Recurrent magma mingling in successive ignimbrites from Amealco caldera, central Mexico

The Amealco Tuff is a widespread (>2880 km²), trachyandesitic to rhyolitic pyroclastic deposit in the central Mexican Volcanic Belt that was erupted from the Amealco caldera at 4.7ǂ.1 Ma. It includes three major ignimbrites, each showing complex mingling of pumice fragments and matrix glass with andesitic to rhyolitic compositions. The different glasses are well mingled throughout each of the pyroclastic-flow deposits. Mingling of glasses may have occurred just before and during the explosive eruptions that produced the pyroclastic flows, as the distinct melts had insufficient time to homogenize. Mingling of glasses is evident in each of the three separate major ignimbrites of the Amealco Tuff; thus, the processes that caused it were repetitive. It is infered that the repetitive mingling of melts was due to repeated mafic magma inputs to an evolved magma chamber.     

Study of a few palaeozoic ignimbrites in the armorican massif and in the central Massif,France

A detailed study of the petrographical characteristics of the acid palaeovolcanic formations in the Armorican Massif and in part of the Central Massif (France) reveals important ignimbritic provinces ranging from Precambrian to Carboniferous age. Close comparisons show that the ignimbrites, though varied in the details, form a whole which has constant characteristics. They can thus be related or opposed to ignimbrites of more recent origin. Field data, essential for the identification of recent ignimbrites, are of little practical help when dealing with old ones. But the study of macroscopic samples proves highly effective: heterogeneity, pseudo-fluidality, spindle-shaped structures, etc…, are among the relevant features. Only a microscopic study, however, can show vitroclastic structures and reveal the striking similarity between spindle-shaped structures and the « fiammae » of more recent ignimbrites.     

Rare earth and strontium isotope evidence concerning the petrogenesis of North Chilean ignimbrites

New rare earth element (REE) data, Rb and Sr analyses and Sr isotope measurements are presented for pumice clasts collected from some North Chilean ignimbrites of dacite and rhyolite composition. The samples are light-REE enriched with chondrite-normalised Ce (Ce_N) of 17–98, Yb_N of 4–14 and Ce_N/Yb_N of 2.6–15. While some samples have no Eu anomalies, others do have anomalies with inferred Eu/Eu^* values of down to ca. 0.4. Eleven samples have present-day⁸⁷Sr/⁸⁶Sr ratios between 0.7053 and 0.7100, and noting that some samples are up to 12 Ma old, initial⁸⁷Sr/⁸⁶Sr ratios are below ca. 0.709. These trace element and Sr isotope characteristics resemble those of contemporaneous andesite and dacite lavas, suggesting a common origin for all these rock types. The higher Rb/Sr ratios and larger Eu anomalies in most of the dacitic and rhyolitic ignimbrites are consistent with an origin by plagioclase-dominated fractional crystallization of mantle-derived andesite magma.     

Water content of ignimbrites: Lunar implications

The most important lunar resource may be ignimbrite because of its water content. The variation of water in tuff as a function of degree of welding, oxidation and flow thickness is of direct significance to lunar basing in volcanic terrains on the moon. The fact that many calderas on earth have ignimbritic « aprons » extending from them is correlatable with the apron-type ray patterns around certain post-mare and highland craters on the moon. If lunar craters can be positively identified with associated ignimbritic flows then such areas are potentially water richer than impacted environments. Electrolytic hygrometry and static dehydration analysis of the Tumulo Creek, Wineglass, Bishop, Pelée and Gharvegh tuffs indicate a wide span in water content from 0.2 to 4.1 % even within one flow unit. Devitrification of the mesostasis of these tuffs expels water. Water content apparently varies as a function of flow geometry and possibly thickness. The lower portions of the Tumulo Creek tuff are more hydrous than the upper portions. A slight increase in water content is observed with increase in oxidation. Most of the water in all welded tuffs analysed with the hygrometer is driven off at around 300°C suggesting polymeric hydroxyl bonding with dissociation energies ranging from 2 to 8 kilocalories per mole. However, other types of bonding — Van der Waal, coordinated hydroxyl, free monomeric hydroxyl, etc. — are probably also present. No single, simple bond type controls water retention in rocks. Lunar exploration techniques for hydrous portions of ignimbrite flows include the lateral and vertical application of the epithermal log.     

