Geology topics

James G. Moore

Publications and source records attributed to James G. Moore.

78 records · Page 5Linked to original sources

USGS1969

Volcanic substructure inferred from dredge samples and ocean-bottom photographs, Hawaii

Ocean-bottom photographs from 18 stations and dredge hauls from 35 stations adjacent to the Island of Hawaii indicate that basaltic pillow lava and pillow fragments are the dominant rock type on the crest and flanks of the submarine rift zone ridges, whereas glassy basalt sand and scoria are the dominant type on the submarine flanks of the volcanoes directly downslope from land. These relations indicate that three major rock units comprise different levels of the volcanoes depending on the site of eruption: (1) pillow lavas and pillow fragments are dominant below sea level and are erupted from deep-water vents; (2) hyaloclastite rocks (vitric explosion debris, littoral cone ash, and flow-foot breccias) mantle the pillowed base of the volcano, and are erupted from shallow-water vents, subaerial vents in water-soaked ground, or are produced where subaerial lava flows cross the shoreline; and (3) thin subaerial lava flows make up the visible, subaerial shield volcano, are built atop the clastic layer, and are erupted from subaerial vents. This three-fold structure is similar to the table mountains of Iceland that are built by eruption beneath glacial ice. Large-scale slumping in the clastic layer may modify the submarine slopes of the volcanoes as well as produce faulting and downslope movement of parts of the overlying shield volcano. The slope change produced where the gentler shield meets the steeper pillowed pile can be recognized beneath sea level in the older volcanoes, where it has been submerged by regional subsidence.

Hawaii↗

USGS1968

Argon-40: Excess in submarine pillow basalts from Kilauea Volcano, Hawaii

Submarine pillow basalts from Kilauea Volcano contain excess radiogenic argon-40 and give anomalously high potassium-argon ages. Glassy rims of pillows show a systematic increase in radiogenic argon-40 with depth, and a pillow from a depth of 2590 meters shows a decrease in radiogenic argon-40 inward from the pillow rim. The data indicate that the amount of excess radiogenic argon-40 is a direct function of both hydrostatic pressure and rate of cooling, and that many submarine basalts are not suitable for potassium-argon dating.

Hawaii↗

USGS1967

The role of olivine in the crystallization of the prehistoric Makaopuhi tholeiitic lava lake, Hawaii

On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa 14 composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000° C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800° C was followed by a 40 year period before the temperature reached 700° C. The olivine phenocrysts settled at an average rate of about 4 × 10 −6 cm Sec −1 to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa 55 ) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa 20 Fa 48 ) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000° C to at least as low as 800° C, the olivine phenocrysts were converted to Fa 30–40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant ground mass olivine. Poikilitic hypersthene grew at the expense of pre-existing ferromagnesian minerals in the cumulate zone.

Hawaii↗

USGS1966

The September 28–30, 1965 eruption of Taal Volcano, Philippines

A moderately violent phreatomagmatic explosive eruption of Taal Volcano, Philippines, occurred from 28 to 30 September, 1965. The main phreatic explosions, which were preceded by ejection of basaltic spatter, opened a new crater 1.5 km long and 0.3 km wide on the southwest side of Volcano Island in Lake Taal. The eruption covered an area of about 60 square kilometers with a blanket of ash more than 25 cm thick and killed approximately 200 persons. The clouds that formed during the explosive eruption rose to heights of 15 to 20 km and deposited fine ash as far as 80 km west of the vent. At the base of the main explosion column, flat, turbulent clouds spread radially, with hurricane velocity, transporting ash, mud, lapilli and blocks. The horizontally moving, debris-laden clouds, sandblasted trees, coated the blast side of trees and houses with mud, and deposited coarse ejecta with dune-type bedding in a zone roughly 4 km in all directions from the explosion crater.

Bulletin Volcanologique↗

USGS1966

Gravity slide origin of rift zones of some Hawaiian volcanoes

The east-trending east rift zone of Kilauea volcano on the island of Hawaii is 50 km long and up to 3 km wide. It consists of three elements arranged roughly in three belts from north to south: 1) eruptive fissures, cracks, faults, and narrow grabens, 2) cinder cones (produced by eruptions more localised than the fissure eruptions), and 3) pit craters. Eruptive vents, either fissure or cone, do not occur south of pit craters; vents occur on the floor of some pit craters but are conlined to the north half. Most earthquakes near the rift zone are shallow; they are abundant south of the rift zone but rare north of it. Precise levelling over a 6-year period shows elevation changes of up to 1 metre. Profiles of elevation change across the rift zone are asymmetrically steep on the north side. Precise triangulation shows that points south of the rift have been moving southward at right angles to the rift zone at rates of as much as 10 cm per year. During the major earthquake of 1868, the south coast of the island subsided as much as 2 metres, and abundant evidence indicates other recent subsidence of the south coast. The above facts suggest that the rift zone dips south and that it bounds a large segment of the volcano which is sliding down the steep southern flank. Tensional cracks at the head of this slide tap the shallow central reservoir of the volcano at a depth of a few kilometres. The resulting dikes may feed eruptive fissures in the tensional zone at the head (northernmost part) of the slide, or they may pierce the hanging wall of the south-dipping rift zone through more confined conduits and feed the cinder cones. Likewise, shallow collapse into the rift zone on the north produces narrow grabens, whereas deeper collapse farther south (perhaps aided by magma stoping upward) produces circular pit craters. Submarine topography south of Kilauea caldera indicates a submarine landslide on the south slope of the volcano. The landslide tongue is more than 25 kilometres long and is bounded upslope by a concave escarpment. On land, the northern rim of this escarpment is formed by a series of faults down-dropped on the south, called the Hilina fault system. Dredge hauls from a 300-metre hill on the crest of the landslide tongue at a water depth of 800 metres consists of angular fragments of fresh, glassy, tholeiitic basalt. The high vesicularity of this basalt suggests that it was erupted at a water depth several hundred metres less than that at which it was collected. Presumably, landsliding has carried the lava downward into deeper water.

Bulletin Volcanologique↗

USGS1962

Lava tree molds of the September 1961 eruption, Kilauea Volcano, Hawaii

Well-developed lava tree molds were formed during the September 1961 eruption along the east rift zone of Kilauea Volcano. The upright molds were produced where fluid lava, flowing through dense tropical forest, became chilled against the larger trees and tree ferns and later drained away. Where the lava ponded temporarily in a structural valley, tree molds more than 14 feet high mark the high level attained by the flow.

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