Capelinhos 1957–1958, Faial, Azores: deposits formed by an emergent surtseyan eruption

The 1957–1958 eruption of Capelinhos, Faial island, Azores, involved three periods of surtseyan, hydromagmatic activity: two in 1957 and one in 1958. Deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally >1 km from the vent. Five lithofacies are identified: lithofacies I is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout. Lithofacies II consists of beds with internally discontinuous lenses, and has sand-wave structures that increase in abundance toward the outer margins of the tuff cone. This lithofacies is interpreted as having been deposited from pyroclastic surges. Lithofacies III is composed of mantle-bedded deposits with laterally discontinuous internal stratigraphy. This lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges. In the outer flanks of the tuff cone, lithofacies III grades laterally into fallout beds of lithofacies I. Lithofacies IV consists of alternating beds of coarse ash aggregates and non-aggregated fine ash, and is particularly well developed in distal regions. Some of this facies was formed by fallout. Alternating beds also occur plastered against obstacles up to 2 km from the vent, indicating an origin from wet pyroclastic surges. The orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated. The distinction between surge and fallout in distal regions is uncertain because wind-drifted fallout and decelerating surge clouds can generate similar deposits. Lithofacies V consists of scoria lapilli beds interpreted to be fallout from hawaiian-style fire-fountaining in the later stages of the eruption. Juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular (25–60% of clasts <30% vesicles). However, on both micro- and macroscopic scales, there is a wide range in clast vesicularity (up to 70% vesicles) indicating that, although fragmentation was predominantly hydromagmatic, vesiculation and magmatic-volatile-driven fragmentation operated simultaneously.     

An unconditionally stable hybrid pseudodynamic algorithm

There is a significant motivation to implement an unconditionally stable scheme in the pseudodynamic test method. As more complex experiments with many degrees of freedom are tested, explicit time integration methods limit the size of time step on the basis of the highest natural frequency of the system. This is true even though the response of the structure may be dominated by a few lower frequency modes. The limit on step size is undesirable because it physically increases the duration of a test, but more importantly, because the number of steps to completion increases and error propagation problems increase with the number of steps in a test. In addition, incremental displacements within each step become smaller, introducing the potential for problems associated with stress relaxation. An unconditionally stable algorithm allows the time step to be selected to give accurate response in the modes of interest without regard for higher mode characteristics.     

Shear zones within the Bloody Bluff fault zone: Analysis and tectonic implications

The Bloody Bluff fault zone, which divides the New England Avalon zone and Nashoba zone, contains at least two shear zones that are within Avalonian rocks. The Rice Road shear zone (sinistral, strike-slip) affects the Westboro Formation and is intruded by the 630 Ma Dedham Granite. The Rice Road shear zone, and equivalent pre-granite mylonites appearing in drill cores, parallel the terrane boundary, and may have controlled the later mylonitization. The Nobscot shear zone (dextral, strike-slip) is a prograde shear zone cutting a granite assumed to be related to the surrounding 630 Ma plutons. Similar shear zones have been seen cutting Late Proterozoic plutons in the New England Avalon zone, and represent a series of en echelon strike-slip shears. The Burlington mylonite zone (shear sense equivocal) is part of the terrane boundary. This is a retrograde shear zone that forms the southeastern border of the Wolfpen lens, a lenticular body of sheared and altered metamorphic and intrusive rock that has been assumed to be part of the New England Avalon zone. Microstructural characteristics indicate that the Burlington mylonite zone was active after the Nobscot shear zone. In particular, quartz in the Nobscot shear zone was dynamically recrystallized by a combination of grain boundary migration and rotation recrystallization processes, thought to occur during shearing at upper-greenschist conditions. In contrast, quartz in the Burlington mylonite zone was recrystallized predominantly by rotation recrystallization, indicating lower-greenschist, retrograde, deformation. The two shear zones are too close for these differences to be a result of a simple thermal field gradient.While mineral assemblages in most of the study area indicate no metamorphic grade higher than upper-greenschist temperatures, the Wolfpen lens contains amphibolites with assemblages formed at temperatures above the oligoclase isograd, indicating mid-amphibolite facies metamorphism. As metamorphic contrast is one of the key features differentiating the Nashoba zone from the New England Avalon zone, the Wolfpen lens cannot be assumed to be part of Avalon. It may be a small block of rocks of intermediate grade between the two terranes.     

Effect of salinity and temperature on survival and development of young zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels

We reared larval zebra mussels,Dreissena polymorpha, and quagga mussels,D. bugensis, through and beyond metamorphosis (settlement) at salinities of 0–8‰. Juvenile zebra mussels gradually acclimated to 8‰ and 10‰ have been reared at these salinities for over 8 mo. Tolerance to both higher temperatures and higher salinities increases with larval age in both species (though zebra mussel embryos and larvae have a greater degree of salinity tolerance than quagga mussel embryos and larvae). Thus, only 6% of 3-day-old zebra mussel veligers survived after exposure to 4‰ for 8 additional days, whereas there was 22% survival of veligers placed in 4‰ at day 13 and grown to settlement 11 d later. Zebra mussel pediveligers, acclimated to increasing salinity in 2‰ increments beginning at day 23, continued to survive and grow in 8‰ after 5-mo exposure, though the growth rates of these juveniles were significantly less than those of juveniles reared in lower salinities. Quagga mussels did not metamorphose and settle as quickly as zebra mussel pediveligers. No quagga mussel pediveligers had settled before exposure to artificial fresh water (AFW), 2‰ 4‰, 6‰, and 8‰ on day 30. Percent settlement of these quagga mussel juveniles (based on 100% survival at the start of experiments on day 30) was 90% in AFW, 67% at 2‰, 69% at 4‰, 46% at 6‰, and 0.1% at 8‰.     

