Smectites, a group of hydrated phyllosilicate minerals, are often a main component of sediments entering a subduction zone. They are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component. Smectite-rich strata are structurally weak, retain water, and are highly porous. A review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite-rich horizons. One consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted. At deeper structural levels smectite is transformed to illite, a mechanically stronger mineral. Estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces. I speculate that the smectite-illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity.