Pyroclastic geology of the rhyolitic volcano of La Primavera,Mexico

Explosive eruptions of this large Quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites. A stratigraphie study reveals more than 30 fall deposits, totalling more than 90 km³, which have come from many different vents. Some are of plinian type, and one is among the largest known in the world. The main ignimbrite (Rio Caliente) is of intra-plinian type. In all, 140 km³ of rhyolitic material were erupted during the past 10⁵ years, of which 50 km⁵ are dispersed well outside the volcano. The dense rock equivalent volume is 60 km³, of which fall deposits, ignimbrites and lava bodies comprise 45 %, 25 % and 30 % respectively. The average output rate of 0.06 km³ per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known. A circular area 10 km across in the centre of La Primavera contains updomed lacustrine ashes and associated sediments, including a remarkable giant pumice bed, which probably accumulated in a caldera lake. La Primavera has no record of historical eruptions, and hot springs are the only signs of present activity, but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable.

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Zusammenfassung Explosionsartige Eruptionen des gro\en quartÄren Rhyolith-Vulkans La Primavera haben mÄchtige Bimstuffe und Ignimbrite aufgehÄuft. Eine stratigraphische Untersuchung hat über 30 Aschelagen erkennen lassen, die aus verschiedenen Schloten stammen und insgesamt mehr als 90 km³ an Volumen ausmachen. Einige davon gehören zum plimanischen Typ, von denen eine sogar die grö\te bisher bekannte Lage der Erde ist. Der Haupt-Ignimbrit (Rio Caliente) gehört zum intraplimanischen Typ. Insgesamt wurden in den letzten 100 000 Jahren 140 km³ rhyolithisches Material ausgeworfen, von denen 50 km³ deutlich au\erhalb des eigentlichen Vulkans liegen. Das Äquivalentvolumen des dichten Gesteins betrÄgt 60 km³, von dem 45 % auf Tuffe, 25 % auf Ignimbrite und 30 % auf Lavakörper entfallen. Die durchschnittliche Auswurfrate von 0,06 km³ pro Jahrhundert liegt fast eine Grö\enordnung unter den meisten bekannten aktiven Rhyolith-Vulkanen. Ein kreisrundes Gebiet von 10 km Durchmesser im Zentrum des La Primavera zeigt aufgewölbte lakustrine Aschen und damit assoziierte Sedimente, darunter eine bemerkenswerte riesige Bimslage, die wahrscheinlich in einem Caldera-See abgelagert wurde. Der Vulkan La Primavera ist in historischer Zeit nicht ausgebrochen und hei\e Quellen sind die einzigen Anzeichen heutiger AktivitÄt, aber die Aufwölbung wird als Hinweis auf das Hochdringen eines neuen sauren Plutons unter dem Vulkan angesehen, so da\ zukünftige Eruptionen wahrscheinlich sind.

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Résumé Des éruptions explosives de ce grand volcan rhyolitique quaternaire ont produit de volumineux dépÔts de ponces et d'ignimbrites. Une étude stratigraphique fait apparaÎtre plus de 30 dépÔts totalisant plus de 90 km³ provenant de nombreuses bouches volcaniques différentes. Certaines sont du type plinien, dont l'une est parmi les plus grandes connues au monde. L'ignimbrite principale (Rio Caliente) est du type intraplinien. En tout 140 km³ de matériaux rhyolitiques ont été rejetés au cours des 10⁵ dernières années, dont 50 km³ ont été dispersés bien au delà du volcan. Le volume de roche compact équivalent est de 60 km³, parmi lequel les tuffs, les ignimbrites et les laves occupent respectivement 45%, 25% et 30%. L'émission moyenne, de 0,06 km³ par siècle, est d'un ordre de grandeur moindre que pour la plupart des volcans rhyolitiques les plus productifs actuellement connus. Une région circulaire de 10 km de diamètre dans le centre de La Primavera contient des domes de cendres lacustres et autres sédiments associés, et parmi eux une couche géante de ponce remarquable, qui s'est probablement accumulée dans un lac-caldère. On ne connaÎt pas d'éruptions historiques de la Primavera, et seules des sources chaudes témoignent d'une activité actuelle; toutefois on pense que l'allure en dome est due à la poussée d'un nouveau pluton acide sous le volcan, et que des éruptions sont probables dans le futur.

