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.
Though there are no deposits in Japan exactly comparable to the loess deposits, tephra or volcanogenous deposits analogous to the loess in their aeolian origin are widly distributioned, and the chronological studies of them must be urged in Quaternary researches. In this paper, the relations among aeolian deposits, terrace topography, and fluvial or marine deposits are shown; and stratigraphical correlation problems are discussed. In short, horizon markers in tephra deposits (for example, specific pumice or scoria layers, difference in mineral components, plane of unconformity, buried soil bands, etc.) are useful in a relatively small area; but in order to make a chronological table for a wider area, geomorphological correlation of terrace topography is more effective.
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.
The principal clay minerals in the volcanic ash soils in Pleistocene age in Japan are allophane in the younger volcanic ash soils and hydrated halloysite in the older. Examining clay mineralogical properties of the volcanic ash soils by means of X-ray, thermal analysis and electron-micrographic observation, the writers postulate a sequence of crystallization process in the course of weathering of volcanic ashes, and confirm that the clay mineralogical investigations are useful to decide the stratigraphical features of volcanic ash soils. The change from allophane to hydrated halloysite may take place throughout the following several stages of crystallization process. The initial stage is characterized by the forming of allophane with the decomposition of volcanic glass, which corresponds to the weathering process of the upper-most part of the volcanic ash bed in Pleistocene age, e. g. the Tachikawa Loam member in the Kanto Loam consisting of volcanic ash. The second stage is remarkable in the presence of allophane associated with small amounts of the lower crystalline hydrated halloysite, showing the fine rounded grain on the morphological features. As the results of the writers' studies it is no doubt that hydrated halloysite is crystallized out from allophane. This stage is recognized at the weathering process of the younger volcanic ash soils such as the upper part of the Musashino Loam member of the Kanta Loam. In the third stage, large amounts of allophane crystallize out to hydrated halloysiet, revealing morphologically the chestnut-shell shape as pointed out by Sudo. The final stage is characterized by the formation of pure hydrated halloysite which has the higher degree of crystallinity than that of hydrated halloysite in the second or third stage. The particles of pure hydrated halloysite are generally elongated tubular in shape. The final stage progresses in the older volcanic ash soils, e. g. the Tama Loam member in the Kanto Loam. Generally speaking, the crystallization process of volcanic glass may be intimately related to the geological time of volcanic ash soils. It does not, however, necessarily follow that crystallization at the same geological time advances at the similar grade. The black band (presumed as ‘the fossil soil’ in volcanic ash) intercalated into brown soils in the Tachikawa Loam member shows the slightly more progressive stage than the crystallization stage in the brown soil. The marine deposits of volcanic ash have a peculiar clay mineralogical feature as compared with that of aeolian deposits. They generally include random mixed layer minerals of halloysite and hydrated halloysite which is formed by imperfect dehydration of hydrated halloysite. Taking the above mentioned two results into consideration, the writers intend to emphasize the influence of the environments wherein volcanic ashes are deposited. The relation between the starting material and the crystallization process will be lastly discussed. The crystallization process of the pumice soil is harder than that of the brownish volcanic ash soil. This fact may be concerned with the follwing factors: (a) mineral component of pumiceous materials, and (b) their grain size. In the light of above discussion, the writers are in the present step utilizing the results of clay mineralogical studies of volcanic ash soils for the purpose of the relative correlation among several volcanic ash beds.
