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Geology topics

Arthur H. Lachenbruch

Publications and source records attributed to Arthur H. Lachenbruch.

At least 19 recordsLinked to original sources
USGS1990

Continental extension, magmatism and elevation; formal relations and rules of thumb

To investigate simplified relations between elevation and the extensional, magmatic and thermal processes that influence lithosphere buoyancy, we assume that the lithosphere floats on an asthenosphere of uniform density and has no flexural strength. A simple graph relating elevation to lithosphere density and thickness provides an overview of expectable conditions around the earth and a simple test for consistency of continental and oceanic lithosphere models. The mass-balance relations yield simple general rules for estimating elevation changes caused by various tectonic, magmatic and thermal processes without referring to detailed models. The rules are general because they depend principally on buoyancy, which under our assumptions is specified by elevation, a known quantity; they do not generally require a knowledge of lithosphere thickness and density. The elevation of an extended terrain contains important information on its tectonic and magmatic history. In the Great Basin where Cenozoic extension is estimated to be 100%, the present high mean elevation ( ~ 1.75 km) probably requires substantial low-density magmatic contributions to the extending lithosphere. The elevation cannot be reasonably explained solely as the buoyant residue of a very high initial terrane, or of a lithosphere that was initially very thick and subsequently delaminated and heated. Even models with a high initial elevation typically call for 10 km or so of accumulated magmatic material of near-crustal density. To understand the evolution of the Great Basin, it is important to determine whether such intruded material is present; some could replenish the stretching crust by underplating and crustal intrusion and some might reside in the upper mantle. The elevation maintained or approached by an intruded extending lithosphere depends on the ratio B of how fast magma is supplied from the asthenosphere ( b km/Ma) to how fast the lithosphere spreads the magma out by extension (γ Ma −1 ). For a surface maintained 212km below sea level (e.g., an ocean ridge) B is about 5 km; for continental extension the ratio may be much greater. The frequent association of volcanism with continental extension, the high elevation (and buoyancy) of some appreciably extended terrains, and the oceanic spreading analog all suggest that magmatism may play an important role in continental extension. Better estimates of total extension and elevation change in extended regions can help to identify that role.

Tectonophysics↗
USGS1986

Changing climate: Geothermal evidence from permafrost in the Alaskan Arctic

Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

Alaska↗
USGS1982

Depth and temperature of permafrost on the Alaskan Arctic Slope; preliminary results

As permafrost is defined by its temperature, the only way to determine its depth is to monitor the return to equilibrium of temperatures in boreholes that penetrate permafrost. Such measurements are under way in 25 wells on the Alaskan Arctic Slope; 21 are in Naval Petroleum Reserve Alaska (NPRA), and 4 are in the foothills to the east. Near-equilibrium results indicate that permafrost thickness in NPRA generally ranges between 200 and 400 m (compared to 600+ m at Prudhoe Bay); there are large local variations and no conspicuous regional trends. By contrast the long-term mean temperature of the ground surface (one factor determining permafrost depth) varies systematically from north to south in a pattern modified by the regional topography. The observed variation in permafrost temperature and depth cannot result primarily from effects of surface bodies of water or regional variations in heat flow; they are consistent, however, with expectable variations in the thermal conductivity of the sediments. It remains to be determined (with conductivity measurements) whether certain sites with anomalously high local gradients have anomalously high heat flow; if they do, they might indicate upwelling of interstitial fluids in the underlying basin sediments.

Open-File Report↗
USGS1982

Preliminary interpretation of thermal data from the Nevada Test Site

Analysis of data from 60 wells in and around the Nevada Test Site, including 16 in the Yucca Mountain area, indicates a thermal regime characterized by large vertical and lateral gradients in heat flow. Estimates of heat flow indicate considerable variation on both regional and local scales. The variations are attributable primarily to hydrologic processes involving interbasin flow with a vertical component of (seepage) velocity (volume flux) of a few mm/yr. Apart from indicating a general downward movement of water at a few mm/yr, the results from Yucca Mountain are as yet inconclusive.

Open-File Report↗
USGS1977

Sub-sea temperatures and a simple tentative model for offshore permafrost at Prudhoe Bay, Alaska

In this report, we present temperatures measured in three holes drilled into the sea bed in the Prudhoe Bay region and a tentative interpretation of them in terms of the gross thermal regime and shoreline history of the area. The new holes (PB-1, PB-2, and PB-3, Figure 1) were drilled in spring, 1976 (see Sellmann, 1976) as part of a cooperative study of off-shore permafrost by the USGS, CRREL, and the University of Alaska. Results from two of the holes (#190 and #3370, Figure 1) drilled earlier by the University of Alaska (Osterkamp and Harrison, 1976) have been included in our interpretation. The reader not interested in analytical details may wish to examine Figures 1, 2, and 3, and then skip to the concluding section "Summary and Discussion," page 32.

