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佐賀県三養基郡上峰町堤の八藤遺跡において, Aso-4火砕流堆積物中から3本の巨木を含む多数の樹木群が出土した. すべての樹木は, 表面あるいは全体が炭化しており, 火砕流堆積物中には炭化木から派生する「煙の化石」が随所にみられたことから, これらはAso-4火砕流によって倒され, この場で焼かれたものであることが判明した. また, 火砕流堆積物の直下には森林古土壌とともに, 多くの樹根が発見された. このような事実は, 巨大火砕流災害の具体的証拠を示すものとして重要であるばかりか, Aso-4噴出当時の森林植生を直接復元する上でも重要である. また, 巨木は表面のみが焼かれて炭化しているが, 内部の保存は良好である. 樹木の枯死年代はAso-4火砕流の噴出年代と一致するとみられ, Aso-4によりこれらの樹木の年代決定が可能である. さらに, これらの樹木は当時の古環境やAso-4火砕流の性質を知るための分析用試料として価値が高い.

Volcanic eruptions influence land surfaces in various ways and to varying degrees, both direclly and indirectly. Pyroclastic fall and flow deposits fill or covers land surfaces, resulting in damage to vegetation via rapid accumulation of tephra and through thermal effects.Large-scale pyroclastic flows and debris avalanches may cause a variety of topographical changes of extraordinary scale. The former often cover undulating topography with thick flow accumulations, creating flat surfaces that are preserved as pyroclastic flow uplands. The latter may create buried valleys, eroding valley bottoms and walls, and damaging vegetation and soils.The basic properties and characteristics of blast phenomena were first clarified and illustrated in detail by the 1980Mt. St. Helens eruption. When a large scale debris avalanche enters the sea or a lake, it generates a tsunami which affects the coastal areas over wide areas.After rapid deposition of tephra, the surface tephra materials are easily moved by running water and wind action. Sand dunes composed of secondary tephras are common, especially in Hokkaido.Debris flows and mud flows commonly occur in volcanoes such as Usu and Sakurajima which exhibit continuous small scale tephra-producing activity. These flows contribute to the growth of volcanic fans.Strong rainfalls sometimes trigger mud and debris flows, which contribute to developing volcanic fans, on both active volcanoes and older highly dissected volcanoes. Such volcanoes have extensive volcanic fans surrounding their cones.

This paper outlines the previous studies of many Quaternary gigantic pyroclastic flow deposits widely distributed in Kyushu in terms of Quaternary studies: (1) age, distribution, and source, and (2) influence on the Jomon Culture of Kyushu in the Holocene and on late Pleistocene slope erosion of Yaku and Tane islands.Seven gigantic pyroclastic flows are recognized in the late Pleistocene: Koya (source: Kikai caldera, age: 6, 300yBP), Ito (Aira caldera, 21, 000-22, 000yBP), Aso-4 (Aso caldera, 70, 000yBP), Nagase (Kikai caldera, 75, 000yBP), Ata (Ata caldera, 85, 000yBP), Aso-3 (Aso caldera, 105, 000yBP) and Torihama (Ata caldera, 100, 000-150, 000yBP) pyroclastic flows. Co-ignimbrite ash falls associated with all of them are found in distal areas more than 1, 000km distant from their sources. The ages, estimated by stratigraphic positions of those ash falls as well as radiometric datings, indicate that the eruptions of gigantic pyroclastic flows concentrate in the early stage of the late Pleistocene. Those pyroclastic flows showing circular distribution extend to a distance of 100-150km from the source.In contrast, the age, distribution, and source of middle-early Pleistocene gigantic pyroclastic flows are not sufficiently clarified, except for the Aso-2, Aso-1, Kakuto and Shimokado pyroclastic flows in the late stage of the middle Pleistocene.A clear difference in Jomon pottery between the layer above K-Ah ash associated with Koya pyroclastic flows and that beneath it, is widely recognized in Kyushu, suggesting that Koya pyroclastic flows eruption played an important role in the change in Jomon culture.We can often recognize slope deposits, including blocks of Nagase pyroclastic flows deposits in Yaku and Tane islands. This may suggest that unstable conditions occurred on the slopes over a wide area around the Kikai caldera owing to this eruption.

The Myoko volcano, one of the stratovolcanoes in the northern part of the Fossa Magna, Central Japan, has the complicated history (Table 1).The last activity of the Myoko volcano is called the IV stage, and is divided into precaldera, caldera, and central cone substages. The strata formed during this stage are collectively called the Myokosan group which consists of lavas, pyroclastic flow deposits, pyroclastic fall deposits, volcanic mud flow deposits, and lake deposits. They are described in detail and their stratigraphic relations are tabulated in Table 2 and Fig. 4.The rocks of the essential eruptives are basalt, pyroxene andesite and hornblende andesite. They change from basalt to pyroxene andesite and hornblende andesite keeping step with the eruptive order. Quantitatively, the hornblende andesite is predominant.

