Large Vent Research Articles

Seismicity and tremor in a submarine hydrothermal field: the northern Juan De Fuca Ridge

We have made the first multi‐year deployments of a small number of ocean bottom seismometers on a mid‐ocean ridge. We located our instruments in and around a vigorous hydrothermal vent field on the Endeavour Segment of the Juan de Fuca Ridge. The deployments have revealed a consistent pattern of activity comprising several types of events. Most abundant are swarms of extremely small microearthquakes that we suggest are associated with cracking of the oceanic crust. We also observe local tectonic earthquakes that occur about four times per day. Preliminary hypocentral locations place these tectonic events away from the ridge axis and outside the hydrothermal field. The seismicity on the Endeavour Segment is remarkably different from the southern Juan de Fuca Ridge where little, if any, tectonic activity is occurring. We speculate that the Endeavour and southern Juan de Fuca Ridge are in two different phases of their long‐term evolution. The latter has only rare activity that is only beginning to focus hydrothermal circulation along faults. In contrast, the Endeavour Segment is in an active tectonic phase; and the faults have had time to develop robust hydrothermal circulation, resulting in large hydrothermal vent deposits.

Direct Observations of a Large Active Mud Vent on the Louisiana Continental Slope: ABSTRACT

ABSTRACT High resolution geophysical data taken in support of drilling offshore acreage frequently reveal impressive evidence for expulsion of fluids, gases, and sediments on Louisiana's continental slops. Features resulting from extrusion of sedimentary materials at the sea floor range from small pock marks and vents a few meters in diameter to mud diapirs and volcano-shaped cones of sediment that can be greater than 1 km diameter. In September 1989, the Johnson Sea-Link research submersible was used to study a large active mud vent in the Green Canyon Area, Blocks 143-144. This accretionary feature had a positive relief of approximately 70 m and a diameter of about 300 m. Flanks of the volcano-shaped mud vent were composed primarily of fine-grained sediment pocked by small burrows and grooved by gravity-driven downslope sediment transport. Lithification of the cone flanks was evident in isolated areas, many of which are undercut perhaps by escaping fluids and gas, and along ridges oriented down the cone sides. These ridges of the cone flank ridge and groove topography were found to have undergone surface lithification. The lithified materials were composed of both host sediments (terrigenous muds) cemented with aragonite and Mg-calcite and isolated authigenic carbonate buildups ( The crater at the apex of this feature was approximately 40 m diameter and rimmed by levees Beggiatoa (up to 200 um diameter), covered the fluid mud surface within the crater. Gas escaping from the crater floor caused fine-grained sediments to be entrained in the water column. A turbid cloud was formed that subsequently cascaded down the crater flank. Sediments collected from the crater floor contained abundant crude oil and gas. Two dives were made on this mud vent. Gas was observed escaping from the crater only during the second dive. The conditions responsible for development of these impressive features and their frequencies of eruptions are not well-known. End_of_Record - Last_Page 735-------

Phase relations and adiabats in boiling seafloor geothermal systems

Observations of large salinity variations and vent temperatures in the range of 380–400°C suggest that boiling or two-phase separation may be occurring in some seafloor geothermal systems. Consideration of flow rates and the relatively small differences in density between vapors and liquids at the supercritical pressures at depth in these systems suggests that boiling is occurring under closed-system conditions. Salinity and temperature of boiling vents can be used to estimate the pressure-temperature point in the subsurface at which liquid seawater first reached the two-phase boundary. Data are reviewed to construct phase diagrams of coexisting brines and vapors in the two-phase region at pressures corresponding to those of the seafloor geothermal systems. A method is developed for calculating the enthalpy and entropy of the coexisting mixtures, and results are used to construct adiabats from the seafloor to the P-T two-phase boundary. Results for seafloor vents discharging at 2300 m below sea level indicate that a 385°C vent is composed of a brine (7% NaCl equivalent) in equilibrium with a vapor (0.1% NaCl). Brine constitutes 45% by weight of the mixture, and the fluid first boiled at approximately 1 km below the seafloor at 415°C, 330 bar. A 400°C vent is primarily vapor (88 wt.%, 0.044% NaCl) with a small amount of brine (26% NaCl) and first boiled at 2.9 km below the seafloor at 500°C, 520 bar. These results show that adiabatic decompression in the two-phase region results in dramatic cooling of the fluid mixture when there is a large fraction of vapor.

