Morro Rock (Salinan: Le'samo; Chumash: Lisamu'; Spanish: El Morro)[4][5][6] is a volcanic plug in Morro Bay, California, on the Pacific Coast at the entrance to Morro Bay harbor. A causeway connects it with the shore, making it a tied island. The rock is protected as the Morro Rock State Preserve.[7]
The 581-foot (177 m)[1] Morro Rock is one of 13 volcanic plugs (remnant necks of extinct volcanoes), lava domes, and sheetlike intrusions between Morro Bay on the north and Islay Hill on the south, all in San Luis Obispo County.[8]
It is composed mostly of dacite, an igneous, volcanic rock. It is a groundmass of plagioclase, with amphibole (hornblende), biotite, pyroxene (augite), quartz, and glass; it also includes phenocrysts of plagioclase. Its silicon dioxide (SiO2) content ranges from 63 to 69 percent.
The dacite volcanic plugs, lava domes, intrusive sheets, and felsitic rhyolite-dacite between Morro Rock and Islay Hill are part of the Morro Rock-Islay Hill Complex of the Oligocene epoch (from 27 to 23 million years ago). The complex lies east of the San Gregorio-San Simeon-Hosgri fault (the 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 Morro Rock-Islay Hill Complex was rotated 40 to 50 degrees, perhaps during late Miocene or early Pliocene time.[9]
The Salinan and Chumash tribes consider Morro Rock to be a sacred site. The Salinan name for Morro Rock is Le'samo and the Chumash name is Lisamu.[10]
The Chumash had an important nearby prehistoric settlement at least as early as the Millingstone Horizon (6500-2000 B.C.E.), and the village was near the mouth of Morro Creek, at the current site of Morro Bay High School. The right of the Salinan people to climb Morro Rock for their biannual solstice ceremonies has been established, in which they celebrate the time in legend when a hawk and a raven destroyed a two-headed serpent-monster Taliyekatapelta as he wrapped his body around the base of the rock.[11] The established Salinan right to climb the rock has been in legal dispute by the local Chumash tribe, which claims that Morro Rock is Chumash, not Salinan, territory.[12] The Chumash also believe that the rock is so sacred that it should never be climbed. Because of its fragility, it is illegal for the general public to climb it.[13]