Southern California Field Trip

My first geology field trip in autumn of 1994 was to Southern California. We studied coastal processes and geology of the region between San Diego and Santa Barbara. Click here for our Grad Students' Southern California field trip home page, complete with images. On our way up the California coast, we stopped at Ventura to view the Channel Islands, which were the subject of my field trip report:

The Channel Islands

Just off the coast of Southern California, a series of islands stand out majestically from the Pacific. Visible offshore from the city of Ventura, (about 20 kilometers to the southwest) the small island of Anacapa juts up from the sea. Beyond Anacapa, the islands of Santa Cruz, Santa Rosa, and San Miguel follow westward roughly in a line about 100 km long. These four, the northern Channel Islands, are separated from the mainland by the Santa Barbara Channel, from which they draw their name. Further to the southeast lie the islands of Santa Catalina (a popular tourist destination just 40 km southwest of Los Angeles), Santa Barbara, San Nicolas, and San Clemente. The four northern Channel Islands plus Santa Barbara Island were protected by the U.S. Congress in 1980 when they were designated Channel Islands National Park, a unit of the National Park Service.

The Channel Islands exhibit a broad range of topological features. Anacapa, the smallest of the major islands, has the most jagged appearance. Its profile recedes to sea level in two places, dividing the island into three disconnected islets. The most rugged landscape appears on West Anacapa, the largest of the three, while East and Middle Anacapa have lower and mostly flat upper surfaces. The differences in the slopes of the surfaces, in addition to the narrow water passages separating the islets, indicate the presence of faults between the islets.

Anacapa

Santa Cruz is the largest (249 sq. km) and most diverse of the islands. A long central valley running east-west down the center of the island, created by the Santa Cruz Fault, divides the island into two distinct geologic regions.(1) The highest mountain on the islands, Devils Peak (747 m), dominates the northern side of Santa Cruz. The southern side also exhibits high relief terrain, but with the exposed southern coast showing more weathering. Canyons created by water runoff are abundant.

Continuing west, Santa Rosa also shows high relief on its eastern end, with cliffs, mountains, and canyons, but this terrain gradually gives way to rolling hills and grasslands on the western end. The terrain on westernmost San Miguel is nearly flat and mostly barren, weathered by strong winds, terminating with the sandy beach at the western tip of the Channel Islands, Point Bennett.

The origin of the Channel Islands is rooted in plate tectonics. Most of the geology of California is determined by the interaction of two major crustal plates, the Pacific plate and the North American plate. These two plates were once experiencing "Andean" type tectonic motion, with the Pacific plate pushing underneath the North American plate, giving rise to ranges of mountains, including the Sierra Nevadas. Approximately 30 million years ago however, this plate boundary transitioned to a transform margin, whereby the Pacific plate is now sliding northward against the North American plate in a lateral motion, as seen in the San Andreas and its many associated parallel faults.

Early theories of the formation of the northern Channel Islands centered around the idea that they were extensions of the Santa Monica Mountains of the east-west Transverse Ranges.(1) Indeed, they are roughly parallel, and certainly geologically related, containing many of the same rock units. However, the fact that the Santa Barbara Channel is up to 240 meters deep, with significant ocean floor faults, implies that the islands are not simply an extension of the Santa Monica Mountains.

Current theories indicate that the islands were pushed up and torn from the mainland by the lateral motion of the Pacific plate against the North American plate.(2) In addition to uplifting and displacement, this lateral crustal motion would also cause compression, deformation, folding, and rotation of the large mountain blocks which today make up the islands.

As evidence of this theory, the composition of the Channel Islands consists mainly of basaltic material, indicating volcanic origins. In addition however, large amounts of San Onofre Breccia, a sediment characteristic of the Oceanside, CA area over 100 km distant, pervade the islands.(1,2) Other pieces of evidence include the paleomagnetic alignment of Channel Island rocks, which indicate that they have been tectonically rotated by as much as 80 degrees (clockwise, as expected from the lateral motion of the plates) since they were formed.(2) The conclusion then is that the lateral motion of the transform plate margin caused the displacement, uplifting, and rotation of large blocks of material from the mainland to the southeast to create the Channel Islands.

Point Bennett, San Miguel

Anacapa may have been formed as a result of this displacement. It consists mostly of basalt, formed from lava welling up through faults approximately 15 million years ago. San Onofre Breccia is the only sedimentary rock found on Anacapa.(2) Just off the eastern tip of Anacapa is Arch Rock, a picturesque flat-topped basalt stack exhibiting yet another interesting material on its upper surface: guano.

Arch Rock is just one example of the wide variety of geologic features seen the Channel Islands. All of the islands exhibit many erosional features around their shorelines, such as wave-cut platforms and marine terraces, high cliffs, and numerous bays and alcoves. These features (as described elsewhere in this report) are created when weaker rock is eroded away by wave action, leaving behind stronger rock. Landslides caused by pounding waves undercutting seacliffs gradually create wave-cut platforms, which may become marine terraces with changes in sea level. (San Miguel is basically a Pleistocene marine terrace, exhibiting beachrock (water-tablerock) and Late Holocene sand.(1) Wave action is also responsible for the many sea caves around the islands. The largest cave is Painted Cave on Santa Cruz, with a ceiling 40 meters high./p>

The islands are more exposed to open ocean waves on their southern shores than on their relatively protected northern shores. As a result, the southern shores see more turbulent waters, faster wave erosion, and thus steeper cliffs. Despite the prevalence of steep cliff shorelines, there are beaches around the Channel Islands, each consisting of materials which are most prevalent in that particular area. For example, the beaches on Anacapa contain sand mostly consisting of dark basaltic fragments.(2)

Much of the offshore area surrounding the islands consists of wave-cut platforms and large, shallow submarine terraces. This is evidenced by the many tidepools, rocks (sea stacks), and kelp beds that extend far offshore around the islands.(2)

Information about the geologic history of the islands can also be deduced from the abundant marine and non-marine fossils. The nature of fossils found at a particular site may indicate the conditions to which that site has been subjected. For example, Anacapa consists of volcanic rocks about 15 million years old, but the oldest non-marine fossils on the island are between 4 and 6 million years old. Therefore, Anacapa has only existed above sea level for about the past 5 million years.(2) Fossilized remains indicate that the northern Channel Islands were interconnected during the last (Pleistocene) Ice Age when the sea level was considerably lower, but it is not believed that they were connected to the mainland.

The archaeological record of the Channel Islands also includes evidence of about 6000 years of inhabitation by the Chumash Indians, a group of seafaring Native Americans.

Present day animal life on the islands is just as diverse. The Channel Islands are home to a wide variety of species from sea lions to sea gulls, and are the only domain of the rare island fox. In addition, the protected waters surrounding the islands teem with life, from the smallest tidepool inhabitants, to majestic whales and dolphins. In summary, the Channel Islands are indeed a beautiful exhibit of geological and ecological diversity, certainly deserving of their status as a National Park.

Acknowledgement:
Many thanks to the NPS Park Rangers at Channel Islands National Park for their help in gathering reference material.

References:
1 Geology of the Northern Channel Islands, Weaver, Donald W., 1969
2 Geologic Field Guide to Anacapa Island (draft), Sadd, James L., 1990

Aerial photos by Eric Wegryn