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Geology topics

J.G. Moore

Publications and source records attributed to J.G. Moore.

At least 37 records · Page 2Linked to original sources
USGS1991

Degassing and differentiation in subglacial volcanoes, Iceland

Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60??C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation. ?? 1991.

Journal of Volcanology and Geothermal Research↗
USGS1990

Reef growth and volcanism on the submarine southwest rift zone of Mauna Loa, Hawaii

A marine sampling program, utilizing the PISCES-5 submersible operated by the Hawaii Undersea Research Laboratory (NOAA), has confirmed the presence of a major submerged coral reef offshore from Ka Lae (South Point), Hawaii. The top of the reef is now 150–160 m below sea level. Radiocarbon and Useries dating indicates that it drowned about 13.9 ka by the combined effects of island subsidence (2.5 mm/year) and the rapid rise of sea level at the end of the last glaciation so that the relative submergence rate of more than 10 mm/year exceeded the upward growth rate of the reef. The submerged reef caps the offshore part of the southwest rift-zone ridge of Mauna Loa, which has apparently undergone little volcanic activity offshore since 170 ka, and possibly since 270 ka. This fact suggests that rift zone activity is becoming increasingly restricted toward the upper part of the volcano, a condition possibly heralding the end of the shield-building stage.

Hawaii↗
USGS1988

Subsidence of Puna, Hawaii inferred from sulfur content of drilled lava flows

Sulfur was analyzed in more than 200 lava samples from five drill holes located on the east rift zone of Kilauea volcano on the island of Hawaii. The sulfur content is a gage of whether lava was erupted subaerially (low sulfur) or erupted subaqueously (high sulfur). Despite considerable variation, sulfur is generally low (less than 0.025%) in the upper part of the holes, begins to increase at a depth of 250-320 m below sea level, and generally reaches a high level (greater than 0.1%) indicative of steady submarine eruption at 330-450 m below sea level. Assuming that the island is subsiding at 2.4 mm/yr, an analysis of these data indicates that part of the variation in sulfur concentration results from past eustatic oscillation of sea level, and that the volcano (at the drill hole site) finally emerged for the last time about 98 ka. The long-term average rate of lava accumulation is roughly 4.4 mm/yr, and upward growth of the volcano at the drill hole area is about 2 mm/yr in excess of subsidence. ?? 1988.

Journal of Volcanology and Geothermal Research↗
USGS1986

The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens

The initial eruption column of May 18th, 1980 reached nearly 30 km altitude and released 1017 joules of thermal energy into the atmosphere in only a few minutes. Ascent of the cloud resulted in forced intrusion of a giant umbrella-shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m/s. The mushroom cloud expanded 15 km upwind, forming a stagnation point where the radial expansion velocity and wind velocity were equal. The cloud was initiated when the pyroclastic blast flow became buoyant. The flow reduced its density as it moved away from the volcano by decompression, by sedimentation, and by mixing with and heating the surrounding air. Observations indicate that much of the flow, covering an area of 600 km2, became buoyant within 1.5 minutes and abruptly ascended to form the giant cloud. Calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant. Assuming an initial temperature of 450??C and a magmatic origin for the explosion, these calculations indicate that the flow became buoyant when its temperature was approximately 150??C and the flow consisted of a mixture of 3.25 ?? 1011 kg of pyroclasts and 5.0 ?? 1011 kg of air. If sedimentation is considered, these figures reduce to 1.1 ?? 1011 kg of pyroclasts and 1.0 ?? 1011 kg of air. ?? 1986.

Journal of Volcanology and Geothermal Research↗
USGS1984

Carbon and its isotopes in mid-oceanic basaltic glasses

Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO 2 is about 3.8‰ enriched in 13 C, relative to dissolved carbon. Despite this fractionation, δ 13 C PDB values for all spreading ridge glasses lie within the range −5.6 and −7.5, and the δ 13 C PDB of mantle carbon likely lies between −5 and −7. The carbon abundances and δ 13 C PDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using 3 He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 × 10 13 g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory.

