Geological Survey Data Research Articles

Three‐Dimensional Gravity Modeling of the Geologic Structure of Long Valley Caldera

A 48‐mGal gravity low coincides with Long Valley caldera and is mainly attributed to low‐density caldera fill. Gravity measurements by Unocal Geothermal have been integrated with U.S. Geological Survey data, vastly improving gravity station coverage throughout the caldera. A strong regional gravity trend is mainly attributed to isostasy. A “best fitting” (based on regional control of basement densities) Airy‐Heiskanen isostatic model was used for the regional correction. A three‐dimensional, multiple‐unit gravity modeling program with iterative capabilities was developed to model the residual gravity. The density structure of Long Valley caldera and vicinity was modeled with 22 discrete density units, most of which were based on geologic units. Information from drill hole lithologies, surface geology, and structural geology interpretations constrain the model. Some important points revealed by the three‐dimensional gravity modeling are that (1) the volume of ejected magma associated with the Bishop Tuff eruption is greater than previously thought, (2) the caldera structure is strongly influenced by precaldera topography and the extensions of major, active faults, (3) the main west ring fracture is coincident with the Inyo Domes‐Mono Craters fracture system, (4) a relatively low‐density region probably underlies the caldera, and (5) a silicic magma chamber may underlie Devils Postpile.

The Viscoscity of Synthetic and Natural Silicate Melts and Glasses at High Temperatures and 1 Bar (105 Pascals) Pressure and at Higher Pressures, U.S. Geol. Surv. Bull. 1764

Over the past decade, I have routinely collected papers dealing with the physical properties of rocks and other materials. Their dog‐eared and coffee‐stained appearance is just one indication of their continuing value to me. Focusing on viscosity of silicate melts, Michael Ryan and James Blevins have considerably extended and formalized this collection process, resulting in the publication of a massive report containing viscosity data on an extensive variety of melt compositions. According to the authors, this report represents an initial step in establishing a comprehensive U.S. Geological Survey (USGS) data base for the properties of multicomponent silicate melts.

Submersible observations along the southern Juan de Fuca Ridge: 1984 Alvin Program

In September 1984, the research submersible Alvin provided direct observations of three major hydrothermal vent areas along the southernmost segment of the Juan de Fuca Ridge (JFR). The submersible operations focused on specific volcanologie, structural, and hydrothermal problems that had been identified during the preceding 4 years of photographic, dredging, acoustic imaging, and geophysical studies along a 12‐km‐long section of the ridge. A continuously maintained (from 1981 to the present) net of seafloor‐anchored acoustic transponders allowed the observations from Alvin to be directly tied to all previous U.S. Geological Survey data sets and National Oceanic and Atmospheric Administration water column surveys from 1984 to the present. The three vent areas studied are the largest of at least six areas identified by previous deep‐towed camera surveys that lie within a deep cleft, which marks the axis of symmetry of the JFR in this region. The cleft appears to be the locus of eruption for this segment of the JFR. The vent areas, at least in part, are localized near what appear to be previous volcanic eruptive centers marked by extensive lava lake collapse features adjacent to the cleft at these sites. Each hydrothermal area has several active discharge sites, and sulfide deposits occur as clusters (15–100 m2) of small chimneys, individual large chimneys, or clusters of large branched chimneys. We review the dive program and present a brief synthesis of the geology of the vent sites together with sample and track line compilations.

Aerial and Buried Wires above Stratified Dispersive Media

This paper discusses plane wave reflection and transmission characteristics for soils which are modeled as plane, stratified media with frequency-dependent constitutive parameters. Values selected for soil conductivity and electrical permittivity in each layer of soil were based upon measurements of water content as a function of depth obtained from U.S. Geological Survey data. Currents induced by plane wave illumination are calculated for long wires above and below ground. The results are compared to those obtained from simplified soil models which ignore the frequency-dependence of the constitutive parameters. Currents calculated for buried and aerial wires with near grazing incidence were found to display a strong dependence upon details of the soil model due to attenuation and reflections which occur during wave propagation through stratified, conductive media.

The evolution of alkali elements and the division of geochemical process for pegmatite Vein No. 3, Altai, Xinjiang, China

According to the evolution of alkali elements (e.g. K, Na, Li) and the relationship among them in the melt-solution of pegmatitic magma, the author divides the pegmatitic process for Vein No. 3 into three main geochemical stages associated with mineralization, i.e., K-Na, Na-Li and K stages. The above division scheme has been proved to be valid by field geological survey and laboratory data from thermodynamic analyses. Additionally, the related thermodynamic analyses are given herein on (1) the albitization of microcline formed at the earliest stage, (2) the hydrolysis of microcline (greisenization), and (3) the formation and stability of spodumene.

Streamflow Fluctuation, Bar Roughness, and Bed Load Movement:A Hypothesis

A relation between streamflow variables and bar conditions on the Lower Columbia River derived by linear regression analysis on U.S. Army Corps of Engineer and U.S. Geological Survey data leads to the hypothesis that bed‐load movement and bar scour are increased by streamflow fluctuation.

Sediment-Transporting Characteristics of Streams

Natural streams may have an ill-defined relationship between sediment load and discharge. United States Geological Survey data for several western streams included information on temperature, velocity, depth, width, and bed material, as well as concentration and discharge. The variability of the load-discharge characteristics of these streams was assessed by the use of a relationship for the sediment-transporting capacity of streams and the measured variability of the factors determining the load. Although a direct comparison between prediction and observation was not feasible for every set of measurements, the predicted range of variation in sediment load was more than sufficient to explain the observed variation.