New estimates of rapid sea-floor spreading rates and the identification of young magnetic anomalies on the East Pacific rise, 6° and 11° S

Detailed surveys of the crest of the East Pacific Rise at 6° and 11°S form the basis for estimates of sea-floor spreading rates of 8.2 and 8.3 cm/yr, respectively. These estimates are significantly higher than previous ones of 6.5 to 7.5 cm/yr. The high quality of the magnetic profile at 6°S allows identification of shorter magnetic events such as the two Olduvai events of Cox. DSDP drill-hole data indicate that the present spreading rates have persisted throughout the Cenozoic.     

Mechanisms of high-temperature, solid-state flow in minerals and ceramics and their bearing on the creep behavior of the mantle

The problem of applying laboratory silicate-flow data to the mantle, where conditions can be vastly different, is approached through a critical review of high-temperature flow mechanisms in ceramics and their relation to empirical flow laws. The intimate association of solid-state diffusion and high-temperature creep in pure metals is found to apply to ceramics as well. It is shown that in ceramics of moderate grain size, compared on the basis of self-diffusivity and elastic modulus, normalized creep rates compare remarkably well. This comparison is paralleled by the near universal occurrence of similar creep-induced structures, and it is thought that the derived empirical flow laws can be associated with dislocation creep. Creep data in fine-grained ceramics, on the other hand, are found to compare poorly with theories involving the stress-directed diffusion of point defects and have not been successfully correlated by self-diffusion rates.We conclude that these fine-grained materials creep primarily by a quasi-viscous grain-boundary sliding mechanism which is unlikely to predominate in the earth's deep interior. Creep predictions for the mantle reveal that under most conditions the empirical dislocation creep behavior predominates over the mechanisms involving the stress-directed diffusion of point defects. The probable role of polymorphic transformations in the transition zone is also discussed.     

Scale Dependencies of Vegetation and Topography in a Mountainous Environment of Montana

This research examines the effects of spatial scale on estimating the relationship between vegetation biomass and topography within a portion of Glacier National Park, Montana. The Reflectance/Absorptance vegetation index, developed from processed Landsat Thematic Mapper digital data, is related to three topographic variables obtained through processed Digital Elevation Models: elevation, slope angle, and slope aspect. R² values between the vegetation index and the topographic variables are obtained from regression analyses at a series of aggregated spatial scales. The effective range of spatial scales within which the two sets of variables are spatially dependent and the degree of the spatial dependencies are characterized through semivariance and fractal analyses.     

Ground-Water Tracing with Injected Helium

Helium has several characteristics that make it attractive for use as a tracer in hydrological studies. Two types of experiments were conducted to investigate applicability of helium as a tracer of ground-water movement. The experiments included studies using laboratory sand and soil columns and field ground-water tracing in a basaltic aquifer. A water helium analyzer comprised of a thin quartz glass membrane and diode ion pump (making use of the preferential permeation of helium through the quartz glass into an evacuated space) was developed and used for the experiments. Results of our studies demonstrated that breakthrough curves of specific conductance and helium were similar under saturated conditions. In the unsaturated sand/soil columns, breakthrough curves of helium were retarded relative to specific conductance reducing the usefulness of helium as a tracer.     

An annual fossil record of production,planktivory and piscivory during whole-lake manipulations

Annual and subannual paleolimnological records of pigments and zooplankton were used to analyze three whole-lake manipulations. The relative abundance of cladoceran remains in recent seasonal laminae was significantly correlated with the relative abundance of species in the plankton (r ²=0.59,P<0.001). Comparison of food-web structure after piscivore introduction showed that there are multiple outcomes of predation and that final food-web structure depended on the strength of interaction between piscivorous and planktivorous fish. Intense predation on cyprinids (Phoxinus eos, P. neogaeus, Umbra limi) by largemouth bass (Micropterus salmoides) allowed large herbivores (Daphnia pulex) and invertebrate predators (Chaoborus punctipennis) to dominate. Analysis of fossil invertebrate morphology suggested that small grazers (Bosmina longirostris, Diaphanosoma birgei) were eliminated by invertebrate predators. Under moderate predation by rainbow trout (Oncorhynchus mykiss), cyprinids remained, only intermediate-size herbivores (D. rosea) increased in abundance, andBosmina persisted. In contrast to food-web manipulations, increased algal abundance resulting from watershed disturbance (road construction) did not alter the species composition or size-structure of fossil Cladocera.