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Oxygen and hydrogen in the primitive atmosphere

From time to time there appears in the literature the assertion that photolysis of water vapor could have maintained an appreciable concentration of oxygen in the primitive (prebiological) atmosphere. The implausibility of this assertion is argued in this paper.By itself, photolysis does not provide a source of oxygen because it is usually followed by recombination of the products of photolysis. Only the escape to space (at a much smaller rate) of the hydrogen produced by photolysis of water results in a net source of oxygen. The oxidation state of the primitive atmosphere depended on the relative magnitudes of this net source of oxygen and a volcanic source of hydrogen and other reduced gases. Today the volcanic source of reduced gases is approximately equal to the oxygen source provided by photolysis followed by escape. The oxygen source depends on the mixing ratio of water vapor in the stratosphere, which ultimately determines the rate of escape of hydrogen produced from water vapor. Its magnitude may not have been very different in the past. The volcanic source of hydrogen, on the other hand, is likely to have been much larger when the earth was tectonically young. Hydrogen was therefore released to the primitive atmosphere more rapidly than oxygen, probably. Photochemical reactions with the excess hydrogen maintained oxygen mixing ratios at negligibly small levels. The hydrogen mixing ratio was determined by a balance between the volcanic source (reduced by recombination with oxygen) and escape to space.In time, either because of decline of the volcanic source of hydrogen or because of addition of a biological source of oxygen, the input of oxygen to the atmosphere rose above the input of hydrogen. The oxidation state of the atmosphere changed rapidly. Volcanic hydrogen was now consumed by photochemical reactions with excess oxygen, while the oxygen mixing ratio was determined by a balance between the source (reduced by recombination with volcanic hydrogen) and consumption in reactions with reduced material at the surface.     

Melt density and the average composition of basalt

Densities of residual liquids produced by low pressure fractionation of olivine-rich melts pass through a minimum when pyroxene and plagioclase joint the crystallization sequence. The observation that erupted basalt compositions cluster around the degree of fractionation from picritic liquids corresponding to the density minimum in the liquid line of descent may thus suggest that the earth's crust imposes a density filter on the liquids that pass through it, favoring the eruption of the light liquids at the density minimum over the eruption of denser more fractionated and less fractionated liquids.     

Relation of lava compositions to volcano size and structure in El Salvador

Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.The incompatible element (Na₂O, K₂O, Rb, Ba) contents of lavas vary systematically with the volume of the volcanic centers. At the same level of SiO₂, large volcanic centers have higher incompatible element contents than small volcanic centers. This suggests that open system fractionation in a periodically refilled chamber is the controlling factor. The large difference in Ba contents of lavas between eastern (low) and western (high) El Salvador suggests a difference in the mantle source region.     

Generation and dispersal of fine ash and dust by volcanic eruptions

The volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite-forming, plinian, vulcanian and phreatomagmatic types; these are also the eruptions which produce the widest dispersal of this material, attributed to the superior height attained by their eruptive columns. However, much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water, particularly in phreatomagmatic eruptions. Recent studies made on the dispersal and grain-size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types, have yielded estimates of the quantities of material generated in each grain-size class, besides the extent of their dispersal. Not all of the fine volcanic particles are produced by fragmentation at the eruptive vent; in ignimbrite eruptions, there is good evidence for their large-scale generation in and loss from the moving ash flows.     

Effects of petroleum on estuarine bacteria

The effects of petroleum on the natural microbial flora of a given environment, that is, on the bacteria, yeasts, filamentous fungi, and achlorophyllous algae, have largely been ignored. This paper represents the first recorded investigation of the impact of petroleum on ecologically important groups of bacteria.     

A study of late Pleistocene loess deposits,South Canterbury,New Zealand: Part I. Forms and amounts of phosphorus compared with other techniques for identifying paleosols

Four paleosols, and soil horizons within paleosols, were clearly identified in the thick calcium carbonate-free loess sections at Timaru, South Island, New Zealand, by changes in the distribution of total phosphorus and calcium phosphate in the upper 2 m to each paleosol. Extractable manganese was also sensitive in identifying paleosols, particularly the upper horizons. The distribution of bulk density values was useful in identifying paleosols; however, the maximum bulk density (>1.7 g/cc) occurred in horizons identified as B2 rather than fragipan horizons in three of four cases. The distribution of clay particles was useful in understanding the genesis of the modern soil and paleosols, but not in identifying paleosols.     

Radiocarbon dating the Devensian Lateglacial in Britain: New evidence from Llanilid,South Wales

Radiocarbon dates are described from a section through Lateglacial and early Flandrian sediments at Llanilid, Mid-Glamorgan, South Wales. Comparisons between age determinations on the alkali soluble (humic) and alkali insoluble (humin) organic fractions from 12 biostratigraphic horizons reveal the extent of contamination by both older and younger carbon residues. The Llanilid time-scale suggests that for the Lateglacial, the earliest organic sediments date from around 13 200 yr BP, the early Interstadial Juniperus maximum occurred at ca. 12 400-12 500 yr BP with a marked decline some 200 years later, the main Betula phase lasted only from ca. 11 700 to 11 400 yr BP and the end of the Interstadial occurred around 11 100 yr BP. The beginning of the Flandrian dates from ca. 10 000 yr BP, the Juniperus maximum occurred approximately 200 years later, the expansion of birch woodland began around 9600 yr BP, while the first hazel arrived in the area at ca. 9300 yr BP. These age determinations are discussed in the context of radiocarbon dates from comparable biostratigraphic horizons in western Britain and the dating of Lateglacial events in the ocean core records from the North Atlantic.