Problem in the diagnostics and petrogenesis of ignimbrites

Correct diagnostics of rocks requires the 3D study of the structure of their basis, since a splinter-like basis structure may occur only in a single observed section of a specimen of a rock that has a more complex structure of the plane-parallel or linear type. Ignimbrites differ from lavas with a similar silica content primarily in the presence of fiamme, a smooth outline of the phenocrysts, and their constant cataclasis; they differ from pyroclastic lavas by having broken-down phenocrysts enclosed in an undisturbed groundmass with smooth and undulating outlines of the volcanic particles and by the presence of fiamme showing a regular pattern of evolving composition. The study of the morphological and material features of all components of ignimbrites in their relationships with the silica content of the melt and the attitude of the ignimbrite geologic bodies they compose corroborates the hypothesis that ignimbrites are generated by the complex evolution of a gas-saturated melt of the emulsified type rather than by baking (welding) of ash-fall particles.     

Relationship of voluminous ignimbrites to continental arc plutons: Petrology of Jurassic ignimbrites and contemporaneous plutons in southern California

Volcanism was broadly associated in both space and time with Mesozoic plutonism in the Cordillera continental margin arc, but the precise petrogenetic relationships between volcanic rocks and adjacent zoned plutons are not known. Igneous rocks in a tilted crustal section in California include four laterally extensive Jurassic ash flow tuffs from 550 to >1100 m thick underlain at deeper structural levels by Jurassic plutons. Zircon geochronology confirms previous correlations of individual tuffs, suggesting ignimbrites with eruptive volumes up to 800 km³ were deposited both during the apparent Early Jurassic plutonic lull as well as contemporaneous with solidification of regionally widespread Middle and Late Jurassic plutons. The tuffs are weakly to strongly porphyritic (5 to 55% phenocrysts) monotonous intermediate porphyritic dacite to low-silica rhyolite and show strong bulk rock chemical affinity to contemporaneous plutons. Trace element compositions of zircons from the tuffs and contemporaneous plutonic rocks record large and consistent differences in Hf/Zr and REE over similar ranges in Ti abundances, supporting bulk compositional similarities and illuminating similarities and variations in thermal histories despite the effects of hydrothermal alteration.     

On the genesis of ignimbrites how ignimbrites and other pyroclastic products originate from a flowing melt

All the textural and structural characteristics of the alkaline ignimbrites of Northern Latium have been studied in detail. The study clearly indicates that these «ignimbrites» are the products of differential gas exolution from gas-oversaturated melts during and after their moving on the surface. It was also found that the processes of differential gas expansion, depending on the initial melt viscosity, mechanical stresses, etc., result in the whole series of features recognised as typical of ignimbrites. A careful re-examination of the «classic» Italian acid calc-alkaline ignimbrites, and a critical review of analogous materials known from the international literature allowed the Authors to extend the genetic trend of the alkaline ignimbrites of Latium to all deposits having ignimbritic character, and to rearrange their broad range of variability into a comprehensive genetic picture.     

