A series of seismic refraction profiles has been made across a flat bank at Gardner Pinnacles (a pair of volcanic islets on the western Hawaiian Ridge) down the side of the ridge and across the adjacent deep to the floor of the Pacific basin. The ridge is composed principally of material with velocities typical of volcanic islands. The high-velocity oceanic crust, found in the oceanic areas adjacent, extends beneath the ridge and up into the center of the rise. The total crustal section is thickened and the Mohorovičić discontinuity depressed beneath the deep as well as beneath the ridge.
The smooth “archipelagic apron” on the north side of the ridge has at most 20 meters of sediment over a layer with a velocity of 3 km/sec, which could be volcanic and is definitely of higher velocity than normal oceanic sediments.