Vertical and lateral propagation of radial dikes inferred from the flow-direction analysis of the radial dike swarm in Komochi Volcano, Central Japan

Abstract

Dikes with lateral and outward intrusion directions (named L-type dike) and dikes with vertical and upward intrusion directions (V-type dike) are recognized in a radial dike swarm of Komochi Volcano, Japan, by the combined analysis of the preferred orientations of deformed vesicles and anisotropy of magnetic susceptibility (AMS). Intrusion directions of magmas were examined for 41 dike outcrops. Among them, 19 dikes were classified as L-type dike and 15 dikes were as V-type. The L-type dikes distribute over the area of the dike swarm radiating from the central conduit, which is named Daikokuiwa stock, and the V-type dikes occur mainly in the peripheral portion of the volcano. The L-type dikes are characterized with evolved whole-rock compositions similar to those of rocks consisting of the central conduit, whereas the V-type dikes have less-fractionated composition and are enrich in mafic phenocryst as compared to the L-type dikes. The outward intrusion directions of the L-type dikes and their petrological similarity to the rock of the central conduit indicate that these dikes intruded from the shallow part of the central conduit where the magmas underwent fractional crystallization and degassing. The petrological characteristics of the V-type dikes suggest that the less-fractionated magma intruded directly from a magma chamber, in which mafic phenocryst crystals accumulated. The larger dike thickness and higher magnetic foliation of the V-type dike as compared to the L-type dike indicate higher magmatic overpressure, which was possibly result of the direct connection to the pressurized magma chamber and vertical-growth of the dikes with buoyant magma. Solidification of the central conduit is favorable for the accumulation of internal excess pressure in the magma chamber to break the wall rock. Fissure eruptions independent of the central conduit have occurred in many volcanoes and these fissure eruptions might be fed by the dikes directly propagated from the deeper magma chamber.

Introduction

Eruptions of stratovolcanoes are commonly divided into central summit and flank fissure eruptions. Summit eruptions are a consequence of the extrusion of magma through a stable central conduit beneath the summit. In many stratovolcanoes, eruptions have recurred from a fixed vent at the structural center of the volcano and the pile of eruptive materials forms a conical volcanic edifice around the vent. By contrast, flank eruptions are commonly observed as fissure eruptions fed by dikes. Most of the flank eruptions are fed by an ephemeral conduit with dike-like shape.

In many cases, it is believed that the feeder dikes of flank fissure eruptions propagate laterally from the central conduit. Rupturing of wall rock of a conduit or magma reservoir in the shallow depth leads to the formation of radial fissures through which the magma is supplied to the flank vents. This mechanism has been observed in Hawaiian volcanoes (Swanson et al., 1976, Decker, 1987), Mount Etna (McClelland et al., 1989), Stromboli (Acocella et al., 2006a), Vesuvio (Acocella et al., 2006b), Piton de la Fournaise (Michon et al., 2007, Urai et al., 2007), and many other volcanoes. However, some stratovolcanoes repeated flank fissure eruptions without an active central conduit suggesting that their feeder dikes are discrete from the central conduit system. In the case of Miyakejima, Japan, more than 10 fissure eruptions have repeatedly occurred without a major central eruption over the last 10 centuries (Tsukui et al., 2005). In Mount Fuji, more than 15 flank fissure eruptions occurred repeatedly without a central summit eruption during the last 2200 years (Miyaji, 1988, Yamamoto et al., 2005). In the case of Mount Etna in 1974, 2001 and 2002–2003, the eruption sequence and petrological characteristics of the flank ejecta indicate that the feeding systems are discrete from the central conduit system (Behncke and Neri, 2003, Acocella and Neri, 2003). In Mount Etna, Acocella and Neri (2003) named the flank fissure eruption with a feeding system isolated from the central conduit as “peripheral eruption”. In Island volcanoes, regional fissure eruptions were fed by “regional dike” independent on the magma system of central volcanoes which have magma reservoirs and local radial or concentric dike and sheet swarm in shallow depth (Gautneb and Gudmundsson, 1992, Gudmundsson, 1995, Gudmundsson, 2006).

These examples indicate that the feeding mechanism for the flank fissure eruptions can be divided into two types: eruptions fed from the ruptured central conduit and those fed directly from the deeper magma chamber. Variations in the feeding system for the flank fissure eruption may reflect the tectonic control on the growth of dikes in the volcano. Therefore, investigations on the propagation process of dike and migration of magmas inside the stratovolcanoes can provide crucial information on the development of a feeding system for the fissure eruptions. Direct and detailed observations of the active dikes are, however, difficult because they are hidden in the volcanic edifice. Instead, the radial dike swarms exposed in some dissected stratovolcanoes are fossils of the conduit system for the flank fissure eruptions, and we can directly examine the intrusion process from the structural and petrological characteristics of these dikes. In particular, the propagation direction of the dikes is one of the important observations for understanding the development of the feeding system for the fissure eruptions.

In this study, the intrusion directions of the radial dikes observed in the Middle Pleistocene Komochi Volcano in Central Japan are investigated from the view points of structural geology and petrology in order to obtain further insight into their intrusion mechanism controlling factor.