The cenozoic rhyolite-andesite association of the chilean andes

The eruption centres of late volcanism in Chile are situated in two separate areas in the northern and southern High Cordillera. In the north, the ignimbrites of the Rhyolite Formation and the rocks of the « Andesite » Formation occur in about equal proportions, and recent activity is meagre. In the south, the rocks of the « Andesite » Formation predominate, and many volcanoes are in a highly explosive phase of activity. Field relationships, petrological and geochemical data show that the rocks of both Formations are closely related to each other. There is evidence that the magmas of the Rhyolite Formation were formed by fusion of sialic material in the upper parts of the crust. The data for the volcanics of the « Andesite » Formation are inconsistent with their derivation by fractional crystallization of a basaltic parent or by direct mantle derivation involving a single stage process. The authors suggest that the « andesitic » magmas are products of a primary andesitic magma originated by partial fusion of material of the lower crust. Assuming that the « andesitic » magmas of the central parts of the Andes are derived from the upper mantle, this would mean — in the light of the Sr⁸⁷/Sr⁸⁶ data — that the upper mantle in the central region of the Andes is essentially more radiogenic than in other orogenic areas; moroever, it should be very similar in its chemical and Sr⁸⁷/Sr⁸⁶ composition to that of the lower crust.     

Internal tides in the central Mediterranean Sea: observational evidence and numerical studies

Ocean Dynamics - Internal tides are studied in the central Mediterranean Sea using observational data and numerical experiments. Both numerical results and observations indicate that the baroclinic...     

Some observations on the ignimbrites,lava domes and lava flows of M. Cimino (Central Italy)

Cimino Volcano, in Northern Latium, is one of the most characteristic centers of the early Quaternary acid post-orogenetic volcanism in Central Italy. Three stages in the volcanic activity are distinguished: extrusion of several lava domes; ignimbrite cruptions; effusion of more or less differentiated lava flows. Ignimbrites, that form a shield of over 300 sq. km, show characters of welding variable with the distance from the source area and, in a single section, with the level in the sheet. Most part of the Cimino volcanics (domes and ignimbrites) have a quartzlatite composition. The olivine-trachyte (selagite) inclusions in the domes and ignimbrites, and the dark quartz-latite to olivine-trachyte lava flows, could be the products of a pneumatolitic differentiation in the various stages of the magmatic cycle.     

Rhyolitic ignimbrites in the region of Afyon (Central Anatolia)

Occurrence and field relations of an extensive ignimbrite sheet near Afyon in Central Anatolia are described. These rhyolitic ignimbrites are part of the important Neogene volcanic activity in Turkey and belong to the alignment of volcanic complexes along the inner border of the Taurian ranges. In close stratigraphical connection and in the same tectonic position as the rhyolites there occurs an assemblage of high-potassic, intermediate to basic volcanic rocks (alkali trachytes, mela-trachytes, latites and leucite-bearing rocks). Petrological and magmatological considerations led to the conclusion that the rhyolites and the potassic series, in spite of the close geological connection, are not related by processes of magmatic differentiation. Arguments in favour of an anatectic origin of the rhyolitic melt are presented. The occurrence of garnet and allanite as accessory minerals and as inclusions in the salic minerals of the ignimbrite are interpreted as relictic witnesses of a sialic parent rock. However, the trace elements, especially high Rb connected with low Sr, Ba and Zr and K/Rb ratios below 100 give a pattern generally explained by strong fractionation processes.     

Identification of eruption clouds with the landsat satellites

Landsat satellite images were selected for the analysis of a tephraladen eruption cloud and a volcanic fume cloud. A 35 km long eruption plume from Sakurazima Volcano, Kyushu, Japan was viewed by the satellite on December 2, 1972. Multispectral Scanner (MSS) band 4 was density sliced into eight levels. Grey levels over the tephra-laden cumulus, which had formed at the terminus of the eruption plume, were distinct from most of the nearby cumulus clouds. MSS band 4 is the key band for identifying eruption clouds in overcast volcanic regions. A lume cloud from Stromboli, Italy was studied in the same manner. It is easily identified over land areas and for 8 km over water in areas of clear sky, but cannot be distinguished from banks of cumulus clouds.     

