Subduction zones are known to be segmented with clusters of volcanoes separated by gaps in volcanism. A mechanism proposed initially to explain the clustering of volcanoes, or “hot fingers”, along the Japan arc is arc-normal, small-scale convection facilitated by low viscosity in a hydrated mantle wedge. To test this hypothesis, we carried out full-wave ambient noise tomography to image the structure beneath the Cascadia arc, in the Pacific Northwest of the United States. The resulting model shows low-velocity bodies in the mantle beneath the arc and back-arc that are spatially correlated with the Cascadia volcano clusters. A more detailed study at the crustal level further delineates the magma plumbing system and the importance of deep crustal magma chambers in the segmentation of arc volcanism.