Seiches induced by storms in the English Channel

Investigation of an extreme storm surge event revealed the presence of an oscillation in sea level with a period of 2-6 h. The English Channel is well known for its quarter- and sixth-diurnal tides, and it was expected that these oscillations were simply the result of tide-surge interactions. However, results of model experiments permitted the reproduction of oscillations from wind forcing alone. The forcing used was from an extreme wind event in December 1989. In this article we describe these oscillations, and in particular their spatial characteristics. It is shown that the dominant response is a transverse mode of the English Channel, with minimum amplitude in the central Channel and maximum amplitude in the Baie de Seine and the Golfe de St Malo.     

Permo–Triassic unconformity-related Au-Pd mineralisation, South Devon, UK: new insights and the European perspective

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (¹³C and ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, <0.5 NaCl/(NaCl+CaCl₂)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl₂)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity). The sodic brine is reasoned to be the parent metalliferous fluid and to have acquired its enrichment in Au and Pd by the leaching of immature sediments and intra-rift volcanic rocks within the local Permo–Triassic basins. Metal precipitation is linked to the destabilisation of Au and Pd chloride complexes by either mixing with calcic brines, dilution by groundwaters or interaction with reduced lithologies. This explains the diversity of mineralised settings below and above the unconformity and their affinity with red bed brines. The paucity of sulphide minerals, the development of selenides (as ore minerals and as mineral inclusion in gold grains), the presence of rhodochrosite and manganoan calcites (up to 2.5 wt% Mn in calcite) and the co-precipitation of hematite and manganese oxides are consistent with the overall high oxidation state of the ore fluids. A genetic model is proposed linking Permo–Triassic red beds, the mixing of oxidising and reducing brines, and the development of unconformity-related precious metal mineralisation. Comparison with other European Permo–Triassic basins reveals striking similarities in geological setting, mineralogy and geochemistry with Au, Au-Pd and selenide occurrences in Germany (Tilkerode, Korbach-Goldhausen), Poland (Lubin) and the Czech Republic (Svoboda nad Úpou and Stupná). Though the known Au-Pd occurrences are sub-economic, several predictive criteria are proposed for further exploration.Editorial handling: B. Lehmann     

Geochronological Constraints Using 40Ar/39Ar Dating on the Mineralization of the Hishikari Epithermal Gold Deposit, Japan

Abstract. Ages for thirty adularia samples collected from various veins were in the Hishikari gold deposit determined by ⁴⁰Ar/³⁹Ar dating to constrain the timing of adularia‐quartz vein formation and to determine the temporal change in temperature of hydrothermal fluid. Plateau ages were obtained from all adularia samples, and significant excess ⁴⁰Ar is not recognized from inverse isochrones. The duration of mineralization within individual veins was determined by adularia ages from the early and late stages of mineralization within the same vein. The durations of mineralization in the Daisen‐1, Daisen‐3, Hosen‐2 and Keisen‐3 veins in the Honko‐Sanjin zone were 7,000, 140,000, 160,000 and 170,000 years, respectively. The durations of mineralization in the Seisen‐2 and Yusen‐1–2 veins in the Yamada zones were 360,000 and 320,000 years, respectively. Mineralization lasted for a relatively longer period in individual veins at the Yamada zone. Mineralization ages from the Honko‐Sanjin zone range from 1.04 to 0.75 Ma, and most mineralization ages are concentrated in a short period from 1.01 to 0.88 Ma. In contrast, mineralization ages for the Yamada zone range from 1.21 to 0.64 Ma. These results indicate that fracturing and subsequent vein formation lasted for a longer period in the Yamada zone (about 570,000 years) compared with those events in the Honko‐Sanjin zone (about 290,000 years). The homogenization temperatures of liquid‐rich fluid inclusions in columnar adularia used for age determination were determined to be 223°C on average, and most of these temperatures range from 180 to 258d?C. No significant temporal change in homogenization temperature is recognized in this study. However, adularia in the Keisen veins indicated higher homogenization temperatures compared with elsewhere in the deposit, suggesting that the principal ascent of mineralizing hydrothermal fluid was via the Keisen veins.     

Impact of a rock avalanche on a moraine‐dammed proglacial lake: Laguna Safuna Alta,Cordillera Blanca,Peru

Moraines that dam proglacial lakes pose an increasing hazard to communities in the Andes and other mountain ranges. The moraines are prone to failure through collapse, overtopping by lake waters or the effect of displacement waves resulting from ice and rock avalanches. Resulting floods have led to the loss of thousands of lives in the Cordillera Blanca mountains of Peru alone in the last 100 years. On 22 April 2002 a rock avalanche occurred immediately to the south‐west of Laguna Safuna Alta, in the Cordillera Blanca. The geomorphic evidence for the nature, magnitude and consequences of this event was investigated in August 2002. Field mapping indicated that the avalanche deposited 8–20 × 10⁶ m³ of rock into the lake and onto the surface of the frontal region of Glaciar Pucajirca, which flows into the lake. Repeated bathymetric surveying indicated that ～5 × 10⁶ m³ of this material was deposited directly into the lake. The immediate effect of this event was to create a displacement wave that gained in height as it travelled along the lake basin, overtopping the impounding moraine at the lake's northern end. To achieve overtopping, the maximum wave height must have been greater than 100 m. This, and subsequent seiche waves, caused extensive erosion of both the proximal and distal faces of the impounding terminal moraine. Further deep gullying of the distal face of this moraine resulted from the supply of pressurized water to the face via a relief overflow tunnel constructed in 1978. Two‐dimensional, steady‐state analysis of the stability of the post‐avalanche moraine rampart indicates that its proximal face remains susceptible to major large‐scale rotational failure. Copyright © 2005 John Wiley & Sons, Ltd.     

