Erosion calderas: origins, processes, structural and climatic control

The origin and development of erosion-modified, erosion-transformed, and er osion-induced depressions in volcanic terrains are reviewed and systematize d. A proposed classification, addressing terminology issues, considers stru ctural, geomorphic, and climatic factors that contribute to the topographic modification of summit or flank depressions on volcanoes. Breaching of a c losed crater or caldera generated by volcanic or non-volcanic processes res ults in an outlet valley. Under climates-with up to similar to 2000-2500 mm annual rainfall, craters, and calderas are commonly drained by a single ou tlet. The outlet valley can maintain its dominant downcutting position beca use it quickly enlarges its drainage basin by capturing the area of the pri mary depression. Multi-drained volcanic depressions can form if special fac tors, e.g., high-rate geological processes, such as faulting or glaciation, suppress fluvial erosion. Normal (fluvial) erosion-modified volcanic depre ssions the circular rim of which is derived from the original rim are terme d erosion craters or erosion calderas, depending on the pre-existing depres sion. The resulting landform should be classed as an erosion-induced volcan ic depression if the degradation of a cluster of craters produces a single- drained, irregular-shaped basin, or if flank erosion results in a quasi-clo sed depression. Under humid climates, craters and calderas degrade at a fas ter rate. Mostly at subtropical and tropical ocean-island and island-are vo lcanoes, their erosion results in so-called amphitheater valleys that devel op under heavy rainfall (> similar to 2500 mm/year), rainstorms, and high-e levation differences. Structural and lithological control, and groundwater in ocean islands, may in turn preform and guide development of high-energy valleys through rockfalls, landsliding, mud-flows, and mass wasting. Given the intense erosion, amphitheater valleys are able to breach a primary depr ession-from several directions and degrade the summit region at a high rate . Occasionally, amphitheater valleys may create summit depressions without a pre-existing crater or caldera. The resulting, negative landforms, which may drain in several directions and the primary origin of which is commonly unrecognizable, should be included-in erosion-transformed volcanic depress ions.