The authors tried to review scientific works on volcanic ash soils in Japan from the pedogenetical point of view. Morphology: The soils consist of the thick humified loose A horizon and brownish structural B to BC horizon, though in younger soils A-C sequence is common, lacking B horizon. Neither any sign of podzolization nor laterization is recognized. Such a normal profile is often disturbed by denudation, accumulation of surface humic soil materials or by successive addition of recent pyroclastic fall. Mechanical, chemical and mineralogical composition: Though younger soils mainly consist of sand and gravel in texture, most of matured soils are so highly weathered that the clay content often amounts to about one-half of the total soil mass. Chemical analysis reveals that matured soils have been suffered from severe weathering which results in marked loss of silica and bases leaving clay complex rich in hydrous sesquioxides. The principal mineral of the clay fraction in volcanic ash soils is allophane with some quantity of various layer silicates, gibbsite and other sesquioxide minerals. Allophane is considered to be formed by co-precipitation of hydrous silica and alumina gels both of which are regarded as end products of severe weathering of volcanic glass and plagioclase dominant in original volcanic ash. The genesis of allophane is mainly conditoned by such peculiar physical properties of the parent material as homogeneous fine grain size, extraordinally broad specific surface area and rapid permeability. And it seems that the genesis is accelerated by the warm and superhumid climate in Japan. Accumulation of humus: The characteristic thick humus layer of volcanic ash soils contains very high amounts of organic carbon (8-30%), and its almost real black colour looks just like as in Chernozem. The raw material must be derived from grass vegetation seeing from the predominace of the phytolith or plantopal in the very fine sand fraction of the humus layer. The chernozemic black colour may be due to highly condensed and polimerized humic acid, which may have been formed in the rather aerobic soil condition of porous volcanic ash materials persisting even under humid monsoon climate. Nevertheless, judging from the fact that more content of the fulvic acid is determined than that of the humic acid in most cases, and that both acids are combined with sesquioxides or allophane and not with Ca, the humus of the soils should have also podzolic character which may be related to superhumid climate and aerobic soil condition. And allophane must be responsible for such a marked accumulation of humus in the volcanic ash soils. Physical and chemical characteristics: The volume of the solid phase of the soils occupies only 20-30% of the whole soil mass, in other words, the porosity and water holding capacity are extremely high and apparent density is very low. It is the main reason why the volcanic ash soils are not only subjected from rapid weathering and specific humification, but also suffered from wind and sheet erosion, as well as frost damage. Clods of the soils are apt to be broken down into water-stable micro-aggregate (0.05-0.02mm in dia.), which is thought to be bound by loose hydrogen bond in the swelling water with Fe or Al bridge. The soils contain a lot of swelling and hygroscopic water, both of which may be mostly combined with allophane. Besides, such structural OH is released continuously by heating or an ion exchange treatments. The reaction of surface horizon of the soils is generally acid based on humic acid, but very weakly acid to nearly neutral in subsoil in which allophane is dominant component in the clay fraction, in spite of extremely low degree of base saturation caused by severe leaching under the superhumid climate. The cation exchange capacity of the soils is rather high, though the determination tends to be ill-reproducible
A great many examples of soils have been known to contain siliceous phytogenic particles called, also, plant opal, grass opal, phytolith, etc., which are originated from siliceous fillings of cells in plant grasses. The contents of the siliceous phytogenic particles in soils are in the positive relation to the humus contents. The same siliceous particles as in soils are also found in the various Quaternary volcanic ash deposits in which zones of buried soils (dark zones or cracky zones) are more abundant in the siliceous particles. The siliceous phytogenic particles are low (2.10-2.15) in specific density, very low (1.44-1.45) in refractive index, and are optically isotropic. They reveal only very broad diffraction near 4Å in X-ray diffraction diagram. Silica is predominant component of them. All above characteristics prove the particles to be opal. The sizes and shapes of the siliceous phytogenic particles seem to be controlled by the cells wherin they deposited. Thus, the fan-shaped ones are about 0.04mm. large, the rod-shaped ones are 0.02mm. wide with maximum length of 0.5mm., and the dumbell-shared ones are about 0.01mm. large (Fig. 1). The rather common occurrence of the siliceous phytogenic particles in the Quaternary volcanic ash deposits calls our attention in the following two aspects. First, the siliceous phytogenic particle is a possible indicator of a buried soil in the volcanic ash deposits with supplemental field and in-door evidences such as content of organic matter, degree of weathering, etc. (Fig. 2). Second, they must suggest the presence of plant grasses which grew in situ on the subaerial ground where volcanic ash deposited. Although some promissing results are in hand, it is riserved for further precise study whether species or genus names of the plant grasses, otherwise only the family names can be identified by them.
