that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system "filter" resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.

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This article presents new field, geochronological, and geochemical data for the Late Miocene Oki-dozen dike swarm (ODS), southwest Japan. This swarm is part of a volcanic suite comprising mafic and silicic dikes, sills, and pyroclastic cones from which we obtained structural measurements at a various genetic orders and scales. The mafic magmas generated three dike swarms with dikes oriented to NW-SE, N-S, and NE-SW. In comparison, the silicic intrusions do not have a preferred orientation but instead appear to radiate from the center of the volcanic suite. Comparison of the maximum thickness of 37 dikes with SiO 2 content (wt%) yielded a critical thickness (T cr) value of T cr = 0.2 × (SiO 2 − 40). These data indicate that the orientations of dikes were controlled by the magnitude of dike tip pressure and magma overpressure, both of which positively correlate with SiO 2 concentrations. The silicic units yield estimated pressures (up to 15-60 MPa) that are large enough to have counteracted the regional stress field, whereas the mafic dike swarm only yielded lower pressures. This result suggests that comparative analysis at a range of scales is essential for the accurate determination on the tectonic stress field by igneous rocks.

We have undertaken a detailed field, petro logical, geochronological, and modeling study of Esan volcanic complex, northern Japan , in order to establish its eruptive history over the past 50,000 yr. The Esan volcanic complex includes seven lava domes, which are endogenous domes developed without basal edifi ces. Each dome was produced by intermediate to silicic magmas with 57-67 wt% SiO 2 that were erupted in magnitude 4-5 eruptions. Five explosively erupted pyroclastic units cover the aprons of these lava domes. The spatial and temporal relationships of the domes and the pyroclastic units have been determined in order to constrain the eruptive sequence of the Esan volcanic complex. A simple elastic model, assuming a hydraulic connection state, was utilized to study variations in long-term magma discharge at the Esan volcanic complex, which have been attributed to changes in magma storage conditions. The stepwise change in magma discharge with time, which is 4.2-5.3 × 10 4 m 3 /yr, is interpreted to refl ect a change between magma chamber radius (R c ) and magma depth (H c ) caused by an upward migration of magma. Assuming a continuous input of magma from a deep source at a constant supply rate, the magma storage system that existed during the early stage of the Esan volcanic complex has been renewed, which will result in a shorter recurrence interval between eruptions rather than a long period of dormancy (22,400 yr). Given this fi nding, and the fact that the most recent eruption occurred 9000 yr ago, there is currently a risk of a large eruption at the Esan volcanic complex.