Morro Rock

• Landmark Historical Place
• Landmark & Historical Place

Morro Rock is a 581ft volcanic plug located just offshore from Morro Bay, California, at the entrance to Morro Bay Harbor. A causeway connects it with the shore, making it a tied island. The area surrounding the base of Morro Rock can be visited. The rock is protected as the Morro Rock State Preserve. Climbing on the rock or disturbing the bird life is forbidden by law.HistoryGeologically, Morro Rock, located in San Luis Obispo County, in the town of Morro Bay, California, is one of 13 volcanic, intrusive units that form volcanic plugs, lava domes, and sheetlike intrusions from Morro Rock southeast 29 km or 18 mi to Islay Hill. The plugs form the distinctive peaks along U.S. Highway 101 just south of San Luis Obispo and south of State Highway 1 from San Luis Obispo to Morro Bay. The rock is composed of dacite, a felsic extrusive rock, intermediate in composition between andesite and rhyolite, and is a member of the igneous family. Its lithology and composition is a groundmass of plagioclase with amphibole, biotite, pyroxene, quartz, and glass; phenocrysts of plagioclase, amphibole and often quartz. Its silica content can range from 63%-69%. The dacite volcanic plugs, lava domes, intrusive sheets, and felsitic rhyolite-dacite between Morro Rock and Islay Hill are part of the Oligocene Mono Rock-Islay Hill Complex. The complex lies east of the San Gregorio-San Simeon-Hosgri fault . This complex is one of three probable sources of the volcanic clasts within the sandstone and conglomerate of the Miguelito and Edna members of the Pismo Formation in the Point Sur area 145–160 km to the north and west of the SG-SS-H fault. Based on paleomagnetic signatures, the 27-23 Ma Morro Rock-Islay Hill Complex was rotated 40'-50',perhaps during late Miocene or early Pliocene time.