ISLA SAN LUIS VOLCANO, BAJA CALIFORNIA, MEXICO - LATE HOLOCENE ERUPTIONS

The Holocene island volcano of Isla San Luis (ISL) lies in the northern Gulf of California, only 5 km from the Baja California coast. Volcanism there is highly differentiated for a small eruptive center (3.5 by 2.5 km) and the compositional variation has been surprisingly regular, ranging from early basaltic andesite to the latest eruptions of rhyolite. Eruptions have varied from mildly effusive to powerfully explosive hydrovolcanic events.

Seven marine mollusk shells were analyzed by ¹⁴C acceleration methods to date beach deposits at three stratigraphic levels interbedded with the volcanic layers on ISL. Three shells were dated in beach sands directly below the youngest of the pyroclastic deposits on the island: coarse, rhyolitic breadcrust-bomb-containing tephras associated with the early eruptive stages of the growth of the central rhyolite domes. These shells yield a maximum age of 2888 to 3289 years before present for the latest rhyolite eruptions. Additionally, three mollusks were dated in beach deposits below the intermediate lavas exposed along the NW beach cliffs. These give a maximum age of 3036 to 3289 ybp for the intermediate lavas. One shell was dated at 4469-4725 ybp from the oldest palagonitic pyroclastic deposits exposed near sea level.

We attempted to use obsidian hydration analysis to date the youngest, coarse, rhyolitic breadcrust-bomb-containing tephras associated with the early eruptive stages of the growth of the central rhyolite domes. Hydration rinds of 3.6 and 3.7 microns thickness were measured for two separate bombs (averages of 5 measurements each). The hydration rate for this obsidian has not been established, however, a typical hydration rate of 82.74 (microns)^2.06 yields an age of 1225 years for this tephra.

The beach sands dated at 2888 to 3289 ybp lie approximately 21m above sea level. The elevated deposits indicate an average uplift rate of 6.38 to 7.27 mm/yr, if sea level remained unchanged since deposition. This is the highest uplift rate along the Gulf of California and appears to be localized at ISL, probably the result of inflation due to upper crustal magmatic intrusion.

These dates indicate that volcanism on ISL began at least 4725 ybp and continued to perhaps 1200 ybp. These recent deposits and mildly active fumarolic activity suggest that this volcano remains potentially active.