Contribution of landsat imagery to the study of volcanic structures

The principal uses of Landsat imagery in volcanological studies are for regional reconnaisance, for the interpretation of large volcanic structures and to facilitate the comparison of structures in different parts of the world. Standard black and white single band prints and standard false colour composites are the cheapest and most readily available forms of Landsat imagery. However, standard Landsat images have a poorer resolution and lower information content than enhanced images. The most generally useful enhancement techniques for volcanic studies have proved to be destriping. contrast stretching and edge enhancement. Enhancement techniques are illustrated with examples of young volcanic structures from the Central Andes. The next few years should see significant advances in satellite remote sensing technology with higher resolution imagery (down to 10–30m) and imagery in a wider range of spectral bands becoming available.Paper presented at the Symposium Volcanoes of the Earth and Planets, held at the University of Lancaster, March 17, 1981.     

The isotopic composition of strontium in Central American ignimbrites

Voluminous sheets of rhyolitic ignimbrites were crupted during Miocene time in a region of Central America that is underlain by a thick sequence of middle Paleozoic and older metamorphic and plutonic rocks. Strontium isotopic ratios of fifteen ignimbrites range from 0.7035 to as high as 0.7175. These values are markedly higher than those measured for cale-alkaline lavas of the same province, but overlap the range found in basement rocks that may have served as source rocks for anateetic magmas. This relationship is in contrast to that found in the western United States where siliceous ignimbrites are not significantly richer in radiogenic strontium than are the basalt erupted through the same basement series. Several possible models for the origin of the large volumes of siliceous magma are examined in terms of major-element and isotopic relations, experimental studies of phase relations, and the thermal requirements of melting or assimilating basement rocks. A mathematical model for the effects of assimilation in open and closed systems permits a comparison of predicted chemical and isotopic compositions with those observed and places limits on the plausibility of various schemes of contamination with or without concurrent crystal fractionation. None of the models is without its flaws. Recent suggestions that large volumes of siliceous magma may be derived from the mantle or lower crust explain certain aspects of the Central American ignimbrites very well if one postulates that the high strontium ratios resulted from contamination of the magma with radiogenic strontium released by the break-down of mieas in basement rocks through which the magmas rose. Such a model fails to explain the apparent restriction of large rhyolitic ignimbrite cruptions to areas underlain by thick continental crust.     

Evolution of Icelandic central volcanoes: evidence from the Austurhorn intrusion,southeastern Iceland

The Austurhorn intrusive complex in southeastern Iceland represents an exhumed Tertiary central volcano. The geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate Austurhorn was a volcanic center analogous to Eyjafjallajökull and Torfajökull in Iceland's eastern neovolcanic zone (EVZ). Early transitional tholeiitic basalt magmatism at Austurhorn formed a shallow crustal chamber 5 km in diameter. Apparent rhythmic modal layering of, and intrusive contacts within, the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized. Felsic activity postdated near-solidification of the gabbro; numerous granitic magmas intruded along gabbro margins and within the adjacent crust. Field relations indicate that infrequent felsic replenishment prevented convective mixing of the Austurhorn chamber during this time, although commingled mafic and felsic magmas are observed in an extensive net veined complex. Late stage mafic dikes intrude the entire complex, suggesting that magmatic heat was abundantly available throughout the evolution of the Austurhorn system. Plagioclase and clinopyroxene compositions in mafic through felsic rocks, including gabbros, support a model of progressive differentiation. Field relations constrain the felsic magmas to originate at P1 kbar, presumably by fractional crystallization. The structure and geochemistry of the Austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern EVZ. The proposed rift segment was parallel to the western and eastern neovolcanic zones, and probably resulted from a reorganization of plate boundaries 7 Ma (Saemundsson 1979; Helgason 1985; Jancin et al. 1985) triggered by activity of the Iceland mantle plume.     

Some Problems of Reducing the Pollution of Water Bodies from Diffuse Sources

Water Resources - The role of diffuse pollution of water bodies in the formation of water quality in them is characterized. Some features of diffuse pollution, which are of importance for managing...