FACTORS INFLUENCING THE HEIGHT OF HAWAIIAN LAVA FOUNTAINS - IMPLICATIONS FOR THE USE OF FOUNTAIN HEIGHT AS AN INDICATOR OF MAGMA GAS CONTENT

The heights of lava fountains formed in Hawaiian-style eruptions are c ontrolled by magma gas content, volume flux and the amounts of lava re -entrainment and gas bubble coalescence. Theoretical models of lava fo untaining are used to analyse data on lava fountain height variations collected during the 1983-1986 Pu 'u 'O'o vent of Kilauea volcano, Haw aii. The results show that the variable fountain heights can be largel y explained by the impact of variations in volume flux and amount of l ava re-entrainment on erupting magmas with a constant gas content of s imilar to 0.32 wt.% H2O. However, the gas content of the magma apparen tly declined by similar to 0.05 wt.% during the last 10 episodes of th e eruption series and this decline is attributed to more extensive pre -eruption degassing due to a shallowing of the sub-vent feeder dike. I t is concluded that variations in lava fountain height cannot be simpl y interpreted as variations in gas content, as has previously been sug gested, but that fountain height can still be a useful guide to minimu m gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height.