Catalog Preparation of Tephras in Kyushu
By MIO YOSHIDA
Second Environment Division, Kyushu National Agricultural Experiment Station (Chikugo, Fukuoka, 833 Japan)
There are more than ten volcanoes, active inherited by soils. Aomine3> made a review on and extinct, including three major ones, Aso, the distribution and limiting factors for Kirishima, and Sakurajima, in Kyushu. A agricultural productivity of pyroclastic soils variety of tephras, i.e. volcanic ash, pumice, in Kyushu. scoria, and pyroclastic flow deposit erupted For the reclamation of areas covered with from them, cover an area of 8800 km2, which various layers of tephras, therefore, the know is approximately 20% of the whole area of ledges on distribution of tephras are especially Kyushu except Okinawa.o Especially in important in predicting the properties of the southern half of Kyushu, about 30% of total soil which will become a plowed horizon after land area is covered with some kinds of the reclamation. tephras. Thus the identification of tephras and Properties of tephras are diffe1•ent with knowledges on their distribution and charac sources of eruption. Ejecta from Kuju or teristics are indispensable for increasing agri Unzen, for example, has andesitic composition cultural productivity. with biotite and hornblende,8> that from Aso, Sakurajima, and Kaimon is also andesitic but with augite,s> and highly glassy volcanic ash Problems related to tephra from the Kikai caldera called Akahoya is identification rhyolitic.6> On the other hand, although the composition of both ashes erupted from Saku Stratigraphic survey with the aid of some rajima and Aso are augite-andesitic, the latter additional techniques such as primary mineral contains more heavy minerals and brown inspection has been the principal means for glasses with higher refractive indices. Ac tephra iden tification. There are, however, cordingly, the latter contains more cations about 200 volcanoes in Japan and more than and the acidity is less than the former.6> 10 in Kyushu and numerous kinds of tephras Furthermore, properties of deposits from the from them exist complicatedly. It is probable same source change with different stages of that different tephras have similar features eruption as described on Sakurajima lavas2 1J or the same tephra shows different appear Generally, tephras are to be considered not ances because of differences in grain size, as soils but as parent materials of soils. In environments of deposition, and/or the degree many cases, however, agriculture is performed of weathering. Disturbance by erosion and directly on tephra layers because they form translocation also causes more difficulties in plowed horizons, and the properties of tephras identifying them. Exact identification by 'the affect on the agricultural techniques and routine method is thus rather difficult. productivities of the areas. Also, characteris In Kyushu, stratigraphic survey of tephras tics of tephras exert definite effects on pattern has already been accomplished in fields of of weathering, and hence, on properties of geology, tephrochronology, soil science, vol soils derived from them: the chemical and canology, etc. Description and c14 dating of physical peculiarities of tephras are strongly individual tephras at certain sites have also 217 been reported. Despite of these efforts, a number of tephra layers have not been welJ Method of thermomagnetic identified yet. The reason might be that the analysis for tephra identification detailed description of the features of individ ual tephra layers had received much less at Advantages of using the thermomagnetic tention as compared with the stratigraphic analysis for identification of tephras are: (1) field survey. Machida10> emphasized the im ferromagnetic minerals are commonly in portance of specification of features on each volved in volcanic materials. (2) they are tephra