The writer summarizes some features of the Quaternary volcanic ash layers in the Kanto and Hokuriku regions of central Japan from palynological point of view. He intends to aquire informations of the difference in the contents of pollen and spore grains between the volcanic ash layers and the normal sediments above and below them. (A) Pollen and spore grains found in volcanic ash layer sedimented in the sea. 1) The absolute quantity of pollen and spore grains is generally less in the volcanic ash layer than in the overlying and underlying deposits. 2) In the case of a thin ash layer, about 5 to 20cm in thickness, the relative frequency of pollen and spore grains in the ash layer is similar to that in normal sediments. 3) In the case of a thick volcanic ash layer, about 20 to 50cm in thickness the contents of pollen and spore grains are not uniform throughout the layer. 4) Even in the case mentioned in 3), the relative frequency of pollen and spore grains included in the normal sediment above the volcanic ash layer is similar to that in normal sediment below the ash layer. (B) Pollen and spore grains found in volcanic ash fall deposits on the land. 1) Although many pollen of such needle-leaf trees as Pinus, Picea, and Abies and pollen of such broad-leaved trees as Alnus, Betula, Fagus and Quercus are found in the lowermost and uppermost parts of volcanic ash layer, they are scarcely found in the middle part. 2) The absolute quantity of pollen and spore grains found in normal sediment below the ash layer is always more than that in normal sediment above the ash layer. 3) The relative frequency of pollen of such woody plants as Pinus, Picea, Abies, Quercus and Alnus is higher than that of herbaceous plants. 4) In the normal sediments above and below the volcanic ash layer, the relative frequency as absolute quantity of the pollen of such needle-leaf trees as Pinus, Picea and Abies is higher than those of such broad-leaved trees as Quercus and Alnus. 5) Concerning the amount of spore grains, they are frequently found in the underlying normal deposits, but are hardly found in the volcanic ash layer and in the overlying normal sediment. The information about the difference in the contents of each layer mentioned above are summarized in the text-figure. These data may give a hint as to the influence of volcanic ash fall on the land flora in the environs of a volcano and the floral change after the fall of volcanic ash.
In the Northern Kanto Plain, the stone artifacts excavated from the volcanic ash layer (Kanto Loam Bed) are classified typologically into the following several industries; a. the industry characterized by pebble-tools or hand-axes; b. the industry characterized by crudes and backed blades; c. the industry characterized by points; d. the industry characterized by microblades and microcores, and these are correlated with the geological sequence as follows:
The Japanese Committee on Tephrochronology made a distribution map of the Quaternary volcanic products in Japan (Fig. 1, original scale :1/2, 000, 000). As the compiler of this map, the author gives some descriptive notes about the distribution and the related problems. 1. The main sources of this map are geological and soil maps made by various scholars and organizations, and unpublished tephra distribution maps made by the committee members. The degree of accuracy of tephra distribution on this map varies for different regions; more reliable for Hokkaido, Central Honshu and Southern Kyushu, and less for Japan Sea side and for Shikoku. In general, under 10cm thick tephra layers are not illustrated. 2. In this map, not only tephra layers, but also pyroclastic flow deposits and lavas are shown. Moreover, both of them are classified chronologically into those of the Holocene and the Pleistocene. It is noticeable that almost all Pleistocene tephra layers in this map belong to the upper Pleistocene age, because the greater part of tephra layers of the middle and lower Pleistocene have been eroded away. 3. The pattern of the tephra layer distribution suggests that the prevailing wind direction during both the Holocene and the Pleistocene was from the west. Furthermore, the total thickness of the tephra layers called Tachikawa and Musashino Loam (fig. 2), which were derived from Mt. Fuji during the Würm glacial age suggests that the average wind direction of that age was similar to that of the present mean direction, i. e. W10°S in 2 to 20km high in the upper atmosphere over the Japanese Islands. From the stand-point of tephrochronology, the types of tephra distribution in Japan-zonally extended tephra layers on the meridionally elongataed islands-have both week and strong points in determining the Quaternaty geochronology. The former lies in difficulty in the correlation of tephra layers in the north-south direction, because of few cases which show the overlapping relations between tephra layers of north and south regions; the latter is the case of correlation between the west volcanic and the east coastal regions by the use of east and west extended tephra layers. 4. The volumes of some remarkable tephra layers and the above-mentioned Loam from Mt. Fuji are cited in table. From these several examples, the total volume of the Quaternary tephra layers in Japan are roughly estimated over 1, 000km3.
