Toothpaste lava: Characteristics and origin of a lava structural type transitional between pahoehoe and aa

Toothpaste lava, an important basalt structural type which illustrates the transition from pahoehoe to aa, is particularly well displayed on the 1960 Kapoho lava of Kilauea Volcano. Its transitional features stem from a viscosity higher than that of pahoehoe and a rate of flow slower than that of aa. Viscosity can be quantified by the limited settling of olivine phenocrysts and rate of flow by field observations related to the low-angle slope on which the lava flowed. Much can be learned about the viscosity, rheologic condition, and flow velocity of lavas long after solidification by analyses of their structural characteristics, and it is possible to make at least a semiquantitative assessment of the numerical values of these parameters.     

Climatic oscillations during the Precambrian era

A remarkably regular cyclicity with a fundamental period of ~11–12 cycles is preserved in the 680 million year old Elatina formation of South Australia. All but one of the many periods present can be interpreted as resulting from the combined influences of the sunspot cycle and the lunar nodal tide - in particular, beating between these two cycles gives rise to a long period phase alternation. Available paleontological evidence is used to constrain the lunar distance 680 Ma ago, thereby constraining the length of the lunar nodal tide. We then infer from the beat period that the sunspot cycle was 10.8 ± 0.2 years, which is in agreement with independent astronomical evidence suggesting that the sunspot cycle would then have been some 3–10% shorter than it is at present. Although this interpretation is not consistent with the 12.0 year sunspot cycle counted by Williams and Sonett (1985), we demonstrate that unavoidable random errors made in discriminating unusually indistinct varves gives rise to a systematic overcount of varves of this magnitude. The clarity of this evidence for solar and lunar signals in the climate 680 Ma ago lends support to reports of their importance today.National Research Council Postoctoral Associate.     

Characteristics of dune-bedded pyroclastic surge bedsets

The grainsize characteristics of dune-bedded pyroclastic surge bedsets are surveyed. The variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets, and it increases as the grainsize of the coarsest bed increases, reflecting an increasing velocity of emplacement. Deposits of wet surges, identified as those which contain accretionary and ash-coated lapilli, tend to be finer and show less variance, this partly because wet ash is cohesive, but mainly because wet surges tend to be weaker. Dry surge bedsets are strongly fines-depleted, wet ones less so. The lack of erosion of underlying ash layers shows that the environment is a strongly depositional one. Individual bedsets are demarcated by thin intervening fine ash-fall layers, which are the complementary ash-cloud deposits settled or flushed out after the passage and decay of each turbulent surge. Surge deposits are generally less coarse than the coarsest associated airfall deposits, which shows that they are formed by generally weaker events.This study helps interpret the dune-bedded parts of the landscape-mantling May 18th 1980 “blast” deposit of Mount St. Helens. The blast was a very violent event, but the variance and the grainsize of the coarsest bed are those of a relatively weak surge. This suggests that the dune-bedding was produced by a weak effect, such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs.     

Comparison of spawning seasons,age, growth rates,and food of two sympatric species of searobins,Prionotus carolinus and Prionotus evolans,from Long Island Sound

