Morro Rock

Morro Rock is a 581-foot (177 m)volcanic plug located just offshore from Morro Bay, California, at the entrance to Morro Bay Harbor. A causeway connects it with the shore, effectively 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.

History

Geologically, 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 (remnant volcanic necks of volcanoes), 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 (hornblende), biotite, pyroxene (augite), quartz, and glass; phenocrysts of plagioclase, amphibole and often quartz. Its silica (SiO2) 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 (27-23 Ma) Mono Rock-Islay Hill Complex. The complex lies east of the San Gregorio-San Simeon-Hosgri fault (SG-SS-H 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 (90-100 mi) 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.

Morro Bay, California

Morro Bay is a waterfront city in San Luis Obispo County, California. As of the 2010 census, the city population was 10,234, down from 10,350 at the 2000 census.

History

The prehistory of Morro Bay relates to Chumash settlement, particularly near the mouth of Morro Creek. At least as early as the Millingstone Horizon thousands of years before present, there was an extensive settlement along the banks and terraces above Morro Creek.

The first European land exploration of Alta California, the Spanish Portola expedition, came down Los Osos Valley and camped near today's Morro Bay on September 8, 1769. Franciscan missionary and expedition member Juan Crespi noted in his diary that "we saw a great rock in the form of a round morro".

Morro Rock later gave its name to the town. The descriptive term morro is common to the Spanish, Portuguese and Italian languages, and the word is part of many place names where there is a distinctive and prominent rock formation. Note that the similar Spanish descriptive word "moro" indicates a bluish color rather than a shape.

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