Location Of Eruption Research Articles

New age determinations and the geology of the Kenya Rift-Kavirondo Rift junction, W Kenya

The area of the triple-rift junction in W Kenya, between the E-W Kavirondo Rift, the N-S Northern Kenya Rift and the NW-SE Central Kenya Rift, has been the site of continuous volcanic and tectonic activity since the Middle Miocene. From about 20 to 7 Ma the area was dominated by the accumulation of a thick pile of nephelinitic and phonolitic volcanics in the Timboroa area. From 7 Ma to the present day, apart from local eruptions of basanite in the W, the volcanic and tectonic activity has been progressively concentrated towards the axial zone of the Kenya Rift with alkali basalt-trachyte sequences predominating. During this period the Kavirondo Rift was inactive. The stratigraphic sequence in the area has been dated by 19 new K-Ar age determinations. These enable the absolute and relative ages of the Tinderet, Timboroa, Kapkut, Londiani and Kilombe volcanoes to be established for the first time. Topographical and structural considerations show that the Kavirondo Rift largely fades out 50 km W of the Kenya Rift and is barely perceptible at its edge. The W wall of the Kenya Rift adjacent to the Kavirondo Rift is a monocline and is very similar to the South Turkana region. Although the area is adjacent to major rift-bounding faults there is little faulting within it, the tectonic pattern being dominated by monoclines and volcano-tectonic structures.

A geochemical and mineralogical investigation of some British and other European tonsteins

ABSTRACTThe volcanic origin of a number of major tonstein horizons has been established from mineralogical compositions and from trace elements that are quantitatively retained during diagenesis. Discriminant function analysis using the variables Ti/Al, Cr/Al, Zr/Al and Ni/Al allows the tonsteins to be classified according to original composition. Two main groups are recognized amongst the British tonsteins. Those formed from acid volcanic ash are comparable in composition with French and German tonsteins and a common source is postulated, whereas tonsteins formed from basic volcanic ash contain variable amounts of detrital sediment and are thought to originate from local eruptions. Possible lateral equivalents between tonstein horizons in Britain and elsewhere in Europe are suggested.

Therapeutic aspects of herpes genitalis. A short review.

The treatment of manifestations of infection by herpes simplex hominis virus must be aimed at both the local eruptions and the mechanism of recurrence. Recently, some success has been achieved with the use of vaccines and other agents to stimulate a cell-mediated immunity in patients with recurrent herpes infection.

Atmospherically transported volcanic glass in deep-sea sediments: Theoretical considerations

Measurement of long-term fluctuation in the volcanically derived atmospheric dust load requires isolation of the regional accumulations caused by relatively local eruptions. Only then will it be possible to evaluate the contribution of volcanic dust to past global climatic changes and to changes in marine fauna and chemistry. As a means of delineating the regional effects of such eruptions, a series of experiments have been initiated involving the separation and counting of volcanic dust particles deposited in deep-sea sediments downwind from known eruptions during the past 2 m.y. To provide working limits for the series of analyses and to facilitate the design of efficient associated piston coring programs, a simple model has been developed relating distance from source to volcanic dust particle size for volcanic eruptions of various explosive intensities and for various upper atmospheric transporting powers (wind strengths). It is shown that paleowind velocity determinations can be made by measuring the downwind increase in the width of the dust cloud fallout pattern. By using the measured paleowind it is then possible to employ measurable critical distances of selected size fractions from known eruptive sources to derive the height of the associated volcanic cloud top and base, from which the net paleoexplosivity can be estimated. Less precise estimates can be made if it is necessary to assume paleowind velocities. Given a traverse of cores taken at unknown distances from the parent volcanos, it is possible to derive the distances to the sources. For a single sampling locality, relative eruptive intensities of a known source over a long period can be estimated. It is shown that the most efficient use of ship time would be the taking of two coring traverses broadside to the fallout direction at distances up to 3000 km from the source (to facilitate paleowind velocity estimates) and a single coring traverse along the fallout axis (to derive the height of the volcanic cloud top and base).

Morphology and Structure of the Western Galápagos Volcanoes and a Model for Their Origin

The two westernmost Galapagos Islands, Isabela and Fernandina, comprise six large, active, basaltic shield volcanoes, which are closely related in space and in the time of their formation. The volcanoes have marked morphological and structural similarities, but gradational differences exist which show relative volcano ages. These features also indicate the mechanisms of volcano origin and development. The unusual subaerial profiles are typified by steep upper slopes and a broad, flat top. Profiles, including submarine portions, show that all six volcanoes have four slope segments: two submarine and two subaerial. Distinct profile differences provide evidence for various formational processes; the subaerial upper segments are sensitive indicators of these processes. Superimposed profiles show that erupted lavas move far down the volcano slopes to be deposited on the subaerial lower segment and especially on the submarine segments. Relative slope maturities on a volcano can be determined that depend upon the volume of lava added to different profile directions. Different ranges of slope maturities indicate the relative volcano ages. Subaerial profiles are influenced by the submarine base which formed prior to subaerial accumulation. The “flattish” volcano tops are mostly large calderas, although some calderas are surrounded by a wide, flat rim area. Caldera characteristics, such as diameter, circularity, and depth, show trends which agree with the age sequence determined from the profiles. Volcano elevations decrease regularly with increasing caldera diameter. A radial fissure pattern is well developed on the younger volcanoes but becomes less well defined with age. When present, this pattern is restricted to the subaerial upper segment of the volcano. Fissures on subaerial lower segments have a linear nature; they extend upslope to the calderas and downslope to link adjacent volcanoes. A circumferential fissure pattern around the caldera is well displayed by some volcanoes. The diameter of the circumferential fissure pattern increases with age; on the youngest volcano, circumferential fissures have a strong linear nature. Coalescence of circumferential and linear fissures, caldera elongation, and eruption locations define dominant, throughgoing lineaments which cross each volcano and link the six volcanic centers. Subordinate lineaments can also be defined which intersect dominant lineaments at each caldera. Two lineament orientations are present which correspond to previously proposed trends for the Galapagos Islands. A model for the origin and development of the volcanoes outlines five main periods: (1) submarine base construction, (2) subaerial construction, (3) tumescence and radial fracturing, (4) caldera development, and (5) morphological decline. A key concept in the model is the formation of a dome by tumescence and associated radial fracturing. The dome is supported by radial dikes so that a steep upper slope segment is formed and maintained during caldera development. Circumferential fissures are viewed as “passive” features which form during collapse of the central part of the dome. Calderas are initially grabenlike because of the control of the throughgoing dominant lineament. Caldera growth is accompanied by increasing circumferential fissure diameter; radial fissures become less distinct as tumescence decreases in importance. During morphological decline, the caldera consumes the upper portion of the volcano while eruptions broaden and flatten the profile. The resulting landform is similar to shield volcanoes of other areas and to the older Galapagos Islands.