Large Volcanoes Research Articles

Outline of the petrology of the Kenya rift alkaline province

Summary The Kenyan and N Tanzanian volcanic province contains sodic alkaline rocks ranging from melilitites and melanephelinites to transitional alkali basalts and their differentiates. Individual volcanoes display three principal magmatic suites: (i) nephelinitic; (ii) alkali basaltic; (iii) transitional basaltic. However, some large volcanoes contain more than one of these suites implying that parental magmas of variable alkalinity were available at certain times and places. A general decrease in alkalinity with time is detectable in the rift zone and for any time period there was a tendency for the least alkaline magmas to be erupted within the central and deepest part of the rift zone. Compositional variation within the suites was largely controlled by low-pressure crystalliquid fractionation. Extended fractionation produced salic differentiates. Liquid fractionation caused upward segregation of phonolitic and trachytic magmas which were erupted in preference to more mafic magmas. Isotopic data suggest that crustal contamination did not occur on a large scale.

Volcanic chronology of the Men–Kilimanjaro region, Northern Tanzania

New conventional K-Ar dates for volcanic rocks from a recently mapped area in northern Tanzania, which includes the large volcano Meru, confirm rift-faulting episodes around 2.1 Ma and 1.2 Ma. Alkaline volcanic activity in the Meru area occurred in two main episodes, the earliest around 2.0–1.5 Ma. The later, main phase began about 0.3 Ma and peaked between 0.15 Ma and 0.06 Ma, although minor activity continued into the present century. Available data suggests that Meru and the neighbouring Kilimanjaro volcanic massif were never substantially active during the same periods.

The western Anahim Belt: root zone of a peralkaline magma system

The Anahim Volcanic Belt extends easterly across central British Columbia, where it includes three large shield volcanoes and numerous small monogenetic cones and lava flows. Potassium–argon dates on the lavas suggest that the onset of volcanic activity progressed from approximately 8 Ma in the west to less than 1 Ma in the east. A western extension of the belt into the strongly uplifted and deeply dissected Coast Mountains is defined by an east-trending zone of chemically distinctive, high-level plutons, dyke swarms, and remnants of eruptive breccia that yield K–Ar dates of 10–14 Ma. The lavas and dykes include both basic (alkali basalt and hawaiite) and salic (trachyte and sodic rhyolite) suites, which have remarkably similar major-element abundances. In addition, moderately to strongly peralkaline rocks (comendite and pantellerite) constitute a major part of the volcanic piles and a lesser proportion of the dykes. Epizonal plutons show an upward zonation from hypersolvus syenite to miarolitic, alkaline to mildly peralkaline soda granite. The plutonic, hypabyssal, and eruptive rocks of the western Anahim Belt are considered to represent different levels of exposure in the root zone of an alkaline magma system that gave rise to peralkaline end members. Uplift rates, based on fission track dates, suggest that fractionation leading to mildly peralkaline soda granite took place in crustal reservoirs emplaced at depths of 2.5–4.5 km below the surface and that additional enrichment in alkalis took place at still shallower depths.

The martian hemispheric dichotomy may be due to a giant impact

Mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1–3. The more southerly province is heavily cratered, contains relatively old geological units, and superficially resembles the lunar and mercurian highlands. The northern province is relatively lightly cratered and contains younger geological units, including extensive plains, volcanic edifices, and volcanic calderas. Except for the Tharsis and Elysium regions and other large volcanoes, most of the younger, northern province consists of lowlands, which lie an average of 3 km below the highlands. Lowlands occupy about one-third of Mars. They are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits (Fig. 1). The transition is characterized by a variety of landforms unknown on other planets. The highlands and lowlands are in isostatic equilibrium across the transitional zone4. No generally accepted explanation for the cause of the highland-lowland dichotomy has been proposed although thinning of the lithosph¨re in the northern hemisphere by internal processes has been suggested5. Large impacts, which were a feature of early Solar System history, have also been suggested as an important cause of such early planetary-surface heterogeneities3,6,7. Subcircular lowlands such as Argyre, Chryse, Hellas, and Isidis Planitia and the plains south of Tharsis occupy previously recognized impact basins8–10. We propose here that the largest expanse of lowlands also results from formation of a large impact basin early in Mars' history that has profoundly shaped the planet's surface.

