Periods Of Volcanic Quiescence Research Articles

Geological evolution of a pleistocene rhyolitic center — Sierra La Primavera, Jalisco, México

The Sierra La Primavera volcanic complex consists of late Pleistocene comenditic lava flows and domes. ash-flow tuff, air-fall pumice, and cold caldera-lake sediments. The earliest lavas were erupted about 120,000 years ago, and were followed approximately 95,000 years ago by the eruption of about 20 km 3 of magma as ash flows that form the compositionally-zoned Tala Tuff. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11-km-diameter caldera. It soon filled with water, forming a caldera lake in which sediment began to collect. At about the same time, two central domes erupted through the middle of the lake and a “giant pumice horizon”, an important stratigraphic marker, was deposited. Shortly thereafter ring domes erupted along two parallel arcs: one along the northeast portion of the ring fracture, and the other crossing the middle of the lake. All these events occurred during a period of approximately 5,000–10,000 years. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine-grained ashy sediments virtually free from pumice lapilli. Approximately 75,000 years ago, a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10–20 m of sediments, as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 20,000–30,000 years ago. The four major fault systems in the Sierra La Primavera are related to caldera collapse or to uplift caused by the insurgence of the southern are magma. Steam vents and larga-discharge 65°C hot springs are associated with the faulting. Calculated equilibrium temperatures of the geothermal fluids are ∼170°C, but temperatures in excess of 240°C have been encountered in an exploratory drill hole. A seismic survey showed attenuation of both S and P waves within the caldera, P waves attenuated more severely than S waves. The greatest attenuation is associated with an area of steam vents, and the rapid lateral variations in attenuation suggest that they are produced by a shallow geothermal system rather than by underlying magma.

The magmatic evolution of the Vermilion greenstone belt, NE Minnesota

Abstract The Vermilion greenstone belt, a nearly linear belt of steeply dipping, low grade, metavolcanic and metasedimentary rocks and adjacent areas in northeastern Minnesota compose a typical Archean (∼2.75 Ga) greenstone—granite association. In the western part of the belt, four formally designated units have been defined, which together form a complex volcanic—sedimentary pile that apparently formed through the coalescence of petrologically distinct volcanic centers. The oldest supracrustal unit, the lower Ely Greenstone, consists of calc-alkaline basalts, andesites, heterolithic agglomerates and tuffs deposited in a relatively shallow subaqueous environment. The lower part of this unit apparently was destroyed by emplacement of a granitic batholith. Following a period of volcanic quiescence, during which the Soudan Iron Formation and associated rocks were deposited, basaltic (tholeiitic) volcanism commenced and produced a subaqueous shield volcano (upper Ely Greenstone); the basalt interfingered laterally with calc-alkaline, dominantly dacitic material. The termination of basaltic volcanism was followed by a period of eruption of largely dacitic volcanic material accompanied by rapid erosion and turbidite deposition represented by the Lake Vermilion Formation and Knife Lake Group. This period was followed by renewed volcanism that changed laterally from dominantly calc-alkaline lava, tuff and breccia (felsic member of Newton Lake Formation) to dominantly basaltic lava and mafic—ultramafic intrusions (mafic member of Newton Lake Formation). Within the mafic sequence five petrologically distinct basalt types were erupted. Assuming that the present erosion surface in the Vermilion greenstone belt is a representative cross-section of the original supracrustal sequence, the volume of calc-alkaline, dacitic volcanic material appears to exceed that of basaltic volcanic rocks. If current petrologic models proposed for these rocks are valid, a dynamic tectonic environment capable of generating and rapidly recycling large volumes of volcanic and derived sedimentary material is required to account for the development of the Vermilion greenstone belt.

Umkehr vertical ozone profile errors caused by the presence of stratospheric aerosols

The unusually large increase in stratospheric aerosol concentration resulting from the eruption of Agung in 1963 is shown to produce fictitious anomalies in the record of vertical ozone profiles determined from Umkehr observations at Aspendale, Australia. The anomalies are of sufficient magnitude that an empirical haze error to the profiles can be extracted from a comparison between seasonal averages of ozone profiles observed shortly after the Agung event and ozone profiles observed during periods of volcanic quiescence. Haze errors to the ozone profiles are also theoretically calculated for various models of vertical profiles of aerosols, ranging from relatively large concentrations of tropospheric aerosols to large concentrations located exclusively in the stratosphere. It is shown that the theoretically determined haze errors are essentially of the same character as those determined empirically. Haze error corrections are applied to a long‐term Umkehr record of north temperate upper stratospheric ozone concentration following Agung. The results imply that the anomalously low ozone concentrations in the upper stratosphere, noted after the eruption of Agung and Fuego in 1974, may not be real.

Plate Tectonics and Regional Stratigraphic-Metamorphic Relations in the Southern Part of the New Zealand Geosyncline

The evolution of the New Zealand geosyncline is examined in terms of both the plate tectonics model and such geological constraints as regional metamorphic and biostratigraphic patterns and interrelations between paired sedimentary facies. A common western provenance is tentatively concluded for both major sedimentary facies belts of the geosyncline; the inner (Hokonui) belt acted as a volcanotectonic sediment trap during times of increased volcanic activity, while during periods of volcanic quiescence, quartzo-feldspathic debris from the adjacent “continental” foreland was transported across the inner belt into the outer (Torlesse) basin. In general the Torlesse rocks appear to decrease in age but increase in metamorphic grade toward the inferred western (continental) source, and it is suggested that the older (Carboniferous and Permian) rocks were never overlain by substantial thicknesses of younger strata. Development of an early extensional basin, by strike-slip displacements, in Carboniferous to Jurassic times is consistent with both this concept and with the apparently anomalous regional biostratigraphic zonation of the Torlesse rocks. Differences in geosynclinal evolution between New Zealand and California appear to be accountable in terms of the greater age of the New Zealand geosyncline and the early period of extensional growth. The Late Jurassic-Cretaceous Rangitata orogeny was an important event occurring relatively late in the depositional history of the New Zealand geosyncline, and is interpreted as representing the initiation of the present system of Pacific basin sea-floor spreading. A corresponding early extensional event is not recognized in the Jurassic-Cretaceous Coast Range geosyncline of California, the evolution of which appears to be entirely related to subduction and arc activity related to an East Pacific rise spreading axis. Differences in detailed deformational style and metamorphism between the two areas may result from differences in spreading rates and from the steeper dip of the Jurassic-Cretaceous subduction zone in New Zealand.

II. The Geology of a Composite Intrusion at Bennan, South Arran

The Composite Dyke at Bennan, South Arran. There occurs in the extreme south of the island of Arran a composite intrusion composed of three sheets. The prominent cliffs produced by the weathering out of the intrusion have been remarked, in their references to Bennan Head, by most writers on Arran for a century. Inland from the sea the position of the dyke is marked by raised barren ground, for the intrusion is more resistant than the sandstones and marls through which it cuts. It is accurately mapped on the colour-printed geological map of Arran published by the Geological Survey of Great Britain in 1910. The intrusion cuts steeply through the Keuper marls and sandstones (Gregory, page 179), which are gently baked, whilst it is cut in turn by numerous basic dykes which throughout the Western Isles are regarded as being of late Tertiary Age. Since the Mesozoic was a period of volcanic quiescence in Scotland, the dyke is a product of early Tertiary volcanic activity. The two outer sheets are quartz “dolerite” and the central one enclosed between them is quartz porphyry. At its northern termination where the dyke approaches the main road the central sheet pinches out and the two other sheets coalesce into one. The “dolerite” thus forms a sheath enclosing the porphyry. The outer margins of the sheath show chilled edges due to rapid cooling, but the junction between the “dolerite” and the porphyry is ragged, with apophyses of the latter intruded into the former. Fragments of This 250-word extract was created in the absence of an abstract