanches may on occasion supply more extensive block-and-ash flows. The domes are commonly active, growing by slow addition of magma to the interior and periodically gravitational collapsewhich may or may not be accompanied by explosions. Lava domes, a common feature of evolved composition volcanoes, may dominate vent region morphologies over long periods of timeoccupying either craters (e.g., Soufrière and Montserrat) or sector-collapse scar amphitheaters (e.g., Mount St. The morphology of the summit region can have a significant effect in directing volcanogenic flows, particularly if there are distinct crater breaches. 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.įIGURE 8 Broad classification of volcanogenic products (lavas vs volcaniclastic rock types, distinguished by shading of boxes) at composite volcanoes. ![]() ![]() In the latter category, the surface morphology controls the distribution of lava flows. 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 this respect, the role of volatiles is critical ("Volatiles in Magmas"). 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 propertiesdensity and viscosityof the magma. The controls on the nature and distribution of erupted products are twofold-magmatic differentiation processes and syneruptive modulation by interaction with the surface environment. Characteristics and Distribution of Volcanogenic Products at Composite VolcanoesĬomposite volcanoes are characterized by a greater diversity of volcanogenic products than any other volcanic landform (Fig. The Encyclopedia of Volcanoes - Chapter 43 ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |