Isla Roca Partida, located southwest of Baja California, Mexico, is probably a remnant of the volcanic neck of a large composite volcano, the summit of which has been eroded by wave action. A broad insular shelf, with shelf break occurring at about 80 fathoms, indicates wave erosion during a past eustatic sea-level lowering; it also indicates that the volcano is Late Pleistocene or older in age. The island consists of trachyandesite, which is similar in chemical composition to the alkali basalt-sodic rhyolite associations found on the other three Islas Revillagigedo.

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