Prionotus carolinus and Prionotus evolans were collected from many locations within Long Island Sound in 1971–1973, and in 1976–1977. Data from earlier collections in Block Island Sound (1943–1945) were also included. A total of 1751 specimens, 960 P. carolinus and 791 P. evolans was examined within these two time periods. Both species entered the Sound in April and spawned during June and July. P. evolans appeared to spawn slightly earlier in summer than P. carolinus. Adults began to leave the Sound after spawning and were usually absent after November. Young-of-the-year were taken regularly from August to November and, occasionally in water over 20 m deep, into February when the bottom water temperature was 1.4°C. At the end of the first growing seasons both species exhibited large variations in standard lengths. Back-calculations from scale annuli measurements indicated that linear growth rates during the juvenile years were similar in both species. However, P. evolans was considerably heavier than P. carolinus. During adulthood P. evolans was not only longer and heavier than P. carolinus, but lived longer. Growth rates are described by the following equations: P. carolinus L_t+1=9.60+0.68 (L_t), and p. evolans L_t+1=7.70+0.80 (L_t). Both species were opportunistic feeders, and crustaceans were clearly the dominant group of prey. Young-of-the-year, between 3–6 cm, ate copepods. As they grew they ate larger prey, such as Neomysis americana, Diastylis quadrispinosus, various species of amphipods of small sizes, and juvenile Crangon septemspinosus. Older fish ate larger sizes of these same prey, a number of species of crabs, juvenile Homarus americanus, and Squilla empusa. Occasionally they ate polychaetes, molluscs, and juvenile fish. Partitioning of the resources of Long Island Sound by these two species appeared to be by prey size. P. evolans ate prey that, on the average, were slightly larger than those eaten by P. carolinus. Furthermore, P. evolans ate a greater amount of nektonic species than P. carolinus, which appeared to prefer benthonic invertebrates.     

Abyssal tholeiites from the Oceanographer Fracture Zone

Unusual effects have been discovered in the major element phase relations of basalts from the Oceanographer Fracture Zone (OFZ) which suggest that magma mixing of primitive basalt with the differentiation residue of a previous batch of primitive magma has occurred. These effects include a reversal in mineral crystallization sequence which cannot happen during normal differentiation processes or be explained by any plausible change in physical conditions. This unusual effect is encountered as a result of curvature in composition space of the liquid-line-of-descent equilibria involving olivine, plagioclase, and high-calcium clinopyroxene. Mixing of magmas at different stages of their evolution produces mixtures that do not lie on the curved liquid-line-of-descent. Observation of such anomalous compositions in the OFZ suite supplements accumulating petrographic and trace-element geochemistry evidence that magma mixing is an important petrogenetic process. Mixing of fractionated residual liquids can produce mixtures which are either superheated or supercooled depending on the sense of ‘thermal curvature’ of the liquid-line-of-descent. Both senses are encountered in the tholeiitic system, and this effect may exert a qualitative control on the crystallization texture of the mixture. A comparison of approximately 2,000 abyssal tholeiite compositions to the experimental liquid-line-of-descent reveals that erupted differentiates which would be expected from advanced fractionation are scarce. Just this sort of phenomenon (the ‘perched’ steady state) was proposed by O'Hara (1977) as an earmark of the operation of a continuously fractionating magma chamber into which fresh magma is periodically remixed.     

Effect of quaternary ammonium compounds on some aquatic plants

The toxic effects of various types of quaternary ammonium compound (QAC) upon a unicellular green alga (Chlorella sp.) and a duckweed (Spirodela oligorhiza) have been investigated. In general all types of QAC suppressed plant growth when present at concentrations above 10^?5 M (ca 3–5 ppm). With the duckweed sublethal levels of QAC caused yellowing or browning of the frond margins and the production of smaller sized fronds whilst with Chlorella QACs affected both the size, shape and internal organization of the cell; death appeared to be due to disruption of the chloroplast structure.     

