Sakurajima(2)：Eruption History of Sakurajima Volcano

Sakurajima Volcano - 2nd Edition -

Introduction / Geology of the Area around Sakurajima Volcano / Summary of the Topography and Geology of Sakurajima Volcano

Eruption History of Sakurajima Volcano

Historical Eruptions

Volcanic Rocks of Sakurajima Volcano

Monitoring and Observation of Volcanic Activity / Forecasting of Future Activity

Appendix: Revisions from First Edition

References

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Fig.2 Stratigraphy, ages and volumes of the tephra from Sakurajima Volcano
Eruption ages are taken from Okuno (2002) and Kobayashi (2010), and cal ka means a calibrated calendar age in 1,000 years ago. The age with * is inconsistent with the stratigraphy. This volume is apparent one for tephra.

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Fig.3 Distributions of the P14 (Sakurajima-Satsuma) tephra from Younger Kitadake Volcano
Modified from Kobayashi and Tameike (2002). Numerals are thickness in cm. S = Sakurajima Volcano.

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Fig.4 Cumulative tephra volume of Plinian eruptions vs. age for Sakurajima Volcano
See Fig.2 for name of tephra. This volume is apparent one for tephra.

Eruption History of Sakurajima Volcano

Figure 2 is a summary of the eruption history of Sakurajima Volcano based on tephra stratigraphy. Plinian pumice fall layers from Sakurajima Volcano have been classified into 17 layers in total, of which the upper four layers (P1 to P4) are comprised of historical period eruption ejecta. The eruption history of Sakurajima Volcano based on tephra stratigraphy and radioactive dating can be divided into four stages (Older Kitadake, Younger Kitadake, Older Minamidake, Younger Minamidake).

Soon after the Aira-Tn eruption (29,000 years ago), the period of volcanic activity for the Older Kitadake began (24,000 to 26,000 years ago), and can be confirmed by three layers of tephra (P15 to P17) deposited outside of the island. The ejecta determined to be that of the Older Kitadake is not distributed over the surface of the island, but the K-Ar age of andesite lava boulders collected from the Kurokami observation well approximately 350 m underground are 30±5 ka. This means that there is the products of the Older Kitadake beneath the subaerial edifice (Uto et al., 1999). Volcanic activity for the Younger Kitadake began 13,000 years ago. There was about a 10,000 year dormant period between the end of the Older Kitadake and the start of the Younger Kitadake. During this dormant period, the Takano Base Surge (A-Tkn: approx. 19,000 years ago) and the Shinjima Pyroclastic Flow Deposits (A-Sj: approx.16,000 years ago) off the northeast coast were expelled. However, the composition of both of the deposits were different than the ejecta from Sakurajima Volcano, but were similar to the magma of Aira Caldera. Therefore it has been assumed that both deposits erupted from the Wakamiki Caldera ( Fig. 1) or from somewhere nearby.

There were a number of large-scale pumice eruptions in the Younger Kitadake' s earliest stage. P14 (Sakurajima-Satsuma Tephra), which occurred 13,000 years ago, was the largest eruption ever at Sakurajima (the volume of pyroclastic material was approx. 11 km³). The volume of pyroclastic material within the tephra from other Sakurajima eruptions do not exceed 2 km³, so the P14 tephra is an order of magnitude larger than all other tephras. In addition, the base surge fanned out over an area 10 km from Sakurajima, and pyroclastic material was deposited not only over southern Kyushu but also offshore on Satsuma-Iojima and Takeshima ( Fig. 3)

The lava on the flank of Gongenyama is covered with pumice fall deposits (P11) and it may have appeared in the P12 eruption. The SiO2 content of the lava on the flank, including that of the Gongenyama lavas (Harutayama and Hikinohira lavas), is 66 to 68 wt.%; this lava is more felsic than the 64 to 65 wt.% of Younger Kitadake Lavas.

In all likelihood these lavas were ejected around the same time as the Gongenyama Lava. On the other hand, the majority of the Younger Kitadake Lavas is younger than 10,000 years old and the lava on the north flank is directly covered by the last tephra (P5) of the younger Kitadake.

This P5 eruption generated the Take Pyroclastic Flow from the summit and deposits from the eruption were widely distributed at the foot of the mountain. Non- to weakly welded pumice flow deposits are at the foot; above them from the upper flank are strongly welded deposits.

Activity at the Older Minamidake started around 4500 years ago. This time was marked by the deposition of Minamidake Volcanic Sand around the volcano body due to the volcanic ash falling from vulcanian-style eruptions (Sz-Mn; Fig. 2). Most of the main body of the Minamidake main stratovolcano was formed in this period and the 4000 year old Miyamoto Lava and 3000 year old Kannonzaki Lava flowed down to the foot of the mountain (Miki, 1999). The deposition of Minamidake Volcanic Sand continued up until around 1600 years ago.

The Younger Minamidake activity began with the Tenpyo-Hoji eruption (P4) from 764 to 766 AD, which resumed plinian-style eruptive activity that had not occurred during the Older Minamidake eruptions. As described in a later section, when comparing the same SiO2 content of ejecta, the chemical composition of all other ejecta after P4 are systematically different than the other compounds found in the Younger Kitadake/Older Minamidake ejecta. Considering the eruptions currently happening at Minamidake, this younger period of activity is continuing.

Figure 4 is a chart of the tephra eruption volume vs. age of plinian-style pyroclastic fall from Sakurajima Volcano. There is a long period of dormancy between the Older and Younger Kitadake. The eruption volume (P14) immediately following this inactivity is extraordibary larger at approximately 11 km³. However, examined broadly, the tephra eruption rate since that event is maintaining a constant trend. Especially during the recent 500 years since P3, it is clear that this a very active period for tephra discharge by plinian-style eruptions.

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