The dynamics of expanding domes of isothermal lava are studied by treating
the lava as a viscoplastic material with the Herschel-Bulkley constitutive
law. Thin-layer theory is developed for radially symmetric extrusions onto
horizontal plates. This provides an evolution equation for the thickness of
the fluid that can be used to model expanding isothermal lava domes. Numer
ical and analytical solutions are derived that explore the effects of yield
stress, shear thinning and basal sliding on the dome evolution. The result
s are briefly compared with an experimental study. It is found that it is d
ifficult to unravel the combined effects of shear thinning and yield stress
; this may prove important to studies that attempt to infer yield stress fr
om morphology of flowing lava.