Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing-induced crystallization

Lava solidification is controlled by two mechanisms: external cooling and g as exsolution, the latter inducing crystallization due to increasing liquid us temperature. The andesite lava dome of the Soufriere Hills Volcano, Mont serrat, is an extrusion dominated by crystallization caused by gas exsoluti on where cooling is unimportant in controlling emplacement. In the magma ch amber the magma has an estimated viscosity of 7 x 10(6) Pa s. During ascent , gas exsolution caused the magma to extrude in a highly crystalline state, with only 5-15% residual melt, viscosities in the range 10(13)-10(14) Pa s and mechanical strength > 1 MPa. Deformation can be heterogeneous with ext rusion along shear zones. Rheological stiffening in the upper conduit also causes large overpressures, shallow seismicity, and cyclic patterns of dome extrusion. Gas-rich porphyritic andesites tend to be the least mobile kind of lava, because transition from magma into hot crystalline material was r eached during ascent.