Bull Volcanol (1999) 61:174–193 Q Springer-Verlag 1999 ORIGINAL PAPER D. KarÁtson 7 J.-C. Thouret 7 I. Moriya A. Lomoschitz Erosion calderas: origins, processes, structural and climatic control Received: 4 January 1998 / Accepted: 18 January 1999 Abstract The origin and development of erosion-mod- sion. The resulting landform should be classed as an ified, erosion-transformed, and erosion-induced de- erosion-induced volcanic depression if the degradation pressions in volcanic terrains are reviewed and syste- of a cluster of craters produces a single-drained, irregu- matized. A proposed classification, addressing termino- lar-shaped basin, or if flank erosion results in a quasi- logy issues, considers structural, geomorphic, and cli- closed depression. Under humid climates, craters and matic factors that contribute to the topographic modifi- calderas degrade at a faster rate. Mostly at subtropical cation of summit or flank depressions on volcanoes. and tropical ocean-island and island-arc volcanoes, Breaching of a closed crater or caldera generated by their erosion results in so-called amphitheater valleys volcanic or non-volcanic processes results in an outlet that develop under heavy rainfall (1F2500 mm/year), valley. Under climates with up to F2000–2500 mm an- rainstorms, and high-elevation differences. Structural nual rainfall, craters, and calderas are commonly and lithological control, and groundwater in ocean is- drained by a single outlet. The outlet valley can main- lands, may in turn preform and guide development of tain its dominant downcutting position because it high-energy valleys through rockfalls, landsliding, mud- quickly enlarges its drainage basin by capturing the flows, and mass wasting. Given the intense erosion, am- area of the primary depression. Multi-drained volcanic phitheater valleys are able to breach a primary depres- depressions can form if special factors, e.g., high-rate sion from several directions and degrade the summit re- geological processes, such as faulting or glaciation, sup- gion at a high rate. Occasionally, amphitheater valleys press fluvial erosion. Normal (fluvial) erosion-modified may create summit depressions without a pre-existing volcanic depressions the circular rim of which is derived crater or caldera. The resulting, negative landforms, from the original rim are termed erosion craters or ero- which may drain in several directions and the primary sion calderas, depending on the pre-existing depres- origin of which is commonly unrecognizable, should be included in erosion-transformed volcanic depressions. Key words Erosion caldera 7 Erosion crater 7 Editorial responsibility: C. Newhall Degradation and drainage of volcanoes 7 Volcanic DÁvid KarÁtson (Y) geomorphology Department of Physical Geography, Eötvös University, H-1083 Budapest, Ludovika tÉr 2, Hungary [email protected] Introduction Jean-Claude Thouret1 UniversitÉ Blaise Pascal, Center de Recherches Volcanologiques, 5 Rue Kessler, Clermont-Ferrand Cedex 1, Erosionally formed volcanic depressions, which may be France several kilometers in diameter, are traditionally refer- red to as erosion calderas. The origin and development Ichio Moriya Department of Geography, Kanazawa University, of such depressions have not been studied in detail by Kakuma-machi, 920-11 Kanazawa, Japan volcanologists. Even geomorphologists have paid little attention to the formation and evolution of erosion cal- Alejandro Lomoschitz Departamento de IngenierÍa Civil, Universidad de Las Palmas deras. In general, volcanic landscapes and their evolu- de Gran Canaria, E-35017 Las Palmas, Spain tion are studied infrequently (cf. Francis 1993). As a Present address: consequence, several terms for erosional depressions in 1ORSTOM Instituto Geofísico del Perú, Calle Calatrava 216, volcanic terrains are used in the literature (Thouret Urb. Camino Real, La Molina, Lima 12, Peru 1993, in press). 175 A traditional explanation of erosion calderas, widely ic. For example, a wide range of volcanic, tectonic, and accepted presently, has been given by Ollier (1988). His erosional processes may modify the subsequent shape description and examples of erosion calderas are al- of a volcanic depression. most the same as those in the most comprehensive In this paper we review the common ways in which book on volcanic landforms by Cotton (1952). This ex- erosion calderas form and evolve, the factors that con- planation envisions a pre-existing, closed crater or cal- trol their creation, and how can they be classified. dera that is later breached and eventually enlarged by a radial valley that develops on the external slopes of the volcano. Such an interpretation implies that fluvial ero- sion is the major process determining the subsequent Degradation and