The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits

The twin 6000 m volcanoes San Pedro and San Pablo rise 2000 m above the extensive ignimbrite plateau of northern Chile. San Pablo is extinct and quite deeply eroded, San Pedro has an active fumarole and a record of several historic volcanic eruptions. Activity on both has on the whole migrated westwards with time. The volcanoes consist of pyroxene and hornblende andesite lavas with broad aprons of volcaniclastic rocks, including hot avalanche deposits, on their flanks. 30 new chemical analyses show that the composition range of the rocks composing the volcanoes is small, with silica varying between 56 and 66%, significantly higher than that of Quaternary volcanic suites of the western Pacific. The hornblende andesite lavas are very thick (commonly 100 to 200 m) indicating a high viscosity. Where such flows were erupted high on San Pedro, large portions of them collapsed when they became mechanically unstable and the resulting avalanche deposits form an important part of the volcano. Most avalanches took place when the source-lava was still hot, though so viscous that it broke by brittle fracture when it collapsed. The larger blocks have a prismatic jointing normal to their outer surface, showing that they cooled down in place after collapse. Evidence is presented that the mechanism of transport of these hot avalanches is not likely to be different from that of non-volcanic rock avalanches. Pumiceous pyroclastic flows also occur, one on the flanks of San Pedro, while others comprise the ignimbrite plateau. The latter is made up of many pyroclastic flows different from the San Pedro example only in size, and there are good grounds for believing that the ignimbrites likewise originated by eruptions on the andesitic volcanoes. Many of the ignimbrites are rhyolites, much more acidic than the andesite stratovolcanoes and a process of surface transport separation has operated whereby the andesites are concentrated on and near their source, while most of the rhyolites, because of their different style of eruption, have spread widely from it.

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Zusammenfassung Die beiden 6000 m hohen Vulkane San Pedro und San Pablo erheben sich etwa 2000 m über der breiten, von Ignimbrit gebildeten Hochebene Nordchiles. San Pablo ist längst erloschen und tief erodiert; dagegen weist San Pedro mehrere historische Eruptionen und dazu eine noch aktive Fumarole auf. Bei beiden hat sich die vulkanische Tätigkeit im Laufe der Zeit nach Westen verlagert. Die Vulkane sind aus Pyroxen- und Hornblende-Andesit-Laven gebildet; ihre äußeren Flanken bestehen aus breiten vulkanoklastischen Ablagerungen, die heiße Erdrutsch-Ablagerungen einschließen. Chemische Analysen von 30 Gesteinsproben ergeben, daß die Produkte der Vulkane nur geringe chemische Schwankungen zeigen und daß die SiO₂-Werte zwischen 56% und 66% liegen; deutlich höher als die SiO₂-Werte der quartären westpazifischen vulkanischen Serien. Ihrer Mächtigkeit nach (100–200 m) müssen die Hornblende-Andesit-Laven eine hohe Viskosität gehabt haben. Wo solche Laven hoch auf San Pedro ausflossen, verloren größere Teile der Lava ihr Gleichgewicht und rutschten ab. Die so gebildeten Erdrutsch-Ablagerungen sind ein wichtiger Teil des ganzen Vulkans geworden. Die meisten Erdrutsche entstanden, als die Lava noch in heißem Zustand war, aber schon so hohe Viskosität hatte, daß sie mit sprödem Bruch abbrach und niederstürzte. Die größeren Brocken davon zeigen radiale, säulige Klüfte, die senkrecht zur Oberfläche laufen. Dies zeigt, daß sie nach dem Abrutschen als einzelne große Brocken gekühlt sind. Es wurde bewiesen, daß der Transportmechanismus dieser heißen Erdrutsche wahrscheinlich sehr ähnlich den nicht-vulkanischen Erdrutschen ist. Bimsstein-pyroklastische Lavadecken sind auch vorhanden, eine befindet sich an der Flanke von San Pedro, während andere die Ignimbrit-Hochebene bilden. Die letztgenannte erhält ebenfalls viele pyroklastische Lavadecken von geringerer Mächtigkeit als jene von San Pedro. Es ist sehr wahrscheinlich, daß auch diese Ignimbrite ihren Ursprung an den andesitischen Vulkanen fanden. Manche von den Ignimbriten sind Rhyolithe, viel saurer als die andesitischen Stratovulkane. Die Trennung der rhyolitischen und andesitischen Laven wurde auf der Erdoberfläche durch Transport vollzogen: die Andesite wurden nahe am Vulkan angehäuft, während die meisten der Rhyolite wegen ihres völlig anderen Eruptionsvorganges weiter entfernt vom Vulkanzentrum abgelagert sind.

