Since 1950 six expeditions from the U. S. Navy Electronics Laboratory and Scripps Institution of Oceanography have added detailed information to the marine geology of the Hawaiian area.
The Hawaiian Islands are peaks on a great ridge which is flanked, in the southern part, by the Hawaiian Deep beyond which is the Hawaiian Arch. The Deep is about 400 fathoms deeper on the east side. Echo-sounder profiles across the Arch on the east side indicate that it is faulted and more asymmetrical than at first supposed (it is steeper to the west) and has a width of about 200 nautical miles and a relief above the Deep of 300–500 fathoms. Downslope northeast of Oahu the normal structure is interrupted by linear volcanics along the same trends as the Honolulu Series formed by Pleistocene and Recent volcanism on Oahu. The Hawaiian Deep and Arch continue to the northwest past Oahu. Off Molokai and Hawaii the Arch has been faulted.
The sediments off Oahu are mixtures of volcanic sand and silt and planktonic Foraminifera; the latter are dominant down to about 2300 fathoms where the sediment is a clayey silt with relatively large amounts of volcanic sand. The sedimentation off Oahu is apparently controlled by submarine canyons which funnel the material to form delta-like features.
The structure around the southern end of the Hawaiian Islands is due to the response of the earth's crust to the great load of the Hawaiian Ridge; this response is thought to be due mainly to elastic downbowing, but actual foundation failure cannot be excluded from consideration.