The questions of how and where abyssal hills are formed were investigated by means of a detailed areal survey of two north-south trending abyssal hills in the northeast Pacific (vicinity 32° 25′N, 125° 45′W) using an accurately navigated, (±15m) near-bottom geophysical instrument package. The western hill is an elongate shield volcano, and the eastern hill is a block fault (horst) structure. Both hills are approximately 40 km by 10 km and have several hundred meters of relief. The western hill was formed by volcanic processes about 30 m.y. ago at the crest of the East Pacific Rise. Episodes of faulting initiated after accumulation of tens of meters of pelagic sediments, downdropped the central valley and created the horst. At the same time lava flows covered wide areas of the sediment surface. Heat-flow measurements show that the area may be volcanically active.
The proposed mode of formation of the western volcanic hill, and other hills formed at the crest of a fast spreading rise, is by accumulation of lava flows and small, conical volcanic knobs. These knobs are less than a kilometer in diameter and tens of meters high. Abyssal hills, like the horst, that formed in other areas of the northeast Pacific may have been caused by faulting of the oceanic crust as it was deformed during a change in sea-floor spreading direction. Because of a closely spaced, two-dimensional structural grain of weakness that is frozen in the oceanic crust at a rise crest, both the younger and older topography have the same strike.