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.
The Colima volcanic complex, Mexico:III
A 34 meter section of ash- and scoria-fall units has been studied on the upper NE flank of the active Volcán Colima. Charcoal and soil horizons are restricted to the topmost 12 m. Nine 14C dates show a smooth progression from 235 years near the surface to 8,300 years at 10 meters depth, indicating a post-Pleistocene accumulation rate of 1.3 m/1,000 years at 6 km from the vent. This figure allows an estimate of the magmatic eruption rate for fallmaterial of 0.31 km3/1,000 years, less than 15% of Colima's lava eruption rate. An unusually thick and coarse-grained scoria-fall horizon at about 4 m depth in the section appears to have been produced by the caldera-forming eruption of Colima some 4,300 years ago.The majority (40) of 46 analyzed scoria and ash horizons are typical Colima hornblende-andesites with an average SiO2 content of 58%, nearly identical to the scoriae of Colima's 1913 pyroclastic flows. The scoriae are significantly more basic than Colima's andesitic lavas, which average 61% SiO2. The six remaining scoria horizons are anomalously alkalic and rich in incompatible elements. Five coarse alkaline scoria layers occur in sequence just below the 8,300 year level. They show progressive upward increases in K, P, Ba, Sr, Zr, La, Ce, and related elements, culminating in a phlogopite-bearing scoria horizon.Over the last 20,000 years or so, a group of cinder cones erupted 20–35 km to the north of Volcán Colima, producing basic alkalic magmas including basanites and phlogopite-bearing minettes. The alkaline scoriae of the studied sections probably record pre-eruptive injections of minette magma into the subvolcanic, calc-alkaline system of V. Colima. The age, composition, and mineralogy of the alkaline scoriae are consistent with this interpretation. Least squares mixing models suggest 60 wt.% minette component in the phlogopite-bearing horizon.
The Colima volcanic complex, Mexico: Part II. Late-quaternary cinder cones
Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO2, 7.4–15.3% MgO, 2.5–4.4% K2O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba≦ 4,200 ppm, Sr≦3,100 ppm, Zr≦ 550 ppm, Ce≦190 ppm, Hf ≦18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes.Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO2, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO2). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend.The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events.During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcán Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcán Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.