Volcanological studies require dating of volcanic ejecta to within several tens of kiloyears (ka). However, such dating presents difficulties because of adequate methods are few and sampling problems are inherent. Radiocarbon (14C) dating is applicable for ages from several hundred years to a few tens of thousands of years. Nevertheless, the possible occurrence of contaminants such as mold, mildew, and fungus on samples complicates the interpretation of dating results. Moreover, during 14C dating, one frequently encounters difficulties in collecting datable organic material in volcanic contexts. Although thermoluminescence (TL) and optically stimulated luminescence (OSL) dating have been used widely for archaeological studies, this method entails difficulties because it requires quartz for accurate dating (e.g., Tsukamoto
Paleomagnetic dating represents an alternative. Volcanic materials deriving from eruption can acquire stable thermoremanent magnetization (TRM) during their initial cooling. The Earth’s magnetic field changes its direction by several degrees to a few tens of degrees over timescales of 100–1,000 years, which represents secular variation. Paleosecular variation (PSV) can thereby be reconstructed, as described in numerous earlier reports of the literature (e.g., Jackson and Finlay, 2007). Comparison of the PSV with the paleomagnetic direction inferred from TRM of volcanic materials can suggest the age at which magnetic minerals in the materials acquired the remanence. Paleomagnetic age determination can be done directly from the material for which the age is sought.
One shortcoming inherent in the method, however, has often troubled us and other researchers: PSV varies at most by a few tens of degrees around a direction that is generally derived from the axial dipole component of the Earth’s magnetic field. Even if the paleomagnetic direction could be determined with high accuracy, the paleomagnetic age could not be identified uniquely because several candidates for age can be inferred from paleomagnetic data. Ascertaining the correct age using paleomagnetic methods alone is difficult. Age estimation based on PSV must be combined with other dating techniques.
An almost perfect target as an attempt for a combined age estimation is Kannabe volcano (
Fig. 1
Index (a) and locality (b) maps. Sampling locations of Kannabe scoria cone in southwestern Japan for paleomagnetic samples (locs. 1–6) and OSL dating (loc. a). Outcrop of loc. b is shown as AT tephra under Kannabe scoria (see Fig. 2).

We attempted paleomagnetic dating to estimate the Kannabe volcano eruption age. Furthermore, we applied OSL dating to sediments associated with the Kannabe scoria cone.
Kannabe volcano in southwestern Japan (N 35°30’20.1”, E 134°40’29.0”), the youngest volcano in Kannabe monogenetic volcano group (
The Kannabe monogenetic volcano group comprises seven scoria cones (Nishiki, Yamanomiya, Buri, Otsukue, Tada, Kiyotaki, and Kannabe volcanoes) and one scoria bed (Nishibashi-kita) (Kawamoto, 1990; Furuyama
These volcanoes erupted alkali olivine basalts with phenocrysts of olivine and plagioclase (e.g., Kawamoto, 1990; Furuyama
Fig. 2
Geological columnar sections in locs. a and b of Fig. 1. Closed circle show sampling positions for OSL dating. The estimated OSL age is also shown.

The Kannabe lava sample 4 (
Fig. 3
Optical photomicrograph of a Kannabe lava sample 4 under crossed polarizer: Ol, olivine; Pl, plagioclase.

Paleomagnetic samples were taken from six outcrops of Kannabe lava flows. Sampling locations are presented in
component was defined as follows: (1) a linear segment consisting of more than three vector end points; and (2) maximum angular deviation (MAD) of less than 5°.
An OSL sample was collected at loc. a (N 35°30’20.9”, E 134°41’02.2”) in
The OSL measurements were taken using an OSL reader (NRL-99-OSTL; Neoark Corp.) equipped with an array of 32 blue LEDs (470 nm, 18.1 mW/cm2) (Nagatomo
Fig. 4
Preheat temperature plateau measurement. Averages of three aliquots are shown at the respective temperatures. A preheating plateau is apparent at 220–260°C.

