Showing papers on "Phreatomagmatic eruption published in 1984"

Caldera Development During the Minoan Eruption, Thira, Cyclades, Greece

Abstract: The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodiacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minormore » eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lave shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large-scale base surge deposits during vent widening by phreatomagnetic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding.« less

Climatic impact of explosive volcanic eruptions

Abstract: Major explosive volcanic eruptions inject ash and gas into the upper atmosphere, producing aerosol layers which can affect the global energy balance and climate1. Empirical studies have shown that major eruptions can produce a decrease in surface air temperature of up to a few tenths of a degree Celsius over the Northern Hemisphere land masses and that the effects may last for 2 or 3 yr (refs 2–4). This temperature decrease has been simulated by numerical models using realistic estimates of the nature of the aerosol cloud1. Previous empirical studies of volcanic effects have examined fluctuations in monthly, seasonal or annual climate data, but generally only at a frequency of one observation per year. This has rendered determination of the timing of the onset of effects during the first year impossible. Using continuous monthly surface air temperature for the Northern Hemisphere land masses, we resolve the month-by-month development and decay of the initial climatic impact. In the case of Northern Hemisphere eruptions, abrupt cooling occurs during the first two to three months, which is more rapid than previously assumed.

Explosive Volcanism of the West Eifel Volcanic Field/Germany

Abstract: The Quarternary volcanic field of the West Eifel is located on the Rhenish Massif which is presently rising above an anomalous mantle structure. Magmas of nephelinitic, leucititic, basanitic, tephritic, and phonolitic composition reached the surface in about 240 volcanoes. About 60 maars occur in this classic maar region and the remaining 180 volcanoes are mostly scoria cones. Nearly all maars formed in valleys where abundant groundwater was able to circulate through zones of structural weakness in bedrock beneath the valley floors. The rising magma generally had access to this ground water during the entire period of phreatomagmatic maar eruptions. In contrast most of the scoria cones erupted within small maars (initial maars), which suggests that the magma rising underneath these volcanoes must have contacted only limited amounts of groundwater circulating along hydraulically less active zones of structural weakness. The available water was shut off when the initial maar collapsed and then the magma could rise, intrude the diatreme, and erupt on the maar floor forming a scoria cone in a second eruptive phase. The hydrogeological situation in the Eifel is characterized by zones of structural weakness that exhibit different or no hydraulic activity and it clearly controls formation of the various West Eifel colcano types (maars, scoria cones with initial maars, and scoria cones).

Geothermal Energy Release at the Solfatara of Pozzuoli (Phlegraean Fields): Phreatic and Phreatomagmatic Explosion Risk Implications

Abstract: The H2O, CO2 and H2S outputs at the Solfatara of Pozzuoli have been measured and a map of the exhaling areas has also been made. The energy released at the surface by the fluids has been estimated to be 1019 ergs/day. The presence of aquifers at Phlegraean Fields increases the phreatic and phreatomagmatic explosion risk. Our results suggest that even if an uprising magma may interact with water at depth, an explosion could occur only at the shallow levels of a few hundred meters. Since the transfer of energy toward the surface is favoured by the presence of fractures, a detailed analysis of the deep fracture network would help to evaluate the risk levels of the various areas of Phlegraean Fields.

Estudio vulcanológico y qeoquímico del maar de la Caldera del Rey. Tenerife (Canarias)

Abstract: In this study a cartographic, morphological, geochemical and petrographic study is made of the ''Caldera del Rey". The "Caldera del Rey" is a volcanic structure formed by two overlapping maars, The second one (maar) that was formed is of greaten dimensions and destroyed part of the first one, Both maars erupted throught a possible fracture N 35 E, which is one of the directions with regional importance in the Archipelago. The eruption, which was very explosive, has been thought to be due in part to the great importance of the gaseous phase of the salic magma and also to the steam produced during the interaction of the magma with underground water. This explosivity can be clearly seen in the cleaf resalte cul out in the "Serie Basaltica Antigua" to some extent penetrated and fragmented by the eruption, The phreatomagmatic character of the eruption is evident because of the existence of accretionary lapilli. The materials emitted are exc1usively of aerial projection: agglomerates, tuffs cinerites. There was no flow of lava. ' Some of the fragments of tuffs as cinerites and pumice are comagmatic. These correspond to salic trachytic phonolitic rocks, which represent one of the last stages of differentiation of the alkaline oceanic magmas. The geochemical character of the materials of the "Caldera del Rey" is characteristic and can be easily distinguished from other nearly salic deposits formed in different cycles.

