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181.
182.
El Misti is a 5822 m strato volcano in southern Peru. The mineralogy of the fumaroles is essentially sulfur, anhydrite, gypsum and ralstonite which are precipitated by acid vapors at 100°C–125°C. There are 2 to 6 parts per thousand chloride concentrations and several parts per million sodium, potassium, magnesium, calcium, iron and zinc concentrations in the condensed vapors. Elemental ratios indicate near complete leaching of the wall rock by the vapors. Arsenic and selenium are contaminants of the sulfur but are absent from the other minerals. Both the gypsum and anhydrite contain strontium and lead and show no tendency to fractionate these metals. The anhydrite alone contains significant thallium and bismuth. Bromine, strontium, thallium and lead are present in the ralstonite. The average δ D and δ O18 ratios from the fumarolic condensates are ?31.4 ‰ and +4.8 ‰ respectively. A marine contaminated heavy water source at depth is indicated as the parent water for the vapors. 相似文献
183.
The inner bay of Lourenço Marques receives the untreated sewage of the city, the run-off from five rivers and metal and hydrocarbon pollution from the activities of the port. Iron ore handling results in an increase in iron and copper concentrations in the water, particularly since loading by slurry was introduced. Large quantities of metal pollutants are probably accumulating in muds and are likely to contaminate shellfish which are an important food source in the area. Untreated sewage discharges are at such a level that eutrophication of the bay is threatened, besides constituting a public health risk on bathing beaches. 相似文献
184.
A three-dimensional model covering the northwest European Shelf and part of the adjacent Atlantic Ocean is used to examine the influence of water depth change upon the distribution of maximum tidal bed stress. The direction of bed stress is an indicator of sediment movement as bed load and various regions of convergence and divergence in good agreement with observations are identified. Calculations are performed with water depths reduced by 35 m, corresponding to 10 000 years before present (B.P.). Initially, the model is forced by only the M2 tide, although subsequently five constituents, namely M2, S2, N2, K1 and O1, are used for tidal forcing. Although the distribution of extreme bed stresses computed with only M2 tidal forcing is comparable to that computed with five tides, the additional tidal constituents modify the magnitude of the bed stress. In particular the diurnal tides show regions of local enhanced current amplitude in the shelf-edge region with corresponding changes in bed stress. When water depths are reduced such that the North Sea and English Channel are separated, then there is a significant change in the tidal distribution in the shallow Southern Bight which influences bed-stress distributions and hence bed-load sediment transport in the area. Besides changes in shallow regions, the distribution of tides at the shelf edge is affected. A discussion of the limitations of the present coarse-grid model in shelf-edge regions and how it can be used to provide boundary conditions for limited-area three-dimensional models that can include stratification is presented. Also the importance of stratification for sediment movement at the shelf edge is briefly discussed.Responsible Editor: Phil Dyke 相似文献
185.
An accelerometer array at Pacoima Dam with three locations along the base and abutments recorded ground motion from a magnitude 4.3 earthquake on 13 January 2001. These records present an opportunity to study spatial nonuniformity for the motion in a canyon. Topographic amplification is characterized by ratios of response spectral displacement between locations, and seismic wave travel times are studied using cross‐correlation functions to obtain delays. Results of the analysis of the 2001 earthquake records are used to generate ground motion for the 1994 Northridge earthquake to replace records that were not able to be fully digitized. The ground motion generated for the Northridge earthquake is used as input to a finite element model of Pacoima Dam. The response of the model is consistent with observations of Pacoima Dam after the Northridge earthquake. Comparison of the response due to nonuniform input with the response due to uniform input demonstrates the importance of accounting for spatial nonuniformity because of the significance that the pseudostatic component has for the response to nonuniform input. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
186.
Four Late Holocene pyroclastic units composed of block and ash flows, surges, ashfalls of silicic andesite and dacite composition,
and associated lahar deposits represent the recent products emitted by domes on the upper part of Nevado Cayambe, a large
ice-capped volcano 60 km northeast of Quito. These units are correlated stratigraphically with fallout deposits (ash and lapilli)
exposed in a peat bog. Based on 14C dating of the peat and charcoal, the following ages were obtained: ∼910 years BP for the oldest unit, 680–650 years BP for
the second, and 400–360 years BP for the two youngest units. Moreover, the detailed tephrochronology observed in the peat
bog and in other sections implies at least 21 volcanic events during the last 4000 years, comprising three principal eruptive
phases of activity that are ∼300, 800, and 900 years in duration and separated by repose intervals of 600–1000 years. The
last phase, to which the four pyroclastic units belong, has probably not ended, as suggested by an eruption in 1785–1786.
Thus, Cayambe, previously thought to have been dormant for a long time, should be considered active and potentially dangerous
to the nearby population of the Interandean Valley.
Received: 5 July 1997 / Accepted: 21 October 1997 相似文献
187.
The spring of 1997 has represented a stable period of operation for the joint University of Tromsø/University of Saskatchewan MF radar, being between refurbishment and upgrades. We examine the horizontal winds from the February to June inclusive and also include estimates of energy dissipation rates derived from signal fading times and presented as upper limits on the turbulent energy dissipation rate, . Here we address the periodicity in the dynamics of the upper mesosphere for time scales from hours to one month. Thus, we are able to examine the changes in the spectral signature of the mesospheric dynamics during the transition from winter to summer states. 相似文献
188.
Derek R. Hall 《GeoJournal》1998,46(3):283-287
This paper examines the prospects for Albanian rural diversification with particular reference to rural tourism. In the early
1990s, tourism was identified by the Albanian government and Western development consultants as a likely vehicle for an appropriate
and sustainable development strategy, given the relative wealth generated by international tourism in neighbouring Greece,
Italy and Croatia. Pressure has been exerted by commercial developers for mass coastal tourism and permission for around 40 new
tourist development has been granted. But Albania embraces a considerable wealth of natural and cultural heritage, which provides
opportunities for the development of small scale rural tourism carefully integrated into the local economy with respect to
organic farming, quality wine production and sustainable fishing. But sensitive social integration is vital where it is difficult
to identify rural communities where there is a conspicuous absence of local consensus. The resurgence of traditional social
systems and the empowerment of local actors in cases where organised crime has taken root pose major problems for rural diversification.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
189.
190.
A laboratory simulation of rock breakdown due to freeze-thaw in a maritime Antarctic environment 总被引:1,自引:0,他引:1
Kevin Hall 《地球表面变化过程与地形》1988,13(4):369-382
Results of freeze-thaw simulations on three large blocks of quartz-micaschist are presented. Three types of water to ice phase change were identified from temperature and ultrasonic measurements. It is suggested that the type of phase change results from a particular combination of rock moisture content, solute concentration, freeze amplitude, and rate of fall of temperature. The temperature at which ice thawed inside the rock (?0.7 to ?1.9°C) was also found, and this indicates the possibility of freeze-thaw effects without positive temperatures. Approximately 80 per cent of the water that will freeze under natural conditions, in the Maritime Antarctic environment under study, appears to have done so by ?6°. 相似文献