Protocol for SAR measurement.
Step | Treatment | Observed |
---|---|---|
1(a) | Give dose, | – |
2 | Preheat (240°C for 60 s) | – |
3 | Stimulate for 100 s at 125°C | |
4 | Give test dose, 5 Gy | – |
5 | Cut heat (160°C) | – |
6 | Stimulate for 100 s at 125°C | |
7 | Stimulate for 40 s at 250°C | – |
8 | Return to 1 | – |
The annual dose (dose rate) was measured using a high-resolution gamma-ray spectrometer. The concentrations of 238U, 232Th, and 40K in the soil sample were analyzed using a Ge gamma-ray detector (EGSP 8785; Eurisys Mesures). The annual dose was calculated using the dose-rate conversion factors reported by Guérin
Results of demagnetization experiments show that all specimens had strong and stable magnetizations, with NRM intensities of 1.0–5.0 × 10–3 Am2/kg. Magnetization of ten samples decayed linearly toward the origin. Single magnetic components with low MAD of less than 0.5° were obtained (
Fig. 5
Vector end-point diagrams of demagnetization results for typical Kannabe samples. Solid and open circles respectively represent projections onto the horizontal and N–S vertical planes: (a) one magnetic component with smooth decay to the origin; (b) noisy trajectories with scatter; and (c) two magnetic components.

directions are almost parallel to the directions of magnetic components from the other samples. Therefore, the components removed at higher demagnetization levels are adopted as TRM (
Sampling localities and demagnetization results.
Sampling locality | Sample name | Demagnetization methods | Demagnetization results | ||||
---|---|---|---|---|---|---|---|
Number | Latitude | Longitude | Dec (°) | Inc (°) | MAD(°) | ||
1 | 35°29’9.599” | 134°41’45.999” | 264 | thermally | 5.6 | 68.2 | 0.3 |
1 | 272 | thermally | 12.8 | 64.7 | 0.4 | ||
282 | thermally | –23.3 | 65.5 | 0.2 | |||
291 | thermally | –11.3 | 66.3 | 0.3 | |||
301 | thermally | –4.8 | 67.8 | 0.8 | |||
2 | 35°29’27.599” | 134°41’34.800” | 191 | thermally | 22.9 | 51.3 | 0.4 |
201 | thermally | –3.8 | 63.7 | 1.4 | |||
211 | thermally | 2.8 | 65.9 | 0.5 | |||
3 | 35°29’31.199” | 134°41’34.800” | 222 | thermally | –5.9 | 68.3 | 0.3 |
231 | thermally | –14.7 | 67.5 | 0.4 | |||
241 | thermally | 12.9 | 68.7 | 0.2 | |||
251 | thermally | –23.7 | 61.6 | 1.8 | |||
4 | 35°29’16.799” | 134°42’3.599” | 41 | magnetically | 2.0 | 63.4 | 1.0 |
161 | thermally | 6.2 | 62.1 | 1.8 | |||
171 | thermally | –10.1 | 64.5 | 1.3 | |||
181 | thermally | 6.0 | 68.9 | 1.2 | |||
5 | 35°28’30.000” | 134°42’46.799” | 12 | magnetically | 4.9 | 69.1 | 0.9 |
31 | magnetically | 4.7 | 66.2 | 0.8 | |||
131 | thermally | 8.0 | 69.1 | 0.3 | |||
145 | thermally | 2.9 | 76.3 | 0.2 | |||
151 | thermally | 10.3 | 71.2 | 0.4 | |||
6 | 35°28’8.400” | 134°46’37.199” | 113 | thermally | 0.8 | 53.5 | 5.0 |
121 | thermally | –2.1 | 61.0 | 2.6 |
Magnetic components derived from TRM at the origin show better grouping of the direction (
Fig. 6
Equal-area projection onto the lower hemisphere of magnetic components (solid circles) showing the mean direction (cross), its 95% confidence circle and present Earth’s magnetic field (square) around the studied area are shown. The mean direction and its 95% confidence limit from Torii et al. (1978) and from Morinaga et al. (2000) are also shown respectively as open triangles and open circles.

Sensitivity-corrected OSL signals were calculated by dividing
Fig. 7
a) OSL decay curve of natural and 45 Gy as regeneration dose. b) Growth curve of an aliquot for regenerative doses of 0, 15, 25, 35, and 45 Gy.

Fig. 8
Dose distributions (n = 9) are shown as the radial plots.

9.2%. Following procedures reported by Olley
The OSL age, 21 ± 6 ka, is presented in
Results of OSL dating.
Paleodose (Gy) | Aliquots (n) | U (ppm) | Th (ppm) | K (wt%) | Water content (%) | Annual dose (mGy/a) | OSL age (ka) |
---|---|---|---|---|---|---|---|
34.7±1.6 | 9 / 15 | 1.25 ± 0.09 | 8.26 ± 0.42 | 0.74 ± 0.05 | 62 ± 6 | 1.62 ± 0.42 | 21 ± 6 |
We investigated the eruption age of the Kannabe volcano in Japan. The Kannabe scoria lies between widespread tephras of K-Ah (ca 7.2–7.3 ka) and AT (
The paleomagnetic age was estimated by comparing our results with PSV data obtained from Lake Biwa sediments (Hayashida
Fig. 9
Paleomagnetic direction from Kannabe lava shown on paleosecular variations from Lake Biwa after Hayashida et al. (2007). Bold lines and gray bars respectively represent the paleomagnetic direction and its 95% confidence angle.

