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At least 145 records · Page 8Linked to original sources
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

Thermal and mineral waters of nonmeteoric origin, California Coast Ranges

Recent isotope studies show that the waters involved in a variety of geologic processes are dominantly the local meteoric water of each area. In most active geothermal systems, the D/H ratio of the hot water is nearly identical with the local cold meteoric water, but the O 18 /O 16 ratio has been shifted to a more positive value because of subsurface exchange with rocks. The numerous thermal springs of the Wilbur Springs mercury district, although rich in CO 2 , are otherwise similar in Cl content and isotopic composition to analyzed California oil-field waters. Some of the springs discharge near the tops of ridges. These relations cannot be explained by normal meteoric recharge. Water of isotopic composition similar to that of Wilbur Springs occurs in the Sulphur Bank mercury district 15 mi west of Wilbur Springs, but the Sulphur Bank water is higher in B and NH 3 and much lower in Cl than are the Wilbur and most oil-field waters. The Wilbur Springs and Sulphur Bank waters are enriched by ∼40‰ in δD and ∼13‰ in δO 18 relative to local meteoric waters of each area, and thus require processes that differ, at least in part, from most previously studied geothermal systems. The D enrichment, chemical composition, and ridge-top discharge are best explained by large proportions of nonmeteoric water. Wilbur Springs and Sulphur Bank may be dominated, respectively, by waters of connate and metamorphic origin, derived from reaction of ancient ocean waters and marine sediments, and now being forced out by pressures that are higher than hydrostatic. Present data indicate that the most saline of each of these types is more restricted in range of δD than are present-day meteoric waters of the same areas; complete flushing by existing or ancient meteoric waters is unlikely. Many springs in the region are chemically intermediate between the high- and low-chloride types and commonly mix near the surface in different proportions with local meteoric water. Many of these springs are associated with mercury deposits and Alpine serpentinites.

California↗
USGS1973

Magma Mixing as Illustrated by the 1959 Eruption, Kilauea Volcano, Hawaii

The 1959 eruption of Kilauea volcano is unique among recent Kilauea summit eruptions (1952 to 1968) in at least two respects: (1) a large collapse of Kilauea summit accompanied the eruption, and (2) the erupted lavas show a complex variation in their bulk chemical composition. Both features suggest that the 1959 eruption was fed from a source different from that which fed other summit eruptions in this period. The variations in chemical composition can be interpreted in terms of mixing chromite-bearing olivine with variable proportions of two “end-member” magmas represented by the composition of samples of pumice erupted during the first phase of the eruption. Mixing calculations show that the MgO content of olivine varies with the MgO content of the lava and thus with olivine percentage in the same manner as previously determined by Richter and Murata (1966) from petrographic study of hand samples. The calculations also show that the proportion of the two end-member magma types varies throughout the eruption. These results suggest that the eruption was fed from two separate magma batches, each of which was held in a reservoir with an olivine-poor top and olivine accumulation toward the bottom. There is also evidence that the two end-member magmas may be related by redistribution of clinopyroxene in a third hypothetical magma. The complex pre-eruption history implied by the chemical data is at present not satisfactorily explained by any physical model of fractionation, storage, and mixing that can be inferred from data on other well-studied Kilauea eruptions. The average MgO content (15.5 percent) of the 1959 eruption is estimated to be in minimum MgO content of magma produced by partial melting in the mantle beneath Kilauea.

