GEOLOGY OF RICHARDSON ROCK, NORTHERN CHANNEL ISLANDS, SANTA BARBARA COUNTY, CALIFORNIA
Richardson Rock, 10 km west-northwest of San Miguel Island, represents the westernmost visible extension of the broad insular ridge beneath the Northern Channel Islands off southern California. The rock is composed of porphyritic dacite, and its structure and orientation suggest that it is the exposed summit of one of many dikes cutting the submerged insular shelf west of San Miguel Island.