MORRO ROCK, CA
FIELD TRIP STOP - MORRO BAY STATE PARK AND NATURAL PRESERVE (An Oligocene Volcanic Plug)
LOCATION: Morro Rock is located in the town of Morro Bay, CA which is found north of Pismo Beach and south of San Simeon.
GEOLOGIC FEATURES: Volcanic Plug; Dacite; Phenocrysts; Porphyritic Texture; Sheeting
DESCRIPTION: Morro Rock is the northernmost volcano in a chain of volcanoes (“The Nine Sisters”) that are 27 myo (Tertiary: Oligocene) and extend from Morro Bay southeast to San Luis Obispo. It rises about 576 ft above the ocean surface.
Morro Rock is a volcanic plug formed when lava hardens in a volcanic vent. Differential erosion of softer volcanic rock results in exposure of the more resistant domal shaped plug. Part of the rock was quarried between about 1889 and 1969 to aid in the creation of breakwaters and the construction of a causeway between the Rock and the mainland. Weathering and earthquake activity continue to break down the rock.
The volcanic plug is composed of Dacite, an extrusive gray-colored igneous rock composed largely of Plagioclase Feldspar with lesser amounts of Hornblende (Amphibole), Biotite (Mica), Augite (Pyroxene) and Quartz. Phenocrysts of Plagioclase are present and distinctive throughout. Chemically, a dacite is somewhat intermediate in composition between an andesite and rhyolite.
The existence of Morro Rock and the remaining Nine Sisters (The Morro-Islay Volcanic Chain) is thought to have been related to the subduction of a spreading center (divergent boundary) underneath the edge of the North American Plate. This process not only accounted for the source of rising magma that would form Morro Rock but also to the incipient development of the San Andreas Fault (which occurs when a divergent boundary, such as a Mid-Ocean Ridge, meets a subduction zone).
(1) What is postulated to have been the source of the magma that would eventually form Morro Rock? Why does magma rise from depth to eventually reach the surface?
(2) If larger crystals are indicative of slow-cooling at depth and small crystals are indicative of rapid cooling at or near the surface, how can Morro Rock have both large crystals (Phenocrysts) as well as small, non-visible crystals?
(3) CHALLENGE: How can a Divergent Boundary get to be located within in a Subduction Zone as has been postulated in the formation of Morro Rock?
--Morro Rock – The Gibraltar of The Pacific And Anchor Of The Central Coast. Accessed on Aug. 5, 2023 from
--San Luis Obispo Coast District: Virtual Minds Walks (July 20, 2021). The Morro-Islay Volcanic Chain: And What's A Slab Window Anyway? [44:38]. (YouTube Presentation by Scott Johnston). Accessed on Aug. 5, 2023 from https://www.youtube.com/watch?v=SvKcWXit9ng
--SeeCalifornia.com. Morro Rock in Morro Bay. Accessed on Aug. 5, 2023 from http://www.seecalifornia.com/attractions/morro-rock.html
TEXT-FIGURE 1 - FORMATION OF MORRO ROCK AND PINNACLES NATIONAL PARK
In Figure B, Spreading Center "S" has subducted under the North American Plate and led to the formation of a Transform Fault (San Andreas). Volcanism produced by Spreading Center "S" forms the California Coastal Volcanics ('v") - Morro Rock and Pinnacles National Park.
Figure 1 - Morro Rock, a 27 myo (Oligocene) Volcanic Plug.
Figure 2 - The outer surface of Morro Rock suggests Sheeting due to Unloading.
Figure 3 - Morro Rock has an overall domal shape. Parts of the rock were quarried for nearly 80 years between 1889 and 1969, somewhat reducing its conical appearance. (View is westward from the very south end of the parking area at the base of the Rock.)
Figure 4 - Morro Rock is composed of Dacite, an extrusive igneous rock whose overall composition lies somewhere between an Andesite and Rhyolite. Large white crystals are Phenocrysts of Plagioclase Feldspar floating in a groundmass (fine-grained background) of microscopic mineral crystals.