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Resumen Los volcanes gemelos de San Pedro y San Pablo se elevan 2000 m. sobre la extensiva altiplanicie ignimbrítica del norte de Chile. El San Pablo yace extínto y fuertemente erosionado, mientras que el San Pedro demuestra actividad fumarólica y ha registrado erupciones en tiempos históricos. En ambos volcanes la actividad se ha desplazado prpgresivamente hacia el occidente. Los volcanes se hallan constituídos por lavas de andesita piroxénica y hornbléndica. Tienen amplias falds de rocas volcanoclásticas, con depósitos de tipo hot avalanche. Treinta nuevos analisis demuestran que las rocas no varían mucho en cuanto a composición química. El porcentaje de sílice es de 56% a 66%, cantidad que supera la de de las rocas volcánicas cuaternarias del mergen pacífico occidental. Las lavas de andesita homblendica alcanzan grandes espesores (de entre 100 y 200 m.), lo que indicaría una alta viscosidad. Cuando una lava de estas brotara en la superficie alta del San Pedro, gran parte de ella devendría inestable, deslizándose y originanado depósitos que ahora constituyen una importante fracción del edificio volcánico. En su mayoriál los depósitos provinieron por disagregación mecánica de la lava paterna, la que fracturó formando bloques mientras se hallaba aun cálida. La presencia de un sistema de un sistema de fracturas perpendicular a la superficie externa de los bloques mas grandes indicaría que estos alcanzaron temperaturas ambientes en el propio sito donde ahora se hallan. El mecanismo de transporte de estos deslizamientos volcanicos no sería muy differente al de los deslizamientos formados por transporte Únicamente terrestre. Flujos piroclásticos con piedra pómez se hallan, uno sobre las faldas del San Pedro, otros constituyendo la altiplanicie ignimbrítica. Estos Últimos se distnguen del primero Únicamente por su mayor volÚmen. Uno que otros probablemente originaron por erupción en los propios volcanes. En su mayoria las ignimbritas son riolíticas, es decir, mas acídas que los estratovolcanes andesíticos. Por lo tanto, ha operado un proceso de separación en transporte superficial, cual proceso ha concentrado las andesitas cerca de su origien, mientras que las riolitas, con su propio estilo eruptivo, se han dispersado más, alcanzando grandes distancias.

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Résumé Les deux volcans San Pedro et San Pablo, hauts de 6000 m, du Chili Septentrional, s'élèvent à 2000 m au-dessus d'un vaste plateau d'ignimbrites. San Pablo est éteint et profondément érodé, tandis que San Pedro montre une activité fumerollienne après plusieurs éruptions survenues au cours de la période historique. L'activité volcanique chez les deux volcans s'est déplacée dans le temps vers l'ouest. Les volcans sont composés de laves d'andésite à pyroxène et à hornblende, avec, sur leurs flancs, des roches volcanoclastiques comprenant des dépÔts dûs à des éboulements chauds. 30 nouvelles analyses chimiques montrent que les limites de composition sont étroites, avec de 56 à 66% de silice, soit une composition nettement plus élevée que celle des roches volcaniques quaternaires du Pacifique Occidental. Etant donné leur épaisseur (100 à 200 m), les laves andésitiques à hornblende doivent avoir eu une viscosité très grande. Dans les endroits élevés de San Pedro où de telles coulées se sont épanchées, une grande partie d'entre elles se sont effondrées. Les produits de ces éboulements forment une partie importante du volcan. La plupart des éboulements se sont produits quand la lave était encore chaude, mais si visqueuse qu'elle s'est fracturée avec une cassure nette pendant l'effondrement. Les blocs les plus grands sont cassés le long de lignes perpendiculaires à leur surface extérieure, ce qui montre qu'ils se sont refroidis sur place après leur effondrement. Cela montre avec évidence que le mécanisme de transport de ces «éboulements chauds» n'est vraisemblablement pas différent de celui des éboulements non-volcaniques. Il y a aussi des coulées de laves pyroclastiques ponceuses, dont une sur les flancs du San Pedro, tandis que d'autres forment le plateau ignimbritique. Le plateau est aussi composé de nombreuses coulées pyroclastiques d'épaisseur moindre que celles du San Pedro. Il est très vraisemblable que les ignimbrites aussi proviennent des volcans andésitiques. Beaucoup d'ignimbrites sont rhyolitiques, encore plus acides que les andésites des stratovolcans. Un processus de séparation au cours du transport a concentré les andésites près du volcan, tandis que la plupart des rhyolites, à cause d'un style d'éruption différent, se sont répandues à plus grande distance.

