Geology topics

David L. Williams

Publications and source records attributed to David L. Williams.

5 recordsLinked to original sources

USGS2012

Evidence for a shallow pluton beneath the Goat Rocks Wilderness, Washington, from gravity and magnetic data

A gravity and aeromagnetic study was conducted in and adjacent to the Goat Rocks Wilderness area, Washington. This work was done in conjunction with geologic and geochemical studies which were critically important to our interpretation. The Bouguer gravity anomaly map displays a large central positive gravity anomaly and three superimposed smaller positive anomalies. The main anomaly, about 26 by 11 km with an amplitude of about 16 mGal seems to delineate a dense large pluton or sheeted dike complex intruding less dense, older volcanic and sedimentary rocks lying directly below the volcano. The pluton or sheeted dike complex may have fed the Goat Rocks volcano throughout its 1–2 m.y. history. Three small feeders, mapped on the surface as andesite intrusions, may be the cause of three smaller gravity anomalies superimposed on the main anomaly. The magnetic anomaly maps also exhibit a large central anomaly whose source is the Goat Rocks pluton. The magnetic data suggest that the three shallow intrusive cupolas have undergone a significant amount of hydrothermal alteration. In addition, to some extent the magnetic data reflect the terrain of the area. Topographic highs produce magnetic highs, and topographic lows produce magnetic lows. The deviations from this pattern delineate subsurface geology. Magnetic lows may be of particular interest in locating zones of mineralization where magnetic minerals have been altered and have become significantly less magnetic.

Washington↗

USGS2012

An aeromagnetic study of Mount St. Helens

Aeromagnetic data from surveys flown by the U.S. Geological Survey over Mount St. Helens, Washington, before and after the climactic May 18, 1980, eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source. We assumed that most of the edifice of preeruption and posteruption Mount St. Helens has a magnetization direction near the present earth's field of 69° inclination and 20° declination and calculated its intensity as 4.1 A/m. After subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity, the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows, indicative that their sources were not altered or removed by the May 1980 eruption. The residual highs were explained by a 200-m-deep source lying mostly within the edifice of Mount St. Helens. The source could be terrain that predates Mount St. Helens, such as a buried ridge or a cone or a valley filled with lava. We calculated the magnetization of the material removed by the May 1980 eruption and found its intensity to be 4.2 A/m in a direction near the present earth's field, similar to that assumed for the volcano as a whole. This similar result confirmed the validity of the magnetization vector assumed for the entire edifice.

Washington↗

USGS2012

Evidence from gravity data for an intrusive complex beneath Mount St. Helens

On the basis of gravity data, aided by aeromagnetic, magnetotelluric, side-looking radar, and geologic information, we tentatively identify a large, shallow intrusive complex beneath Mount St. Helens. The complex is roughly 5–6 km thick and has apparently intruded a buried and compressed Mesozoic forearc sedimentary sequence. The lateral extent of the intrusive complex is uncertain, and we give three alternative models varying from about 18 by 10 km to as much as 18 by 22 km. The western boundary of the inferred Mount St. Helens intrusion abuts several of a number of Tertiary and younger plutons that crop out in the area. The Mount St. Helens intrusion apparently is adjacent to sedimentary rock or shallow volcanic rock along most of the remainder of its boundary. A ringlike drainage pattern around Mount St. Helens suggests subsidence caused by removing magma from deep beneath Mount St. Helens and adding this mass to the Mount St. Helens edifice and its underlying intrusion. Our analysis indicates that the average bulk density of the volcanic rock comprising Mount St. Helens is about 2.15 g/cm 3 .

Washington↗

USGS2012

Analysis of gravity data in volcanic terrain and gravity anomalies and subvolcanic intrusions in the Cascade Range, U.S.A., and at other selected volcanoes

Gravity data were investigated to reveal the presence of subvolcanic intrusions. With few exceptions, these intrusions produce a detectable gravity anomaly. In the past, these gravity anomalies have often been overlooked or misinterpreted because the data reduction procedure was inadequate. A pragmatic method for reducing and interpreting reconnaissance gravity data from volcanoes as well as gravity models of a variety of volcanoes is developed. Large calderas (diameters greater than 15 km) have relatively low-density intrusions beneath them. All other large volcanic systems that would include small calderas (diameters less than 15 km) have relatively high-density intrusions beneath them. The density contrasts that produce the observed anomalies occur between the intrusion, whose density is usually greater than 2.6 g/cm 3 , and the country rock. Commonly, the shallow country rock is an older volcanic layer with a density less than 2.5 g/cm 3 . The result of the contrast is a positive anomaly over the intrusion. For larger calderas, the surrounding volcanic layer is usually thin and overlies dense metamorphic and plutonic country rocks. In this case, we find the intrusion commonly less dense than country rock. The result is a negative anomaly. In modeling volcanoes of the Cascade Range, gravity data and geologic considerations required a bottom on the intrusion. This may be an actual bottom or the depth at which the density contrast between the intrusion and the country rock disappears. The tops of the intrusions are usually shallow and are significantly wider than overlying craters or calderas. Calderas are associated with wider intrusions. Some intrusions are single cooling units, but more commonly they are an accumulation of the unerupted portions of individual magmatic injections. These injections could occur periodically throughout the life of the volcano, and would generally be accompanied by eruption. Comparing the volume of the intrusion and the volume of the volcanic edifice indicates that only a small part of a magma injection erupts, although some of the apparent intrusive material may be reworked older volcanics. Exceptions to the general discussion presented tend to be related to the nature of the country rock.

California, Oregon, Washington↗

USGS1975

Assessment of geothermal resources of the United States, 1975

This assessment of geothermal resources of the United States consists of two major parts: (1) estimates of total heat in the ground to a depth of 10 km and (2) estimates of the part of this total heat that is recoverable with present technology, regardless of price. No attempt has been made to consider most aspects of the legal, environmental, and institutional limitations in exploiting these resouces. In general, the average heat content of rocks is considerably higher in the Western United States than in the East. This also helps to explain why the most favorable hydrothermal convection systems and the hot young igneous systems occur in the West. Resources of the most attractive identified convection systems (excluding national parks) with predicted reservoir temperatures above 150 deg C have an estimated electrical production potential of about 8,000 megawatt century, or about 26,000 megawatt for 30 years. Assumptions in this conversion are: (1) one-half of the volume of the heat reservoirs is porous and permeable, (2) one-half of the heat of the porous, permeable parts is recoverable in fluids at the wellheads, and (3) the conversion efficiency of heat in wellhead fluids to electricity ranges from about 8 to 20 percent , depending on temperature and kind of fluid (hot water or steam). The estimated overall efficiency of conversion of heat in the ground to electrical energy generally ranges from less than 2 to 5 percent, depending on type of system and reservoir temperature. (See also W77-07477)

Circular↗