Earth and Planetary Science Letters↗
USGS1984

Concentrations and isotope ratios of carbon, nitrogen and sulfur in ocean-floor basalts

Fresh submarine basalt glasses from Galapagos Ridge, FAMOUS area, Cayman Trough and Kilauea east rift contain 22 to 160 ppm carbon and 0.3 to 2.8 ppm nitrogen, respectively, as the sums of dissolved species and vesicle-filling gases (CO 2 and N 2 ). The large range of variation in carbon content is due to combined effect of depth-dependency of the solubility of carbon in basalt melt and varying extents of vapour loss during magma emplacement as well as in sample crushing. The isotopic ratios of indigenous carbon and nitrogen are in very narrow ranges,−6.2 ± 0.2% relative to PDB and +0.2 ± 0.6 %. relative to atmospheric nitrogen, respectively. In basalt samples from Juan de Fuca Ridge, however, isotopically light carbon ( δ 13 C = around −24%.) predominates over the indigenous carbon; no indigenous heavy carbon was found. Except for Galapagos Ridge samples, these ocean-floor basalts contain 670 to 1100 ppm sulfur, averaging 810 ppm, in the form of both sulfide and sulfate, whereas basalts from Galapagos Ridge are higher in both sulfur (1490 and 1570 ppm) and iron (11.08% total iron as FeO). The δ 34 S values average +0.3 ± 0.5%. with average fractionation factor between sulfate and sulfide of +7.4 ± 1.6%.. The sulfate/sulfide ratios tend to increase with increasing water content of basalt, probably because the oxygen fugacity increases with increasing water content in basalt melt.

Geochimica et Cosmochimica Acta↗
USGS1984

Drowned reefs as indicators of the rate of subsidence of the Island of Hawaii

A major submerged terrace whose seaward edge is at about 150 m depth was investigated in 1983 during 10 dives with the Hawaii Undersea Research Laboratory (HURL) submersible Makali'i off Kealakekua Bay, Hawaii. The seaward termination of the terrace is a steep wall of reef limestone that extends from 150 to 250 m below sea level. Three samples of limestone collected from the reef face at depths of 204-219 m yield a weighted average age of 13,250 (S.D. 380) years before present. The -150 m terrace as well as two deeper terraces off northwestern Hawaii at depths of and apparently were formed by the interaction of glacioeustatic sea-level changes and island subsidence. The best fit of depth, age, and subsidence data indicates that the northwestern coast of Hawaii has subsided at an absolute rate of 1.8 to 3 + mm/yr and that the rate of subsidence has generally increased over the past 0.3 m.y. Each reef terrace grew intermittently for nearly 0.1 m.y.; the -150 m reef terrace was drowned about 13,000 years ago, the -390 m terrace about 145,000 years ago, and the -580 m terrace about 255,000 years ago. Warping of the two older terraces may have resulted from crustal loading caused by the major growth period of Mauna Loa volcano.

Journal of Geology↗
USGS1982

Chemistry and isotope ratios of sulfur in basalts and volcanic gases at Kilauea volcano, Hawaii