from the standpoints of field survey, stable against weathering. (3) they can be petrography, and chemistry. He proposed to easily separated with a hand magnet. (4) sum up and compile the results into catalogs thermomagnetic analysis is a fairly sensitive of tephras, which will facilitate the pursuit method and a few milligrams of specimen are of tephras to a great extent. In the catalog enough for one determination (5) magnetic preparation, studies on components or factors properties cah be revealed by a simple magne which are stable against disturbance or tic balance. weathering can only provide a clue to identify In ferromagnetic minerals, heating causes a tephra among many layers. a gradual decrease of spontaneous magnetiza Recently, the techniques to determine re tion, i.e. attractive force to a magnet, until fractive indices of primary minerals has the temperature reaches a certain critical advanced, and those of hyperthenes and temperature, which is called the Curie tem volcanic glasses in tephras have been revealed perature (Tc). Above this temperature, the to be specific to each tephra.9> Two wide minerals lose all their spontaneous magnetiza spread tephras were identified with the aid of tion. The curve of the spontaneous magnetiza this method.9, 11 > Chemical and magnetophysi tion vs. temperature as shown in Fig. f is cal properties of ferromagnetic minerals in called a thermomagnetic curve or a Js-T curve. volved in tephras have also been known to be Tc values and configurations of Js-T curves specific to sources and eruption stages of host depend on the chemical composition of fer tephras. Chemical composition of ferromagne romagnetic minerals when the applied magne tic minerals has been studied and applied to tic field is sufficiently high. identify a number of tephras.1,is,20> The structure of a vertical type magnetic A method of tephra identification by ther balance is shown in Fig. 2. The temperature momagnetic analysis of ferromagnetic min of the sample room changes after set program erals has been developed by M:omose et al.17> and the attractive force of the sample toward They carried out a series of the analyses on the magnet at every temperature is balanced Plistocene pumice deposits and reported that to the rest position by a feed-back coil. The each ferromagnetic minerals had a definite current of the coil for compensation is re property peculiar to the pumice, which was coraed against the corresponding temperature. confirmed also by Maenaka and Yokoyama.13> The present auther attempted to apply Momose et al.'s thermomagnetic method17> for Distribution and description of the identification of tephras in Kyushu, and several standard tephras in Kyushu the information so far obtained by the use of this method has proved to be highly help Several standard tephras whose sou1·ces and ful.2·1 , 2r. > The method employed and the pre eruption ages are known are selected and pared catalogs of some of standard tephras in their outer limit of distribution are shown in Kyushu will be presented below. Fig. 3,12.14> Akahoya is highly glassy rhyolitic volcanic ash deposit with yellowish orange in color. It is known as one of wide-spread tephras in 218 JARQ Vol. 14, No. 4, 1981
Jt/Jo ------Akahoya ---- Taisho pumice --- Psc11do-Akahoya ash 1.0 ---- Miike pumice ------An-ei pumice ·-.. -----·-Kaimon C pumice ·,,;s, ------Bunmei pumice ~ ~------Unidentified pumice ~ -0 "\ . ,,"'-, ~ ~,~ \ \ \-...~ \\ \ 0.5 ', \\ \\ \\ \\·"'--, \ ~ t, ·, "'\\ \ ; i ' ',._ - (b) ' \ (c) I\ k / .. ,, (a) \ \ "',\~\ 0 .___.__~_,..,.,_ ....- ,---.c.. ' ...._. .... V N~±:.!_.. .- ···· ..\ 200 400 It,,_ 6ooc •c > I . I ll515 · 270 (500j ) ! f ~10 (4~5) 323 413 (499) 51 2 220 225 295 Fig. 1. Normalized Js-'1' curves of ferromagnetic minerals. (a) Glassy volcanic ashes, (b) Pumices from Kirishima and Kaimon, (c) Sakurajima pumices. Jt : spontaneous saturation magnetization intensity at determination temperatures, Jo : spontaneous saturation magnetization intensity at room tempernture H = 2500 re and in vacuo (10· 2 mmHg)
