As magma ascends and decompresses, volatile exsolution not only produces bubbles, but increases the liquidus temperature of the residual melt, resulting in an undercooling that can trigger crystallisation. In volcanic systems of intermediate composition, late-stage crystallisation and vesiculation in the shallow conduit have been shown to exert a strong control on eruptive style. These processes may be similarly important during subsurface and surface transport of basaltic melts. In recent experiments we demonstrated that the lag between degassing and crystallisation is sufficiently short that crystallisation as a consequence of degassing can be expected to occur in the conduit, depending on ascent rates. Up to 35% volume crystals were observed to grow as a result of the degassing of <1 wt% water. Degassing-induced crystallisation therefore has the potential to rapidly and profoundly change magma rheology before and during eruption, and so have a strong influence on the eruptive style.
The effects of degassing-induced crystallisation on rheology depend on crystal fraction, morphology and size distribution. Timescales of rheology changes also depend on crystal growth rates. Here we report on experiments designed to quantify these characteristics. We use a microscope with a heated stage to directly observe crystallisation events and record crystal growth at temperatures up to 1300 °C. Experiments are conducted on quenched (i.e. with near-eruptive volatile content) samples from Mt. Etna, Sicily, and Mauna Loa, Hawaii, and recorded with time lapse imaging. From these images, crystal growth rates as a result of degassing are measured, and the crystal contents, morphologies and size distributions at different stages of degassing determined. The undercooling experienced by the samples as a result of degassing can be estimated from the crystal morphology. Crystal contents on eruption are much higher at Etna (~30%) than Hawaii (~2%), meaning the effects of degassing on samples with radically different initial textures can be observed. Comparing textures produced during degassing with those produced during cooling at different rates allows assessment of the contribution of degassing to textural evolution of the lava, and hence could provide a means of estimating the effect of degassing on magma rheology. This work has implications for the modelling of magma flow in conduits, and of the flow of lava after eruption.