Hokkaido lsland has been a site of intense volcanic activity during Quaternary period through which 36 volcanoes have erupted along the inner zones of the Kurile arc and the Honshu arc. A number of Quaternary pyroclastic are widely distrbuted in this island, and vast pyroclastic plateaux are also developed around calderas. As shown in Fig. 1, most of the ash-fall deposits of Pleistocene as well as those of Holocene were accumulated on the eastern side of the volcanoes, which may give an important information on the atmospheric circulation in the past. Pyroclastic flow deposits around the calderas are so widely developed that thay are useful in Quaternary geology not only as important time-makers like ash-fall deposits but also as excellent indicators of old topographies. Tephrochronological studies on these pyroclatic deposits have been carried on by pedologists, geologists, archaeologists and geophysicists, since 1933. This paper is a summary of these studies. (Fig. 1 and Table 1) In early Pleistocene, a tremendous amount of rhyolite pumice-flow were erupted from the central highland of Hokkaido where a major volcano-tectonic depression was formed through this violent activity. Then, a number of dacite pumice-flows accompanied by ash-falls was issued from several calderas. Most of them were erupted in glacial periods of middle Pleistocene. During the last interglacial period, volcanic activities were rather quiescent. Then, in the last glacial period, violent activities of dacite pumice-eruptions took place again, and most of the calderas in Hokkaido completed their formation in this period. Some pyroclastic falls of this period embedded fossil forests composed of Picea jezoensis, and covered lower terrace deposits, from which Mammonteous primigenius primigenius was discovered. Some pumice-flow deposits swept over the middle and lower terraces and buried pre-existing valleys, which were formed by a glaclo-eustatic regression and reached scores of meters or more below the present sea level. Volcanic activity in Holocene has been confined to small areas near the Pleistocene volcanoes, especially within calderas, and it was interrupted by two quiescent periods, 5000-2000 years B. P. and 1000-500 years B. P. respectively. Most of the pyroclastic materials of Holocene are of andesite. Owing to the Jomon transgression which reached about 10m high from the sea level, ash-falls of early Holocene were not accumulated as a primary deposit in alluvial plains near the sea coast. Most of the volcanoes constructed in Holocene erupted during the last 500 years. Detailed tephrochronology of the younger pyroclastic deposits may substitute for historic records of volcanic activity, because no reliable records older than 300 years B. P. are available in this island.
The volcanoes of Quaternary age in Northeast Honshu are distributed along two belts which are named the Nasu and Chokai Volcanic Belts. The Nasu Volcanic Belt is situated along the Backbone Ranges of Northeast Honshu and the Chokai in the Dewa Hilly Lands. Northeast Houshu is divided into five sections, each has its own geologic and geomorphic characteristics. There are, from east to west, Kitakami-Abukuma Massif, lowland on the western side of the massif. Backbone Ranges, lowland on the western side of the ranges, and Dewa Hillylands. The Kitakami-Abukuma Massif consists largely of pre-Tertiary and granitic rocks. Along the eastern foot of the Abukuma Massif and in the areas between the Kitakami and Abukuma Massifs, and between the Kitakami Massif and Cape Shiriya, there are hillylands mainly consisting of Cenozoic sediments. The lowlands between the Kitakami-Abukuma Massif and the Backbone Ranges comprise several basins which are distributed along the main valleys of the Kitakami, and other trunk rivers. The Backbone Ranges consist mainly of Tertiary rocks and comprise the central belt of the Tertiary of the region. The lowlands between the Backbone Ranges and the Dewa Hilly Lands comprise several basins. The Dewa Hilly Lands consist largely of Tertiary rocks which rest upon the pre-Tertiary and granitic rocks with unconformity. Kitamura (1959) summarized the Tertiary orogenesis in this region as shown in Figure 2. Most of the Quaternary sediments and terraces of Northeast Honshu are developed in the mentioned two lowlands and coastal areas and distributed in the north to south direction in belts parallel to the main structural trend of the region. An analogous trend is found in the distribution of the volcanoes which are located near the tectonic lines. They have a northeast trend, obliquely crossing the main structural trend. The tectonic lines with a NW-SE trend were important throughout the pre-Quaternary history of the region, and at least, during the Pleistcene time, they controlled the northeastwards tilting blocks and determined the sitesof the basins within the above cited two lowland areas. For determination of the Quaternary tephrochronology in Notheast Honshu, available are such data as shown by the structural control on the distribution of the volcanoes, the sediments and terraces of the Quaternary age. In the middle latitudes of Japan, ash is generally spread eastwards from