Within the age range, the remanent magnetization (declination and inclination) of the Kannabe lava accords exactly with the environmental magnetic record at around 22 ka. Consequently, we can infer the eruption age of the Kannabe scoria cone as
Identification of active volcanoes is extremely important not only for volcanology but also for disaster prevention. A method for precise age determination of young volcanic rocks must be established. In this study, the eruption age of young basaltic lava was ascertained using paleomagnetic dating, OSL dating, and tephrochronology. Tephrochronological estimation results indicate to us that the eruption occurred between 7.2–7.3 and 30 ka. The OSL age of sediment on Kannabe scoria was
21 ± 6 ka. The paleomagnetic age was found to be
This study demonstrated that multi-method dating is an effective tool for the age determination of young volcanic rocks.
Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Results of OSL dating.
Paleodose (Gy) | Aliquots (n) | U (ppm) | Th (ppm) | K (wt%) | Water content (%) | Annual dose (mGy/a) | OSL age (ka) |
---|---|---|---|---|---|---|---|
34.7±1.6 | 9 / 15 | 1.25 ± 0.09 | 8.26 ± 0.42 | 0.74 ± 0.05 | 62 ± 6 | 1.62 ± 0.42 | 21 ± 6 |
Sampling localities and demagnetization results.
Sampling locality | Sample name | Demagnetization methods | Demagnetization results | ||||
---|---|---|---|---|---|---|---|
Number | Latitude | Longitude | Dec (°) | Inc (°) | MAD(°) | ||
1 | 35°29’9.599” | 134°41’45.999” | 264 | thermally | 5.6 | 68.2 | 0.3 |
1 | 272 | thermally | 12.8 | 64.7 | 0.4 | ||
282 | thermally | –23.3 | 65.5 | 0.2 | |||
291 | thermally | –11.3 | 66.3 | 0.3 | |||
301 | thermally | –4.8 | 67.8 | 0.8 | |||
2 | 35°29’27.599” | 134°41’34.800” | 191 | thermally | 22.9 | 51.3 | 0.4 |
201 | thermally | –3.8 | 63.7 | 1.4 | |||
211 | thermally | 2.8 | 65.9 | 0.5 | |||
3 | 35°29’31.199” | 134°41’34.800” | 222 | thermally | –5.9 | 68.3 | 0.3 |
231 | thermally | –14.7 | 67.5 | 0.4 | |||
241 | thermally | 12.9 | 68.7 | 0.2 | |||
251 | thermally | –23.7 | 61.6 | 1.8 | |||
4 | 35°29’16.799” | 134°42’3.599” | 41 | magnetically | 2.0 | 63.4 | 1.0 |
161 | thermally | 6.2 | 62.1 | 1.8 | |||
171 | thermally | –10.1 | 64.5 | 1.3 | |||
181 | thermally | 6.0 | 68.9 | 1.2 | |||
5 | 35°28’30.000” | 134°42’46.799” | 12 | magnetically | 4.9 | 69.1 | 0.9 |
31 | magnetically | 4.7 | 66.2 | 0.8 | |||
131 | thermally | 8.0 | 69.1 | 0.3 | |||
145 | thermally | 2.9 | 76.3 | 0.2 | |||
151 | thermally | 10.3 | 71.2 | 0.4 | |||
6 | 35°28’8.400” | 134°46’37.199” | 113 | thermally | 0.8 | 53.5 | 5.0 |
121 | thermally | –2.1 | 61.0 | 2.6 |
Protocol for SAR measurement.
Step | Treatment | Observed |
---|---|---|
1(a) | Give dose, |
– |
2 | Preheat (240°C for 60 s) | – |
3 | Stimulate for 100 s at 125°C | |
4 | Give test dose, 5 Gy | – |
5 | Cut heat (160°C) | – |
6 | Stimulate for 100 s at 125°C | |
7 | Stimulate for 40 s at 250°C | – |
8 | Return to 1 | – |