Observations show that, when a viscous fluid flows vertically through a bed of settled flocculated aggregates, particles are emerged from locations on the upper surface of the bed, as a volcano process, and hollow tubes are created in the bed though the latter remains basically stable. A theoretical model is presented to explain this odd phenomenon from the onset of particle motion to the development of the tubular cavities and the particles' deposits around them. Analytical and numerical calculations are performed to show fluid streamlines and particle trajectories. (C) 2008 Elsevier B.V. All rights reserved.