Piercement structures such as mud volcanoes, hydrothermal vent
complexes, pockmarks and kimberlite pipes, form during the release of
pressurized fluids. The goal of this work is to predict under which
conditions piercement structures form from the insights gained by sand
box experiments injecting compressed air through an inlet of width w at
the base of a bed of glass beads of height h. At an imposed critical
velocity v(f), a fluidized zone consisting of a diverging cone-like
structure formed with morphological similarities to those observed in
nature. Dimensional analysis showed that v(f) is correlated to the ratio
of h over w. In addition, we derived an analytical model for v(f) which
is compared to the experimental data. The model consists of a force
balance between the weight and the seepage forces imparted to the bed by
the flowing gas. The analytic model reproduces the observed correlation
between v(f) and h/w, although a slight underestimate was obtained. The
results suggest that the gas-particle seepage force is the main
triggering factor for fluidization and that the commonly used proxy,
which the fluid pressure must equal or exceed the lithostatic weight,
needs to be reconsidered. By combining the experiments and the model, we
derived critical pressure estimates which were employed to a variety of
geological environments. Comparing the estimated and measured pressures
prior to the Lusi mud volcano shows that the presented model
overestimates the critical pressures. The model paves the way for
further investigations of the critical conditions for fluidization in
Earth systems.