Tomographic image of the ratio between P velocity and S velocity in a subduction zone beneath Nicaragua. The highest ratio (darkest red area) indicates the presence of melts rising from near the subducted oceanic plate. The sinking plate is indicated by earthquakes extending to depths of 175 km. Fluids released from the sinking plate lower the melting temperature of material in the mantle wedge, resulting in partial melting and ascent of magmas to the volcanic arc. (Image from E.M. Syracuse, G.A. Abers, K.M. Fischer, L.G. MacKenzie, C.A. Rychert, J.M. Protti, V. Gonzalez, and W. Strauch, 2008. Seismic tomography and earthquake locations in the Nicaraguan and Costa Rican upper mantle, Geochemistry, Geophysics, Geosystems, 9, Q07S08, doi:10.1029/2008GC001963.)