Transition in eruption style during the 2011 eruption and subsequent activity of Shinmoe-dake, Japan; Implications from a steady conduit flow model.

Abstract:

Quantitative discussion regarding the state within the conduit and the magma chamber is important in considering subsequent eruption scenarios, since it is known that the mechanism and efficiency of gas escape from the conduit control the eruption style. The 2011 eruption of Shinmoe-dake is a suitable example for a quantitative examination using physical models of magma flow in a conduit. Mount Shinmoe-dake, in the Kirishima volcanic group (southwest, Japan), resumed its activity with a small phreatomagmatic explosion on January 19, 2011. The eruption style then underwent a series of sub-plinian explosions, shifted to lava extrusion that half-filled the summit crater, and to subsequent magmatic/phreatomagmatic activity including several vigorous vulcanian explosions that lasted until September 2011. Among this eruption sequence, we focused on the transition from the explosive (sub-plinian) stage to the effusive (lava extrusion) stage that apparently ceased after four days. To examine the conditions in the conduit and the magma chamber, numerical code is devised, based on the one-dimensional steady flow model of Kozono and Koyaguchi (2010), who modeled a dome-forming eruption. We systematically searched for a condition under which the magma would not be fragmented, keeping the initial volatile content in the magma chamber constant. We found that high magma permeability allowing the vertical gas escape in the conduit and/or considerable amount of lateral gas escape through the conduit wall were needed for the eruption for the eruption to effusive. The termination of lava extrusion seemed to have been brought about by successive reduction of the chamber pressure in the course of the explosive phase. We based on the above conduit flow model, estimated the amount of the pressure reduction until the termination of lava extrusion as 30 MPa. Applying a Mogi model, the total volume of the modeled magma chamber was estimated as 10 km³.