Magnetization of Ash-Fall Tuffs of Oshima Volcano, Izu, II Application to Archaeomagnetism and Volcanology
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Magnetization of Ash-fall Tuffs of Oshima Volcano, Izu, Ⅱ Application to Archaeomagnetism and Volcanology By T. YUKUTAKE, K. NAKAMURA* and K. HORAI Earthquake ResearchInstitute, Universityof Tokyo (Read Nov. 27, 1963;Received May 3, 1963) Abstract The validity of making use of subaerial ash-fall tuffs in archaeomagnetism,was examinedby measuring samplescollected from the same ash-falltuff bed at different parts of Izu-OshimaIsland. Magnetizationof these tuffs agrees with each other quite well. Moreover,it has turned out that the magnetizationof ash-fulltuffs is so stable and strong that even a small fluctuation of the magneticfield due to the local ano- maly at the time of deposition,may be detected. All the results so far obtained by archaeomagneticstudy on lavas and pyroclas- tics of OshimaVolcano are summarized. 1. Introduction In volcanic regions, rock samples have usually been collected from lavas for the palaeo- magnetic and archaeomagnetic study, because they have strong and stable T. R. M, and the process of their magnetization is also well known. Lava flows, however, are not always issued even at the eruptions of large scale, especially with the volcanoes of explosive nature. On the other hand, ash-fall deposits are much commonly formed at eruptions. Moreover, the distribution of lavas is often limited to narrow depressed area or valley at the time of eruption, while ash-falls are distributed over much wider area irrespective of topography. Collection of successive samples from different horizons (i.e. ages) at a single locality may also be possible for fall deposits. If we succeed in making use of ash-fall tuffs for archaeomagnetic study, it will certainly add a lot of information to the present knowledge of the secular variation in the geomag- netic field, and in turn to the history of volcanic activities. 2. A Brief Summary of Recent Activities of Oshima Volcano Recent activities of Oshima Volcano, Izu are briefly summarized in this section (Naka- mura, 1960, 1961, 1963, and M S). Oshima Volcano is a basaltic insular stratovolcano and is composed of the main cone with a summit caldera and a central cone, Mihara-yama inside the caldera. They consist of subaerial pyroclastic fall deposits and lavas. The volcanic products which consist of the * former address: Institute of Earth Science , College of General Education, University of Tokyo. (183) 184 T. YUKUTAKE, K. NAKAMURA and K. HORAI uppermost part of the volcanic edifice, are stratigraphically divided into twelve eruptive cycle units. Each unit, expressed as Y1, Y2 etc., includes pyroclastic falls and or lava flows which are conformably accumulated without any weathering breaks. Between any two of the units, on the other hand, there is a weathered ash zone representing a quiet interval of eruptive activity (Table 1). Table 1. Recent Activities of Oshima Volcano. *) Dates of eruptions are revised on the basis of new data. The beginning dates of the activity of individual unit are inferred either by the corre- lation of the deposits with documents or by pottery remains excavated or radiocarbon measurements. The time acquired for deposition of each unit is estimated to be less than, say, ten years. Summit caldera is found to have been formed after the deposition of the S2 and before that of the N4. Pyroclastic fall deposits of individual units cover all over the island (the N1 in Fig, l is an example), so that ejecta from parasitic volcanoes are assigned into the sequence of volcanic products of summit craters. Thus, lava flows and driblet agglutinates from parasi- tic vents were used for archaeomagnetic study (Table 5). It was not until the Y4 time that lava flows from summit craters overspread the lowest rim of the caldera. Earleir lavas have accumulated inside the caldera depression and have now been concealed under the later deposits. Thus, Qshima Volcano has repeated major eruptions with intervals of about a hundred years, during the last 1500 years, with which we are concerned. Every major eruption has formed a considerable amount of ash-fall deposits over the island. While such is not the Magnetization of Ash-fall Tuffs of Oshima Volcano, Izu, Ⅱ 185 case with lava flows. 