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James G. Moore

Publications and source records attributed to James G. Moore.

At least 73 records · Page 4Linked to original sources
USGS1984

Ultrathin lava layers exposed near San Luis Obispo Bay, California

Sequences of extraordinarily thin (1–5 cm thick) lava layers, resembling individual lava flows, are interbedded with Jurassic and Cretaceous pillowed lava flows near San Luis Obispo Bay on the California coast. Such layers are formed inside submarine pillowed lava pipes or flow lobes. As the lava surface in a pillow pipe falls to a lower level owing to diminished supply entering the pipe, water enters the upper compartment through cracks in the outer crust and chills a new crust on top of the lava stream. Repeated lowerings of the lava level in the pipe create a series of discrete lava shelves, each of which represents the upper crust of the lava stream flowing within the pipe. These crusts are supported at different levels on their edges at the side of the pipe. The weight of subsequent overlying lava flows collapses the partly hollow tube, creating a stacked sequence of ultrathin lava layers progressively younger downward.

California↗
USGS1982

Diverse basalt types from Loihi seamount, Hawaii

Loihi seamount is the southeasternmost active volcano in the Hawaiian-Emperor volcanic chain. The seamount is considered representative of the early phase of Hawaiian volcanism because of its youth, small size, and location near the melting anomaly. Seventeen dredge stations recovered transitional basalt, alkalic basalt, and basanite, in addition to the expected tholeiitic basalt. Four flows of alkalic basalt contain common small dunite xenoliths. The recovered samples have thin palagonite rinds and almost no manganese on the glassy surfaces; we estimate that the lavas are less than about 4,000 yr old, and many are less than 1,000 yr old. Loihi seamount is apparently in a transitional growth phase between the early eruption of alkalic lavas and the commonly observed (subaerial) tholeiitic eruptive phase, previously thought to dominate Hawaiian volcanism from inception until the postcaldera collapse, alkalic stage.

Hawaii↗
USGS1979

Vesicularity and CO2 in mid-ocean ridge basalt

Vesicles and included CO 2 are enriched in deep-sea basalts that are also enriched in light rare earth and incompatible elements. This enrichment probably results from a unique deep mantle origin of such melts but may have been modified by CO 2 bubbles rising in shallow magma chambers.

Nature↗
USGS1979

Late Jurassic Independence dike swarm in eastern California

The Independence dike swarm in eastern California, more than 250 km long, extends from the eastern Sierra Nevada and Inyo Mountains through the Argus Range, Alabama Hills, and Spangler Hills to the Garlock fault, where it is offset 64 km before continuing into the Mojave Desert. The dike swarm includes a wide variety of rock types, from lamprophyre to granite porphyry belonging to a calc-alkalic suite. U-Pb dating of three silicic dikes gives concordant ages of 148 m.y., which probably indicates the time of intrusion of the entire swarm. Analyses of rare zircons in the mafic dikes yield discordant, inordinately old ages that suggest entrainment of Pre-cambrian zircons in the dike magma at depth. The regional fracture system intruded by the dikes is believed to have been produced by a crustal extension event that occurred after the Nevadan orogeny during Late Jurassic time, when subduction beneath the Sierra Nevada foothill belt jumped westward and subduction of Franciscan rocks began along the Coast Range thrust.

California↗
USGS1978

Mapped offset on the right-lateral Kern Canyon fault, southern Sierra Nevada, California

The north-trending Kern Canyon fault, the longest fault within the southern Sierra Nevada, has been mapped from lat 36°00′N to its northern end near lat 36°40′N. The fault cuts and offsets granitic plutons as young as 80 m.y., but despite the fact that many recent earthquake foci plot close to the fault, it does not appear to offset an overlying 3.5-m.y.-old basalt flow. Seven granitic plutons are clearly offset by the fault in a right-lateral sense. In the area mapped, offset of plutonic contacts is 6.5 to 13 km and increases southward by 0.2 km/km.

