EFFECTS OF ERUPTION HISTORY AND COOLING RATE ON LAVA DOME GROWTH

To better understand the factors controlling the shapes of lava domes, laboratory simulations, measurements from active and prehistoric flow s and dimensional analysis were used to explore how effusion history a nd cooling rate affect the final geometry of a dome. Fifty experiments were conducted in which a fixed volume of polyethylene glycol wax was injected into a tank of cold sucrose solution, either as one continuo us event or as a series of shorter pulses separated by repose periods. When the wax cooling rates exceeded a critical minimum value, the dom e aspect ratios (height/diameter) increased steadily with erupted volu me over the course of a single experiment and the rate at which height increased with volume depended linearly on the time-averaged effusion rate. Thus the average effusion rate could be estimated from observat ions of how the dome shape changed with time. Our experimental re suit s and dimensional analyses were compared with several groups of natura l lava flows: the recently emplaced Mount St Helens and Soufriere dome s, which had been carefully monitored while active; three sets of preh istoric rhyolite domes that varied in eruptive style and shape; and tw o sets of Holocene domes with similar shapes, but different compositio ns. Geometric measurements suggest that dome morphology can be directl y correlated with effusion rate for domes of similar composition from the same locality, and that shape alone can be related to a dimensionl ess number comparing effusion rate and cooling rate. Extrapolation to the venusian 'pancake domes' suggests that they formed from relatively viscous lavas extruded either episodically or at average effusion rat es low enough to allow solidified surface crust to exert a dominating influence on the final morphology.