Arenal Volcano is composed of a hierarchical series of geologic units: unit flow, composite flow, lava field, and lava armor. Volume-limited unit flows are emplaced at short time intervals to make up composite flows. Composite flows form lava fields, and lava fields in turn, constitute the lava armor (the volcano). Tephra and lava breccias are selectively eroded from the steep slopes of the volcano by heavy rains and contribute little to the actual shape of the cone. This constructive process has important consequences for the distribution of the age of lava on a composite cone. We show that lava of significantly different ages may be juxtaposed at all scales from the unit flow, to the composite flow, to the lava field, and to the lava armor. These relations are applicable to the time sequential sampling of a composite volcano.Detailed observations of the dynamic behavior of unit flows indicate that two dimensionless parameters determine the distribution of lava between an active, flowing component and a passive, stationary component. The first parameter, f, is a measure of how much lava the front uses to advance relative to how much it uses to build up levees. The second parameter, q, is the fraction of lava that is able to drain out of the channel when no more lava from the vent feeds the flow. Both parameters have a primary role in determining the final dimensions of a lava flow. These parameters may be calculated from observations of lava flowing onto different topography and at different times after effusion. This data set may allow the prediction of f and q for future flows, and as a consequence, the final flow length along possible flow paths is also predictable.The development of a thermal structure within the flow plays a critical role in the dynamic evolution of a unit flow. The weight of a cold, highly viscous crust at the surface of the flow actively modifies the stress distribution in the flow and controls the rate of processes such as front velocity, levee formation, and growth of surges. We propose that for a given flux of lava there is a critical channel length beyond which the flow accelerates triggering the separation of the flow from its source near the vent. Thus, the unit flows are volume-limited. Based on this hypothesis we derive a relation for the velocity and position of the flow front at any time after effusion has started, assuming the time functions of f, q, and flow rate are known. We find that the length of a unit flow is directly proportional to f, q, and the flow rate and it is inversely proportional to the cross-sectional area of the channel and to the sine of the slope. These relations also hold for composite flows.Finally, by making the approximation that a composite flow grows to a constant slope we derive equations for the evolution of lava fields and the growth of the volcanic structure. These relations explain the asymmetric distribution, areal extent, and slope of the various lava fields at Arenal and allow us to infer the position of buried craters and contacts. Re...
The 1350 years B.P. Big Obsidian Flow (BOF) at Newberry Volcano in central Oregon contains a wide variety of mafic magmatic inclusions. Although little grain size variation is observed within single inclusions, within the suite textures vary continuously from fine‐grained (“quench”) to coarse‐grained (“cumulate”). The BOF inclusion suite, therefore, defies conventional genetic classifications. Detailed petrographic and geochemical observations require a new model for inclusion formation. Only a few of the finer‐grained inclusions have margins suggestive of liquid‐liquid interaction. Interstitial glass content (≈ 20 vol %) and composition (high‐silica rhyolite) are approximately constant throughout the suite. Mineral compositional zoning is greater in the coarse‐grained than fine‐grained inclusions. Whole inclusion compositions vary significantly but do not correlate well with textural diversity. We interpret the BOF inclusion suite to be a heterogeneous sample set of whole inclusions and parts of disaggregated inclusions. The compositional variability of the suite can be explained by a combination of preentrapment (hybridization) and postentrapment (interstitial liquid loss/gain) processes. Preentrapment hybridization involved mixing of basaltic andesite magma similar to Holocene flank flows and up to 30% rhyolitic liquid. The BOF inclusions may demonstrate the recent existence of mafic magma in the Newberry system. Inclusion features characteristic of small ΔT are consistent with the crystal‐free nature of the BOF, suggesting that the rhyolite may have been superheated. If true, a crystal fractionation origin for the BOF rhyolite is unlikely. The disaggregation of grain size and compositionally zoned inclusions, which produced the observed textural diversity in the BOF suite, compounds problems of interpreting mafic inclusions in silicic volcanic rocks and, more so, in plutonic equivalents.
Extreme sedimentation in Swift Creek, located in the Cascades foothills in NW Washington (48°55′N, 122°16′W), results from erosion of the oversteepened, unvegetated toe of a large (55 hectares) active landslide. Deposition of landslide-derived sediment has necessitated costly mitigation projects in the channel including annual dredging and temporary sediment traps in an attempt to reduce the risk of flooding and damage to man-made structures downstream. This study attempts to understand the process of sediment production along with the corresponding erosion rates of the sediment source to help with the development of mitigation plans and construction of optimal sediment reservoirs.The bedload and suspended sediment in the creek are a direct result of the weathering process of the serpentinitic bedrock underlying the landslide. The serpentinite does not weather to smectite clay, as previously thought. Instead, it weathers to asbestiform chrysotile with minor amounts of chlorite, illite and hydrotalcite, all of which occur in clay seeps on the unvegetated surface of the landslide. The chrysotile fibers average 2 mm in length and make up at least 50%, by volume, of the suspended load transported in Swift Creek. This study does not address the environmental or health implications of the asbestiform chrysotile transport or deposition.During the sampled time between February 2005 and February 2006, 127 discrete suspended sediment samples were collected and discharge was measured 66 times. The suspended sediment concentrations ranged from 0·02 g L -1 to 41·6 g L -1 and the discharge ranged from 0·0 m 3 s -1 to 0·5 m 3 s -1. A nonlinear functional model estimated the total suspended sediment flux from detailed precipitation records and discrete suspended sediment concentration and discharge measurements to be 910 t km -2 yr -1 . When the suspended sediment flux is coupled with estimates of downstream deposition of coarse sediment, the estimated erosion rate for the entire Swift Creek landslide is 158 mm yr -1. The majority of the material entering Swift Creek is presumed to originate on the unvegetated toe of the landslide, for which the erosion rate is thus approximately 1 m yr -1 .
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