PYROCLASTIC FLOWS

Pyroclastic Flows



Abstract

A bomb-rich basaltic pyroclastic flow deposit, the Izumikawa pyroclastic flow deposit, occurs at the northeastern foot of Nakadake Volcano, which is the only active central cone of Aso caldera, southwestern Japan. The pyroclastic flow deposit covers a fan-shaped area of about 1.9 km2, and the bulk volume is estimated at 9.4 × 106 m3. The deposit is poorly sorted, and consists of subangular faceted clasts and spherical cauliflower bombs set in a sandy non-cohesive matrix. The deposit forms two different facies: a black reversely graded lower unit; and a reddish-gray reversely graded upper unit. The cauliflower bombs, which have slightly vesiculated crusts (1.0-1.5 cm thick) and denser interiors, are more abundant in the lower unit than in the upper unit. The presence of cauliflower bombs suggests that the Izumikawa pyroclastic flow was generated by an explosion at the source lava lake or conduit, which was filled with mixture of solidified and molten lavas. We propose the term “bomb-and-ash flow deposit” to describe this type of deposit. The age of the Izumikawa pyroclastic flow deposit was estimated at ca. 19 cal ka, based on a 14C age obtained from charred wood fragments in the deposit. Although all recent activity of Nakadake has been ash eruptions, strombolian eruptions and phreatomagmatic explosions, the Izumikawa pyroclastic flow deposit underscores the hazard of bomb-rich pyroclastic flows that rush down the flanks of the volcano.

Introduction

Pyroclastic flows constitute one of the most lethal volcanic phenomena and can severely damage life and property. Small-volume pyroclastic flows, sometimes called nuée ardentes or pyroclastic avalanches, have occurred at several volcanoes in recent decades. They are generated mostly by gravitational or explosive dome collapse, and occasionally by column collapse. Historic examples of gravitational collapse pyroclastic flows include: Merapi, Indonesia (Boudon et al., 1993), Mount St. Helens (Mellors et al., 1988), Unzen, Japan (Miyabuchi, 1999) and Colima, Mexico (Rodríguez-Elizarrarás et al., 1991 and Saucedo et al., 2004). Pyroclastic flows due to explosive lava dome or lava-flow collapse have occurred at Mt. Pelée, Martinique (Fisher and Heiken, 1982), and Santiaguito, Guatemala (Rose et al., 1977). Pyroclastic flows caused by dome collapse, explosive or non-explosive, occurred at Redoubt, Alaska in 1989-1990 (Gardner et al., 1994), and Soufriere Hills, Montserrat in 1995-1999 (Cole et al., 1998 and Calder et al., 1999). Examples of nuée ardentes, pyroclastic avalanches and flows formed by eruption column collapse were reported at Mayon, Philippines in 1968 (Moore and Melson, 1969), Ngauruhoe, New Zealand in 1975 (Nairn and Self, 1978), Colima in 1999 (Saucedo et al., 2002) and Soufriere Hills, Montserrat (Cole et al., 2002 and Druitt et al., 2002). These examples are mostly restricted to dacite or andesite. However, basaltic pyroclastic flows and their deposits are less common.

We focus here on a basaltic pyroclastic flow deposit at the northeastern foot of Nakadake, Aso Volcano, southwestern Japan. We describe the distribution, textures, grain-size characteristics and petrography of the deposit, and discuss possible eruptive mechanisms that formed the deposit. This paper provides further understanding of mafic eruptions that produce pyroclastic ...