Abstract
To constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity, we have investigated a basaltic sub-Plinian eruption that produced Yufune-2 scoria in Fuji volcano 2200 years ago. We deduced magmatic decompression conditions from groundmass microlite textures, including decompression path (i.e. evolution in decompression rate) and approximate decompression rate, in order to relate them to eruption intensity. The overall decompression rate in each eruptive unit (from a to e) has a positive correlation with eruption intensity. The variation in decompression rate was enlarged in the final units, due to 1) variation in flow velocity across conduit and 2) part of the erupted magma in unit d experienced remarkably slow decompression resulting from decreased overpressure in the reservoir following the major eruption of unit b. Two factors had a major impact on eruption intensity. First, magma decompression rate determined the degree of gas-phase separation from ascending magma. The second factor is the conduit radius that, in combination with magma ascent rate, controlled the magma discharge rate.