Hawaii↗
USGS1974

Structure of the continental margin of Liberia, West Africa

Geophysical surveys made by R/V Unitedgeo I (USGS–IDOE Cruise Leg 5), combined with earlier surveys and available geologic information, provide the basis for interpreting the structure of the continental margin of Liberia. This area lies at the junction of the Americas and Africa in published reconstructions of Gondwanaland prior to the opening of the North and South Atlantic in Jurassic and Cretaceous time, respectively. Three fracture zones (St. Paul, Cape Palmas, and Grand Cess) are inferred in the area southeast of 9°30′ W. on the basis of magnetic and gravity data, which is supported by bathymetric and seismic reflection data. The three fracture zones appear to exist as separate lineaments near the African coast. Farther seaward, they may be part of the same transform fault crossing the Atlantic (St. Paul fracture zone). The magnetic anomalies associated with these fracture zones, which may have originated in Cretaceous time at the opening of the South Atlantic, are continuous with magnetic anomalies over crust of Eburnean age (∼2,000 m.y.) in southeast Liberia and its continental shelf. This suggests that Eburnean age structures may have been zones of weakness that were reactivated in Cretaceous time. A positive gravity anomaly (∼50 mgal) along the coast and continental shelf of Liberia is attributed to deep crustal rocks that were uplifted and exposed in Pan-African time (∼550 m.y.). The land boundary of this anomaly coincides with a shear zone that marks the boundary between the Pan-African and the Liberian age province (∼2,700 m.y.); the shearing (in a thrust-fault sense) may be the result of compressive stress associated with the closing of a proto-Atlantic ocean. Liberian age magnetic anomalies in the area northwest of 9°30′ W. cross the Pan-African province (and the positive coastal gravity anomaly) and continue over the continental shelf and slope to about the 3,000-m bathymetric contour; the seaward limit of the anomalies is interpreted as representing the seaward limit of the old continental crust. This westward extension of the continental crust does not completely fill the gap in fit in various published reconstructions of Gondwanaland, and we suggest that the northern Florida block may have been located near the Liberian margin at one time. Magnetic data indicate a thick section of sedimentary rock, possibly as great as 8 km, on the continental slope. Comparison of gravity data over magnetically inferred basins in the shelf, slope, and rise suggests that low-density sedimentary rocks constitute a greater proportion of the section in basins beneath the shelf and beneath the slope and rise northwest of 9°30' W. than beneath the slope and rise in the area of the fracture zones. The gravitational attraction that corresponds to a crust-mantle boundary dipping 45° to 60° can be computed to fit observed data – as might be expected at a rifted continental margin. A shallow high-density block beneath the coast and continental shelf is required to fit the coastal positive anomaly; this block is represented by exposures on land of granulite-grade metamorphic rock of the Pan-African province.

Atlantic Ocean↗
USGS1974

Geologic interpretation of gravity profiles in the western Marquette district, northern Michigan

The presence of elongate troughs of Precambrian X (middle Precambrian) rocks in Precambrian W (lower Precambrian) rocks in the western Marquette district of northern Michigan has been known since the late 1800s. However, little data can be brought to bear on estimates of the depth and cross-sectional configuration of these features. For this reason, gravity models and geologic interpretations were made from gravity profiles measured over the Marquette Trough, Republic Trough, and Mitchigan River Trough. Gravity-model studies, combined with geologic studies, indicate that near Humboldt, Michigan, the Marquette Trough is ∼2,438 m deep and asymetrically shaped, the deepest section being near the southern edge; near the west end of Lake Michigamme, the Marquette Trough is about 1,097 m deep at its northern edge; the Republic Trough is about 1,524 m deep at a point ∼2.4 km northwest of its southeastern end; and the Mitchigan River Trough is probably a fault-bounded westward-dipping monocline ∼610 m deep.

Michigan↗
USGS1974

Garnet-clinopyroxenite from the Red Mountain pluton, Alaska

Several lens-shaped and irregular masses of garnet-clinopyroxenite occur along the north margin of the Red Mountain pluton, near Seldovia, Alaska. The pluton is composed of intercalated dunite, clinopyroxenite, and banded chromite layers. The chromite layers appear to dip toward the center of the mass, but they have been deformed, and in some places display isoclinal overturned folds. The garnet-clinopyroxenite bodies are compositionally layered and appear to have been deformed along with the host peridotite. The garnet-clinopyroxenite bodies have whole-rock compositions analogous to alkali-poor anorthite gabbro or mafic anorthosite. Garnet compositions are dominated by the pyrope, grossularite, and almandine components, and the clinopyroxene bodies are rich in Ca-Tschermaks molecule (CaAl 2 SiO 6 ). Although crystallization from the liquidus is a possibility, textures, structures, and the chemical composition of the constituent mineral phases indicate that the garnet-clinopyroxenite has equilibrated in the solid state at pressures of 15 to 25 kb and temperatures of 1100°C to 1300°C. Such P-T parameters suggest an upper-mantle origin for the garnet-clinopyroxenite and the enclosing pluton.

Alaska↗
USGS1974

Buried Triassic basin in the central Savannah River area, South Carolina and Georgia

A basin filled with Triassic red beds, located on the South Carolina–Georgia line ∼32 km southeast of Augusta, Georgia, is buried beneath ∼350 m of Coastal Plain sediments. An extensive aeromagnetic survey, seismic refraction and reflection surveys, and geophysical logs and samples from three wells define the extent and character of the basin. This basin, herein named the Dunbarton Triassic basin, is ∼50 km long, 10 km wide, and trends northeast. The northwest margin of the basin is well defined by the aeromagnetic survey, a seismic reflection traverse, and a well that passed through 485 m of Triassic fanglomerate before entering the crystalline metamorphic rocks below. Near the center of the Triassic basin, a well passed through 902 m of maroon Triassic mudstone and sandstone of fluvial origin without penetrating the bottom of the basin. The permeability of the Triassic rock is extremely low, and water-transmitting fractures were not penetrated. Even slight water-level disturbances in the Triassic wells require many years to recover. Total dissolved solids of the water from the Triassic basin are about twice that in the crystalline metamorphic rocks that surround it. The inferred geologic history of this basin is similar to the history of Triassic basins that crop out in North Carolina.