the volcanoes because it was transported by the prevailing westerly wind. In Northeast Honshu, the Quaternary sediments are developed more or less continuously and the terraces with a N-S trend are crossed longitudinally by the distribution of the volcanic ashes. The volcanic ash layers which originated from the Bandai-Azuma Volcanoes are distributed in the area between Koriyama and Fukushima along the Abukuma River, and between Hisanohama and Soma along the Pacific Coast. The ashes from the Zao Volcano are distributed between Watari and Shiogama along the Pacific Coast and are separated from those of the Bandai-Azuma Volcanoes. The ash layers derived from the Kurikoma Volcano are distributed east and northeastward and remote from those of the Iwate Volcano. Except for the most extensively distributed ash layers originated from the Towada-Hakkoda Volcanoes whch extend southward to the northern area of ash distrisbuted from the Iwate Volcano, and covered in part by the eastern part of the ash derived from the Iwaki Volcano, the ash layers from one volcano or group of volcanoes in Northeast Honshu are distributed in areas separated and at some distance from those of the neighbouring volcanoes. A discontinuity in the distribution of the Quaternary sediments and terraces is also found in this region.
1. Kanto Plain is thickly covered by the Quaternary volcanic ashes supplied from the volcanic chain running from the north to the west of the region, and offers one of the type areas in the tephrochronologiocal study in Japan as well as the type areas of the Japanese marine Pleistocene. Most of the Quaternary volcanic ashes distributed in the Kanto region are composed of the weathered ashes in the Pleistocene epoch, and have long been called “the Kanto Loam” by many authors. A discovery of stone implements from the Kanto Loam at Iwajuku in 1949 stimulated a close investigation of Loam, and knowledges about Kanto Loam have remarkably advanced for these ten years in such fields as stratigraphy, geomorphology, archeology under the co-operation of many investigators. 2. Kanto Loam formation is generally thick on higher terraces and thin on lower ones. Therefore, we can establish the stratigraphic division of the formation to terrace topography. In South Kanto, the Loam member which conformably covers the Tachikawa terrace gravel is named Tachikawa Loam, and what merges downward into the constituents of the Musashino terrace is named Musashino Loam. What conformably lies on the marine Shimosueyoshi formation (upper Pleistocene) is Shimosueyoshi Loam, and the lowest division of Kanto Loam is Tama Loam which is defined as the loam which covers the Byobugaura formation (middle Pleistocene) or its correlatives on the Tama terraces. 3. Tachikawa Loam member, the uppermost of the Kanto Loam formation, is brilliant brown in color, 3 to 4 meters thick in Tokyo, and is mainly composed of weathered scoriae. Distribution of the loam is very wide, covering the present landsurface. Two dark colored bands made up of buried soil which are recognized in the middle part of the loam can be pursued extensively as key beds. Tachikawa Loam can be divided into the upper and lower submembers at the surface of the upper dark band, and the upper part of the Tachikawa is the stratigraphic equivalent to the Egota conifer bed. Musashino Loam member is composed of weathered scoriaceous ashes of brown color, 5m thick in Tokyo, and more clayey than the Tachikawa. The Tokyo pumice bed interbedded about 1m above the base is well-known as the most important key bed of Kanto Loam in South Kanto. Shimosueyoshi Loam is clayey with pebbles and pumices in the whole section. White pumice beds are generally interbedded in the lowest horizon of the loam. Lithologic facies of the loam is very variable and is at times represented by tuffaceous clay or sand only. Tama Lam is a cracky, browh tuff-clay about 3m thick. Yellow pumicts and brown weathered scoriae are commonly comprised in the whole section, and several pumiceous bands are also interbedded. 4. Kanto Loam formation which covers the terraces and skirts of the volcanoes in Northwest Kanto can be divided into three members; upper, middle and lower with remarkable unconformity each other. Upper loam member, 1 to 2m thick, covers the whole area in agreement with the present topography. It is colored light yellowish brown, the lower half of which is pumiceous. Upper Loam is farther characterized by two pumice beds, the Itahana yellow pumice in the upper and the Itahana brown pumice in the lower. Middle Loam is brown or dark drown, 4m thick, and more or less clayey. The Kanuma pumice, the Hassaki pumice and the Yunokuchi pumice bed mark the upper, the middle and the lower part of Loam respectively. Iwajuku Culture Bed is contained in the dark band developed in the uppermost horizon of the loam, while Gongenyama I Culture Bed is in the dark band right below the Hassaki pumice. Lower Loam is very clayey occasionally containing pebbles and granules. Pumice are often intercalated in various horizons of the Loam, but none is developed in beds wide extent. Fujiyama Culture Bed at the foot of Mt. Akagi is contained in the upper part of Loam.
