Mode Of Eruption Research Articles

40. Mode of eruption of Ata pyroclastic flow estimated from the existence of free mould(Abstracts of Papers submitted for the Meetings of the Society)

40. Mode of eruption of Ata pyroclastic flow estimated from the existence of free mould(Abstracts of Papers submitted for the Meetings of the Society)

Mode of eruption of the deccan trap basalts

The Deccan Trap basalts have long been considered to be products of fissure eruptions and the dykes intruding them have been supposed to represent the fissures of eruption. However, the question of the mode of eruption of Deccan Trap lavas seems to need more careful consideration in view of the features brought to light by detailed field work in Western Maharashtra. Detailed studies of dykes suggest that majority of the dykes could not have acted as feeders as previously supposed. When examined in detail the basalt flows have more often been found to have only a limited lateral extent and are not always quite horizontal. Thin irregular flows with ropy surfaces, dipping in different directions and piled up into a chaotic mass are frequently met with, indicating eruption from local vents of the central type. Volcanic vents are found at a number of widely separated localities. All this suggests that many lavas are products of central type of volcanicity. However, central type of volcanoes would be inadequate to account for the vast amounts of lavas, and as the known dykes are not likely to have acted as feeders, and dykes still remain to be reported from large portions of the Deccan Trap area, the question of how the lavas came to the surface largely remains unanswered. Extensive beds of volcanic breccia traceable over a few miles and upto 50 feet thick are met with. As such extensive beds are more likely to be associated with fissure cruptions the question arises whether these fissure eruptions were accompanied by considerable explosive activity.

Early Tertiary Geomagnetic Polarity Reversals in India

THE Deccan Trap flood basalts of peninsular India are believed to be early Tertiary formations, although the age, duration and the mode of eruption of this volcanism are still debated1–3. Earlier palaeomagnetic investigations have led to their classification into only two polarity zones4 (that is, one geomagnetic field reversal) with the elevation of 2,000 ±200 feet m.s.l. dividing the upper normal (N) and the lower reversed (R) horizons. Inversions of the geomagnetic polarity, however, are known to have been frequent during the late Mesozoic and the Tertiary5 and this implies that several geomagnetic field inversions occurred during these lava extrusions. Pal6 has suggested, on the basis of some recent observations, that further intensive palaeomagnetic studies on these lava sequences are necessary to establish evidence of such reversals.

Evolution of Mt. St. Joseph—an Archaean Volcano

A 6650-m thickness of volcanic rocks at Lake St. Joseph is here interpreted as an Archaean composite strato-volcano. Despite severe deformation, primary mesoscopic fabrics are well-preserved. They permit the inference of a physical eruptive history, to which the chemistry can be related.The lowermost 2700-m of flows, pillow breccias, and autobreccias are exclusively basaltic, and indicate quiet, probably semi-continuous activity. Interruption of this activity is shown by an intraformational conglomerate developed on a metadiorite, which has thermally metamorphosed argillite lenses intercalated with the flows. Subsequently, activity became more episodic and violent, yielding, first, 750-m of mixed flows and fragmental rocks, substantially andesitic; and then 3200-m of fragmental rocks, substantially salic. Basaltic dikes ramify through the volcanic pile. Several rhyolitic or dacitic flows are regarded as flank eruptions.Generally, these volcanic rocks have trends somewhat similar to trends for other Archaean volcanics. However, there is an iron enrichment trend which we suggest indicates an initially low pO2 increasing as the volcano ages. This increase may indicate an increase in total gas pressure, compatible with the change from quiet to violent eruptive mode.