3. The Collection of Specimens and the Results of the Measurements. As was shown in the previous paper (T. Yukutake et al. 1964), it seems very hopeful to measure the magnetization of ash-fall tuffs for the purpose of pursuing the secular variation of the magnetic field of the earth. The results previously shown, however, were obtained from the samples collected succcessively at a single locality. It seems necessary to examine whether the seemingly plausible results are accidental one or not and whether or not the samples collected at a single locality may really be representative for the magnetization of the extensive ash-fall tuff sheet. Besides, it is highly desirable to compare the magnetization of tuffs with that of lavas which are included in the same eruptive cycle unit. For this purpose, a single tuff bed of the N1 has been selected for the testing ash-fall deposits on the ground that the fall deposits are one of most widely distributed and it is easier to identify the bed of different locality because of the charac- teristic features of the bed, such as lithology, colour etc. Thus from the same, single tuff bed even in the N1, samples N1-1, N1-2, N1-4 and N1-5 were collected as is shown in Fig. 1. The site of Nl-l, is chosen for comparison, very close to that (solid circle in Fig. 1) of the samples previously collected from a lava flow which occupies the Fig. 1 Sampling sites and distribution basal horizon of the N1. The positions of the of ash-fall tuff beds, N1. Solid circle sampled tuff bed in the Nl are shown in Table 2 shows the site where lava specimens were collected previously. Broken lines and Fig, 2, together with the thicknesses of the are the lines of equal thickness of the Nl and of the sampled tuff beds. N1 Table 2. Thickness and position of the sampled tuff in the N1 at the sampling sites. Fig. 2 Position of sam- pled tuff bed in the N1. 186 T. YUKUTAKE, K. NAKAMURA and K. HORAI Table 3. Direction of magnetization of the N1-tuffs and the N1-lavas collected from different parts of the island. The results of measurements are shown by the equal area projection in Fig. 5. Table 3 shows the average of these results together with a 95% level of confidence by Fisher's method. Average direction of magnetization of the tuff collected at the same locality as the lava does not agree with that of the lava; that is, the mean direction of the tuff is D= 9.6°E, I=39.6° while the lava shows the mean value of D=1.0°E and I=46.2°. As in Fig. 5-a, the results of measuring the tuff, Nl-1, are so dispersioe that the great significance should not be attached to the above discrepancy. Such large dispersion seems to be caused by bad condition of the sampling site where the exposure is a little overhanged. 4. Relation between the Present Local Anomaly and the Results of Measurement. As is well known, the magnetic field changes from place to place in the volcanic region. If the ash-fall tuffs really preserve the correct direction of the geomagnetic field at the time of its deposition, magentization of the tuffs should be different from place to place, too. Local anomaly of the geomageetic field sometimes amounts to several degrees of arc in Oshima Island. It is the most desirable that the measurement of the geomagnetic field should also be conducted at the same place where samples are collected. Such measurement, however, was not carried out this time. Therefore, the results of old masurement which were conducted at the nearest point to the sampling site, or field values read from the local anomaly map by I. Yokoyama (1957), are shown in Table 3 together with the results of measuring tuffs. Taking these values in abscissas, and the results of measurements of tuffs in ordinates, relationship between the local anomaly and the direction of magnetization of tuffs is shown in Fig. 3. The results obtained out of specimens from the lava are also plotted in the figure by solid circles. If the distribution of the magnetic field at the time of the deposition of the N1, had been similar to the present one, inclination obtained from the magnetization of tuffs should in- crease with the increase in the present value of inclination at a sampling site. Moreover, if both distribution had been identical. the measured point should he aligned in a line with agradient of 45 degrees passing through the present normal inclination value 49°. If the pattern of the local anomaly at the N1 time had been the same as the present one and had Magnetization of Ash-fall Tuffs of Oshima Volcano, Izu, Ⅱ 187 Fig. 3 Relation between the local anomaly and the direction of magnetization of ash-fall tuff. Field values read from the present anomaly map are taken in abscissas and declination and inclination obtained by measuring tuffs are shown in ordinates. Solid circles are the results from lavas. merely differed by constant values caused by the secular variation, the points should be aligned parallel to the above line. On the other hand, declination obtained from samples, wherever they may be collected, should be zero or constant according as the local anomaly was identical to the present one or merely different by constant values, because the oriented samples were not referred to the true north but to the magnetic north at the sampling sites.