Cyclic magma storages and transfers at Piton de La Fournaise volcano (La Réunion hotspot) inferred from deformation and geochemical data
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Abstract Several recent studies have argued that large, long-lived and molten magma chambers 1–10 may not occur in the shallow Earth’s crust 11–23 . Here we present, however, field-based observations from the Bushveld Complex 24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km 3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history 25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces 26 . This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’ 1–10 remains a viable research concept for some of Earth’s magmatic provinces.
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Determining the volume of the various types of products of a highly frequent active volcano can be very difficult, especially if most of them are deposited on a growing volcanic cone. The New South-East Crater (NSEC) of Mt Etna, Italy, may be considered one of the best case studies because of tens of paroxysmal episodes which it produced in the last few years. On 25-26 October 2013, a lava fountain at the NSEC produced magma jets up to 500 m high, a maximum ~8 km high column, a multilobate lava flow field 1.3 to 1.5 km long, and almost 30 m of growth in height of the NSEC cone. Mapping of explosive and effusive deposits allowed us to calculate the total volume of erupted products, including lava flows, proximal and distal tephra fallout, and the amount of coarse pyroclastics on the cone. The estimation of the latter products was also confirmed subtracting digital elevation models (DEMs) obtained at different stages of the NSEC growth. Results show that the volume of tephra fallout away from the cone was only <5 % of the total erupted magma, while the total volume of pyroclasts (distal plus proximal fallout) is about a third of the lava volume. Our analysis suggests that, at least for the studied event, three fourth of the involved magma was already partially degassed and thus emitted as lava flows. Hence, the main distinctive character of lava fountains at Etna, i.e. formation of eruption column and propagation of tephra-laden volcanic plumes to tens of km away from the volcano, would not contribute significantly to the final budget of erupted magma of the 25-26 October 2013 eruption. We finally propose that the same magma dynamics probably occur also during most of the common lava fountain episodes.
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The “drumbeats” mode is a sequence of volcanic earthquakes with similar waveforms (multiplets) that are recorded from tens of minutes to months. As a rule, the “drumbeats” mode accompanies the squeezing of lava blocks on extrusive domes of andesite-dacite volcanoes. Squeezing and movement of a viscous lava flow during an eruption Kizimen volcano in 2010-2013 was accompanied by multiplets with energy classes K<6 (magnitudes < 2.5), which were recorded from tens of minutes to months. Two series of the strongest multiplets were recorded during the four months from May to mid-October 2011. The volcanic earthquakes of multiplets can be attributed to hybrid and long-period. The coordinates of the foci were identified for the most powerful 35 events of two multiplets of “drumbeats” mode and were localized in the frontal part of the lava flow. The location of the volcanic earthquakes hypocenters suggests that they are generated during the motion of a viscous lava flow the volcano slope. The emergence of multiplets, apparently, can be considered as a process of self-oscillations of the relaxation type.
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Scientists generally believe that the Hawaiian Islands formed primarily through endogenous growth, or intrusion, in which hot magma intrudes into a rock and then solidifies before it reaches the surface. However, a new study suggests that the islands may actually have formed primarily through extrusion, which occurs when a volcano erupts and magma reaches the surface and flows away from the eruption site before cooling and solidifying.
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Hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the East Pacific Rise 15°30′–17°N are used to assess the relationship between crustal structure, morphological indicators of magma supply, and ridge segmentation. From stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust, layer 2A. In contrast to other studies, correlation between axial morphology and magma lens characteristics is found with a lens that, on average, is twice as wide and slightly shallower (100–200 m) beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment. However, large local‐scale variations in lens depth and width are also observed that are not related to morphology. We conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment, with large variations within a segment due to local processes of magma eruption and delivery. A negative correlation between zero‐age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment. In this innermost axial region above the magma lens, factors such as magma pressure, not magma volume in the lens, may govern the thickness of extrusives that accumulate. The width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment (5–8 km), and a narrower zone (2–5 km) of minor thickening (less than double) along the northern segment and toward ridge segment ends. This relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply. We find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry, indicating a genetic link between the finest‐scale tectonic segmentation of the ridge and segmentation of the magma lens. In several locations, discrete magma lenses at different levels within the crust (offset by several hundred meters) are imaged beneath adjacent fourth‐order ridge segments.
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Abstract Several recent studies have argued that large, long-lived and molten magma chambers may not occur in the shallow Earth’s crust. Here we present, however, field-based observations from the Bushveld Complex that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering continuously drapes across a ~ 4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within this thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated at > 5-km in thickness and > 380,000 km 3 in volume. This volume of magma is three orders of magnitude larger than any known super-eruption in the Earth’s history and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces. This suggests that super-large, entirely molten, and long-lived magma chambers occur, at least occasionally, in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’ remains a viable research concept for some of Earth’s magmatic provinces.
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