Late Tertiary date from the East Pacific Rise

A potassium-argon date from a basalt boulder dredged from a seamount near the crest of the northern extension of the East Pacific rise is 27±6 m.y. This note presents evidence for the association of Cobb seamount and its geologic age with the axial zone of the East Pacific rise. The theory of sea floor spreading by Hess and Dietz and the associated hypotheses of rise formation and magnetic anomalies [Vine and Matthews, 1963; Wilson, 1963a, b; and Vine and Wilson, 1965] would lead one to expect a date of 1 m.y. [Hess, 1965] or, according to the most recent analysis, 3.5 m.y. [Vine, 1966] for this portion of the East Pacific rise. Vacquier et al. [1961] lean toward a pre-Mesozoic age for the region immediately south of Figure 1, and Menard [1964] suggests pre-Tertiary volcanoes and the development of the East Pacific rise at the beginning of Tertiary time. The age of Cobb seamount is in conflict with these speculations.