7. A Study of the Textural Characteristics of Pyroclastic Flow Deposits in Japan
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概要
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Phenomena of pyroclastic flows (nuees ardentes or glowing avalanches in a broad sense) involve all kinds of flows of incandescent fragmental materials of juvenile origin, graduating from the Pelean type eruptions attending protrusion of domes to eruptions that produce vast ash-flow sheets. Textural and constituent characteristics vary widely in accordance with differences in the types of pyroclastic flows. It is possible to consider that the nature and mode of origin of pyroclastic flows are precisely reflected in these characteristics of the deposits. The writer tried a quantitative study of these characteristics of pyroclastic flow deposits in Japan. His study was carried out on samples collected from non-welded deposits or non-welded parts of welded deposits. 192 samples of pyroclastic flow deposits and 35 samples of pyroclastic fall deposits were collected from various volcanic regions in Japan, and mechanical analyses of these samples were performed. Among these samples, 59 samples of pyroclastic flow deposits were selected as representative of every unit of flows. Moreover, 5 samples of pyroclastic fall deposits which were caused by preceding eruptions immediately before the discharge of pyroclastic flows were selected to compare their characteristics with those of pyroclastic flows. On these samples, measurements of the roundness of constituent particles, the bulk density of essential fragments, the relative abundance of a peculiar type of glass shards and pumice or scoria crowded with fine tubular vesicles, and of the relative proportion of each element of the constituents were carried out. Besides these, 15 samples of dry mud-flow deposits and 5 samples of auto-brecciated lava flows were collected, and their size characteristics were analysed in order to compare them with those of pyroclastic flow deposits. The most distinct features of pyroclastic flow deposits are the poorly sorted size characteristics. Data of mechanical analyses of every kind of pyroclastic flow deposits usually show a chaotic mingling of particles graduating from large blocks to fine dust, contrasting strikingly with the well-sorted size characteristics of pyroclastic fall deposits. The shape of the particle size distribution curves of every kind of pyroclastic flow deposit shows a close resemblance, each with the other. Each curve has tailingout parts both in coarser and finer fractions, a gentle mode in medium fractions, and a sub-mode, conspicuous or faint, in coarser fractions. Such size characteristics are always found in all deposits of pyroclastic flows. There exists, however, considerable room for variation within the framework of size distribution curves. Pyroclastic flows are grouped into several subdivisions based upon the differences in the mode of origin and mechanism of emplacement as well as the differences in size characteristics and other textural characteristics of their deposits as follows:(1) nuee ardente of Pelee type and Sakurajima type, (2) intermediate type pyroclastic flow, (3) St. Vincent ash-flow, (4) Krakatoa type ash-flow (fine-grained and medium-grained), (5) Valley of Ten Thousand Smokes type ash-flow (or fissure eruption type ash-flow). The size characteristics of dry mud-flow deposits and auto-brecciated lava flows are similar to those of pyroclastic flow deposits, but usually more poorly sorted and of coarser grain. Their size distribution curves tend to conform to Rosin's law of crushing. On the contrary, those of pyroclastic flow deposits show some disparities from Rosin's law, indicating the efficiency of sorting at the time of eruption and during flowage. It is possible to consider that sorting in a vertically rising