Haruna volcano, situated in the central part of Japan, erupted twice in the 6th century, from Futatsu-dake crater. The first eruption, which occurred in the early part of the 6th century, was set set off by a low-temperature phreatomagmatic eruption. The initially ejected very fine ash accumulated as accretionary lapilli and muddy rainfalls. Later, the eruption changed to hot pyroclastic flow effusions, which contained many essential lithics. These pyroclastic flow effusions included small-scale phreatic eruptions. The ash had formed ash clouds that then accumulated on each pyroclastic flow deposit. This tephra sequence was named the Haruna-Shibukawa tephra formation(Hr-S).These pyroclastic flow encroached on an older village, Nakasuji, situated on the eastern flank of Haruna volcano. The pyroclastic flow (S-5) burned and destroyed many houses. Because its deposit was very thinly laminated, it took the form of a hot pyroclastic surge, which spread over the eastern side of Haruna volcano, causing widespread damage there before changing to mud flows and floods and damaging rice fields in the area.The second eruption, which occurred in the middle or later part of the 6th century, is characterized by plinian eruptions and pyroclastic flow effusions. This tephra sequence was named the Haruna-lkaho tephra formation (Hr-I). The pumice ejected in the plinian eruptions was deposited, in a layer about 3cm thick, on Soma city, 200km from the vent.An older village, Kuroimine, situated about 10km from the vent, was buried by a layer of pumice about 200cm thick. Because pumice oxidized by the flames of burning houses is observed from the bottom to near the top of the pumice fall deposit, we can confirm that the greater part of the pumice accumulated during a period of hours. A house was crushed by the coarser part of the pumice fall deposit (1-6). The pyroclastic flows, which caused columns to collapse, moved and accumulated along the valleys before changing into mud flows and floods. They also caused heavy damage to rice fields and farms.In Gunma Prefecture, it may well be that villages, rice fields, and farms damaged by volcanic eruptions in the same way as Nakasuji village and Kuroimine village were damaged will be discovered. The data in relation to past volcanic hazards, obtained by joint research between archaeology and volcanology, will contribute to predicting volcanic disasters.

Fragments of earthenwares of the Earliest Jomon age are found on a sea-cliff at Shimotakabora on the west coast of O-shima Island, Izu Islands (34°44.3′N., 139°21.6′E.). They are unearthed from the upper brown weathered ash of a unit layer constituting pyroclastic fall deposits exposed there. Associated with them, are found obsidian flakes, an angular block of biotite rhyolite pumice, several round pebbles of compact basalt, and a small amount of charred wood and bone fragments.All the earthenware fragments are identified to be of the Hirasaka type of the Earliest Jomon age whose type locality is in the Miura Peninsula, South Kanto. Two samples of the obsidian flakes have been determined by fission track method to have the same uranium content and age as obsidian exposed on Kozu-shima Island about 60km south-southwest of O-shima. The biotite rhyolite pumice block is, in petrographic characters, similar to the biotite rhyolite from any one of Nii-jima, Shikine-jima or Kozu-shima Islands, all of which lie to the south-southwest. These obsidian and rhyolite pumice were brought by the Earliest Jomon (Hirasaka) men to O-shima Island for making living tools.On the sea-cliffs at Onoue and Tatsunokuchi to the south of Shimotakabora, earthenware fragments of the Kayama, Kijima plus Sekiyama, Moroiso (?) and Odoriba types have been unearthed from the particular stratigraphic levels of superimposed pyroclastic deposits. The earthenware fragments of the Hirasaka type are found at the layer lying nine fall units below the level which contains the earthenware fragment of the Kayama type about seven thousand years old. As the time represented by a unit pyroclastic layer is thought to be a hundred and several tens of years, the earthenwares of the Hirasaka type may be brought to O-shima eight to nine thousands of years ago. This estimated age well coincides with that given so far to the type.

The ancient sand dune deposits in the Tokachi plain, east Hokkaido, which were constructed by reworked pyroclastic fall materials as a result of the eolian flowing, was studied. These deposits, 1-10m in thickness, are mainly composed of pumiceous fine sand, being covered by the younger volcanic ashes and soils. More than 300 dunes are distributed on the Pleistocene terraces named as “Kamisatsunai I Plain” (Fig. 4).Tephrochronologic and topographic investigation revealed the date of the dune sands to be older than the Tarumai pumice-fall deposit “d” (T-d) in about 9, 000 years B.P. and younger than the Shikotsu pumice-fall deposit 1 (Spfa-1) dated as 32, 000 years B.P. (Fig. 1). The ancient dunes can be classified into three types from the view point of stratigraphic features in the deposits; namely, the first type dune derived from the Spfa-1 deposit, the second type dune derived from the Spfa-1 and Eniwa pumice-fall deposit “a” (E-a), and the third type dune, the newest one of them, derived from the E-a deposit. The third type of them is predominated in numbers.As shown in Figs. 4, 5 and 6, most of them had an oval form, and the long-axis pointed to the northwest. This may give an important information on the prevailing wind in the past. They are supplied with sand carried by the prevailing wind from the unconsolidated pyroclastic deposits (E-a and Spfa-1) covered with poor vegetation, probably under the arid condition. Accordingly, the formation of those ancient dune in this plain might have indicated the palaeoclimatic change in the end of Wurm glacial age.

