Applied Geology of So-called “Shirasu”, Non-welded Ignimbrite

Abstract

“Shirasu” is a local name of pumiceous sediments in Kagoshima Prefecture, Japan. It contains non-welded ignimbrite, their secondary deposited sediments (secondary shirasu) and pumice fall deposits and so on. But the usage of the word in such a broad sense has thrown the civil engineering into confusion. So we propose that shirasu must be restricted to non-welded ignimbrite. Typical shirasu in the vicinity of Kagoshima city is the Ito ignimbrite erupted out of the Aira caldera in 22, 000 y.b.p.
Shirasu covers the wide area, about 50%, of the Prefecture and forms extensive ignimbrite plateaus. Because of high permeability, surface water is so poor on the plateaus that farmers have to get water from very deep wells of about a hundred meter, or from springs under the marginal cliffs. While it has so high water content that the plateau plays a role of aquifer. Therefore water is supplied constantly to the river even in summer.
Landslides, often called “shirasu disasters”, frequently occur at the marginal cliffs in the rainy season. They include surfacial sliding of weathered shirasu and overlying the Satsuma pumice fall deposits (11, 000y. b. p.). City planning on the plateau should include provisions for mitigating potential slope stability and erosion problems.
Shirasu is composed of volcanic glass, mineral grains and pumice blocks. The chemical composition shows a high content of SiO₂ (70%) and a low one of Al₂O₃ (14%) and alkali oxides (8%). Various products as follows are manufactured using these physicochemical properties. Pumice is used as light aggregate for concrete. Dressed volcanic glass is applied as grinding powder for optical lenses and Braun tubes and processed into glass microballoon by rapid heating at about 1, 000°C. Zeolites and porous glasses such as vicor glass are also produced.