Rapid formation of river terraces in non-welded ignimbrite along the Hishida River, Kyushu, Japan

River terraces are well preserved in the middle to lower reaches of the Hishida River, that flows through the extensive ignimbrite field produced by the large-scale Ito eruption about 24,500 years ago from Aira caldera in southern Kyushu, Japan. The terraces are erosional and formed during dissection of the initial topography of the Ito ignimbrite. These occur within the upper non-welded zone of the Ito ignimbrite, about 20–60 m below the depositional surface and usually more than 20 m above the welded zone. Streams associated with terrace formation were ephemeral, because perennial streams are considered highly unlikely within the porous non-welded ignimbrite. All tephra layers on the original ignimbrite depositional surface also cover the terraces. This indicates that the formation of terraces preceded the accumulation of the oldest tephra, the age of which is about 23,000 years old. High erodibility of the non-welded Ito ignimbrite and absence of vegetation on the initial surface would have permitted a rapid dissection of the initial topography and the formation of the river terraces. Thus, it is interpreted that the river terraces were formed very rapidly in a period of far less than 1500 years immediately after the deposition of the Ito ignimbrite and before cover of new vegetation on the depositional surface.