Orientation of the Eruption Fissures Controlled by a Shallow Magma Chamber in Miyakejima

Nobuo Geshi,Teruki Oikawa

doi:10.3389/feart.2016.00099

Abstract

Orientation of the eruption fissures and composition of the lavas of the Miyakejima volcano indicate tectonic influence of a shallow magma chamber on the distribution of eruption fissures. We examined the distributions and magmatic compositions of 23 fissures that formed within the last 2800 years, based on a field survey and a new dataset of 14C ages. The dominant orientation of the eruption fissures in the central portion of the volcano was found to be NE-SW, which is perpendicular to the direction of regional maximum horizontal compressive stress (σHmax). Magmas that show evidences of magma mixing between basaltic and andesitic magmas erupted mainly from the eruption fissures with a higher offset angle from the regional σHmax direction. The presence of a shallow dike-shaped magma chamber controls the distribution of the eruption fissures. The injection of basaltic magma into the shallow andesitic magma chamber caused the temporal rise of internal magmatic pressure in the shallow magma chamber. Dikes extending from the andesitic magma chamber intrude along the local compressive stress field which is generated by the internal excess pressure of the andesitic magma chamber. As the result, the eruption fissures trend parallel to the elongation direction of the shallow magma chamber. Injection of basaltic magma into the shallow andesitic magma chamber caused the magma mixing. Some basaltic dikes from the deep-seated magma chamber reach the ground surface without intersection with the andesitic magma chamber. The patterns of the eruption fissures can be modified in the future as was observed in the case of the destruction of the shallow magma chamber during the 2000 AD eruption.

Highlights

Propagation of a dike is controlled by the stress field in its host rock; the orientation of a dike is basically parallel to the maximum compressive axis (σ1) and perpendicular to the minimum compressive axis (σ3)
The distribution of dikes and eruption fissures in volcanoes display radial, circumferential, and curvature patterns reflecting the local stress in the volcanic edifice (e.g., Chadwick and Howard, 1991)
Combined analyses of the distribution patterns of the eruption fissures and the magmatic composition of the lavas within the last ∼2800 years reveal that a shallow andesitic magma chamber beneath the summit of Miyakejima volcano controls the orientation of eruption fissures

Summary

INTRODUCTION

Propagation of a dike is controlled by the stress field in its host rock; the orientation of a dike is basically parallel to the maximum compressive axis (σ1) and perpendicular to the minimum compressive axis (σ3). The distribution of dikes and eruption fissures in volcanoes display radial, circumferential, and curvature patterns reflecting the local stress in the volcanic edifice (e.g., Chadwick and Howard, 1991). The distribution of the flank fissure eruptions in Miyakejima Island is indicative of the competitive processes of the regional tectonic stress and the local stress generated by the activity of a magma plumbing system beneath the volcano (Nakamura, 1977). The development of a rift zone perpendicular to the regional horizontal compressive axis (σHmax) provides a good example of the role of a local stress field within the volcanic edifice on the distribution of the eruption fissures. A combination of the inferences made from geological and petrological investigations and the observations from a recent caldera collapse event reveal the influence of shallow magma chambers on the development of eruption fissures. We examined the distribution patterns of recent eruption fissures, with a new dataset of 14C ages and petrological analysis to reveal the influence of a shallow magma chamber

BACKGROUND

DISCUSSION

CONCLUSIONS