to vacuum pump
Thermo couple
Compensating coil
soil scientists have 1·epeatedly discussed about this tephra and its source has been deduced as the Kikai caldera recently .11 , 1n> Aira Tn ash erupted from the Aira caldera is also highly glassy ash and is the widest Sample spread tephra in Japan. This was first dis covered in the layers of Tokyo pumice at Weight Tanzawa, after which the name Tn was given, and its origin was pursued to the Aira caldera by Machida and Arai.0 > Pseudo-Akahoya ash has been considered as Fig. 2. Schematic diagram of a vertical marginal facies of welded tuff of Aso1s>, type magnetic balance named Aso-423>, and corresponds to Tosu5' and Ube16> loams. The appearance is very similar Japan and have several local names, "Onji", to Akahoya but this ash contains less volcanic "Imogo", etc. Many tephrochronologists and glasses. 219 N
Kyushu I
O Lake Miikc Mp ( b) ,;-'Q.:-·Anp Kagoshima .~~-B;·:,.., •• T 1> ;...,1-..I 1> Mt. Sakurajnna / :_;:,.. ... ::::e- ····· KmC Lak
ML Kannon
0,..__.___, 20 km N '
(a)
O 200 km
:,~ Kikai "aldcra
Fig. 3. Distribution of several tephras selected (a) Glassy volcanic ashes"> (b) Pumices of Kirishima, Kaimon, and Sakurajima,.> Mp : Miike pumice Km C : Kaimon C pumice Tp : Taisho pumice Anp : An-ei pumice Bp : Bunmei pumice Xp : pumice of unknown old age in Holocene
Table 1. Catalog of several standard tephras in Kyushu
Curie temp. Lattice Fe/Fe+Ti No. of Age Tephra name Source Tc(°C) c;(A~· (molar rntio) ~~:;
Akahoya ash Kikai calderan>l9> 5-6X103y.B.P.22> 323 (498) 8.435 0.890 15 Ail'a Tn ash Aira caldera9> 21- 22X 103y. B. p,10> 413, 512 not dete1·mined 3 Pseudo-Akahoya ash AsolS> 30-40X 103y.B. P.10> 499 not dete1·mined 7 Miike pumice Miike, Kirishimal4> 3X 103y.B.P.14> 270, 607 8.446 0.872 8 Kaimon C pumice Ikedako, Kaimon14> 4Xl03y,B.P.lO> 602, 517 8.423 0.928 6 Taisho pumice Sakurajima14> 1914 A.D.14> 295 8.432 0.885 5 An-ei pumice Sakurajimal4> 1778-1780 A.D.14> 220 8.453 0.863 8 Bunmei pumice Sakurajimal4> 1471-1476 A.D.14) 255 8.443 0.865 3 unidentified pumice Sakurajimal4> unknown old age 310 (495) 8.430 0.882 3 in Holocenel4> 220 JARQ Vol. 14, No. 4, 1981
Miike and Kaimon C pumices are common magnetite.2 J staqdard pumices in southern Kyushu erupted Tc values differ clearly among different from Lake Miike and Lake Ikeda, respectively. tephras and differentiation among them can Miike pumice forms thick l11yers around easily be made. Difference in Tc values also Miyakonojo, a few killometers south of the well corresponds to difference in values of the source. Kaimon C, light yellowish brown lattice constant and chemical compositions of pumice, is a pumice of hornblende-quartz ferromagnetic minerals. andesitic composition and forms thick layers Ferromagnetic minerals in pumices of dif fo southern part of the Osumi Peninsula. ferent stages of Sakurajima show different Taisho, An-ei, Bunmei, and unidentified properties indicating that characteristics of pumices are ejecta from Sakurajima. The magma had changed with successive eruption names were given after the era names at the stages. Similarity between ferromagnetic eruptions respectively. Taisho pumice is ex minerals of Taisho and unidentified pumices tremely fresh and hard two-pyroxene andesite was, however, seen, which might indicate that pumice with light gray color. An-ei pumice is magma properties were similar at the stages grayish yellow orange with slight stain but of these pumices. This is probable because still fairly fresh and hard. The profile near Si02 content of Sakurajima lavas of different the source consists of several fall units. stages was reported to have changed gradually Though it is still under discussion if the and periodically between 59 and 67%.21> pumice was erupted during Bunmei eruption, Accumulation of catalogs on standard Matsui and Wajimal4> considered that the tephras as exemplified in Table 1, will make it certain pumice layers were the ejecta during possible to compare properties of unidentified Bunmei period. 