Large seamounts occur on the outer half of the Mariana forearc. They represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material. The degree of metamorphism in this material depends on the amount of water available and the pressure-temperature regime of the forearc wedge. The major source for the water involved in the metamorphism is most likely the descending slab. Theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc. Zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the Mariana forearc. Thermal models indicate that higher grade greenschist facies should occur farther from the trench. Seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench. Uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the Mariana forearc.