Volcanic activity related to the formation of the Kuroko-type deposits in the Kosaka district, Japan

In the Miocene Kosaka formation of NE-Japan, submarine volcanic sedimentary deposits of the Kuroko-type occur. This formation consists mainly of volcanics which erupted in a submarine environment. The “Kosaka Volcano” was built up by nine or more volcanic events of a single eruptive cycle. The mode of eruption during a representative single eruptive cycle changed as follows: The Uwamuki tuff breccia is a contact product between dacitic magma and the sea water. Dacitic magma pushed the Motoyama dacite dome upward, as a result of the decrease in vesicularity and perhaps also in temperature. Next, a steam explosion occurred at a flank of this lava dome. The hydrothermal activity which began in this steam explosion center is responsible for the formation of the Kuroko-type Cu-Zn-Pb-mineral deposits. Similar examples of a single eruptive cycle as at Kosaka are also found in Quaternary terrestrial volcanoes of Japan.

Palaeomagnetism of the Deccan Flood Basalts

THE Deccan Trap formations, of late Mesozoic–early Tertiary age, encompass a considerable part of peninsular India. These colossal lava flows are believed, much like the Tertiary lavas of Iceland, Mull and Skye, to have extruded sub-aerially. But the questions of their source and mode of eruption, and of their age and the time span of their eruption, are still debated1–4. Their palaeomagnetism has also been extensively studied5. But even the results of these studies have no simple interpretation.

41. A drastic change of mode of eruption of Miyake-jima volcano in the 15th century or thereabouts(Abstracts of Papers submitted for the Meetings of the Society)

41. A drastic change of mode of eruption of Miyake-jima volcano in the 15th century or thereabouts(Abstracts of Papers submitted for the Meetings of the Society)

Mode of eruption of Miyake-jima volcano in historic times

Miyake-jima, an island volcano, resting on the Izu-Mariana arc is a basaltic stratovolcano dotted with many parasitic cones and craters. Literatures tell us thirteen eruptions which took place in A.D. 1085, 1154, 1469, 1535, 1595, 1643, 1712–1714, 1763–1769, 1811, 1835, 1874, 1940 and 1962. The last two eruptions were studied in detail by using modern scientific techniques after the surface activities had almost died down. Of the thirteen eruptions, the earlier five are only described as «eruption» and no detailed accounts are written in a document. The eruptions in and after 1643 all occurred from parasitic craters or fissure vents and are classified into the following two types: Most of these eruptions are simply flank activity, but some are preceded or followed by summit activity. In either type of eruption, the activity begins with the destruction and ejection of pre-existing rocks and then the violent ejection of juvenile scoriae follows. Spindle-shaped bombs seem to be thrown during the later, rather calm stage of the scoria ejection or from lava-filled vent or vents different from the center or centers of the scoria ejection. During the culminating or declining stage of the activity, lava flows down along valley as a torrent and often reaches the sea. In case of the historic eruptions of this volcano, time interval between the start of the activity and the lava outflow is very short, generally less than several hours. The activity abruptly comes to an end soon after the lava outflow. Total volume of the materials erupted in each activity which took place every twenty to seventy years is of the order of 0.01 to 0.02 cubic kilometers. The case history of this volcano as revealed by investigations of the literatures describing the volcanic activity and the basic field survey will give us what a proper means of the prediction of volcanic eruption is.

Welded pyroclastic rock at Tongariro

Abstract Although the rocks of Tongariro National Park have been mapped and their petrograplty is reasonably well known, little attention has so far been paid to their physical character and modes of eruption. Lava, scoria, and ash are the most common types, though hot avalanches that erupted in 1949 and 1954 have been described by Battey (1949) and Gregg (1960). These left round the base of Ngauruhoe deposits that are still recognisable. Gregg (1960) has also referred to a spatter-cinder cone built within the crater of Ngauruhoe by lava fountaining.