'rhe term, Bunmei pumice, is tephras with the catalog and correlate them. used here after them. Unidentified pumice is In Kyushu, tephra identification has not slightly weathered and yellow in color and has been promoted so systematically as in Hok been considered as ejecta at unknown old age kaido yet and numbers of tephras are still in Holocene. remained unidentified despite of many field surveys done till now. Comprehensive and systematic studies including petrographic, Catalog of several standard chemical, and magnetophysical investigatfons tephras in Kyushu for catalog preparation are desired in this region. Js-T curves of ferromagnetic minerals ex tracted from tephras are illustrated in Fig. 1. Differences in thermomagnetic features References among tephras can be seen clearly. Table 1 is the catalog of these teph1·as. It 1) Adachi, T.: On the area and humus content of Ando Soils in Japan. J. Soc. Sci. Soil includes source and age already reported by Mmmre, Javcin, 42, 309-313 (1971) [In others, in addition to Tc values and a few Japanese with English summary]. other properties of fenomagnetic minerals. 2) Akimoto, S. et al.: Magnetic properties of The Tc values of titanomagnetite have been TiFe,01-Fe30., system and their changes revealed to change almost linearly with Ti with oxidation. J. Geomag. Geoelelect., 9, 165-178 (1957) . contents from high to low tempel'atures2> so 3) Aomine, S. : Some soil layers of pyroclastic long as the oxidation degree is the same. The materials in Kyushu, Agr. & Hort. 28, 699- lattice constant, (~CA), is a length of an edge 702 (1953) [In Japanese]. of cubic spinel crystals of titanomagnetite. 4) Ariake Bay Research Gl'Oup: M onogrcivhl It can be obtained by X-ray diffraction 11. The Association fo1· the Geological Col laboration in Japan. (1965). analysis and is known to correspond to Ti 5) Go hara, Y. et al.: Some problems on the content and degree of oxidation of titano- Pleistocene formations of North Kyushu, 221
Japan. Misc. Re7>t. Research Inst. f01· (1963) [In Japanese with English sum Natiwal Resources, 62, 83-108 (1964) maryl. [In Japanese with English summary]. 16) Matsui, T. & Kato, Y.: Preliminary reports 6) Kanno, I.: Genesis and classification of main on modes of occurrence and paleopedology genetic soil types in Japan. 1, Introduction of Red Soils in Chikugo and Shikoku dis and Humic Allophane Soils. Biill. J(yiishu tricts including their neighobouring area Ag?·. Exp. Sta., 7, 1-185 (1961) [In Japanese Paleopedological studies on the pedogenesis with English summary]. of Red Soils in South-west Japan. II. Misc. 7) Kohn, B. P.: Identification of NewZealand Revt. Resecl!l"ch Inst. for NCLtu?·al Reso1wces, tephra-layers by emission spectrographic 64, 31-48 (1965) [In Japanese with English analysis of their titanomagnetite. Lithos, 3, summary]. 361- 368 ( 1970) . 17) Momose, K. et al. : Identification of tephra 8) Kuno, H.: Distribution of volcanoes in by means of ferromagnetic minerals in Japan and its vicinity, in Volcanoes and pumice. Bull. Ecwthq. Res. lnst., 46, 1275- Rocks, 85-87, Iwanami, Tokyo (1954) [In 1292 (1968) . Japanese]. 18) Nagatomo, Y. et al.: Chemical composition 9) Machida, H. & A1·ai, F · A wide-spread of ferromagnetic minerals in volcanic ash teph1·a-Aira Tn ash. Tfogcik:u, 46, 339- 347 layers in South Kyushu. J. Soc. Sci. Soil (1976) [In Japanese] . Mannre, Javan, 47, 25-32 (1976) [In Japa 10) Machida, H.: Tephrochronology and Ap nese with English summary]. pendix, in The quaternary 1>eriod: Recent 19) Nagatomo, Y. & Shoji, S.: Conelation and studies in Japan (Japanese Association for origin of Al