火山砕屑物の沈降速度とその意義

Volcaniclastics falling velocity is estimated by measurements with settling tube．Samples are obtained from the Pliocene Ohtani and Souri volcanic ash layers in the Tokai Group．ln terms of content，volcanic glass and pumice predominate in these layers．In the Ohtani volcanic ash，mean specific gravity of volcanic glass is 2.25 which is higher than that of pumice （1.15）．In the Souri volcanic ash，mean specific gravity of volcanic glass and pumice suggests 2.25 and 1.50 respectively．Particle falling velocity of the Ohtani and Souri volcanic ash can be described by following formulas．
Cd＝26＋24／Re，W＝[−9η＋｛81η^<2>＋156pa^<3>（σ−p）g｝^<0.5>]／39pa
Cd＝3.4＋24／Re，W＝[−18η＋{324η^<2>＋81.6pa^<3>（σ−p）g｝^<0.5>］／10.2pa
whereσand p are the particle and fluid densities respectively，g is the acceleration due to gravity，ηis fluid dynamic viscosity，Re is grain Reynolds number，a is the particle radius，Cd is non-dimensional drag coefncient，and W is the falling velocity．These formulas mean the transition to turbulence is promoted at
a lower Reynolds number than for the corresponding smooth spherical particles．More poorly sorted bedforms can be explained by these formulas，which suggest that more varied grains in diameter can be deposited under a small range of falling velocity．