The western Pacific region has been refrigerated by the subducting cold oceanic plates since 450 Ma. However, the region is also characterized by the presence of many oceanic microplates less than 1300 km across, as well as active magmatism; the Philippine Sea plate is representative. We have compiled and examined petrochemical characters of drilled basalts of DSDP from the Philippine Sea plate, and conclude that the source mantle for oceanic basalts is rich in water ca. 0.2 wt.%, and is 50-60 degrees C lower than that for MORB. The extensive melting is due to the high water content in the source mantle. It is well known that some marginal basins apparently have greater depths than the major oceans. We calculated the age-depth correlation based on a model of transient half-space cooling at given parameters of temperatures of mantle and surface, 1280 and 0 degrees C, and the thermal diffusivity, 1 mm (super 2) s (super -1) . The calculation shows the correlation of age-residual depth from a mid-oceanic ridge is 367 age(my) for the Philippine Sea, consistent with the bathymetric data. Moreover, the mid-oceanic ridge may be relatively deep because this region is underlain by the cooler mantle. Addition of water to the mantle peridotite lowers the solidus temperature and viscosity. Melting experiments of hydrous peridotite show that addition of 0.2 wt.% H (sub 2) O content lowers the solidus temperature by 150 degrees C. As a result, the mantle under the region may practically correspond to a ca. 90 degrees C hotter mantle than normal MORB-source mantle in terms of magmatism and rheology. Numerical simulation for a hotter mantle suggests that many small plates should be formed because of extensive heat release by active magmatism, consistent with many microplates in this region. The presence of many oceanic microplates may be analogous to Archean plate tectonics, characterized by a hotter mantle.