Vent Alignments Research Articles

Genesis of lavas of the Taupo Volcanic Zone, North Island, New Zealand

The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km3 of high-Al basalt ( 10,000 km3 of rhyolite and ignimbrite (> 97.4%). The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben. Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.

Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California

The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre‐Cenozoic plutonic and metamorphic rocks is partly buried by ∼35 km3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) ≤1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being ≤0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north‐trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east‐west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot‐water geothermal system. This system apparently is heated by a reservoir of silicic magma at ≥8‐km depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle‐derived basaltic magma that intrudes the crust in response to lithospheric extension.

Andesites of the Tongariro volcanic centre, North Island, New Zealand

The Tongariro volcanic centre comprises four major andesite massifs, Kakaramea, Pihanga, Tongariro and Ruapehu, and four smaller cones and flows, Maungakatote, Pukeonake, Hauhungatahi and Ohakune. Older lavas from Kakaramea, Pihanga and Tongariro were erupted from a series of vents aligned NW-SE and more recent lavas from vents aligned NNE-SSW. The most voluminous lava types in the centre are labradorite and labradorite-pyroxene andesite containing phenocrysts of plagioclase (An 68—An 48), orthopyroxene (mainly bronzite) and clinopyroxene, in a fine to medium-grained groundmass. Smaller amounts of pyroxene andesite (from Pihanga), olivine andesite (from NNE-trending vents), and hornblende andesite also occur. Pigeonite surrounds orthopyroxene in some of the pyroxene andesites. Average major and trace element compositions of each petrographic type indicate that all lavas are “normal” calc-alkaline andesites and low-Si andesites. Low-Si andesites, represented mainly by pyroxene and olivine andesites, are typified by higher FeO, MnO, MgO, Sr, Cu, Ni, V, Sc and Cr and lower Al 2O 3, Na 2O, K 2O, Rb, Ba, Zr and Pb than “normal” andesites. Some low-Si andesites are regarded as phenocryst-rich cumulates. The location of the Tongariro andesites and alignments of eruptive vents suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island arc andesites such as those of Tonga by having higher Na 2O, K 2O, Rb, Ba and Zr. This suggests some crustal contamination, but the overall chemistry of the andesites is inconsistent with an origin by mixing, en route to the surface, of high-alumina basalt with either Mesozoic greywacke-argillite or with a partial melt of the greywacke-argillite equivalent in composition to Taupo rhyolite. A model is presented in which oceanic crust assimilates Mesozoic and Cenozoic sediments at the eastern side of the North Island, and is subducted to produce amphibolite. This amphibolite would subsequently break down, hydrating the overlying upper mantle and lower crust, and, in steeply dipping subduction zones such as that under Tongariro, would produce phlogopite eclogite below 90 km. The latter will partially melt at 150–200 km and the resultant magma will fractionate in the upper mantle or lower crust to produce andesite.

Geology of Santiago, Rábida, and Pinzón Islands, Galápagos

Santiago, Pinzon, and Rabida Islands are each single volcanoes that lie along a north-trending line near the center of the Galapagos Archipelago. The core volcano of Santiago Island and its flanking lava fields are composed of basaltic to intermediate lava of alkalic parentage. The smaller islands of Rabida and Pinzon are constructed of tholeiitic lava and tuff, ranging in composition from basalt to siliceous trachyte. On each of these two islands, products of eruptive cycles are preserved as tuff-flow sequences of decreasing degree of differentiation and increasing phenocryst abundance up-section, possibly a consequence of tapping successively deeper levels of compositionally zoned magma chambers. Island age generally increases to the south away from the Galapagos rift. Volcanic activity on Santiago spanned most of the past 0.7 m.y. K-Ar ages of rocks exposed on Rabida range from about 1.0 to 0.7 m.y.; those on Pinzon are approximately 1.2 to 0.8 m.y. in age. A part of the tectonic history of the central and southeastern Galapagos Islands is recorded in conspicuous, east-trending alignments of volcanic vents on Santiago and parallel normal faults and some aligned vents on six other islands to the southeast. The central islands among this group are also characterized by uplifted submarine rocks. On the basis of geological and geophysical observations, we hypothesize that this uplift, north-south distention, and associated volcanism may have resulted when an east-trending curtain of mantle upwelled beneath the center of the archipelago during the past 0.5 m.y. The east-trending tectonic pattern appears to be superimposed on the older northwest and northeast trends that have been cited for the entire island group.