Gasdynamics of vented explosions part I: Experimental studies

An experimental investigation is described of the gasdynamic processes occurring in vented explosions in which two cubical vessels are employed, with internal sides of 18 and 38 cm, repectively. Both vessels are fitted with windows which allow access to the whole field of view for optical observations. The flow field was studied by high-speed framing and streak schlieren cameras and the velocity of the unburned gas by hot-wire anemometry. Pressure signatures were also obtained at the vessel wall by piezoelectric transducers. When venting occurs through large and intermediate vent areas, Helmholtz oscillations are generated within the vessel, and the resulting flame accelerations give rise to Taylor instabilities of the flame front. For smaller vent areas the fundamental acoustic mode of the vessel is excited, which, at higher breaking pressures, is usually coupled with a second pressure peak which can be several times the magnitude of the first peak. Possible mechanisms responsible for this phenomenon are discussed.

Petrography and age of agglomeratic vents near Toscaig, Applecross, Scotland

The occurrence of two small volcanic vents on the Applecross 1:50,000geological sheet 81(E) was recently noticed during a study (Rock, 1981) of the PermoCarboniferous vents of the Lochaber area, further south. The vents were recorded in 1892 by J. Horne and a few thin sections were made (IGS Scotland sliced collection Nos. S5167-72), but they seem never to have been described. The area was therefore revisited briefly and further thin sections (S64942-6) prepared. The vents occur in the Applecross peninsula (Fig. 1a) to the southeast of Cam nan Uaighean, between the gorge of the River Toscaig and Lochan nan Slochd; the actual locality [NG 737398] (Fig. 1b) is named as 'Bruach Bhuidhe' on 6-inch sheet Ross 109SW. One vent lies on each side of a shallow and partly dry valley, containing a small stream which drains Lochan nan Slochd. The site of the larger vent can be recognised by an almost complete absence of exposure, which contrasts strongly with the craggy ground occupied by the Torridonian sandstone (Applecross Formation) country rocks on all sides; it forms a low grassy knoll surrounded by depressions. Exposures of both vents are extremely poor, and no contacts are seen. The larger vent reaches approximately 200 m by 100 m and its rock-type varies considerably; a tiny exposure on the southwest slope of the knoll [NG 73753890] is formed almost entirely of friableweathering volcanic matrix; Horne recorded a 'decomposed basalt' dyke but this is no longer visible. By contrast, a second exposure at the east end of the northern slope isalmost entirely composed of clasts with only very sparse matrix, while a third, slightly further west, is intermediate in texture between these other two. The clasts are of only one type-a grey gritty sandstone-and the larger ones are often much rounded due to extraordinary spheroidal fracturing (noticeable in both vents); smaller clasts may however be angular. The smaller vent is slightly better seen at [NG 73603910], where there are four small exposures (Fig. 1c): (a) Subsoil outcrop of fractured and weathered igneous rock, carrying numerous well-crystallised pseudomorphs of saponite preserving a good basal cleavage and presumed to be after megacrysts of biotite (XRD identification of saponite by courtesy of Mr. R. I. Lawson). (b) Weathered agglomerate with similar sandstone clasts to those in the larger vent. (c) Variably altered agglomerate with two large masses of this sandstone (2 m and 1 m diameter), showing well-developed spheroidal fracture. (d) Agglomerate dominated by spheroidally fractured clasts, with a rather sparse, though somewhat fresher than usual, matrix. These vents are peculiar for their isolation. Igneous activity in the Applecross peninsula is otherwise represented only by N-S trending Tertiary basalt dykes and a few Caledonian felsites and lamprophyres (see geological sheet 81E). The nearest vents elsewhere on the mainland are the Tertiary ones of Ardnamurchan, and the Permo-Carboniferous ones in the Loch Arkaig area, all some 70-80 km distant. Rather nearer are three vents on Raasay in the area around Brochel Castle [NG 584463], 15 km northwest of Toscaig, which were described by Davidson (1935) and Selley (1966). These are far better exposed but of similar size to the Toscaig vents and, like them, penetrate Torridonian strata.

