Shapes of chondrules determined from the petrofabric of the CR2 chondrite NWA 801

The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X-ray computed tomography and best-fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro-deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3-D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation.     

Accuracy of Flowmeters Measuring Horizontal Flow in Fractured-Rock Simulators

Laboratory evaluations of flowmeter response to flow in fractured-rock simulators are needed to improve understanding of data collected in field settings. The ability of flowmeters to accurately measure the velocity and direction of water flowing between parallel plates was used as a surrogate for instrument response in fractured-rock aquifers. A colloidal borescope flowmeter and a heat-pulse flowmeter were deployed in a fractured rock simulator with 4-inch and 6-inch inner-diameter, uncased wells with 0.39- and 1.0-inch fracture apertures and groundwater velocities from 35 to 975 ft/d. The colloidal borescope measurements and applied velocities were positively correlated in all wells and apertures (the coefficient of determination [r²] = 0.61–0.89) and most accurately measured direction at higher velocities. The mean directional error in colloidal borescope measurements was less than 17° in 6-inch wells and 31° in the 4-inch wells at velocities between 92 and 958 ft/d. Heat-pulse flowmeter measurements were 0.001 to 0.004 times less than applied rates and may indicate that water was moving around rather than through the instrument's integrated packer. The mean directional error of heat-pulse flowmeter measurements were about 18 and 42° in the 0.39- and 1.0-inch fractures, respectively, for groundwater velocities within the manufacturer's suggested range of application (0.5–100 ft/d). Measurements made at vertical increments and fracture positions in the well using the colloidal borescope indicate that laminar flow occurs within the central 50% of the fracture but measurements above or below are likely affected by eddy currents.     

Statistical analysis of factors affecting landslide distribution in the new Madrid seismic zone, Tennessee and Kentucky

More than 220 large landslides along the bluffs bordering the Mississippi alluvial plain between Cairo, Ill., and Memphis, Tenn., are analyzed by discriminant analysis and multiple linear regression to determine the relative effects of slope height and steepness, stratigraphic variation, slope aspect, and proximity to the hypocenters of the 1811-12 New Madrid, Mo., earthquakes on the distribution of these landslides. Three types of landslides are analyzed: (1) old, coherent slumps and block slides, which have eroded and revegetated features and no active analogs in the area; (2) old earth flows, which are also eroded and revegetated; and (3) young rotational slumps, which are present only along near-river bluffs, and which are the only young, active landslides in the area.

Discriminant analysis shows that only one characteristic differs significantly between bluffs with and without young rotational slumps: failed bluffs tend to have sand and clay at their base, which may render them more susceptible to fluvial erosion.

Bluffs having old coherent slides are significantly higher, steeper, and closer to the hypocenters of the 1811-12 earthquakes than bluffs without these slides. Bluffs having old earth flows are likewise higher and closer to the earthquake hypocenters.

Multiple regression analysis indicates that the distribution of young rotational slumps is affected most strongly by slope steepness: about one-third of the variation in the distribution is explained by variations in slope steepness. The distribution of old coherent slides and earth flows is affected most strongly by slope height, but the proximity to the hypocenters of the 1811-12 earthquakes also significantly affects the distribution.

The results of the statistical analyses indicate that the only recently active landsliding in the area is along actively eroding river banks, where rotational slumps formed as bluffs are undercut by the river. The analyses further indicate that the old coherent slides and earth flows in the area are spatially related to the 1811-12 earthquake hypocenters and were thus probably triggered by those earthquakes. These results are consistent with findings of other recent investigations of landslides in the area that presented field, historical, and analytical evidence to demonstrate that old landslides in the area formed during the 1811-12 New Madrid earthquakes.

Results of the multiple linear regression can also be used to approximate the relative susceptibility of the bluffs in the study area to seismically induced landsliding.     

Bifurcation of volcanic plumes in a crosswind

Bent-over buoyant jets distorted by a crosscurrent develop a vortex pair structure and can bifurcate to produce two distinct lobes which diverge from one another downwind. The region downwind of the source between the lobes has relatively low proportions of discharged fluid. Factors invoked by previous workers to cause or enhance bifurcation include buoyancy, release of latent heat at the plume edge by evaporating water droplets, geometry and orientation of the source, and the encounter with a density interface on the rising path of the plume. We suggest that the pressure distribution around the vortex pair of a rising plume may initially trigger bifurcation. We also report new experimental observations confirming that bifurcation becomes stronger for stronger bent-over plumes, identifying that bifurcation can also occur for straight-edged plumes but gradually disappears for stronger plumes which form a gravity current at their final level and spread for a significant distance against the current. Observations from satellites and the ground are reviewed and confirm that volcanic plumes can show bifurcation and a large range of bifurcation angles. Many of the bifurcating plumes spread out at the tropopause level and suggest the tropopause may act on the plumes as a density interface enhancing bifurcation. Even for quite moderate bifurcation angles, the two plume lobes become rapidly separated downwind by distances of tens of kilometers. Such bifurcating plumes drifting apart can only result in bilobate tephra fall deposits. The tephra fall deposit from the 16 km elevation, SE spreading, bifurcating volcanic plume erupted on 15 May 1981 from Mt Pagan was sampled by previous workers and clearly displayed bilobate characteristics. Examples of bilobate tephra fall deposits are reviewed and their origin briefly discussed. Bilobate deposits are common and may result from many causes. Plume bifurcation should be considered one of the possible mechanisms which can account for come examples of bilobate tephra fall deposits.     

Biological and physical events involved in the origin,effects, and control of organic matter in solar saltworks

Aspects of communities and events in the concentrating ponds (S.G. 1.130 to 1.214) and salt crystallizing ponds (S.G. 1.215 to 1.264) of solar saltworks pertinent to salt manufacture are described. Communities that aid salt manufacture enable continuous and efficient production of high quality salt at a saltworks' design capacity, and they provide important controls on levels of organic matter in the brine. Fluctuating salinities, high concentrations of nutrients, and petroleum products are disturbances that causeAphanothece halophytica andDunaliella salina to release excessive quantities of organic matter, and that suppress or cause death to nutrient stripping organisms. Disturbances result in decreased quality and quantity of salt and increased costs for salt harvest, washing, and pond upkeep. Organic matter can be controlled by management techniques that keep nutrient stripping communities at proper levels and maintain a narrow and unchanging range of salinities in each pond, by constructing pond dikes able to withstand wind and water erosion, and by preventing spills of petroleum products in the ponds.