An Introductory Review of Complex Rheology of Magma for Rheologists

Abstract

The rheology of magma is complex and similar to that of polymers and suspensions. Most magmas are partial melts of natural rocks, consisting of silicate melts, crystals, and bubbles. A silicate melt has the structure of a 3D network of SiO₄ tetrahedra whose rheology is approximated by a Maxwell fluid. The rheology of silicate melts depends on temperature, composition, and strain rate. Sufficiently large stress on the silicate melt causes rapid strain before structural relaxation, resulting in brittle fragmentation. The crystals and bubbles provide suspension characteristics to the whole magma. Crystals with a volume fraction close to the maximum packing fraction dramatically increase the effective viscosity of the magma. In contrast, bubbles increase and decrease the effective viscosity depending on strain rate. The effective viscosity of magma varies by orders of magnitude owing to various factors such as temperature, composition, and crystal fractions. Such wide-ranging magma rheology characterizes volcanic eruption styles. Low-viscosity magma can flow like a liquid underground and over the earth&spos;s surface as lava flows. By contrast, solid-like viscous magma fragments into small pieces in a brittle manner, causing explosive eruptions. Understanding the rheology of magma is crucial, as it significantly shapes our understanding of volcanic eruptions.