[S-VC29] Volcanic and igneous activities, and these long-term forecasting

[SVC29-03] Paleomagnetic dating of large volcanic bombs around Tsubakurosawa craters, Azuma volcano, NE Japan: Was the 1893 eruption magmatic?

*Takeshi Hasegawa¹, Bunta Kikuchi¹, Shibata Shohei¹, Yuhji Yamamoto², Takumi Imura³, Masao Ban³, Makoto Okada¹, Tatsuro Aibe¹ (1.Department of Earth Sciences, College of Science, Ibaraki University, 2.Center for Advanced Marine Core Research, Kochi University, 3.Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University)

Keywords:Paleomagnetic dating, Azuma volcano, Volcanic bomb

Vulcanian eruptions are some of the most common eruption styles on earth. They eject and deposit volcanic bombs around their source craters. Although paleomagnetic studies of volcanic bombs can provide high-resolution dating of volcanic activity, particularly when compared with known geomagnetic secular variation records, there are only few of such studies. In this contribution, we analyzed paleomagnetic directions of huge (>1m in diameter) volcanic bombs around “Tsubakuroswa craters”, located on Azuma volcano, for evaluating the utility of volcanic bombs in paleomagnetic dating. Oriented samples were collected from the central parts of 4 large volcanic bombs, located on a gentle slope a few hundred meters south from the craters (Fig. 1). More than 4 drill cores were sampled from each bomb and all samples were subjected to thermal demagnetization analysis. The ChRM of specimens from all bombs were well determined as follows: Site mean directions (Dm) of each range in declination (Dec = 350.6 to 358.0º) and inclination (Inc = 48.9 to 50.8º) with small (<2.4) α95, and in close agreement with each other. We obtained consistent thermoremanent magnetization results for the four individual volcanic bombs, thus supporting the utility of volcanic bombs for further paleomagnetic dating research. Accounting for overall mean direction (Dec = 355.3º, Inc = 50.0º, α95 = 1.1º, n=16) and secular variation record in this area most likely suggests that the volcanic bombs were derived from the Meiji Era (AD1893) eruption (Fig. 2). Historic pictures and descriptions are consistent with and support this interpretation. Previous reports suggested that the Meiji Era eruption did not eject magmatic materials and that the last magmatic eruption of this volcano was in AD1331. However, our results suggest that magmatic eruptions might have occurred here only 130 years ago and may be largely affecting the current activity of this crater area.
We also sampled and tried to determine paleomagnetic direction of large volcanic bombs around the summit crater of Nasu Chausudake volcano, NE Japan, by the same methodology. However, the alignment of Dm from 4 bombs were bad, suggesting that some movement of the bombs occurred on the steep slopes during or after cooling. Only one of the four bombs indicates a consistent direction with the age of the AD1410 historic eruption.
Although volcanic bombs are potentially useful materials for paleomagnetic dating, a considerable number of bombs would need to be collected and the topography of their area of deposition carefully checked, especially in case of older eruption products.