Seismic refraction data from 18 traverses along the Hawaiian Ridge from Kauai to Midway Island are presented in this paper. The seismic velocities of the mantle vary from 7.8 km/sec to 8.8 km/sec. The velocity of the lower layer of the crust tends to cluster around 6.5 km/sec. The velocities of the upper crustal layers vary from 4.0 to 5.7 km/sec without tendency to cluster around any value. The depth to the mantle along the Leeward Islands is usually between 10 and 15 km. The mantle is deeper (18 km) near the major islands.
From the data, inference is drawn that the area of subsidence was smaller for older volcanic islands than for the newer ones. An hypothesis based on fluid dynamics of inverted densities is proposed to account for this.