Volcanic Eruption and Nature of the Pyroclastic Deposits

This article deals with the mechanism of eruption and transportation of the pyroclastic material and the nature of the resultant deposits from the geological standpoint.In Japan, the method of tephrochronology is best applied to pyroclastic deposits of the Quaternary central volcanoes and those related to the Krakatoan calderas. Most of the rocks are andesitic in composition with subordinate amount of basalt and dacite.Three modes of volcanic eruption may be distinguished: 1) projection of pyroclastic materials which form pyroclastic fall deposits, 2) eruption of pyroclastic flows, and 3) outflow of lava flows or extrusion of dome and spine. Table 1 shows characteristic features of the deposits formed by the three modes of volcanic eruption.Tephra, as originally defined by Thorarinsson, signifies only the air-fall pyroclastic materials and its relation to pyroclastic flow is not clear. In this article, all the pyroclastic materials directly connected with volcanic eruptions, irrespective of their origin (i. e. essential, accessory, or accidental) and of their mode of emplacement, are included in the term tephra. The chronology using the deposits of pyroclastic flows are included in the tephrochronology.The small-scale vesiculation occurring at or close to the top of the magma column results in the so-called Strombolian and Vulcanian eruptions. Larger scale vesiculation with longer time duration leads to the Plinian eruption. The greatest vesiculation takes place within the magma reservoir resulting in the formation of a depression caldera. The larger the size of eruption column, the more effective the sorting of the erupted pyroclastic fragments. The larger and denser particles fall first and closer to the vent while the smaller and more vesicular fragments fall farther away. Consequently the deposits of pyroclastic falls are well sorted and exhibit pronounced lateral regular grading in texture and composition. This is in strong contrast with the poorly sorted character of pyroclastic flow deposits, in which all particles travel en masse in a state of turbulent flow.Welding of the deposit is not uncommon in the pyroclastic flow deposits while it is rare in pyroclastic fall deposits except those deposited near the vents of basaltic eruptions.To reconstruct past eruptions from volcanic deposits, it may be necessary to establish definite correlation between stratigraphic units by which volcanic deposits are grouped and time duration by which specific eruptive activity is grouped. A single eruptive cycle, the deposits of which represent such a time unit, is defined as a series of eruptive events limited by fairly long intervals of quiescence. Historic examples indicate that the duration of a single eruptive cycle ranges from a day to several years in most cases. The intervening periods are generally far longer than the duration of single eruptive cycle.From many examples of single eruptive cycles, a rule has been established: the degree of vesiculation of magma gradually decreases toward the end of the cycle. This is expressed in successive eruption of pyroclastic fall, pyroclastic flow, and lava flow from the same vent in case of felsic magma, and of pyroclastic fall and lava flow in case of mafic magma, which fact may indicate that the original magma column responsible for the eruptive cycle was more enriched in volatiles in the upper part than the lower.The close correlation between the recorded sequence of single eruptive cycles and the reultant beds of volcanic materials is described for a few examples. The beds produced by a single cycle of witnessed eruption conformably superpose each other and do not include a layer representing weathering break. It is stressed that such a group of beds of volcanic ejecta, volcanic deposits of a single eruptive cycle, should be taken into account as a stratigraphic unit when precise tephrochronology is undertaken.

地震及噴火の発現力として氣壓變動の及ぼす効果

It is generally accepted that one of the secondary causes of earthquakes and volcanic eruption is the variation of atmospheric pressure. The author of the present paper thought that a certain tectonic line or volcano has a special mode of dislocation or eruption, and consequently it is necessary to take a special condition of atmospheric pressure gradient proper to them when a fault or an eruption occurs there. In this paper this working hypothesis is accertained using seismometrical and volcanic data of semidestructive earthquakes and strong eruptions observed at our meteorological stations. The following conclusions are drawn:-(1) Dislocation may occur when the direction of atmospheric pressure gradient becomes perpendicular to the existing fault line and the amount of pressure gradient changes spontaneously from lowest or highest value. Moreover, when a pressure tends to increase in this case there occurs a destructive one, and when it is decreasing, a moderate shock may occur. Following few examples may support this theory;(2) When an extraordinary intense anticyclone covers the region surrounding a certain volcano even a slight variation of pressure may cause in many instances volcanic earthquakes and eruption. The examples of this case are found in the eruptions of Mt. Asama, Mt. Sakurazima, Mt, Bandai and others. All of these eruptions occurred, without exception, in the day of anticyclonic weather.