In the Late Pleistocene of Kyushu, the San'in and Aso-Kirishima volcanic series began to erupt on the Tsukushi type andesites.The former are characterized by the pyroclastic flow of hornblende-biotite andesite, connected with forming the graben, the latter are pumiceous flow including welded tuff, connected with forming the caldera.These volcanic activities are divided into three stages; older, middle and younger.Pyroclastic deposits of the older stage are interbedded with lacustrine deposits and dark gray coloured solid Loam bed (Older Loam). They form higher terraces.In the middle stage, a great volume of pyroclastic flow erupted out from the effusive center, which was depressed and formed a graben or a caldera at a slightly later period. Particularly, Aso and Aira pyroclastic flows covered extensive areas. Their volumes are estimated to be 175.2 and 154.8km3 by T. Matsumoto, (1952), and they form middle terraces. Therefore, they are the best tephrochronological key beds. On the other hand, 14C dates measured by K. Kigoshi show the Wurm glacial stage as follows; black humic clay at the base of Aira pumice fall (Fig. 1, 2): 22, 000±850 yr B. P. (Kigoshi and Endo, 1963), charcoal including in the base of Aso pyroclastic flow: 33, 000 +3, 000 -2, 200 yr B. P. (unpublished).In the younger stage, the central cones began to act a violent explosive eruption by which ejecta was thrown out. The ash, scoria and pumice wafted by wind covered the extensive areas and formed the Younger Loam (Fig. 4). Some of these activities continued to the Recent and supplied the essential part of the Black Volcanic Ash Formation.The Younger Loam Formation is divided into three beds; upper, middle and lower parts (Fig. 3). The lower bed overlies the gravel bed which covers the pyloclastic flow, or lies upon the flow immediately. The upper two beds cover the lower terrace and contain palaeolithic tools.The Black Volcanic Ash contains ceramics of the Jomon, Yayoi and later cultures, by which the chronological division of Ash beds may be classified.

When a volcano eruptes, its pyroclastics are deposited over the surface of the earth, so the depositional features of the pyroclastics tell us the history of volcanic activities. Therefore, if we wish to investigate the tephrochronology of pyroclastics which spread over the surface of the earth, the following works should be done.1) At first, we must classify the sorts of the pyroclastics which spread over the surface of the earth and study the characteristics of each pyroclastic. Then we must research their areal distribution and the sources of their eruptions.2) Next, we must determine the time of eruption of those pyroclastics, and in this case the following principles should be adopted as fundamental ideas.a) When natural trees grow on pyroclastic forming the surface soil, we can estimate the age of the pyroclastic by means of calculation of their annual ring.b) When pyroclastic falls on peat lands, peat begins to develop on the pyroclastic. As the annual rate of increase of the peat layer thickness is considered to be constant, so by determining the depth of the peat layer growing on the pyroclastic, we can estimate the time when the pyroclastic was deposited.c) When we find carbonized trees in a layer of pyroclastic, we can estimate the age of the pyroclastic by means of determining the 14C of those carbonized trees.d) When we find a prehistoric site in a layer of pyroclastic, we can estimate the age of the pyroclastic by determining the age of the prehistoric site by archaeologists.e) When we find obsidian stone implements made by aborigines in a layer of pyroclastic, we can estimate the age of the pyroclastic by determining the thickness of the hydrated surface layer which has been formed outside the obsidian.f) Moreover, if we can correlate the age which has been estimated by means of the above mentioned methods to ancient records of the volcanic activity, we may possibly estimte still more accurately the age of the pyroclastic.

The Ontake pyroclastic fall deposits, which have been accumulated in Pleistocene time, show widespread occurrence in the eastern areas of the Ontake volcano in Honshu. In this study, the weathering sequence of these deposits was given, and it was concluded that the alteration followed the sequence of volcanic ash→allophane→halloysite in the order of progressive alteration.In view of the sedimentary environments, the pyroclastic fall deposits may be divided into water-laid, sea-laid and air-laid sediments.In the case of “Pm I” pumice fall bed which served as a key bed in these pyroclastic fall deposits, the degree of alteration is reflected by the delicate shade of different environmental conditions. That is to say, in air-laid sediment argillization is remakable, where as, in water-laid and sea-laid sediments, the volcanic glasses are usually prevented from conspicuous alteration, and only small amount of allophane is recognized. These facts suggest that the sufficient water supply and the retaining of dead water in the sediments might be essential factors in progressive alteration.In general, during the course of weathering, alkalies and alkaline-earth metals might be removed by slightly acid water. The fact that iron and manganese are concentrated as hydroxide at the base of pumice bed may be explained by more alkaline conditions at this site. Under these physicochemical environments, silica and alumina might be leached out as colloidal forms and precipitate as gels. The usual occurrence of allophane, α-cristobalite and gibbsite in the weathering profile will support this view.The features of alteration mentioned above, may indicate that the hydrogen ion concentration and the redox potential as a function of it, are the dominant factor to controll the mineral species which were formed in the course of weathering of these pyroclastic fall 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.