Submarine central volcanoes on the Nazca Plate — High-resolution sonar observations

This paper describes observations of 214 submarine central volcanoes, made with long-range side-scan sonar and narrow-beam echosounder between Peru and the East Pacific Rise. They are all circular in plan, with basal diameters ranging from less than 1 to over 10 km. Profiles of four volcanoes were obtained with the narrow-beam echosounder, and those of a further nine were inferred from their acoustic shadows. All the profiles show steep, mostly concave, sides. A distinct summit region could be identified on about two-thirds of the volcanoes. In just under half of these the volcano culminates in a simple crater, but the rest have broad, slightly convex summit plateaus, with or without craters. Almost half of all the volcanoes have one or more craters. Summit plateaus are relatively more common on the larger volcanoes, so that for central volcanoes with a basal diameter greater than 4 km, the commonest morphology is a steep-sided, truncated cone with a slightly domed summit plateau. In most cases this form must represent the mode of formation, not the result of subsequent wave erosion. It is suggested that this form is the result of relatively fluid lavas travelling outward on low slopes from a central vent, but terminating in steeply dipping hyaloclastites and volcanic breccias. The spatial distribution of volcanoes is very non-random. In the area studied, the East Pacific Rise out to 6.5 Ma has less than one volcano per 10 4 km 2, whereas a field hundreds of kilometers across with a peak density of over 60 per 10 4 km 2 was seen north of Mendaña Fracture Zone on 6.5–26 Ma-old crust.

Nevado Portugueza volcanic center, central Peru; a Pliocene central volcano-collapse caldera complex with associated silver mineralization

Nevado Portugueza, a major center of calc-alkalic volcanism on the crest of the Western Cordillera of the Andes in central Peru, was active during Pliocene and perhaps Quaternary time. A large central volcano composed mostly of intermediate lavas formed about 3.5 to 4 m.y. ago. Explosive pyroclastic volcanism 2.4 + or - 0.2 m.y. ago produced the Atunsulla Tuff, a rhyodacitic ash-flow sheet of moderate volume, and created an east-northeast elongate collapse caldera about 7 km in maximum diameter.Steeply dipping fractures in the hydrothermally altered central part of the intracaldera tuff prism host thin veins containing base and precious metal mineralization dated at 1.90 + or - 0.08 m.y. This mineralization is probably related to postsubsidence magmatic activity reflected at the surface by a rhyodacite dome emplaced along the ring-fracture system 1.96 + or - 0.1 m.y. ago. Tetrahedrite-bearing veins, which appear to postdate collapse, are exposed at lower elevations directly east of the caldera. Hot water and sulfur present in workings in these veins may reflect episodically continuing activity of the volcanic center.Limited geochemical and Sr isotope data show that the postsubsidence lavas are chemically similar to the Atunsulla Tuff and suggest that these unevolved late-stage silicic rocks were not derived from magmas compositionally similar to the intermediate lavas that make up the earlier central volcano.

Structure of the cantal strato-volcano, French Central Massif, from electric, magnetotelluric soundings and aeromagnetic data

The Neogene composite volcano of Cantal is the largest volcano in Europe. Its area (2500 km 2) is larger than that of Etna (1200 km 2). It was formed between 23 and 3 Ma ago. An aeromagnetic survey, in addition to electrical (ES) and magnetotelluric soundings (MTS) carried out in recent years have made it possible to describe the structure of this volcano. The terrain effects of the various pyroclastics and lavas that make up the volcanic edifice have been computed and removed from the aeromagnetic data; the analysis of the resulting terrain-corrected residual anomalies gives clear evidence for the presence of three internal or deep-seated magnetized bodies which are also revealed by the ES and MTS investigations: 1. (1) The main hypostructure corresponds to a large volcano-tectonic trough filled with low-resistivity (10-5 ωm) and moderately magnetized (1 A/m) volcanic material. However, a part of the trough is filled with non-magnetic formations and it may be an Oligocene sedimentary graben cut across by the present volcanic trough. 2. (2) The hypovolcanic intrusion (monzonites and gabbros) of the Jordanne upper valley was previously known from an outcrop and a borehole (260 m). Its large extent (15 km 2) is shown by magnetic study as well as by ES and MTS. According to the magnetic interpretation, its thickness would be 300 m. 3. (3) The nature of the structure inside the Plomb du Cantal is still uncertain. In view of its high resistivity and high magnetization, it may be an unexposed pile of lavas inside the cone or a large intrusion. In the area where a caldera affects the volcanic cone, ES and MTS underline the importance of a hydrothermal process (propylitization) that lowers and homogenizes the resistivity ( p = 30 ω m) of all volcanic rocks and probably that of the uppermost part of the crystallophyllian basement.