Eighteen basalts and some volcanic gases from the submarine and subaerial parts of Kilauea volcano were analyzed for the concentration and isotope ratios of sulfur. By means of a newly developed technique, sulfide and sulfate sulfur in the basalts were separately but simultaneously determined. The submarine basalt has 700 ± 100 ppm total sulfur with δ 34 S Σs of 0.7 &#xB1; 0.1 &#x2030; ">‰ 0.7 ± 0.1 ‰ . The sulfate/sulfide molar ratio ranges from 0.15 to 0.56 and the fractionation factor between sulfate and sulfide is +7.5 &#xB1; 1.5&#x2030; ">‰ +7.5 ± 1.5‰ . On the other hand, the concentration and δ 34 S Σs values of the total sulfur in the subaerial basalt are reduced to 150 ± 50 ppm and &#x2212;0.8 &#xB1; 0.2&#x2030; ">‰ −0.8 ± 0.2‰ , respectively. The sulfate to sulfide ratio and the fractionation factor between them are also smaller, 0.01 to 0.25 and +3.0‰, respectively. Chemical and isotopic evidence strongly suggests that sulfate and sulfide in the submarine basalt are in chemical and isotopic equilibria with each other at magmatic conditions. Their relative abundance and the isotope fractionation factors may be used to estimate the &#x192;o 2 "> ƒo 2 and temperature of these basalts at the time of their extrusion onto the sea floor. The observed change in sulfur chemistry and isotopic ratios from the submarine to subaerial basalts can be interpreted as degassing of the SO 2 from basalt thereby depleting sulfate and 34 S in basalt. The volcanic sulfur gases, predominantly SO 2 , from the 1971 and 1974 fissures in Kilauea Crater have δ 34 S values of 0.8 to 0.9%., slightly heavier than the total sulfur in the submarine basalts and definitely heavier than the subaerial basalts, in accord with the above model. However, the δ 34 S value of sulfur gases (largely SO 2 ) from Sulfur Bank is 8.0%., implying a secondary origin of the sulfur. The δ 34 S values of native sulfur deposits at various sites of Kilauea and Mauna Loa volcanos, sulfate ions of four deep wells and hydrogen sulfide from a geothermal well along the east rift zone are also reported. The high δ 34 S values (+5 to +6%. o ) found for the hydrogen sulfide might be an indication of hot basalt seawater reaction beneath the east rift zone.

Geochimica et Cosmochimica Acta↗
USGS1980

Late Cenozoic volcanic rocks of the southern Sierra Nevada, California: I. Geology and petrology: Summary

The geology and petrology of the Cenozoic volcanic rocks of the region of California between 38° and 35°45'N latitude and 117°30' and 120°W longitude, including the rocks of a major potassic magmatic province on the west slope of the Sierra Nevada, have been described in Part I (Moore and Dodge, 1980) of this study. The geochemical features of these rocks may provide clues to their origin and aid in comparing and contrasting the Sierra Nevada potassic province with other localized potassic provinces elsewhere in the world. Basaltic rocks occur in numerous small lava-flow remnants, dikes, and plugs that intrude the predominantly granitic terrain of the western slope of the southern Sierra Nevada. Leucite-bearing rocks are present at several localities in this western region. More voluminous basalt, commonly associated with rhyolite, is present east of the range in the Basin and Range province. However, leucite-bearing rocks have been found-at only one locality in this region. Most of the late Cenozoic volcanic rocks in the southern Sierra Nevada eastward through Owens Valley and in the extreme western Basin-Range occur in five rather distinctive areas—the San Joaquin-Kings, Kern, Big Pine, and Coso volcanic fields, and the Mono-Long Valley volcanic center (Fig. 1).

Geological Society of America Bulletin, Part I↗
USGS1979

Trackline chart, bathymetric and seismic profiles acquired on R/V Lee Cruise 9-78-HW, near Hawaii

The U.S. Geological Survey research vessel S.P. Lee conducted a transit line from Honolulu, Hawaii to the big island of Hawaii during the cruise L9-78-HW. The navigational track is shown in plate 1 and the three types of data collected (3.5 khz. and 12 khz. bathymetry plus single channel seismic reflection) are shown in plate 2. The bathymetry records are 400 fathoms (one second sweep) collected by hull mounted transducers. The seismic reflection profiles are from a 148in 3 . Bolt Association Airgun* as energy source with reflectors transmitted via single channel streamer towed approximately 500 feet behind ship. Copies of the data may be obtained from the National Geophysical and Solar-Terrestrial Data Center, U.S. Dept. of Commerce, Boulder, Colorado 80303. *Use of commercial names does not imply endorsement by the U.S. Government.

Hawaii↗