Local, regional and global tomographic studies of mantle plumes and subducting slabs are reviewed. Applications of the well-established local and teleseismic tomography methods to subduction zones have resulted in clear images of subducting slabs and magma chambers in the upper mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with arc magmatism and back-arc spreading are related to deep processes, such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab. Evidence also shows that arc magma and slab dehydration may also contribute to the generation of various types of earthquakes in subduction zones. Most of the slab materials are stagnant in the mantle transition zone before finally collapsing down to the core-mantle boundary as a result of large gravitational instability from phase transitions. Because most hotspots are located in poorly instrumented continental and oceanic regions, 3-D crust and upper mantle structure is determined for only a few hotspots such as Iceland, Yellowstone and Eifel which are covered by seismic networks. Plume-like low-velocity anomalies are revealed in the upper mantle under those hotspots. Global tomographic studies of deep mantle plumes have just started, and more efforts are needed to image the conduits of the lower-mantle plumes. A thorough understanding of the seismic structure of mantle plumes and subducting slabs will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks, particularly in the oceans.
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