The role of lithospheric stress in the support of the Tharsis Rise

The Tharsis rise on Mars is a broad topographic high covered by volcanic plains and capped by large volcanoes. The geoid high associated with Tharsis shows that shallow isostatic compensation is not complete. We hypothesize that the Tharsis rise can be approximated as an axisymmetrical igneous construct. Linear theory for the deflection of planetary lithospheres is used to show that the lithospheric stresses required partially to support the construct are reasonable and consistent with the observed radial grabens around Tharsis. The computed thickness of the elastic lithosphere is between 110 and 260 km depending on the assumed values for crustal thickness and crustal density. The computed thickness of the Tharsis load is between 40 and 70 km. In this model the height of the geoid is not specified a priori. Therefore, the agreement between the observed and computed geoid is evidence for the validity of our model. The contribution of Tharsis to the gravitational flattening of Mars is computed as ΔJ2 = −8.8 × 10−5. After a correction is applied for the flattening, the geoid low around Tharsis is more nearly axisymmetrical. The tectonics of the Tharsis region are briefly reviewed, and it is argued that all observations are consistent with the loading model.

Wind streaks in Tharsis and Elysium: Implications for sediment transport by slope winds

Detailed maps of wind streaks in Tharsis and Elysium have been compiled from Viking Orbiter observations spanning one complete Martian year. The streak pattern is controlled by slope winds on the central volcanoes and on the flanks of the Tharsis bulge, while the global circulation dominates in Elysium. Dust erosion by downslope winds occurs over much of Tharsis and in the vicinity of Elysium Mons; this process is effective even at the low atmospheric pressures found near the summits of the large volcanoes. Erosional streaks are largely absent in Elysium Planitia; net deposition of dust might have occurred during the period of our observations. Surface properties such as slope, thermal inertia, and roughness may influence the efficiency of slope wind production sufficiently to account for the pronounced differences in streak types and patterns present in these two regions.

Segmentation of the Cascade volcanic chain

Aligned volcanoes can be used to divide the Cascade volcanic chain into six segments averaging 175 km long. The petrology and eruptive style of volcanoes vary with their position on the segmented structure. Large volcanoes along the volcanic front and in the interiors of segments have been called “coherent,” relatively uncomplicated composite volcanoes that erupt uniform porphyritic andesite with only a small range of composition. Volcanoes near segment boundaries are “divergent” volcanoes that erupt porphyritic basalts, andesites, dacites, and rhyolites. Volcanoes near the segment boundaries but behind the volcanic front are distinguished by a predominance of basaltic and rhyolitic volcanism and phenocryst-poor lavas, as compared with those near the segment boundary but on the volcanic front.

The morphology of the Martian surface

Most of the southern hemisphere of Mars is densely cratered and stands 1–3 km above the topographic datum. The northern hemisphere is more sparsely cratered and elevations are generally below the datum. A broad rise, the Tharsis bulge, centered at 14° S, 101° W, is 8000 km across and 10 km above the datum at its summit. The densely cratered terrain has two main components; very ancient crust, nearly saturated with large craters, and younger intercrater plains. In many areas the older unit is fractured and extensively dissected by small channels. The younger intercrater plains are distinctly layered in places and less dissected, less fractured, and less cratered. Both units probably date from very early in the planet's history. Cratered plains cover much of the northern hemisphere and are highly variegated. Those around the large volcanoes are covered with numerous volcanic flows whereas in other areas the plains are featureless except for craters and lunar mare-like ridges. Between 40° N and 60° N the plains are complex with various kinds of striped and patterned ground, low escarpments, and isolated irregularly shaped mesas. Their peculiar morphology has been attributed, in part, to the repeated deposition and removal of volatile-rich debris layers. Along the boundary between the northern plains and the densely cratered terrain to the south, the plains and cratered terrain complexly inter-finger. The old terrain forms the high ground and appears to have undergone mass wasting on a large scale. In several areas, particularly south of Chryse Planitia, the old, cratered surface has collapsed to form chaotic terrain. Large channels, tens of kilometers wide and hundreds of kilometers long, with numerous characteristics suggestive of catastrophic flooding, commonly emerge from the chaotic areas. Much of the area between 50° W and 180° W and 50° N and 50° S is cut by fractures radial to the center of the Tharsis bulge. The equatorial canyon system, Valles Marineris, is radial to the bulge and appears to have formed largely by faulting along the radial fractures, although it has also been extensively modified by various mass wasting and fluvial processes. Most but not all volcanoes are in the Tharsis and Elysium regions. The largest resemble terrestrial shield volcanoes except for scale; the edifices, flow features and calderas are all far larger than their terrestrial counterparts. Most impact craters on Mars are surrounded by layers of ejecta, each with a distil ridge. This unique morphology coupled with other surface characteristics suggests large amounts of ground ice. Layered deposits at both poles appear to be relatively young, volatile-rich, aeolian deposits. The north pole is also surrounded by a continuous belt of dunes several tens of kilometers across. In most other places, aeolian modification of the surface at a scale of several tens of meters appears slight despite annual global dust storms.

