Transition from seamount chain to intraplate volcanic ridge at the East Pacific Rise

A number of large submarine intraplate volcanic ridges have been discovered throughout the South Pacific Basin, but their origins are enigmatic. Recent shipboard geophysical surveys reveal that the Sojourn Ridge, one of these large intraplate ridges, becomes a chain of discrete seamount volcanoes that intersects the ridge axis. This seamount chain exhibits several features that suggest that it is directly related to the Sojourn Ridge. The Sojourn Seamount Chain grows continuously in both volume and number of seamounts with distance from the spreading axis; several loci of recent volcanic activity along the chain are evident in the side-scan imagery, and a mantle Bouguer anomaly low underlies the entire length of the chain. This evidence provides new constraints on the origin of intraplate volcanic ridges. The continuation of the Sojourn Ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism, such as flexural stresses induced by the mass of the volcanic pile, as the probable mechanism for developing volcanic ridges and long seamount chains.