CATALINA ISLAND--26 MILES ACROSS THE SEA: Geology Field Trip, Guest Post by Paige Arnold

In August, my granddaughter Paige, a geology student at Amherst College, went on a field trip to Catalina Island. On a clear day, Catalina Island is easily visible from the Southern California coast. Yet, in the more than fifty years that I’ve lived in California I’ve never been there! I thank Paige for her excellent report!

This summer I had the opportunity to go to Catalina Island with my professor, his collaborator, and another undergraduate student to do field work relevant to my senior thesis (a year-long research project). We were there for a little under a week, and in that time I got to see some pretty cool stuff– both geology-related and otherwise!

A picture of the USC Wrigley Marine Science Center, where we stayed in dorms for the duration of the trip. We ate breakfast and dinner here, taking a sack lunch to eat in the field (unfortunately, given that sandwiches are not particularly resistant to getting crushed by rocks, they were always pretty soggy by the time we got to them). Otherwise, their food was surprisingly good and they had 24/7 coffee, exceeding my expectations for the dining situation.

For 3 of the 5 days of field work, we drove to the Catalina Airport and looked at/sampled rocks in the surrounding area. There was an airport store behind me where I took this picture, which provided great relief to us in the form of shade, cold Gatorade, and ice cream, during our lunch breaks.

The Catalina landscape is beautiful, but quite unfriendly. A lack of tall trees means no shade, and most of the plants have sharp qualities that inspire fear. Unfortunately, interesting rock outcrops don’t care about sun, spines, and spikes, and are frequently located in places surrounded by these hostile plants (don’t ask about the cactus incident).

Another obstacle between us and the outcrops was the occasional bison (small black dot above the heads in the first photo). Fourten bison were brought to Catalina for the filming of the 1924 Western The Vanishing American, after which it was determined that removing them from the island would just be too hard, leading to their continued presence on the island today. Despite their intimidating size, they didn’t cause us any problems other than brief moments of panic at rustling sounds in the bushes.

Here is a photo of an outcrop with a couple of the rock types we were focusing on. I won’t go into too much detail, but the big picture idea is that some rocks were part of an old subducting plate, got ripped off the plate in blocks and mixed into a weaker unit of rock, then got real hot and partially melted. In this photo, we’ve inferred that the garnet and hornblende (red and black minerals) are remnants of the original rock that did not melt, while the light minerals are the remnants of melt that did not escape the rock (leucosome). What makes Catalina so special is that these rocks are at the surface (subduction zone rocks very rarely end up at the surface!), allowing us to make direct observations of rocks that were subducted rather than extrapolating from complex geochemical signatures in the corresponding ancient volcanic arc (the Peninsular Ranges).

This is what most of the blocks looked like in the field, though they could range from cat-sized to car-sized. On the rock are our sampling tools– sledgehammer, chisel, and sample bag. Geology can be a brutish science at times.

Check out these cutesy little garnets in a row above my fingers (red dots in white line)! We think these are “peritectic” garnets, meaning they formed along with melt (the white line) as the result of the breakdown of another mineral.

Another one of many beautiful views.

Before Spanish colonizers arrived, Catalina was inhabited by the Tongva, who used the local soapstone (more formally known as talc schist– part of the weaker unit of rock I mentioned earlier) to craft bowls and other cooking ware that they traded along the California coast. This was one of a few soapstone-working sites we passed by on the way to outcrops, where you can see bowls that were never finished still attached to the rock.

In this picture I’m standing on a completely different kind of rock– metamorphosed pillow basalt intruded by blueschist dikes! Despite Catalina’s geology being so varied and complex, we’d been entirely focused on one particular kind of rock up until this day, so it was nice to branch out a little bit and see something new. This picture was taken right after a swimming break in Shark Harbor, which (if you ignore the ominous name) was an amazing place to swim with clear, cool water and waves just high enough to body surf without getting tossed around.

The aforementioned cool, clear water. It made me sad to have to look up from the water and board the ferry back to the mainland, so I took this picture to remember how blue it was.