Monte Vulture Volcano (Basilicata,Italy): an analysis of morphology and volcaniclastic facies

The earliest activity of Monte Vulture, central Italy, included ignimbrites but the bulk of the volcano was built up by plinian airfall deposits. Contemporaneous remobilisation of these deposits formed an apron of lahars around the base of the main cone. The volcano was constructed on a ridge; the valley to the east and tributaries to the north and south became sediment traps for volcaniclastic materials emplaced by fluvial reworking and directly from volcanic activity. To the west the valley was swept clear by active downcutting. Instability of the west flank as a result of this erosion was probably a contributory cause of major gravitational sector collapse on the volcano's flank, terminating the main cone-building phase. The resultant scar is an amphitheatre-shaped hollow called here the Valle dei Grigi. Previous workers have attributed this feature to coalescing calderas formed by engulfment. The last volcanic phase was the production of the Monticchio calderas and associated phreatomagmatic explosions producing airfall and surge deposits. Because most of the activity at Vulture has been repeated plinian eruptions producing similar assemblages of products, detailed stratigraphy of the volcano is difficult to accomplish. To characterise Vulture in terms of its products, various facies are identified and interpreted in relation to volcanic processes, distance from vent and environmental conditions.     

Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation,Namibia

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher ¹⁸O values than normal mantle. The basic rocks (average of 29=8.8) have significantly different ¹⁸O to the acid rocks (average of 10=14.4) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between ¹⁸O, SiO₂ and _Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a ¹⁸O value of 15 which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst ¹⁸O data for the high _Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between _Sr and SiO₂ resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO₂ ¹⁸O because the material with the most Si-O bonds was able to concentrate ¹⁸O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene ¹⁸O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.     

Volcán Ecuador,Galapágos Islands: erosion as a possible mechanism for the generation of steep-sided basaltic volcanoes

Volcán Ecuador (0°02′S, 91°35′W) consists of two strongly contrasting components: the eroded and vegetated remnant of a once-circular main volcano with a probable caldera, and a prominent rift zone extending to the northeast that is neither strongly eroded nor weathered. There are about 20 young-looking flows and vents on this caldera floor but only one on the higher remnant of the main volcano. The southwest half of the main volcano is faulted into the ocean. The main part of Volcán Ecuador possesses steep erosional slopes (average 30–40°) that cut into a sequence of flows that dip radially outward at <10°. In contrast, the northeast rift zone consists entirely of young flows and vents. The upper 10 km of the rift zone forms a peninsula about 7.5 km wide that connects Volcán Ecuador to Volcán Wolf. The rift zone bends to the southeast and the lower 8 km is tangential to the coast of Volcán Wolf. The rift zone axis dips away from the northeast edge of the main volcano, and its flanks slope roughly northwest and southeast at <4°. The rift zone is the Galápagos structure that most closely resembles a Hawaiian rift zone because it is constructed of lavas from subparallel linear vents, shows evidence of a deep feeder conduit, and has changed its direction to avoid a direct intersection with neighboring Volcán Wolf. The steep erosional slopes extending around the perimeter of the main volcano (except to the southwest where slumping occurred) were probably generated by marine erosion during a prolonged period of eruptive inactivity (perhaps 20 000–30 000 years). Only a few post-erosional eruptions have taken place at the main volcano in and near what was once the caldera. The entire rift zone postdates the period of prolonged erosion. Using the evidence for prolonged inactivity at Volcán Ecuador, we propose that erosion may have helped to produce steep slopes on the other western Galápagos volcanoes. On these more active volcanoes, however, numerous subsequent eruptions have completely mantled the erosional slopes with lava. The mechanism by which the volcanoes may shut off for long periods of time is unknown, but the fact that the Galápagos hotspot is presently supplying nine active volcanoes suggests that the magma supply at an individual volcano could vary greatly over periods of (tens of?) thousands of years.     

The Karoo igneous province — A problem area for inferring tectonic setting from basalt geochemistry

Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin.When data for the “southern” Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of “within-plate” basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a “subduction component” has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities.In contrast, the data for “northern” Karoo basalts largely plot in the “within-plate” field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks.     

The evolution of lava flow-fields: observations of the 1981 and 1983 eruptions of Mount Etna,Sicily

The eruptions of Mount Etna in 1981 on the north flank and 1983 on the south flank of the volcano were of strikingly different character. The former was a short duration, high effusion rate eruption producing for the most part a simple flow-field; the latter was of relatively long duration and low effusion rate, producing a compound flow-field of overlapping flows.Despite the differences between the eruptive behaviour of these two events and the way in which the flow-field developed, both the flow-fields achieved about the same maximum length. This is considered fortuitous. The evidence suggests that the main 1981 flow stopped because the lava supply ceased and was thus volume controlled. The 1983 flow-field had a more complex history of branching, but in this case it appears that, for the longest individual flow, cooling played an important role in controlling the maximum extent of the flows.     

The ejection of drops from the sea and their enrichment with bacteria and other materials: A review

This paper is a review of work done on the production of an aerosol by the sea, and on the mechanisms by which bacteria might be enriched on the aerosol. Air bubbles produced in the sea, primarily by breaking waves, burst at the surface to eject both film and jet drops into the atmosphere. These drops are mixed upward by turbulence to produce the well-known marine sea-salt aerosol. In rising through the water, the bubbles may scavenge bacteria which, when the bubble burst, are skimmed off the bubble and ejected upward with the jet drops. Depending on drop size, the distance the bubble moves through the water, and other factors, the concentration of bacteria in jet drops can be several hundred times that in the bulk water film drops can also be enriched with bacteria. The enrichment of jet and film drops with bacteria, viruses, or toxins may at times produce a health hazard for those living along the shore.