Georgia, South Carolilna↗
USGS1974

Tectonic evolution of the southern Gulf of Mexico

A detailed magnetic survey in the southern Gulf of Mexico shows subdued irregular magnetic anomalies that are similar in wave length to those attributed to sea-floor spreading on present-day oceanic rises. The small amplitude of these anomalies, about 75γ, would be compatible with an oceanic basalt source at a depth of 10 km, and previous seismic refraction studies in this area have shown that layer 2 of the oceanic crust (presumed to be basalt) does indeed lie at approximately that depth. Palynomorphs in Deep Sea Drilling Project samples of cap-rock material from the Sigsbee Knolls have shown the associated salt to be Jurassic. A crustal model, based on seismic refraction evidence and on new gravity data, suggests that, whereas the salt-dome belt of the southern gulf is underlain by a thick layer with a density of 2.2 g per cm 3 (presumed to be halite rock), adjacent deeper areas of the basin seem to lack this low density layer. In northeastern Mexico, Triassic red beds fill grabens that are correlative with the Newark Group of the Atlantic Coast and suggest that the Gulf of Mexico originated at the time of the initial rifting of the North Atlantic. When the gulf was about half-opened during the Jurassic, oceanic circulation was restricted; and thick deep-basin evaporite deposits, analogous to those found in the Mediterranean Sea by the Deep Sea Drilling Project, were laid down. Further opening established normal salinity and led to the development of salt-free areas of oceanic crust that separated the Sigsbee Escarpment (together with adjacent ridges of offshore Mexico) from the Sigsbee Knolls and salt domes of Cuba. The subsequent structural evolution of the Gulf of Mexico basin is believed to have been mainly a result of interaction between it and tectonic plates of the Pacific area. After opening of the gulf, subduction began along the Cuban arc, where Atlantic rifting had created a nearly uninterrupted tract of oceanic crust extending from the Pacific between Yucatán and Colombia. Then, folding and gravity sliding associated with Laramide deformation on the west side of the gulf led to salt anticlines that underlie the ridges offshore from Mexico. Except for continued diapirism and subsidence associated with sediment loading, present tectonic activity is confined to the southwestern corner of the gulf, where volcanism and intermediate-focus earthquakes are a distant manifestation of subduction along the Middle America Trench on the Pacific side of Mexico.

Gulf of Mexico↗
USGS1974

Offset plutons and history of movement along the McKinley segment of the Denali Fault system, Alaska

The Foraker and McGonagall plutons, bodies of granodiorite with nearly identical mineralogy and chemistry, are considered to be parts of a single igneous mass that has undergone right-lateral displacement of about 38 km along the McKinley segment of the Denali fault system since the igneous mass crystallized about 38 m.y. ago. These offset plutons place severe constraints on the amount and rate of movement along the McKinley segment since the beginning of the Oligocene Epoch. The 38-km displacement indicates an average rate of 0.1 cm/yr if movement began immediately after crystallization in early Oligocene time or an average rate of 0.4 cm/yr if movement began 10 m.y. ago in late Miocene time. These rates, however, are considerably less than Holocene movement rates measured along the fault further to the east, which suggests that the Holocene displacement rate is greater than the pre-Holocene rate or that right-lateral movement along the fault may diminish to the west.

GSA Bulletin↗
USGS1974

Stratigraphic value of silicoflagellates in nontropical regions

Silicoflagellates are important biostratigraphic markers for age determination in nontropical regions because age-diagnostic calcareous microfossils are sparse. Upper Cretaceous and Cenozoic biostratigraphic zonation is proposed, based on silicoflagellates from Deep Sea Drilling Project sites in the subantarctic region.

