The Elusive Origin of Guadalupe Island

Although the ocean around Guadalupe Island off Baja California, Mexico is well-known as a location brimming with great white sharks, the island’s geology has been obscure for decades. 

To shed light on this geopuzzle, an international team of geologists led by Scripps Institution of Oceanography at UC San Diego spent May and June investigating the isolated ancient oceanic volcano. The recent fieldwork launched an ongoing project that is funded by the National Science Foundation to investigate the rock record on Guadalupe Island with the aim of understanding the processes that drove its emergence from the surrounding seafloor.

Regional seafloor geology provides clues for the origin of Guadalupe Island, especially the distribution of two submarine features: fossil oceanic spreading ridges, where seafloor spreading once occurred, and underwater oceanic volcanoes called seamounts. On the one hand, Guadalupe Island is located on a fossil oceanic spreading ridge and resembles neighboring seamounts located on a discontinuous system of north-south oriented fossil ridges that spans 1,800 kilometers (1,100 miles). On the other hand, the island also defines the end of the linear, 1,000 kilometer (620 mile)-long Fieberling–Guadalupe seamount chain that runs east to west and resembles a smaller version of the Hawaii–Emperor seamount chain. This suggests that Guadalupe Island may either record southward migration of the East Pacific Rise oceanic spreading center or represent a product of mantle plume volcanism.

The island is known among scientists for its unique ecology in addition to its role as a gathering place for great white sharks. In 2005, the Mexican National Commission on Protected Natural Areas (CONANP) designated Guadalupe Island as a biosphere reserve, which establishes land- and ocean-use regulations and related scientific projects in pursuit of ecological sustainability. Besides a small fishing community, the most common visitors to the island today are park rangers with CONANP and biologists and ecologists with the Ensenada-based Island Ecology and Conservation Group (GECI), a nonprofit organization that maintains several field stations throughout the island to conduct ongoing ecological restoration.

Prior to 2024, the most recent field geology work on Guadalupe Island took place in the mid-1970s as a part of Scripps alumnus Rodey Batiza’s dissertation research. Scientific and analytical advances since this time mean the origin and evolution of the island can be significantly better understood today.

Our international field team of “geólogos” included myself and PhD student James Muller from Scripps, along with four geology/volcanology students and faculty from the Autonomous University of Baja California, the University of Colima, and University of Mainz. We sailed to Guadalupe Island as guests of the Mexican Navy, which transports people and supplies from its base in Ensenada. The island itself is about 30 kilometers (19 miles) long from north to south, about the same distance from the Scripps Oceanography campus in La Jolla, Calif. to the U.S.-Mexico border. Its elongated shape reflects two overlapping shield volcanoes (large, low-profile volcanoes), the northern of which is younger and reaches higher elevations (up to 1.3 kilometers or 0.8 miles).

We started our work at GECI’s Campo Bosque field station in the northern, high-elevation, reforested part of the island where a persistent marine layer to the west obscures the Pacific Ocean and conjures a sense of life above the clouds. Looking eastward from the top of the northern shield volcano, one sees a low, flat platform of younger volcanic rocks filling a caldera depression in the foreground. The background reveals a wide-open view of the Pacific Ocean created when the eastern half of the volcano collapsed onto the seafloor long ago. This volcanic landslide left steep cliffs behind, exposing one kilometer (0.6 miles) of volcanic strata (rock layers) that are otherwise buried by younger lava flows and pyroclastic (explosive, high-energy) deposits. 

The enhanced exposure enabled our team to investigate the deeper strata of the volcano to understand its early development, allowing us to determine its petrogenetic evolution over much of its lifespan. The cliffs also expose clear views of volcanic dikes that cut upwards through the entire stratigraphy (sequence of volcanic layers) to the surface, where they fed lava to scoria cones — small, pointy volcanoes that are likely the youngest on the island. Additionally, our group discovered thick pumice and associated pyroclastic deposits along the caldera rim and flank that record explosive volcanism, likely driven by the collapse of the central volcanic caldera as its underlying magma supply dwindled. This alone is a remarkable finding, as explosive volcanism on Guadalupe was previously considered to be insignificant.

Our group migrated southward to lower elevations for the remaining weeks of fieldwork on Guadalupe Island to investigate the southern shield volcano and an array of younger volcanic features. The southern shield-volcano strata are exposed along steep hills that tower over the local fishing cooperative at Campo Oeste, where continual boat activity and active families engender a boisterous, sea-centered environment that is no less welcoming than that above the clouds at Campo Bosque. Much of southern Guadalupe Island is blanketed in pyroclastic deposits including ignimbrite, which record highly explosive eruptions that are deposited from ground-hugging currents of hot, dense ash and volcanic debris. Ignimbrites also transport xenoliths (“foreign rocks”) from the underlying and otherwise inaccessible mantle and lower crust, effectively sampling pieces of the deeper Earth.

As an outcome of our recent fieldwork, hundreds of kilograms of rock samples from Guadalupe Island are now the subject of ongoing collaborative investigation in San Diego, Ensenada, Colima, and Mainz. Our group will conduct analyses over the coming months to understand the age and composition of Guadalupe Island, with the aim of discerning its origin as a product of interaction between the East Pacific Rise and continental North America or as a symptom of mantle-plume activity. Work at Scripps will involve myself, Muller, and petrologist Prof. Emily Chin, in addition to a local community college student with the Scripps-GEOPaths program. Future fieldwork opportunities may focus on paleomagnetic and volcanostratigraphic investigation of Guadalupe Island to further enhance our understanding of this volcanic island’s construction, lifespan, and behavior. Scientific questions are infinite and our collaborative framework among geologists and support personnel from CONANP, GECI, and the Mexican Navy provide a promising outlook for future interdisciplinary exploration and discovery of this special place.

James Worthington is a researcher at Scripps Oceanography and co-principal investigator of the NSF-funded project on Guadalupe Island, along with Scripps Associate Professor and principal investigator Emily Chin.