drainage of volcanic edifices shape of the volcanic depression. A frequently quoted example of such a basin is Caldera La Palma, presently The flanks and summit region of volcanic landforms named Caldera de Taburiente, Canary Islands (Ollier that are positive and concentric are expected to be 1988). This depression, from which Charles Lyell took deeply dissected and degraded by fluvial erosion. Con- the word caldera for general application, was thought structional edifices, such as composite and shield volca- for more than a century to have been enlarged primari- noes, pyroclastic cones, and lava domes, are typical. ly by headward erosion (Lyell 1855; Middlemost 1970). These forms must be built, at least in part, of imperme- Recent studies, however, attribute the origin of Caldera able rocks to enable the drainage network to develop. de Taburiente mainly to giant landslides (Staudigel and Permeability depends on many factors, mostly endog- Schmincke 1984; Ancochea et al. 1994; Masson 1996), enic ones (geological structure, surface texture, rock so that it is no longer the exemplary type of caldera type, welding, etc.), which are accentuated by exogenic formed by fluvial erosion. agents such as rainfall and frost action. Even elementa- Much has been learned recently about the signifi- ry volcanic landforms show great variability. For exam- cance of volcanic slides and debris avalanches that fre- ple, fresh scoria cones, built of permeable material, are quently occur on active and dormant volcanoes all over usually not liable to fluvial erosion (Bloom 1991), the world. As a result, we now can distinguish between whereas spatter cones, or those composed of welded closed or quasi-closed breached depressions called ero- scoria fall or flow deposits, experience intense gullying. sion calderas and the open or half-open avalanche, am- However, gullying can occur even on coarser-grained phitheater, or horseshoe calderas (Siebert 1984). Ero- material, such as scoria, when rainfall rate is greater sion calderas are formed by long-term, small-scale flu- than infiltration, because erosion depends also on rain- vial processes; horseshoe-shaped calderas are formed fall intensity (Collins and Dunne 1986). by short-term, large-scale debris avalanches or may be After cessation of volcanism, a cone or dome is ex- created or transformed under cold and wet climate by posed to rapid fluvial erosion. The typical course of de- glaciers, such as at Mount McLoughlin, Oregon (Smith gradation (Cotton 1952; Macdonald 1972; Ollier 1988; 1990). Francis 1993) is reviewed herein. Significant erosion Despite this restriction, the origins of many erosion can also take place during the eruptive period (Moriya calderas may be uncertain or, when known, polygenet- 1987; Cas and Wright 1988; Carracedo 1994): Fig. 1 Parasol ribbing on the external slopes of the active Sakurajima volcano (Kyushu, Japan) 176 Fig. 2 Minor fissure formed during the 1959 phreatic activ- ity of Mt. Shinmoedake, Kiri- shima volcano (Kyushu, Ja- pan), has initiated gullying in the neighboring slopes. (Cour- tesy R. Imura, Kagoshima University) 1. In the first, “initial” stage, rill erosion of gullies or This process, called channel capture, is controlled in barrancos dissects the surface and creates a radial the short term by rapid valley incision and behead- (centrifugal) drainage pattern on the external volca- ing and by increased permeability of the eroding ma- no slopes by parasol ribbing (Fig. 1; Cotton 1952). terial (Horton 1945; Dunne 1980; Collins and Dunne Ash avalanches (e.g., on Vesuvius; Cotton 1952; 1986; Dohrenwend et al. 1986). Channel capture is Hazlett et al. 1991) can accelerate this process early, well documented either under semiarid climate depending on material, climatic, and topographic (Cima volcanic field in California; Dohrenwend et conditions, or wave erosion such as at Kaho‘olawe al. 1986) or under temperate to tropical climate (the (Hawaiian Islands; GarcÍa 1990) and VolcÁn Ecuad- MichoacÁn-Guanajuato area in central Mexico; Ha- or (GalÁpagos Islands; Rowland et al. 1994). Vol- senaka and Carmichael 1985). In the Cima field, canic initiation of slope transformation can be ob- where vegetation recovery is slow, channel excava- served on Mount Shinmoedake, Kirishima volcano tion is accelerated by debris flows down the initial (Japan). A minor fissure and the deposition of fine- gullies, and piping may also contribute to intense grained ash, generated during phreatic activity in gully development (Dohrenwend et al. 1986). In the 1959 (Imura and Kobayashi 1991), have initiated long term, the reduction of the drainage network is gullying in the surrounding area (Fig. 2). On Lascar related to vegetation recovery, which hinders fluvial volcano, Chile, the