GSA Bulletin↗
USGS1974

Composition and time relations of plutonic and associated volcanic rocks, Boulder Batholith Region, Montana

Comparison of areally weighted bulk compositions for the Boulder batholith and prebatholith volcanic rocks (Elkhorn Mountains Volcanics) shows a close match in terms of K 2 O-Na 2 O-CaO-SiO 2 variations. Detailed examination of available chemical data suggests, however, that the constituent units of the volcanic rocks differ among themselves, as well as from many of the batholith units. Most sampled volcanic rocks are chemically and isotopically similar to the volumetrically minor mafic members of the putonic main series of Tilling (1973). The sodic-series plutonic rocks, which form about 15 percent of the exposed batholith, apparently have no compositionally similar counterparts in the Elkhorn Mountains Volcanics in terms of bulk composition. The Butte Quartz Monzonite, which makes up about three-quarters of the batholith in terms of individual samples and bulk composition, is chemically distinct from most volcanic rocks. This relation suggests that only small amounts of magma of Butte Quartz Monzonite composition erupted onto the surface. The voluminous silicic ash flows of the middle member of the volcanic rocks were probably derived from the same magma that mostly crystallized subsurface to form the Butte Quartz Monzonite. Field, chemical, K-Ar age, and paleomagnetic evidence, however, strongly suggest that a time gap separated the extrusion of the middle-member volcanic rocks from the intrusion of the Butte Quartz Monzonite and related younger silicic variants. Compositional and time relations between the plutonic and genetically associated volcanic rocks are generally compatible with the concept that the volcanic rocks are eruptive equivalents of a shallow evolving batholith but seem incompatible with some specific aspects of the “extrusive complex” or “floored sheet” hypothesis as applied to the Boulder batholith by Hamilton and Myers (1967, 1974).

Montana↗
USGS1974

Nature of the angular unconformity between the Paleozoic metasedimentary rocks and the mesozoic metavolcanic rocks in the eastern Sierra Nevada, California

Two major wall-rock sequences, the Paleozoic metasedimentary rocks and the Mesozoic metavolcanic rocks, in the eastern Sierra Nevada, California, are separated by an angular unconformity rather than by a fault as has been proposed by other investigators. The unconformity is parallel to formation contacts in the younger metavolcanic rocks and crosscuts formation contacts in the older metasedimentary rocks. Locally, basal conglomerate with pebbles derived from the older metasedimentary rocks occurs just above the unconformity. The older metasedimentary rocks consist mostly of former marl, chert, and mudstone; they contain three generations of superposed structures. The younger metavolcanic rocks consist mostly of former andesite flows, tuff, and graywacke; these contain only the younger two generations of superposed structures. The sinuous trace of the unconformity is primarily the result of superposed deformations of regional scale which occur in most roof pendants of the eastern Sierra Nevada.

California↗
USGS1974

Oceanic crust forms basement of eastern Panamá

Basement rocks of parts of eastern Panamá include tholeiitic pillow basalt and diabase overlain by sedimentary rocks typical of deep oceanic environments. Both paleontologic and stratigraphic evidence indicate that some of these rocks are of Late Cretaceous age or older. Regional Bouguer anomalies over the basement terrane exceed +120 mgal, indicating that eastern Panamá is a raised block of oceanic crust. Age relations in the Caribbean region apparently preclude an “Atlantic” or single intra-Caribbean origin for eastern Panamá and the southern Caribbean basin, but multiple intra-Caribbean origins of the basaltic basement rocks are permitted by the age data. An in situ origin of the oceanic basement of Panamá at a position which later became a Cenozoic island arc is likewise consistent with the available geologic and geophysical data. Seismic horizon B″ in the Caribbean is correlative with or overlain by deep-sea sedimentary rocks of Coniacian to Campanian age. This horizon forms the top of the apparent basement of eastern Panamá and, thus, the eastern isthmus is a horstlike block that has been elevated or obducted a minimum of 6 km since Coniacian-Maastrichtian time, partly caused by northeastward underflow of the Pacific plate but possibly related to southwestward movement of the Caribbean plate beneath the isthmus.

GSA Bulletin↗
USGS1974

Contributions to the petrography and geochronology of volcanic rocks from the leeward Hawaiian Islands

Petrographic and chemical analyses of basalt from Nihoa Island, Necker Island, French Frigate Shoals, and Midway Atoll, all in the leeward part of the Hawaiian chain, confirm that these islands are subaerial remnants of tholeiitic shield volcanoes similar to those that form the principal Hawaiian Islands. Chemistry suggests that Gardner Pinnacles may be part of the alkalic cap on a tholeiitic shield. Weighted mean potassium-argon ages of 7.0 ± 0.3 m.y. for Nihoa, 10.0 ± 0.4 m.y. for Necker, 11.7 ± 0.4 m.y. for French Frigate, and 17.9 ± 0.6 m.y. for Midway demonstrate that the ages of these volcanoes increase northwestward, continuing the trend of increasing age away from the active volcano of Kilauea shown by the main islands. The increase in age with distance along the chain, however, appears to be nonlinear. The results support the general hypothesis that the volcanoes of the Hawaiian chain have a common origin and were formed as the Pacific plate moved northwestward over a melting spot in the mantle.

Hawaii↗