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The Tichka Massif,Morocco—an example of contemporaneous acidic and basic plutonism

The Tichka Massif is a shallowly emplaced, domal intrusion that has produced a thermal aurole in surrounding rocks. The interior of the massif consists of a series of leucocratic granitic, tear-shaped pods sorrounded by dioritic rocks that range in composition from gabbro to granodiorite. The orientation of the pods has a strong vertical component. Flow foliation is well-developed in the rocks that surround the pods, whereas foliation is poorly developed in the granite pods. Dikes of andesite emanate from dioritic rocks and crosscut the granite pods. These rock relationships indicate that a highly viscous granitic liquid (pods) coexisted with a less viscous, denser dioritic liquid. Differences in viscosities of the two magmas and low diffusion rates in the granite magma prevented mixing.     

Origin of titaniferous lunar basalts

Delineation of low pressure phase equilibria in the composition space relevant to titaniferous lunar basalts demonstrates a significant degree of control by those equilibria on the compositions of the basalts. The existence of two distinct chemical groups of basalts (high and low-K) which cannot be related one to the other by fractional crystallization at any pressure, suggests that melting is responsible for the two groups. Consideration of the pressure shift required to produce the differences between groups constrains magma segregation to have occurred in the outer 150 km of the Moon. It is difficult to relate low-Ti and high-Ti basalts to the same source region. The preferred source region of high-Ti basalts, based on phase equilibrium considerations, is a late ilmenite-rich cumulate produced from the residual liquid of the primordial differentiation of the outer portions of the Moon. This ilmenite-rich layer is sandwiched between the lunar feldspathic crust and a complementary mafic cumulate.     

Late Quaternary climates of Australia and New Guinea

Between 60,000 and 40,000 B.P., northeastern Queensland, south New South Wales, and southeastern South Australia were drier than at present. From 40,000–30,000 B.P. a colder climate than at present is indicated from one New Guinea area. Dryness became even more accentuated in northeastern Queensland, whereas many lakes filled up in the southern mainland, probably because of increasing precipitation effectiveness there. Before the end of this period colder conditions than now were already giving rise to slope instability in the Snowy Mountains of New South Wales.The period of 25,000–15,000 B.P. saw the greatest lowering of the New Guinea treeline, reaching an extreme at 17,000 B.P. when glaciers also achieved their maximum extent. This was the time of extensive glaciation in Tasmania and small glaciers formed in the Snowy Mountains. Estimates of the lowering of mean annual temperature range from 6°–10°C. Northeastern Queensland experienced its driest Late Quaternary climate; lakes were contracting throughout the southern mainland and the final phase of substantial desert dune building took place before the period ended.In the Snowy Mountains ice retreat began before 20,000 B.P., as did the construction of clay dunes in the southern semi-arid belt, a process demanding higher temperatures. However, in New Guinea and Tasmania ice retreat and treeline rise did not begin till after 15,000 B.P. Temperatures rose rapidly and everywhere most of the ice had gone by 10,000 B.P., when some lakes filled up in southern Australia, implying an increase in absolute precipitation.In the last 10,000 years climate has been relatively stable although there are some indications that temperature and rainfall were marginally higher than now between 8000 and 5000 B.P. Since then, lake levels have oscillated; a brief, limited resumption of periglacial activity took place in the Snowy Mountains and there were small glacier advances in New Guinea.