Probe measurement of SPL in hearing-aid occluded ear canals

A minature probe microphone was developed for use with earmolds to measure eardrum SPL. The probe has been designed using the earmold as an integral part of its acoustic system and no part of the probe itself projects into the ear canal. Previous ear simulator data indicated accurate (±2 dB) eardrum SPL measurements at frequencies up to 5.0 kHz. To determine the effects of ear-canal variations on probe measurement accuracy, tests were performed on normal hearing adults. Assuming that pure tone thresholds are not different for the ear either open or occluded, thresholds measured (tracking procedure) with an earphone were compared to those measured via a hearing-aid receiver and earmold-probe assembly at frequencies between 500 Hz and 6.0 kHz. Both systems were calibrated on the same simulator. No significant differences were observed between the two sets of threshold measurements up to frequencies of 6.0 kHz. Subsequent measurements indicated that a large parallel vent in the earmold had no effect on the probe microphone accuracy. Results suggest that the probe-earmold procedure can be used to measure occluded eardrum SPL up to frequencies of 6.0 kHz.

Mucus production and its role in the feeding behaviour of Diarthrodes nobilis (Copepoda: Harpacticoida)

Using light microscope histology and scanning electron microscopy, the secretion, storage and discharge of copious quantities of mucus has been demonstrated in the algadwelling harpacticoid copepod Diarthrodes nobilis (Baird). The mucus is secreted by cells closely associated with large haemocoelomic reservoirs, located in the cephalothorax and urosome, which store the mucus prior to its discharge through large integumental vents (up to 38 μm in greatest dimension). A denned behavioural sequence is recognized whereby the animal utilizes discharged mucus to construct an enmeshing capsule. Late stage nauplii are also capable of capsule fabrication.The mucus bag presumably, acts as a rich and rapidly degradable substrate to marine prokaryotes and is therefore rapidly colonized; suspended organic and inorganic materials are also trapped by the mucus. Observations on gut contents and browsing activity of capsule inhabitants indicate a direct trophic dependence of D. nobilis on the bacteria and organic agglutinations that adhere to the mucus. Utilization of such nutritive materials supports a ‘mucus-trap’ hypothesis recently proposed as a feeding strategy in meiobenthic nematodes.

Cost of noise control as seen by a public utility

Promulgation of federal, state, and local noise control regulations may require electric utilities to reduce noise emissions. Equipment requiring noise abatement includes central power station, substation, and street excavation operations. Purchasing new noise-controlled equipment, retrofitting existing equipment, and acquiring additional real estate for acoustic buffer zones are discussed relative to noise regulation compliance. Costs to abate major power plant noise sources including large fans, steam vents, exterior walls, and alternate cooling systems (cooling towers) are discussed. Economic trade-offs between sub-station transformer source noise emissions and external noise controls are presented. An example of costs for purchase of new street excavation equipment for compliance with a local noise control code is explored.

Effects of Earmold Modifications as Measured in a 2-cm3 Coupler and upon the Psychophysical Experience of Loudness

This study explored the relationship between physical measurements of intensity and psychophysical measurements of loudness using four standard-type earmolds. Two of the earmolds were unaltered, while a third contained a small vent, and the fourth a large vent. The physical measurements were derived using a 2-cm3 a coupler; psychophysical data were derived with an alternate binaural loudness balance technique using seven normal-hearing subjects. The test signals employed to collect the psychophysical data were pure tones of 300, 500, and 2300 Hz, each of which was presented at 45, 55, and 65 dB SPL. Significant changes in loudness did not occur due to vent size although frequency response changes were apparent for the vented and nonvented earmolds. The data which were obtained suggest that acoustic measurements derived in a laboratory with a 2-cm3 a coupler should not be indiscriminantly applied as an index to the way in which such physical changes in intensity are auditorily perceived.