The Central Region including the so-called “Fossa Magna”-a Tertiary orogenic belt in Central Japan has been a field of intensive volcanisms since Miocene. Many volcanoes inside and outside the belt have supplied a vast amount of tephras through Quaternary times. In this territory, pyroclastc flows and volcanic mudflows as well as tephras of aeolian origin are usable as index horizons for the correlation of geomorphic surfaces and geological sediments and so on. (1) In the valleys of Ina, Kiso, Matsumoto and Suwa is well developed a type of tephraic section of which materials have been supplied from Ontake and Norikura Volcanoes. The section is composed of three or four tephra units as mentioned in the following: i) Hata or the Younger Loam (tephra) unit: The upper part may postdate the Late Würmian glaciation, and the lower part is dated roundly a little younger than 30, 000 14C yr. B. P. The Palaeolithic cultural layers are to be placed in the upper horizons of Hata Loam and its equivalents. ii) Osakada or the Middle Loam unit: Pumiceous, tephrochronologically equivalent to the upper part of the Atsuta (marine) formations which have hitherto been considered as the deposits during the time of Riss/Würm Interglacial higher sea-level. iii) The Older Loam unit: In Ina valley, Older Loam underlies Osakada Loam with less appreciable hiatus. A distinguished red pumice bed called “Pm-0”, which is often recognized within the fluviatile gravel beds in Ina valley, seems to mark the lower horizon of Older Loam. Much clayey. iv) Nishibayashi Loam: At the top is a sign of subaerial weathering giving an evidence of the existence of stratigraphic break before the deposition of Osakada Loam. Much clayey. It is problematic whether the Loam might be a product mainly of tephra falls. Clay mineralogical content consists of the mixed type of halloysite, illite and 14Å minerals. (2) Our knowledges about the tephras from Yatsugatake Volcanoes have not yet been detailed. The author is now examining heavy mineral compositions of the tephras as shown in Fig. 5. The Lower Loam unit is characterized by the excessive amount of magnetite, and presence of hornblende including oxyhornblende. The Middle Loam is pumiceous aud especially so in the lowest horizon. As seen in Fig. 5, a pumiceous horizons with a peculiar appearance and called “Uridane-gata-Fuseki So” (the word Uridane means the seeds of cucumber) occurring the middle part of Middle Loam is doubtlessly considered as exotic material, which is assigned to the pumice grains from Ontake Volcanoes. The modes of occurrence of these Loam units upon fluviatile terraces are discussed in the text. (3) The Late Pleistocene activity of Tateyama Volcanoes supplied tephras in the north-wastern part of Shinshu or Nagano Prefecture. The tephraic section is marked by two pumiceous horizons, of which the lower one is in the lowest part of the Middle Loam unit and contains more amount of hypersthene and less amount of common hornblende and magnetite compared with the upper pumiceous horizon. The tephra covering the mudflows of Midagahara and appearing to be younger in age, is characterized by a moderate amount of oxyhornblende. Our tephrochronologic studies on the glacial age of Mt. Shirouma and the time of terrace formation along the Matsukawa running down the mountain slope are now in progress. (4) Tephraic horizons consisting of tuff and/or pumice grians are recognized in the marine beds on the Pacific coast of Central Japan. Their geologic ages seem to be different with one another but all of them contain much or a moderate amount of hornblede perhaps due to the activity of some acidic volcanisms. For the purpose of correlation, more elaborate inspection of the mineralogical nature of constituent materials of the tephras will be needed.
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 Würm 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.