Distribution Of Volcanic Vents Research Articles

Control of the stress field and rift structures on the distribution and morphology of explosive volcanic craters in the Manyara and Albertine rifts

Monogenetic volcanoes are widespread in the East African Rift. Their morphologies are diverse with positive (scoria and tuff cones) and negative reliefs (tuff rings and maars). The Manyara Rift (Tanzania) and Albertine Rift (Uganda) are characterised by a singular abundance of negative volcanic reliefs with about 300 and 350 maars and tuff rings, respectively, covering a surface of about ∼3000 km2 in each country. Such high density of explosive craters is not observed elsewhere in the East African Rift.We perform spatial distribution and alignment analyses using two different approaches (Two Point Analysis and Point Alignments) to study the link between the distribution of the monogenetic volcanic vents, the rift structures, and the stress field. Both methods give similar results although the Point Alignments perform better for large radius of investigations. Volcanic alignments in the Manyara rift suggest a dominant control of the old basement and rift structures on dyke emplacement and distribution of volcanic vents, under an oblique rifting. However, data further suggest that this influence is reducing, and the rift is entering a more mature phase with orthogonal rifting. On the other hand, volcanic alignments in the Albertine rift imply a dominant control of the stress field on dyke emplacement and on the distribution of volcanic vents under orthogonal rifting. This suggests that the Albertine rift is a mature rift since at least the past 50 000 yrs.We further tested the influence of rift structures and stress field on maar and tuff ring morphologies. Most of the craters are near-circular, or a least not strongly elongated, and show minor complexities in shape which we attribute to lateral migration of explosions. Few maars and tuff rings in Manyara show that lateral migration have followed pre-existing faults. In general, we advise not to fit an ellipse to individual maars and tuff rings to assess the orientation of the feeder intrusions.

Volcanic field elongation, vent distribution, and tectonic evolution of a continental rift: The Main Ethiopian Rift example

Magmatism and faulting operate and interact in continental rifts at a variety of scales. The East African Rift, an example of an active magmatic continental rift, provides the ideal location to study the interplay between these two mechanisms. Previous work has shown that the distribution of volcanic vents and the shape of volcanic fields are linked to their tectonic environment and their magmatic system. In order to distinguish the impact of each mechanism, we analyzed four distinct volcanic fields located in the Main Ethiopian Rift; three of them (Debre Zeyit, Wonji, and Kone) are located within the rift valley, and one (Akaki) lies on the western rift shoulder. The elongation and shape of each field were analyzed based on their vent distribution using three statistical methods: the principal component analysis, the vent-to-vent distance, and the two-dimensional, symmetric Gaussian kernel density estimation method. The results of these analyses show that the elongations of the fields increase from the western shoulder toward the rift axis and are inversely proportional to their angular dispersion. In addition, we observed that none of the analyzed fields have elongation that parallels either the trends of the youngest faults or the volcano alignments. Dike intrusions inferred from the alignment analysis of volcanic centers that are located within the rift axis tend to parallel the orientation of the recent active faults. This parallelism, however, decreases from the rift axis toward the rift shoulder, which has a lower strain rate and shows a larger number of preexisting fault orientations. Our results suggest that the shape of volcanic fields in the Main Ethiopian Rift is controlled mainly by large crustal structures and/or by the lithosphere-asthenosphere–boundary geometry. Diking is principally controlled by the location of the field within the rift zone; intrusions within the rift zone are controlled mostly by the state of stress and strain rate; while intrusions on the rift shoulder are controlled mostly by the presence of preexisting fractures. This study provides a combination of statistical analyses and geological observations to study and differentiate the mechanisms involved in the formation of volcanic fields on Earth and, potentially, other planets.

Seismic activity at Cadamosto seamount near Fogo Island, Cape Verdes-formation of a new ocean island?

SUMMARY In global catalogues only a handful of earthquakes occurred at or near the Cape Verde islands. The two most recent events occurring in 1998 and 2004 were observed in the southwest of the Cape Verde archipelago away from aerial volcanic centres. Event relocation suggests that both earthquakes may occur at the Cadamosto seamount, a 3-km-tall submarine volcanic edifice to the southwest of the active volcanic islands of Fogo. Swath mapping efforts revealed numerous small volcanic cones between the volcanic island of Brava and the seamount and between Fogo and Brava. The distribution of volcanic vents may support two plumbing systems, one feeding Fogo and volcanic cones between Fogo and Brava and another system feeding Cadamosto seamount. Using ocean-bottom seismic stations of opportunity, deployed roughly a month after a mb= 4.3 earthquake occurred in 2004 August near or at the Cadamosto seamount, we were able to locate local seismic activity clustering at the seamount. We propose that these events reflect brittle rock failure due to magma redistribution in or near a central magma reservoir or more likely dyke intrusion. The observations reported here suggest that Cadamosto seamount is the seismically most active feature in the archipelago. It might be reasonable to hypothesis that ongoing activity causes the Cadamosto seamount to grow, forming a new oceanic island in the future.

Late Cenozoic tectonics and volcanism along the North Anatolian Fault: new structural and geochemical data

Different types of volcanic activity have developed along the North Anatolian Fault zone (NAF) in Turkey. Detailed analysis of satellite images and Digital Elevation Models (DEMs), and field observations have been made in order to understand the tectonics and the distribution of volcanic vents. In the Anatolian block, some faults are oblique to the NAF, and delimit extensional escape wedges. Vents rooted on open tension fractures show that emplacement of the volcanism is related to tensional fracturing of the Anatolian continental block, which we attribute to the onset of free borders in the south and west. Along the NAF, earliest extension ( ϑ1) is directed S to SSW and the latest ( ϑ2) towards W to WSW, parallel to the sinistral slip along the NAF. Major and trace element geochemical data indicate that the dominant calc-alkaline rocks are associated with various alkaline lavas. K–Ar ages of the volcanics range from 22 to 8.5 Ma in Galatia Massif, 900 to 100 ka in Niksar and 3 to 12 ka in Erzincan. Isotopic and trace element data are interpreted as reflecting a dominantly lithospheric mantle source, slightly mixed with asthenospheric liquids. Most magmas were fractionated and contaminated by continental crust during their ascent. ϑ1 and ϑ2 tectono-volcanic events have propagated through time and space from west to east. The ϑ1 extension is late Oligocene in the Galatia Massif, late Miocene in Niksar and late Pliocene in Erzincan. The ϑ2 extension and strike–slip event along the NAF began in the late Miocene in the Galatia Massif, early Pliocene in Niksar and Quaternary in Erzincan. Signature from an asthenospheric source in the second event in the three regions suggests that an ancient suture zone (Galatia Massif) and/or the lithospheric NAF transform (Niksar and Erzincan) have served to channel small quantities of asthenospheric melts existing at the base of the lithosphere.

Tectonic controls on magmatism in The Geysers–Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Three composite cones have grown on the southern edge of the previously existing Atitlan Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlan, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Toliman are mantled by more silicic lava flows. On Toliman, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlan’s rocks and late lavas from Toliman have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.