California↗
USGS1977

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

GSA Bulletin↗
USGS1977

Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat

The electron microprobe was used to determine the bulk composition of immiscible sulfide globules trapped in the glass phase of 25 fresh submarine basalt samples from the Mid-Atlantic Ridge. Twenty-three samples represent a spectrum of primitive through differentiated tholeiites from the FAMOUS dive area; two are differentiated basalts from the Reykjanes Ridge. The analyzed globules range in diameter from 11 to 233 µm. On the average, they constitute only 0.0022 volume percent of the rocks and contain less than 1.5 percent of the sulfur. Compositions of the globules change with differentiation as measured by Fe/(Fe+Mg) or TiO 2 content of the host glass. Globules in glass containing 0.66 to 1.0 wt percent TiO 2 typically contain 20 to 26 wt percent Ni + Cu and have an average atomic Ni/Cu of 1.6. With differentiation toward 1.6 wt percent TiO 2 , Ni + Cu content of the globules falls to less than 10 wt percent and atomic Ni/Cu falls to 0.4. Sulfur content of the host glasses shows a strong correlation with FeO content, increasing from 840 ppm to 1,370 ppm as FeO content increases from 8.0 to 12.6 wt percent. Reference to experimental studies shows that this relationship is consistent with sulfur saturation of the host glass at liquidus temperatures. Crystal fractionation is considered to be the dominant factor in keeping the differentiating melt at sulfur saturation. The sulfide globules may have persisted in the basaltic melt from its place of formation by partial melting in the mantle, or they may have exsolved from the melt as it became sulfur-saturated in a high-level magma chamber. Globule abundance and composition indicate adjustment to the composition of the melt in which they were trapped. Material balance calculations suggest that one-third of the Cu and commensurate amounts of S, Ni, and Fe have settled from the magma as immiscible globules. The sulfide globules contain less than 4 wt percent magnetite, compatible with low f o2 in the magma. Three sulfide phases coexisted in the globules at about 600 °C: monosulfide solid solution, intermediate solid solution, and pentlandite. At lower temperatures, the intermediate solid solution has broken down, and the monosulfide solid solution has exsolved a second generation of pentlandite.

GSA Bulletin↗
USGS1977

Petrology of basalt from the East Pacific Rise near 21 degrees North latitude

Four dredge hauls of fresh tholeiitic basalt lava were recovered from a 3.3-kilometer-wide zone at the axis of the East Pacific Rise. Petrologic and major-element chemical studies indicate that the basalt ranges from moderately fractionated varieties to one sample enriched in iron and titanium. The four samples show no symmetrical compositional zonation across the ridge axis, but the two least fractionated and youngest samples occur on the east side of the ridge axis.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Mechanism of Formation of Pillow Lava

Much of the ocean floor is covered by lava of a distinctive character. The lava appears to be made up of closely packed ellipsoidal masses about the size and shape of pillows - hence the term pillow lava. Only within the last few years has the abundance of pillow lava on the ocean floor been fully recognized. Ocean-bottom photographs and dredge samples have shown that the great bulk of new ocean floor created at diverging plate boundaries (such as the Mid-Atlantic Ridge) is composed of pillowed basaltic lava flows. Closeup observations from submarines at depths of 2.7 km in the rift valley of the Mid-Atlantic Ridge have verified that virtually all the lavas erupted at this plate boundary are pillowed. The submarine portions of the great oceanic volcanoes, such as the ridge beneath the Hawaiian Islands, are also known to be built largely of pillow lava, and it is widespread in outcrops of uplifted ancient lava. Pillow lava is probably the most abundant form of volcanic rock on earth, though most of it is hidden beneath the world's oceans and mantled by younger sediments. Most investigators agree that the pillows form when fluid lava chills in contact with water, either when it erupts directly into water (or beneath ice) or when it flows across a shoreline and into a body of water. However, prior to our study, the process of pillow formation had never been directly observed. The recent eruptions of Kilauea Volcano in Hawaii provided an unparalleled opportunity to study the movement and cooling of lava beneath the sea. In June 1969, lava from the new Mauna Ulu vent on the east rift zone of Kilauea spilled into the sea after flowing 12 km down the south flank of the volcano. This pattern was repeated, with lava flowing into the sea for a few weeks each year through 1973. In April 1971 scuba divers for the first time investigated lava flowing underwater and learned that in favorable circumstances the lava could be approached closely. Despite heated water, explosive concussions, vigorous convective currents, and poor visibility due to suspended sediment, valuable observations were made.

American Scientist↗
USGS1974

Preliminary model for extrusion and rifting at the axis of the Mid-Atlantic Ridge, 36°48′ North

The inner rift valley of the Mid-Atlantic Ridge at 36°48′ N. is 1.5 to 3 km wide and 100 to 400 m deep. It is symmetrical in profile with a discontinuous medial ridge 100 to 240 m high and 800 to 1,300 m wide along its axis. The medial ridge is replaced every 1 to 3 km with a central trough 200 to 600 m wide. The medial ridge is apparently built by eruptions of pillow basalt recurring at intervals of roughly 14,000 years at a given point. Between eruptions (and possibly during them), the ridge splits and divides along its axis and subsides, which produces the central trough. As the trough widens and deepens, it eventually taps magma in a shallow reservoir, initiating a new eruption that rebuilds the medial ridge. Outward spreading of the inward-dipping shingled halves of the former medial ridge produces a layer of pillowed basalts about 400 m thick (oceanic layer 2A), in which resides the bulk of the remanant magnetization of the ocean floor. This layer overlies a layer of intrusive rock (layer 2B) composed of a dike complex that feeds eruptions building the medial ridge as well as the outward moving, solidified shells of a shallow magma chamber.

Geology↗
USGS1973

Origin of Comb Layering and Orbicular Structure, Sierra Nevada Batholith, California

A new descriptive term, comb layering , is proposed to replace the informal term Willow Lake-type layering , first introduced by Poldervaart and Taubeneck (1959) to describe layering in granitoid rocks in which constituent crystals are oriented approximately perpendicular to individual layers. The term schlieren layering is proposed to describe the “normal” layering of granitic rocks defined by alternating layers enriched or depleted in the normal mafic minerals. In such layers, elongate minerals commonly lie in the plane of the layering. Comb layering is widespread in plutonic rocks of California and is commonly associated with orbicular diorites. Evidence from a detailed study of three localities in the Sierra Nevada indicates that comb layering forms chiefly in overturned troughs along overhanging walls of plutons or along walls of dikes or pipes that cut country rocks adjoining plutons. Orbicular rocks associated with the comb layering are formed by a nucleus surrounded by multiple comb layers. The growth direction in comb layers can be determined by the branching and widening of plagioclase and hornblende crystals and is invariably toward the parent pluton. Field data indicate that comb layering cannot have formed from silicate magma, and further suggest that the layers have been deposited by large volumes of aqueous fluids that migrated upward along contacts between magma and wallrock or along the interface between magma and previously solidified melt. Comb layering and orbicules are largely restricted to structural traps into which upwardly migrating, solute-rich water was channeled owing to its low density. The comb layers grew on the solid walls of fluid-filled channels, whereas orbicules formed by precipitation of comb layers on hobbling inclusions suspended within the upward-flowing fluid.

California↗
USGS1973

Flow of lava into the sea, 1969–1971, Kilauea Volcano, Hawaii

Lava from the Mauna Ulu eruption on Kilauea Volcano entered the sea on the south coast of the Island of Hawaii three times from 1969 to 1971. Two of these flows were investigated underwater by divers, one while lava was actively flowing. The June 1969 flow entered the sea as a narrow flow of aa. Below sea level, the flow maintained continuity and flowed at least several hundred meters to a depth beyond 70 m. Several cylindrical flow lobes about 1 m in diameter and about 10 to 15 m long emerged from the side of the aa flow at a depth of about 25m. Underwater investigations, combined with subaerial observations, revealed that the March–May 1971 flow produced a distinct lava delta composed of subaerial pahoehoe lava resting on a submarine sequence of steeply dipping foreset-bedded volcanic sand and rubble that includes conformably dipping cylindrical lava tongues. Most of the pahoehoe streams pouring over the sea cliff are quenched and shattered to glassy sand and rubble that in turn is further fragmented by vigorous wave action and avalanching. In some places, however, larger pahoehoe flows maintain coherence across the cliff and through the surf zone to feed submarine lava tongues. Underwater, these active lava tongues emitted a roaring noise as lava flowed inside their outer black glassy walls. Periodically, cracks exposed the brightly incandescent lava within, and pillow-like buds and toes grew from the top and sides of the lava tongue. Only a small amount of steam was generated underwater. Water temperature close to the active tongues was elevated only 2.5°C.

Hawaii↗
USGS1973

Lower Jurassic ammonite from the south-central Sierra Nevada, California

A Lower Jurassic ammonite has been found in metasiltstone of the Boyden Cave roof pendant, south-central Sierra Nevada, Calif. Although too poorly preserved to permit positive generic and specific identification, its general shape, coiling, and ornamentation are characteristic of Early Jurassic forms. Strata associated with the fossiliferous rocks in the pendant include quartzite, andalusite hornfels, and marble. This assemblage differs strikingly from nearby volcanic rocks to the east, some of which in the Ritter Range pendant also contain Lower Jurassic fossils. The presence of nonvolcanic Lower Jurassic rocks of the Boyden Cave pendant lying west of coeval volcanic rocks of the Ritter Range pendant is anomalous and may be the result of large-scale tectonic dislocations.

California↗
USGS1969

Nuées Ardentes of the 1968 Eruption of Mayon Volcano, Philippines

Mayon Volcano, southeastern Luzon, began a series of explosive eruptions at 0900 April 21, 1968, and by May 15 more than 100 explosions had occurred, at least 6 people had been killed, and roughly 100 square km had been covered by more than 5 cm of airfall ash, blocky ash flows, and a lava flow. All material crupted was porphyritic augite-hypersthene andesite. Explosions from the summit crater (elevation 2460 m) ejected large quantities of ash and incandescent blocks to a height exceeding 600 m and produced ash-laden clouds which rose to heights of 3 to 10 km. Backfall of the coarser material fed nuées ardentes which repeatedly swept down ravines on all sides of the volcanic cone. The velocity of one nuée ardente ranged from 9 to 63 m per sec. The largest nuées descended to the southwest and reached as far as 7 km from the summit. An aa lava flow also descended 3 1/2 km down this flank. The nuées ardentes deposited pyroclastic flows that contained large breadcrust-surfaced blocks averaging about 30 cm across, but occasionally reaching 25 m in greatest dimension. These blocks were still very hot in their interiors several days later. Surrounding the pyroclastic flows is a seared zone as much as 2 km wide, but averaging a few hundred meters, in which vegetation is charred and splintered, but over which only a thin layer of airfall ash was deposited.

Mayon Volcano↗