A shallow-marine alkali-basalt tuff cone in the Middle Miocene Jinzai Formation, Izumo, SW Japan

The Jinzai basalt is an alkali-basalt tephra unit erupted on the Japan Sea side of SW Japan immediately after the opening of the Japan Sea ceased 15 Ma. The Jinzai basalt comprises a large volume of glassy alkali basalt clasts of phreatomagmatic origin, and is interpreted to have constituted a tuff cone. The tuff cone grew to a height of 100 m above the 50-to-150-m-deep submarine substrate, but the summit and a part of the flank were lost due to syn- and post-eruptive sliding, slumping and erosion. The restored tuff cone has a crater 400–500 m wide and 100 m high, and the cone rim reaches 1500 m in width, with bedding dipping outward at a maximum angle of 20–30°. The primary deposits include thinly bedded lapillistone to lapilli tuff and tuff, and thickly bedded lapilli tuff and tuff, which are thought to be correlated to a small-volume phreatomagmatic explosion and more voluminous one, respectively. The shallow marine Jinzai basalt tuff cone is essentially similar in constituents, internal structures, configuration, and size to other tuff cones built in shallow surface water. This tuff cone, however, appears dominated by phreatomagmatic products due to dynamic interaction of rapidly ascending magma and water-saturated unconsolidated sediments or vent-filling slurry of water and ejecta, and has been entirely altered through syn- and post-eruptive reactions of hot basalt glass and interstitial seawater.