Hiroshi Machida

The stratigraphy, chronology and distribution of distal marker-tephras in and around Japan

Abstract This paper forms an overview of Japanese tephrochronology with emphasis on the role of distal tephras in Quaternary studies. Recent developments in tephra characterisation techniques and dating methods have provided fruitful results in the correlation and identification of many Japanese tephras. Since the middle 1970s, many distal tephras have been discovered occurring both in the Japan islands and the surrounding seas. This has resulted in the introduction, revision and refinement of regional stratigraphy, the application of tephra in determining the nature and effects of explosive eruptions, and new applications to many aspects of Quaternary studies. Ongoing tephra research is fed into tephra catalogue for the Japan region giving fundamental data for land–sea correlation and for establishment of chronostratigraphic framework for Japanese Quaternary studies.

Holocene explosive eruptions of Witori and Dakataua caldera volcanoes in West New Britain, Papua New Guinea

Abstract Witori and Dakataua caldera volcanoes have been very active in the middle to late Holocene. Using tephrochronology, this paper establishes the chronostratigraphy of these eruptions and their magnitude, and the frequency of explosive volcanism at Witori and Dakataua. After a long dormancy, Witori started explosive activity at ca. 5600 conventional radiocarbon years BP, producing in the next 4500 years five major tephra layers (W-K1 to W-K4, W-G) with VEIs of 5 to 6. After the W-G eruption at around 1200 BP, the activity decreased in magnitude but increased in frequency, with some eruptions forming central cones. The major eruption of Dakataua began with alternating ejections of phreatomagmatic ashfalls and plinian deposits followed by the cataclysmic eruption resulting in lithic-rich pyroclastic flows ca. 1100–1200 BP. The major tephra layers cover extensive areas in West New Britain due to their large volumes and the prevailing easterly winds, providing valuable time markers for establishing Holocene chronology. The largest eruption, the W-K2 event of ca. 3300 BP, shaped much of the present landscape, with an extensive area significantly devastated by tephra falls and pyroclastic flows. Obsidian and other artefacts buried by the tephras indicate that the area was repeatedly occupied. The major tephra events formed new coastal plains favourable for human occupation.

Volcanic glass found in Late Quaternary Chinese loess: A pointer for future studies?

Abstract In a preliminary investigation of Chinese loess from Shandong Peninsula, trace amounts of volcanic glass have been found in the top of Malan loess and in the top of paleosol layers at two sections. Chemical compositions of the glass shards are compared with those of widespread tephras from Mt Changbai and Japanese volcanoes. There was only enough glass to make tentative suggestions as to the source eruptions. The volcanic glass in the top of Malan loess has a similar chemical composition to that of the ca. 22,000-25,000 BP Aira-Tanzawa (AT) tephra erupted from southern Japan, and the glass in the top of one paleosol has a similar chemical composition to that of the ca. 75,000-95,000 BP Kikai-Tozurahara (K-Tz) tephra, also from southern Japan. These possible identifications are consistent with the stratigraphic positions of the glass within the loess. The results suggest that tephrochronology has potential for dating Quaternary deposits in north-eastern China.

Tephrochronology and Quaternary Studies in Japan

This paper gives an overview of the development of tephrochronology in Japan and discusses several problems regarding the Japanese Quaternary. Tephra from Quaternary stratovolcanoes in Japan is mainly of andesitic composition with subordinate amounts of basalt and rhyolite. In contrast, the eruptions causing the formation of calderas are of rhyodacitic magma and were characterised by extraordinarily violent explosions producing widespread tephras, all of which are important for interregional correlations.