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.