Recent progress in Tephra Study

Significant progresses in recent studies on tephra are reviewed. In relation to volcanological aspects of tephra, recent works on mechanism of vesiculations and disruption of magma, formation of eruption column, transportation of tephra, recognition of types and magnitude on eruptions, and roles of interaction between external water and magma are discussed. The progresses are mainly based on the intensive researches on the recent eruptions observed by volcanologists such eruptions as the 1980 St. Helens, the 1977 Usu and so on. Attempts on quantitative understanding of eruption phenomena such as total eruption volume, initial population of grain-size, and dispersal/fragmentation values of tephra are discussed. Recent studies on phreatomagmatic eruptions on the basis of the experimental researches of magma/water interactions and field observations are remarkable progresses in volcanological studies. In relation to tephrochronological aspects, recent works on identification techniques of tephra layers, discovery of wide-spread tephras, dating methods of tephras, and applications of tephrochronology are summarized. Identification techniques of tephra layers based on determinations of refractive indices and major, minor and trace element compositions of glass and/or minerals were highly advanced in the last 10 to 20 years. Those are effective tools for identification of wide-spread tephras. Wide-spread tephras such as AT ash from Aira Caldera, K-Ah ash from Kikai Caldera, B-Tm ash from Baegdusan Volcano and others were recognized in Japanese Islands and also in deep-sea sediments around Japan. They were dated by radiometric and stratigraphic dating methods. Radiometric dating methods applicable to the late Quaternary tephras are ^C, Fisson Track, Ionium, K-Ar, Thermoluminescence, and Electron Spin Resonance methods. Because the dates of tephras were mostly obtained by ^C dating, the radiometric ages of tephras over 40000 years are lacking so far. The accurate radiometric ages of tephras for this range are strongly required for further developments of tephra studies. It is stressed that tephra studies will play great roles on establishment of eruption histories of poligenetic volcanoes, activity aspects of one cycle eruptions, life time of magma and zoning of magma chamber

Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO2 close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas

Tephrochronological study on the 1986-1987 eruptions of Izu-Oshima volcano, Japan

Sequences and products of the Izu-Oshima 1986-1987 eruptions which started on November 15, 1986, were investigated tephrochronologically. The results are summarized as follows : 1) Summit eruptions (Crater A) During 15-20, Nov. 1986, Strombolian eruptions continued to make a lava lake from where lava flows spilt over and went down the slope of the central cone to the caldera floor (LA I～IV). Volcanic ash and scoria (TA-1～4) were dispersed to the eastern and western parts of the island. On 21 Nov., a little after the beginning of the fissure eruption (Craters B), Strombolian eruptions were reactivated and ejected large volcanic bombs and scoria (TA-5) from Crater A. On Dec. 18, 1986, small explosion occurred from the Crater A for three or four hours, ejecting a scoria fall (TA-6) and bomb. The level of the lava lake lowered about 5 meters. On Nov. 16, 1987, a phreatic explosion occurred to break the crust of the lava lake, and the lava drained back to the deep on Nov. 18. 2) Fissure eruptions in the caldera floor (Craters B) At 16 : 15, on Nov. 21, 1986, fissure eruptions (Craters B) started on the caldera floor and extended to the slope of the central cone. The eruptions became explosive one, generating lava fountains with the height of more than 1500 meters, with a high discharge rate of 8×106 ton/hour, producing pyroclastic cones and rootless (clastogenic) lava flows (LB I and III). Subplinian scoria falls were dispersed to west (TB-1) and east (TB-2). About 5 hours after the beginning, the activity waned to produce only volcanic ash (TB-3 and -6) and finer scoria falls (TB-4 and -5) and ceased on Nov. 23. A rheomorphic lava flow (LB II) occurred from the edge of the deformed cone on Nov. 23. 3) Fissure eruptions on the somma slope (Craters C) At 17 : 45, on Nov. 21, 1986, fissure eruptions occurred on the somma slope, and produced two lava flows (LC I and II), scoria cones, and vesicular scoria falls (TC-1 and -3) from the 11 craters. 4) The 1986 eruptions ejected 0.053 km3, 7.9×107 tons of lava and pyroclasts from A, B and C craters (Table 4)

Tephra-stratigraphical study of the 1988-1989 eruption of Tokachi-dake Volcano, central Hokkaido

Twenty-three small-scale eruptions took place at Tokachi-dake from December 16, 1988 to March 5, 1989. The pyroclastic fall deposits, ballistic fragments, and pyroclastic surge and flow deposits were dispersed over the flank and leeward areas of the volcano. Because the pyroclasts of each eruption were well-preserved in snow during the winter, the stratigraphy and distribution of these deposits could be studied in detail. The volume of the pyroclastic fall deposits are nearly equal to those of the pyroclastic surge and flow deposits. The total volume of these pyroclasts is estimated to be 7.4×105 m3. Judging from the sequential changes of the volume and composition of the pyroclasts, the characteristic features of the eruption can be summarized as follows: At first, a vent was opened by ejection of altered rock fragments in December, 1988. Then, essential fragments were ejected in January, 1989. Finally the activity level of magma declined and the altered rock fragments content increased again in February to March, 1989

The 1988-1989 explosive eruption of Tokachi-dake, central Hokkaido, Its sequence and mode

On December 16, 1988, after 26 years of dormancy since the last eruption in 1962, Tokachi-dake began to erupt from the 62-II crater. The eruption started with phreatic explosions. Then, on December 19, the activity changed into phreatomagmatic explosions of Vulcanian type and continued intermittently until March 5, 1989. Although the composition of the essential ejecta, mafic andesite, is similar to those of 1926 and 1962 eruptions, the mode of the present eruption is considerably diffrent The present eruption consists of a series of 23 discrete cannon-like explosions, being frequently accompanied with small-scale pyrcclastic surges and flows. The total volume of ejecta amounts to approximately 6×105 m3, of which about 20% is essential ejecta. A complete sequence of events was compiled and distribution maps of the ash-fall, ballistic blocks, and pyroclastic surges and flows were drawn for each of the larger eruptions. The pyrrolastic surges and flows of the present eruption were small scale, low temperature pyroclastic flows, rich in accessory clasts and unaccompanied by sector collapse. Therefore, the sudden melting of snow causing disastrous mudflows, as in the case of the 1926 eruption, fortunately did not occur

Vulkaninseln als ozeanische Geosysteme

The structural, temporal, compositional and volcanic evolution of oceanic intraplate islands is one of the major research areas in our department. A regional focus is on the island groups and seamounts along the passive margin off Northwest Africa. The Canary Islands which are characterized by an unususally large compositional spectrum of igneous rocks and long magmatic histories, exceeding 20 Ma in some islands, are the main target area for our ongoing combined on- and offshore studies. We here report on specific events and stages in the structural and chemical evolution of the island of Gran Canaria and its sedimentary apron using a variety of methods. Detailed studies of constructive and destructive processes during island evolution have allowed to predict - and verify by deep sea drilling - the submarine and subaerial evolution of Gran Canaria and its surrounding sedimentary basins. Our aim is to develop a globally representative model explaining the evolution of volcanic islands including aspects of volcanic hazards related to explosive eruptions and tsunamis triggered by island flank collapses