Alaska↗
USGS1976

Rise of a variable-viscosity fluid in a steadily spreading wedge-shaped conduit with accreting walls

Relatively rigid plates making up the outer 50 to 100 km of the Earth are steadily separating from one another along narrow globe-circling zones of submarine volcanism, the oceanic spreading centers. Continuity requires that the viscous underlying material rise beneath spreading centers and accrete onto the steadily diverging plates. It is likely that during the rise the viscosity changes systematically and that the viscous tractions exerted on the plates contribute to the unique pattern of submarine mountains and earthquake faults observed at spreading centers. The process is modeled by viscous creep in a wedge-shaped conduit (with apex at the sea floor) in which the viscosity varies as r m where r is distance from the apex and m is a parameter. For these conditions, the governing differential equations take a simple form. The solution for the velocity is independent of r and of the sign of m . As viscous stresses vary as r m-1 , the pattern of stress on the conduit wall is sensitive to viscosity variation. For negative m , the viscous pressure along the base of the conduit is quite uniform; for positive m , it falls toward zero in the axial region as the conduit base widens. For small opening angles, viscous forces push the plates apart, and for large ones, they oppose plate separation. Though highly idealized, the solution provides a tool for investigating tectonic processes at spreading centers.

Journal of Research of the U.S. Geological Survey↗
USGS1974

Thermal data from heat-flow test wells near Long Valley, California

As part of the Geological Survey's study of the Long Valley area, an attempt was made to define the regional thermal setting of the caldera. The first phase of this study involved the drilling of four holes in granitic rocks outside of the caldera and two holes within it (Figure 1). LV was drilled as a hydrologic test well. In Figure 1, the light areas are areas of crystalline rocks and the stippled area, volcanic and sedimentary rocks. (For an explanation of the symbols, see Plate 1 of Bateman and others (1963).) Preliminary temperature measurements and thermal conductivity determinations have been completed and are presented in this report. Also presented herein are data obtained in November 1973 from the shallow holes drilled by Lewis (1974) and some thermal conductivity values from these short holes. These preliminary data are being released now because of intense public interest. The study is continuing, and an interpretive report will be written later this year. The temperature measurements are presented in both graphs and tables. Thermal conductivity data are tabulated as a function of depth for each hole. For locations of the shallow test wells, reference should be made to the quadrangle maps shown in Figures 3 through 13 of Lewis (1974).

California↗
USGS1969

Heat flow in the Arctic

Defines heat flow as the flux at the earth's solid surface of heat conducted from the interior; the heat-flow-unit (hfu) is on the order of 1-millionth calorie through each sq cm of the surface/sec, which is enough to melt a 4-mm layer of ice over the earth's surface/yr. Earth heat originates from radioactive decay of U, Th and K in the crust and mantle. Although land heat-flow measurements in the Arctic are too few for regional interpretation, those from Cape Thompson, Barrow and Cape Simpson, Northern Alaska are discussed and figured to show what they contribute to understanding of permafrost, climatic change and shoreline movements. Measuring thermal conductivity and gradient is much simpler in ocean basins than on land. Locations of such measurements are mapped, the results for the Alaskan quadrant in more detail. The sharp change in heat flow at the edge of the Alpha Cordillera, shown in a geothermal model, suggests that this feature is a huge accumulation of basalt, rather than mantle material or remnant of a foundering continent as previously postulated. Future Arctic heat flow studies are discussed.

Arctic↗
USGS1958

Periodic heat flow in a stratified medium with application to permafrost problems

Solutions to the Fourier heat equation for quasi-steady periodic flow in a stratified semi-infinite medium can be obtained readily by standard methods. The results have wide application to studies of earth-temperature variations induced by diurnal, annual, and other periodic variations in ground surface temperature. Much of the previous work on this subject has been interpreted with reference to the solution for the homogeneous case; and this can be seriously in error when applied to stratified earth materials. One application of the theory is to the important problem of determining the minimum thickness of gravel fill required to maintain the material on which it rests (the subgrade) in a perennially frozen state in permafrost areas. The results indicate that the required fill thickness is quite sensitive to the thermal properties of the subgrade. If a thin layer of material with low thermal contact coefficient, such as spruce logs, is placed between the fill and subgrade, the thickness of fill required to maintain undisturbed permafrost can be greatly reduced. The thermal properties of the soil beneath the layer supporting plant growth can exercise an important influence on the temperature in that layer. This effect, which cannot be explained by studies of the ground surface and the surficial layer, is likely to have important application to plant ecology in the Arctic.

Open-File Report↗
USGS1957

Thermal effects of the ocean on permafrost

In high latitudes the large difference between the mean annual temperature at the ground surface and in the unfrozen sediments beneath bodies of water can affect ground temperatures to depths of several hundred feet. The effect is of particular interest near the edge of the ocean where it depends upon the magnitude of the temperature difference between the land surface and ocean bottom, the thermal properties of the ground materials, and past changes in climate and/or shore-line configuration. Theoretical considerations suggest that, except where there are transgressing shore lines, permafrost to depths greater than about 100 feet beneath the ocean bottom is not to be expected at points farther than a few thousand feet offshore. Similar considerations indicate that geothermal installations along the Arctic coast can give information regarding post-Pleistocene shore-line changes. The geothermal effects of bodies of water offer an explanation for the anomalously large outward earth-heat flow recently reported by A. D. Misener for Resolute Bay, Cornwallis Island, N. W. T., Canada.

GSA Bulletin↗