Late Pleistocene Tephrochronology in the Region from the Osumi Peninsula to the Miyazaki Plain in South Kyushu, Japan

Abstract: Southren Kyushu has been the region of intense volcanism at least since Pliocene time. One of the most characteristic features is the prevalence of the large-scale pyroclastic flow eruptions which originated from such gigantic calderas as Aira, Ata, Kikai and Kakuto.There exist a considerable number of literature on the stratigraphic sequence and distributions of the pyroclastic flow deposits in South Kyushu. However, relatively small number of reports are available on air-fall tephra deposits, which are useful for establishing Quaternary chronology both of source volcanoes and of marine or fluvial sediments in the coastal regions such as the Miyazaki Plain. In this study, each bed of maker-tephras which erupted during the time from ca. 100, 000 to 25, 000y.B.P., is precisely discriminated and described in the northern part of the Osumi Peninsula, Kagoshima Prefecture first. And then each tephra is traced northeastward along the main axis of distributions to the Miyazaki Plain.Of many tephras, the following four well-dated tephras are used as fundamental timemakers because of their widespread occurence; Ata pyroclastic flows, originated from Ata caldera in 95, 000-90, 000y.B.P. ; Kikai-Tozurahara ash falls, originated from Kikai caldera in 75, 000y.B.P. ; Aso-4 pyroclastic flows, originated from Aso caldera in 70, 000y.B.P.; Ito pyroclastic flows and AT ash, originated from Aira caldera in 22, 000-21, 000y.B.P. Several air-fall tephras from the Aira and Kirishima volcanic centers are identified in detail and roughly dated from their stratigraphic positions between these fundamental maker-beds.About 75, 000-70, 000y.B.P., explosive activity of Aira caldera occurred resulting in the formation of plinian pumice fall deposit, Fukuyama pumice falls, which is found from the Osumi Peninsula to the Miyazaki Plain. During ca. 60, 000-25, 000y.B.P., intermittent eruptions occurred forming five sheets of tephras, of which the Iwato eruption was greatest in producing pumice falls, pyroclastic surges and pyroclastic flows. Iwato pumice falls mantle extensive area from the Osumi Peninsula to the Miyazaki Plain. Cataclysmic eruption occurred from Aira caldera, producing Osumi pumice falls, Tsurnaya and Ito pyroclastic flows and AT ash 22, 000-21, 000y.B.P. Most of these eruptions were accompanied with phreatomagmatic ones.Eruptive history of Kirishima volcano is divided into two stages deduced from the tephra sequence. At ca. 40, 000 y.B.P., older stage of activity started with ejection of relatively felsic pumice falls, Iwaokoshi pumice fall, and graded to more mafic and frequent eruptions, Awaokoshi scoria fall. Younger stage began with the plinian eruption of Kobayashi pumice fall at ca. 15, 000y.B.P.Of many terraces in Miyazaki Plain, Sanzaibaru terrace is the most extensive one and is accompanied with transgressive marine deposits. Stratigraphic relation with tephra sequence shows that Sanzaibaru terrace was emerged before the Ata pyroclastic flow eruption, ca. 95, 000y.B.P., probably indicating the Last Interglacial Stage. Most of terraces younger than Sanzaibaru are of fluvial origin, except for Nyutabaru II and probably III terraces which are partly of marine origin, and are largely devided into two groups, older and younger. Older terraces, Nyutabaru terrace group, formed during the time from the Ata eruption to the Aso-4 eruption, were chracterized by the profiles with more gentle gradient. Younger ones which were chracterized by the profiles with steeper gradient, were formed after the Aso-4 eruption and before the Kobayashi pumice fall. The difference of their profiles reflects the sea level after the maximum stage in the Last Interglacial Age.

Deposits of Hydroclastic Eruptions

Abstract: Many volcanic eruptions result from the interaction of magma and external water (Table 9-1), but few volcanologists (e.g., Jaggar, 1949) have emphasized the importance of nonmagmatic water in volcanic eruptions. In our view, the importance of external water in explosive eruptions is still underestimated. Wood (personal communication) even holds that maars, which most commonly develop from hydroclastic eruptions, are the second most common volcanic landform on earth next to scoria cones.