Morphometric evolution of composite volcanoes

Statistical relations have been determined between geometry, volume, slope, and age for 26 circum‐Pacific composite (strato) volcanoes. General trends in eruption characteristics, repose periods, flow lengths, and petrology are also documented. Few examples of the earliest stages of composite volcano activity are known, perhaps because these small volcanoes are indistinguishable from cinder cones. If cinder cones evolve into composite volcanoes a fundamental change in morphometry, eruption style, and petrology occurs at a basal diameter of 2 km.Composite volcanoes (stratovolcanoes), composed of layered deposits of pyroclastics and lavas, are the characteristic volcanic landform at sub‐ducting plate margins, and are the most abundant type of large volcano on the Earth's surface. Composite volcano morphology results from repeated eruptions of pyroclastics and relatively short lava flows from a central vent. By comparison, pyroclastics are insignificant and lava flows tend to be much longer (10‐100 km) for shield volcanoes. Most composite cones are formed of andesites or basaltic andesites, but some are composed of basalts (Fuji, Fuego, Izalco); thus petrology may be less important in determining the morphology of composite cones than eruption style, as is true for shield volcanoes (Wood, 1977a). In this study, observations of eruption characteristics and chemical/petrological trends for a number of composite cones were synthesized with newly determined measurements of cone morphology to document statistically the evolution of composite volcanoes and to examine their relation to cinder cones. Additionally, such quantitative descriptions of terrestrial volcanoes provide basic data for comparisons with volcanic structures on Mars and the Moon.

Theory on Martian volcano sizes

A theory attributing the big difference in size between Martian and terrestrial volcanoes to the fact that the crust of Mars, in contrast to that of earth, remains fixed in place has been developed by U.S. Geological Survey scientist M. H. Carr on the basis of data from the Mariner 9 (1971) probe. For example, Olympus Mons, the largest volcano on Mars, is between 500 and 600 km across and ∼2 4 km high, whereas Mauna Loa, the largest volcano on earth, is 120 km across at its base on the ocean floor and more than 7000 m above the sea floor.According to Carr's theory (see his article in the Scientific American, January 1976 ), volcanoes that develop over ‘hot spots’ in the upper mantle just below the earth's crust are relatively short‐lived because they erupt and grow only so long as they remain over the hot spots in the stationary mantle. They become extinct as the motions of the crustal plates carry them away from the hot spots, and new volcanoes may form in the parts of the plates that drift over the hot spots. But on Mars, whose crust is not divided into moving plates, possibly because it is too thick to fracture, volcanoes remain for long periods over the hot spots and continue to erupt for long periods of time.

Basaltic volcanism in south-east Terceira, Azores

SummaryThe Terceira Rift is a subsidiary zone of crustal generation in the Azores to the E of the Mid-Atlantic Ridge. The geology of the island of Terceira, which straddles the rift, is thought to give an indication of the nature of the volcanic activity along this zone of crustal separation. Terceira is composed of 4 large strato-volcanoes associated with trachytic or trachybasaltic activity which have since been intruded by basaltic magmas to form monogenic cinder cones and lava flows on the flanks of these large volcanoes. There is little direct evidence of eruptions along fissures although lines of cinder cones may indicate zones of weakness in the crust.

Negative magnetic anomaly associated with Mount Kenya

MOUNT Kenya is a large central volcano just south of the equator in central Kenya. It rises to a height of 5,199 m above sea level. The base of the volcano covers an area of approximately 7,000 sq km, and is roughly circular in plan with an average diameter of 105 km. A detailed geological study1 indicated that some 1,500 m of the upper part of the mountain has been removed by erosion since the volcano was formed in the Plio-Pleistocene.

Volcanic Couples and Deep Mantle Plumes

MORGAN1 has convincingly demonstrated a case for the upward rise of deep mantle material along narrow plumes which give rise to positive gravity anomalies and hot spots on the Earth's surface. These hot spots are reflected in large volcanoes and, as Wilson2 suggested, where the lithosphere moves over hot spots, volcanic chains are produced. Perhaps the most striking support of Morgan's hypothesis are the parallel ‘dog legs’ in hot spot trajectories given by the Hawaii-Emperor, Austral-Marshall/Gilbert, and Tuamotu-Line chains, most readily explicable as generated by motion of the Pacific plate over three fixed hot spots.

