The cohesive Naranjo debris-flow deposit (10 km 3):
Mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams. A good example of this occurred 18.5 ka ago when the eastern flank of the Nevado de Colima Volcano collapsed producing a debris-avalanche deposit that was previously described as one of the largest in the world. The deposit extended from the volcanic summit as far as the Pacific coast, 120 km away. New stratigraphic, sedimentological, and componentry data suggest that the volcanic collapse of Nevado de Colima resulted in a debris avalanche that traveled 20 km southeast to the Naranjo River. There it crashed against a topographic barrier consisting of Cretaceous limestones (Cerro la Carbonera) and the flow direction was diverted to the south down the Naranjo River channel for another 25 km before the avalanche came to a halt. The obstruction of the drainage produced a temporary dam that stored ca. 1 km3 of water and deposited fluvial and slack-water sediments. Some time after the damming, the accumulated water-sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3.5 million m3/s. The resulting flood (cohesive debris flow) followed the channel of the Naranjo River and, due to the high erodibility of the channel and introduction of substrate material, the debris flow progressively increased its volume up to 10 km3, six times its initial volume. This study highlights the relevance of evaluating the potential remobilization of debris-avalanche deposits to initiate large magnitude cohesive debris flows. Therefore, the hazard and risk analysis of future potential events of this nature must consider the pre-eruption conditions and the topography surrounding a volcano. 7 2002 Elsevier Science B.V. All rights reserved.
Mixed-magma pyroclastic surge deposits associated with debris avalanche deposits at Colima volcanoes, Mexico
The transition between the terminal cones and the ancestral edifices of Nevado de Colima and Fuego de Colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the Mount St. Helens (MSH) or Bezymianny type. Unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune-bedded pyroclastic-surge deposits directly associated with these catastrophic events at both volcanoes. At Nevado, these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano (SiO2∼52.50%). The lavas directly preceding and following the debris-avalanche event are silicic andesites (SiO2∼59%). At Fuego these juvenile fregments have ∼56% SiO2. The lavas from the upper parts of the caldera wall are dacites (∼65% SiO2), whereas the terminal cone is composed of andesites (57% to 62% SiO2). At Nevado, petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high-level magma chamber, just before the eruption of pyroclastic surge deposits, consists of: (1) banded juvenile bombs of intermediate composition; (2) the range of composition of these bombs from SiO2∼52% to ∼58%; (3) the presence of highly magnesian olivine with reaction rims; (4) inverse zoning in clinopyroxene with strong Mg enrichment towards the rim; (5) resorption of plagioclase; and (6) significant compositional heterogeneity in the vitric phase. Volcanic debris-avalanche events at Nevado and Fuego de Colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle. Injection of mafic magma into the presently perched viscous surface dome of the active Fuego cone, as occurred in 1818 and 1913, could enhance the likelihood of southward collapse of the flank of an already unstable edifice, and it must be considered in future hazard assessment of this active volcano. Risk to life and property for the entire Colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913, which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano.
The genus Pseudolithophyllum (Corallinaceae) in the Gulf of Maine
SummaryThis is the third of a series of papers covering the anatomy, morphology, systematics and ecology of the crustose corallines of the Gulf of Maine. Pseudolithophyllum orbiculatum, the only species of this genus in the region, is treated in this paper.