Some Tectonic Aspects of Quaternary Volcanoes in Japan

Part 1 of this paper is concerned with various types of deformation of volcanic cones as listed in Table 1. In regard to the crustal displacements before and after eruptions, the local upheaval and fracturing near crater remain as a permanent deformed topography, but the areal upheaval and subsidence before and after eruption are believed to be a reiterative movement caused by increase and decrease of the pressure in magma reservoir. Most of collapse calderas in Japan belong to Krakatoan type and some to Kilauean type, but none of resurgent cauldrons is known. These deformations of volcanic origin are generally circular in their horizontal shape and display larger magnitude and rate in vertical displacement than those of non-magmatic gravitational and tectonic origin.Non-magmatic, superficial, gravitational deformation of volcanic cones such as the settlement of cones and landslides of large scale are emphasized, because of their important role to deform volcanic cones on the weak foundations such as Cenozoic altered volcanics. It is also known that non-magmatic tectonic movements such as the active faulting connected with earthquakes, the active folding of the basement etc., deform volcanic cones.In part 2, firstly, horizontal arrangement of volcanoes is separated into three types of different extent. Two volcanic zones, East Japan and West Japan, are recognized. The two display remarkable cross-sectional asymmetry of both volume of volcanic products and nature of magma from oceanic side to continental side. This asymmetry is believed to be due primarily to the continental-ward inclination of the mantle earthquake zone where parental magma generates. Volcanic center alignments are in parallel with mountain ranges of about 50km in width and are related to the pre-existing faults which were formed during the mountain building. Volcanic vent alignments run in perpendicular or diagonal direction to the volcanic center alignments and are inferred to be related to tension cracks parallel to the late Quaternary local principal horizontal stress direction.Secondly, the vertical distribution of volcanoes of different ages in relation to the configurations of their basements is discussed (Figs. 1, 2, 3, 4). Older volcanoes which were formed during middle and early Quaternary usually rest on the crest of the rised or rising mountain ranges. On the contrary, younger volcanoes which are active during late Quaternary tend to occur on the relatively lower places than older volcanoes.The last chapter is devoted to deal with relationship between the distribution of Quaternary volcanoes of different ages and neotectonics in Japan. Fig. 5 shows a possible working hypothesis on the relationship.

Volcanic alignments North and West of New Zealand's Central Volcanic Region

Abstract Three major types of volcanic alignment are recognised. Volcanic vent alignments are inferred to be tensional features usually parallel to the local principal horizontal stress (PHS) direction, and include fissures, elongation of centres, and lines of closely spaced contemporaneous vents. Volcanic centre alignments are most likely related to underlying pre-existing faults that dilated at a point or as a fissure during volcanism. Regional volcanic alignments, which incl ude widely spaced centres and volcanic formations within a narrow belt, imply a fundamental relation between volcanism and structure, and may indicate the location of the major fractures that act as volcanic conduits from depth to near-surface minor fault systems. Regional alignments, and those centre alignments that strike west in Northland and north-west in Taranaki, reflect post-Hokonui structures. They show a 30° clockwise swing of strike in progressively more southern areas from the Bay of Islands to Bombay — North Hauraki, and perhaps a further 30° to Mt Egmont, comparable to that of Macpherson's regional structural arc. Of all alignments, they alone include pre-Pliocene and non-basaltic volcanic formations. On the other hand, vent alignments, and two other sets of centre alignments, striking south and south-west, show no comparable southwards swing in strike, on a regional basis, but have considerable local variations. They appear to reflect Late Cenozoic structures. Inferred PHS directions are: for the post-Hokonui structures, NE in the Bay of Islands swinging steadily to the S of E in Taranaki; and for Late Cenozoic structures between NNE and ENE, averaging NE, in all districts from the Bay of Islands to the South Waikato. Thus the PHS bearing appears to have swung anticlockwise (or the land rotated clockwise) since the folding of the post-Hokonui Orogeny, by a small amount in stable Northland, by 30° in Bombay and North Hauraki, and by possibly as much as 60° in Taranaki.

Ring structures at Mauna Kea, Hawaii

Macdonald briefly discusses the geology of Mauna Kea. The rocks consist of the older Hamakua volcanic series, with a lower member composed largely of primitive basalts and an upper member containing more highly differentiated types, and the younger Laupahoehoe volcanic series composed largely of andesites, with which are interbedded glacial deposits. Besides the radial rift zones, Mauna Kea also shows concentric arcuate alignments of vents, indicating arcuate fractures occupied at depth by ring dikes or cone sheets. The arcuate alignment of vents appears to result from the intersection of shallow-seated radial fissures with the deeper arcuate intrusions.