Digitized outline of plagioclase crystals in a sample of lava from Taranaki

The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from the active volcano Egmont Volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had several components then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had a residence time of ~50 years in the magma chamber for a growth rate of 10-11 cm/sec. Subsequent lavas had slightly longer residence times (50-75 years), but the following series returned to 50 year residence times. The youngest magmas, from both Egmont Summit and the parasitic Fantham's peak, have the shortest residence times of ~30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the surrounding rocks. Crystal shapes and zonation suggest that crystallisation occurred in a bottle-shape magma chamber, and not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions then a delay of about 30 years can be expected between refilling and eruption.

Higgins, M.D., 1996, Crystal size distributions and other quantitative textural measurements in lavas and tuff from Mt Taranaki (Egmont volcano), New Zealand: Bulletin of Volcanology, v. 58, p. 194-204. https://doi.org/10.1007/s004450050135