Paleomagnetism of San Cristobal Island, Galapagos

Isla San Cristobal, the most easterly of the Galapagos Islands, consists of two parts: a large volcano constitutes the southwest half of the island and an irregular apron of small cones and flows makes up the northeast half. As some of the younger flows on the flanks of the large volcano are reversely magnetized, the minimum age of the volcano is 0.7 my, which is the age of the Brunhes-Matuyama reversal boundary. The true age is probably several times greater. The cones and flows to the northeast are all normally magnetized. The between-site angular dispersion of virtual poles is 11.3° - a value consistent with mathematical models for the latitude dependence of geomagnetic secular variation.

Satellite reliability engineering : W. A. Hogg, IEE Conf. Reliab. Electron., Conf. Publ. No. 60, 10–12 December (1969), p. 160

New multibeam mapping and whole-rock geochemistry establish the first order definition of the modern submarine Kermadec arc between 30° and 35° S. Twenty-two volcanoes with basal diameters > 5 km are newly discovered or fully-mapped for the first time; Giggenbach, Macauley, Havre, Haungaroa, Kuiwai, Ngatoroirangi, Sonne, Kibblewhite and Yokosuka. For each large volcano, edifice morphology and structure, surficial deposits, lava fields, distribution of sector collapses, and lava compositions are determined. Macauley and Havre are large silicic intra-oceanic caldera complexes. For both, concentric ridges on the outer flanks are interpreted as recording mega-bedforms associated with pyroclastic density flows and edifice foundering. Other stratovolcanoes reveal complex histories, with repeated cycles of tectonically controlled construction and sector collapse, extensive basaltic flow fields, and the development of summit craters and/or small nested calderas.Combined with existing data for the southernmost arc segment, we provide an overview of the spatial distribution and magmatic heterogeneity along ∼780 km of the Kermadec arc at 30°–36°30′ S. Coincident changes in arc elevation and lava composition define three volcano–tectonic segments. A central deeper segment at 32°20′–34°10′ S has basement elevations of > 3200 m water-depth, and relatively simple stratovolcanoes dominated by low-K series, basalt–basaltic andesite. In contrast, the adjoining arc segments have higher basement elevations (typically < 2500 m water-depth), multi-vent volcanic centres including caldera complexes, and erupt sub-equal proportions of dacite and basalt–basaltic andesite. The association of silicic magmas with higher basement elevations (and hence thicker crust), coupled with significant inter- and intra-volcano heterogeneity of the silicic lavas, but not the mafic lavas, is interpreted as evidence for dehydration melting of the sub-arc crust. Conversely, the crust beneath the deeper arc segments is thinner, initially cooler, and has not yet reached the thermal requirements for anatexis. Silicic calderas with diameters > 3 km coincide with the shallower arc segments. The dominant mode of large caldera formation is interpreted as mass-discharge pyroclastic eruption with syn-eruptive collapse. Hence, the shallower arc segments are characterized by both the generation of volatile-enriched magmas from crustal melting and a reduced hydrostatic load, allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions. Proposed initiation parameters for submarine pyroclastic eruptions are water-depths < 1000 m, magmas with 5–6 wt.% water and > 70 wt.% SiO2, and a high discharge rate.

A source for volcanic material in the Mohakatino beds

Abstract What is presumed to be the covered remnant of a large volcano has been revealed by magnetic studies off the Taranaki coast. Its situation is such that it is probably the source of the andesitic debris in the tuffaceous sediments of the Mohakatino Group.

Alkali Olivine Basalt dredged near St. Paul's Rocks, Mid-Atlantic Ridge

RECENT studies of deep sea basalts have revealed the great predominance of olivine tholeiites over both normative nepheline and normative quartz basalts1. An alkali basalt recently dredged between 2,950 and 1,975 m near St. Peter and St. Paul Rocks (St. Paul's Rocks) consequently seemed worthy of prompt description. Although mentioned in an abstract2, we have not previously published a chemical analysis of this basalt. This alkali basalt flow is of special interest because it is not part of a large submarine volcano, but rather evidently was erupted directly on a floor of spinel peridotite mylonites similar to and contiguous with those described from St. Paul's Rocks, a probable high temperature intrusion derived from the mantle3.