column of ash or during the flowage of expanding cloud, as well as explosive disintegration and attrition or collision on the way of the ascent of frothing magma and during flowage, control the size characteristics of pyroclastic flow deposits. Although particles of all sizes are transported en masse in a state of turbulent flow as indicated by the distinct ill-sorted features of the deposits, the sorting agency has some effects upon their size characteristics. Judging from the results of measurements of the roundness of particles and types of pumice and glass shards, together with the results of inspections of other features of pyroclastic flow deposits, it is clarified that in eruptions of the intermediate type pyroclastic flows and nu6es ardentes, most of the vesiculation takes place during flowage, while in eruptions of the Krakatoa type ash-flows, it occurs within the vent, conduit or chamber, and in eruptions of the St. Vincent type ash-flows, vesiculation begins partly within the vent and occurs mostly at the time of discharge. In general, discharge of pyroclastic flows follows preliminary explosions. It is inferred that eruptions of pyroclastic flows take place after the removal of a confining load at the top of the vent by preliminary explosions. It is possible to consider that magmas which produce pyroclastic flow eruptions have a relatively large amount of volatiles, and differences in the gas-emission process in the erupting magmas cause various types of pyroclastic flows. Rock types of the deposits seem to belong to the hypersthenic rock series which is produced by the contamination of the original magma and granitic substances. Perhaps such contamination may cause concentration of volatiles in magmas which is inferred to be responsible for the eruptions of pyroclastic flows.|1902年Martinique島のMt.Peleeで烈しい噴火が起つた.側方向に噴出された物質が,ガスとともに一団となつて熱い黒雲を作り,山腹に沿つて非常な速度で流れ下つた.噴火はその後もつづき,同様な噴火が繰り返えされた.これらの噴火は,従来知られていなかつた型式のもので,これに対して,Lacroixはnuee ardenteと命名した.同じ1902年,St.VincentのSoufriereでも,同様な噴火が起つた.nuee ardente噴火はこの二つの噴火によつて確認され,その研究が開始された.1929～1932年にもMt.Peleeは同様の噴火を行なつた.Merapi火山(1930～1935),Mt.Lamington(1951～1952)やHibok-Hibok火山(1948～1952)でも同様の噴火が起り,詳しく調査された.このようにして,nuee ardente噴火は今世紀に入つて次々に確認され,その性質はしだいに明瞭になつてきた.一方,火山砕屑堆積物の研究から,nuee ardenteの産物と認められるものが世界各地の火山地域で発見されるようになり,この型式の噴火が地域的にも時代的にもかなり普遍的に起つていることが明らかになつてきた.Fenner(1920, etc.)はKatmai地域で起つた1912年の噴火による噴出物を詳細に調査し,Valley of Ten Thousand Smokesを埋めた火山灰が同様な噴火によるものであることを明らかにした.Moore(1934)はCrater Lake地域に分布する軽石質堆積物の研究を行ない,"Older pumice"と呼んだ軽石層が,nuee ardenteの産物であると結論した.Marshall(1935)はNew-ZealandのTaupo-Rotorua地方に分布するrhyolite tuffが同様にnuee ardente噴火によるものであることを明らかにした.さらに,Williams(1941)はcalderaの成因について詳しい検討を行ない,Krakatoa火山の1883年の噴火のさいに大量の軽石がnueeardenteとなつて噴出し,その結果,カルデラの陥没が起つたことを明らかにした.Katmai地域,Crater Lake地域やKrakatoa火山で見られるこのようなnuee ardente起源の軽石質堆持堆積物は,その後,世界各地の火山地帯で次々と発見されている.日本では,1929年北海道駒ケ岳の噴火のさいに,軽石流の噴出が起つた.久野(1941)はこの堆積物の特徴が一般の降下軽石堆積物と全く異なることを指摘した.さらに久野(1954)は日本の数多くのカルデラがWilliamsの説と同様の機巧で生じたことを指摘した.日本の各地に分布するnuee ardente起原の火山砕屑堆積物の確認と記載は多くの人々の手によつて進められている.Nuee ardente噴火の機巧とその原因については,実際の観察記録や堆積物の研究から,いくつかの推論が加えられている.しかし研究が進むにつれて,nuee ardenteと呼んできた一連の噴火には,規模も型式も,噴出物の性質も著しく異なるものが含まれていることが明らかとなり,この現象の詳しい検討と解析とが必要になつてきた.Lacroix(1930), Escher(1933), Fenner(1937), Mac-Gregor(1952, 1955), Williams(1957),荒牧(1957)らは,nuee ardenteの機巧について解析を試み,その分類を行なつている.このようにして,nuee ardenteの研究はすでに多くの成果をあげているが,しかしこの分野の研究をさらに前進させるためには,実際の噴火の観察記録をさらに充実させるとともに,噴出物の性質について綿密な定量的な調査を進めることが必要となつている.従来の研究により,nuee ardente噴火の原因と機巧は,マグマの粘性,溶存ガス含量およびその放出の条件などに関係しており,その噴火型式と噴出物の組織的性質との間には密接な関係があるということが示唆されている.この事は研究の将来の方向を暗示するものといえる.すなわち,噴出物の化学組成,鉱物組成や岩石学的特徴の研究と並行して,その組織,構造,構成物質についての解析が必要と考えられる.従つてnuee ardente起原の火山砕屑堆積物をsedimentary petrologyの諸方法をもつて研究を進めれることは,多くの成果をもたらすものと期待できる.筆者はそのような観点から,日本の各火山に見られるnuee ardente起源の火山酔屑堆積物について,その組織的諸性質と構成物質について定量的な調査を行なつて,nuee ardente噴火の原因と機巧の解明に役立つような資料を見出そうと考えて研究を進めている.
- 1961-08-25