Numerical simulations of electrokinetic potentials were performed to investigate the mechanisms responsible for the generation of the W-shaped self-potential (SP) profiles observed on volcanoes. The primary cause of the SP pattern is shown to be the drag current associated with downward liquid flows in the unsaturated and underlying saturated fresh-water layers within the volcanic body, which make the shallow and deep regions low and high in electric potential respectively. If a conductive structure extending to deep levels is present below the summit area, SP around the summit is substantially increased, resulting in the characteristic W-shaped SP profile.