Using thermal remanent magnetisation (TRM) to distinguish block and ash flow and debris flow deposits, and to estimate their emplacement temperature: 1991–1995 lava dome eruption at Mt. Unzen Volcano, Japan

Abstract

The 1991–1995 Mt. Unzen eruption (Kyushu, Japan) produced 13 lava domes, approximately 9400 block and ash pyroclastic flows (BAF) resulting from lava dome collapse events and syn- and post-dome collapse debris flow (DF) events. In the field, it can be very difficult to distinguish from field facies characteristics which deposits are primary hot BAF, cold BAF or rock avalanche, or secondary DF deposits. In this study we use a combination of field observations and thermal remanent magnetisation (TRM) analysis of juvenile, lava dome derived clasts from seven deposits of the 1991–1995 Mt. Unzen eruption in order to distinguish between primary BAF deposits and secondary DF deposits and to determine their emplacement temperature.Four major TRM patterns were identified: (1) Type I: clasts with a single magnetic component oriented parallel to the Earth's magnetic field at time and site of emplacement. This indicates that these deposits were deposited at very high temperature, between the Curie temperature of magnetite (~540°C) and the glass transition temperature of the lava dome (~745°C). These clasts are found in high temperature BAF deposits. (2) Type II: clasts with two magnetic components of magnetisation. The lower temperature magnetic components are parallel to the Earth's magnetic field at time of the Unzen eruption. Temperature estimations for these deposits can range from 80 to 540°C. We found this paleomagnetic behaviour in moderate temperature BAF or warm DF deposits. (3) Type III: clasts with three magnetic components, with a lower temperature component oriented parallel to the Earth's magnetic field at Unzen. The individual clast temperatures estimated for this kind of deposit are usually less than 300°C. We interpret this paleomagnetic behaviour as the effect of different thermal events during their emplacement history. There are several interpretations for this paleomagnetic behaviour including remobilisation of moderate temperature BAF, warm DF or syn-eruptive granular flows. (4) Type IV: clasts with single or two magnetic components that are randomly oriented, suggesting an emplacement at ambient temperature. These clasts are found in DF and syn-eruptive rock avalanche deposits containing high proportion of well-rounded clasts and non-carbonised vegetation.We demonstrate that TRM analysis, associated with field observations, can be a viable method for determining the emplacement temperature of hot BAF and cold DF deposits, characterised by similar componentry and sedimentological facies. This has implications for hazard assessment of older, dormant volcanic systems, to reconstruct the eruptive history.