Transitional basalts from the eastern Australian tertiary province

Study of Tertiary volcanic rocks from Mt. Tamborine in S. E. Queensland, Australia, suggests that some basalts from the large Tweed vent are intermediate in character between true tholeiites and well documented alkaline basalts of the Australian Tertiary province. Geochemical evidence and comparison with synthetic systems suggests that the transitional lavas are derived by continuous fractional crystallization during the ascent of a parent magma produced by substantial partial melting of the upper mantle at moderate depths and pressures. Alkaline basalts that underlie the transitional basalts are the result of relatively rapid, intermittent extrusion of magma generated by a smaller degree of partial melting of the upper mantle. This interpretation resolves the difficulties posed by an apparent inversion of the usual stratigraphic relationship of tholeiites capped by alkali basalts, and is in accord with recent seismic studies of the continental margin of eastern Australia.

The venting of pentane vapour-air explosions on a large vessel

Experiments have been carried out on the venting of pentane vapour-air explosions in a 60 ft 3 vessel. The effects of bursting disc operating pressure and vent size were investigated for both stationary and turbulent gas mixtures. For the turbulent mixtures experiments were also carried out on the effect of degree of turbulence on maximum explosion pressure. It was found that turbulence had a profound effect on maximum explosion pressure. In explosions of stationary gas mixtures the bursting of the disc itself caused turbulence and affected the maximum explosion pressure for a given vent size, while for explosions of gas mixtures which were turbulent initially, pressure increased with increase in degree of turbulence. For the smaller vent sizes explosion pressure reached a maximum value with increase in degree of turbulence, but this maximum was not reached for the larger vents with the existing equipment. The operating pressure of the bursting discs was found to vary with rate of application of pressure, and under explosion conditions all the discs used burst at a higher pressure than their static rating.

The Role of Pemmican in the Canadian Northwest Fur Trade

The Role of Pemmican in the Canadian Northwest Fur Trade

STRUCTURAL RELATIONSHIPS OF TUFF-BRECCIA IN ABSAROKA RANGE, WYOMING

Andesitic tuff-breccia showing unusual structural relationships is exposed throughout at least 100 square miles within two river valleys in the Absaroka Range of northwest Wyoming, a dissected mass of essentially horizontal volcanic deposits. The tuff-breccia occurs as sheetlike bodies within an otherwise dominantly detrital volcanic sequence. At least one, and perhaps all, of these sheets merges with thick masses of pyroclastic material. They display both intrusive and apparently gradational relationships to the detrital beds. The tuff-breccia has the composition of pyroxene andesite but contains much foreign material, largely blocks of dacitic detrital volcanic rock, some of which are probably several miles from their source. Pronounced hardening (silicification ?) and greenish-gray coloration characterize dacitic detrital rock adjacent to the tuff-breccia. Most of the tuff-breccia was probably erupted from a northwest-trending series of large vents. The structural relationships can be best explained by post-depositional movement of several sheets of tuff-breccia originally deposited at the surface and buried by detrital sediments. The tuff-breccia seems to have been injected as mud at relatively shallow depth into the detrital sequence, which was poorly consolidated. How this post-depositional emplacement took place is not determined. Neither primary injection nor tectonic activity alone seems adequate to explain the unusual structural relationships; more probably, gravity was the main control, possibly aided by local doming.

Volcanic vents at Longskelly Rocks and at Yellow Craig Plantation, west of North Berwick

On the long stretch of coast between Weak Law and Longskelly Point there are a few interesting sections, but especially at the latter place, where evidence has been found pointing to the presence of a rather large volcanic vent, a great part of the area of which is much obscured by an extensive intrusion of basalt approaching monchiquite in type (see map, Fig. 1). Also at a place called Yellow Craig Plantation, a short distance inland from Longskelly Point, there is good reason to believe that a volcanic vent exists, either connected with the one on the shore, or separated from it (see map, Fig. 2). The deposits in connection with these vents, both on the shore and on the ground round about Yellow Craig Plantation, are mentioned in the Geological Memoir for East Lothian, 1910, p. 82, and are there regarded as beds of conglomerate containing boulders and fragments of trachyte and basalt, having occasionally an ashy appearance, and as being an upper representative of the sedimentary rocks lying above and in contact with the Markle basalt lava at Cowton Rocks. According to the map ( Geological Survey, East Lothian, 1910, p. 59), these rocks occupy in horseshoe form the southern edge of a shallow syncline which extends westwards from the Cowton Rocks, as far as and embracing Yellow Craig Plantation, and on the shore to the west of the Longskelly intrusion. The deposits on the shore at Longskelly, which occupy an area somewhat raised above the level of the