9 Springer-Verlag 1993 Deformation of foamed rhyolites under internal and external stresses: an experimental investigation

Abstract. The style of magma eruption depends trongly on the character of melt degassing and foaming. De-pending on the kinetics of these processes the result can be either explosive or effusive volcanism. In this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been i vesti-gated in time series experiments (2min-24 h) followed by quenching the samples. The volumetric gas-mek ratio has been estimated through the density measurements of foamed samples. The variation of gas volume (per unit or rhyolite melt volume) with time may be described by superposition of two exponentials responsible for gas generation and gas release processes r pectively. An observed ifference in foaming style in this study is interpreted as the result of variations in initial contents of microlites that serve as bubble nucleation centers during devolatilization f the melts. Quantitatively the values of the gas generation rate constants (kg) are more than an order of magnitude higher in microlite-rich obsidian than in microlite-free obsidian. Possible origins of differences in the degassing style of natural magmas are discussed in the light of bubble nucleation kinetics in melts during foaming. In a complementary set of experiments he mechanical re-sponse of vesicular melt to external shear stress has been determined in a concentric ylinder viscometer. The re-sponse of vesicular melt to the pulse of shear deforma-tion depends on the volume fraction of bubbles. The ob-tained response function can be qualitatively described by a Burgers body model. The experimental shear stress response function for bubble-bearing melt has an over-shoot due to the strain-dependent rheology of a two-phase liquid with viscously deformable inclusions