Stratigraphy, grain-size and component characteristics of the 2011 Shinmoedake eruption deposits, Kirishima Volcano, Japan

Abstract

The 2011 eruption of Shinmoedake Volcano, part of Kirishima Volcanic Complex in southern Kyushu, southwestern Japan was characterized by a significant change in eruption style from subplinian eruptions to lava effusion in the summit crater, and subsequent vulcanian eruptions. The stratigraphy, distribution and textures of fallout tephra deposits reveal the character and sequence of the eruption. The tephra-fall deposits distributed southeast of the volcano are divided into five units based on the eruptive events. Unit 1 is a lithic-rich fine ash-fall deposited on 19 January 2011. Unit 2 is a very well to well sorted pumice-fall deposit from the evening of January 26 to early morning of January 27, and is the main product of the 2011 eruption. The unit-2 deposit was dispersed throughout an area extending more than 20km SE of the source crater. Unit 3 comprises tephra-fall deposits related to the January 27 15h 41min explosion, and is subdivided into lower (3L) and upper (3U) parts. Unit 3L is a lithic-rich well-sorted coarse ash-fall, deposited during the initial stage of the January 27 15h 41min eruption, whereas the unit 3U is composed mainly of coarse-grained pumiceous lapilli. Unit 4 is a fine ash-fall deposited on January 28–29, and consists mostly of fresh lava fragments and crystal grains. Unit 5 is a product of the largest vulcanian eruption, on March 13. Unit-5 tephra is a lithic-rich medium to coarse ash-fall deposit. Furthermore, the 31 August 2011 ash-fall deposit extending 19km southwest of the Shinmoedake crater is fine grained and contains abundant lava fragments. Temporal variations in grain size and components of the 2011 eruption deposits reveal the eruption sequence and the conditions of the crater, conduit and magma chamber.