Previous work on surface deformation associated with volcanic activity has revealed significant departures from ideal elastic and homogeneous behavior. To account for the highly heterogeneous and discontinuous mechanical behavior of volcanic edifices, a numerical model named Bloc was used to model surface deformation data. The basic assumption of this model is that the volcanic edifice behaves mechanically as an assemblage of rock blocks that can move in relation to one another. The well-documented eruption of Mount St. Helens in 1980 was used to test this model and to compare the fit between the observed and computed surface deformation. From the north-south cross section of the volcano, several two-dimensional models were developed to study the effect of various parameters (geometry, boundary conditions, coefficient of friction, etc.). The models were loaded with a step-by-step increasing internal pressure simulating the intrusion of magma within the volcano and with a horizontal acceleration simulating an earthquake. The deformation and failure patterns are highly dependent on the coefficient of friction on block boundaries. For the lower value of this coefficient (0.5), a good fit is found between the observed and computed surface displacements. The failure mode by gravitational sliding of the north flank is obtained with internal pressure loading alone as well as with internal pressure and an earthquake trigger. However, the second kind of loading gives a better fit with the particular deformation pattern observed. ¿American Geophysical Union 1987