Volcanic eruptions are well known, although causes of eruptions are poorly understood. Understanding the evolution of magma sources is important to better predict volcanic hazards that may affect dense human populations living near active volcanoes. From an economic perspective, focused magmatic systems have generated some of the largest ore deposits on Earth and thus exploring and recognizing more focused systems may lead to further discoveries.

Continental arcs consist of long chains of volcanoes and huge (10-30 times volumetrically larger than volcanic material) underlying columns of so-called plutons (former magma storage sites) representing the now frozen volcano feeders. A detailed understanding is lacking of how exactly magma collects into these voluminous magma storage sites and pathways. Granitic rocks of Yosemite National Park represent such a system that over time was geologically uplifted to the surface. In contrast to active volcanoes which have their magmas stored deep underground and thus inaccessible, this ancient magma system in Yosemite called the Tuolumne pluton could be studied petrologically, structurally, and geochronologically directly, a particularly useful approach since these rocks hold information about how plutons form and evolve along magma pathways.

The results of studying the Tuolumne pluton suggest that small batches of hot andesitic magma ascended through the Earth?s crust and pre-heated the pathway for larger volumes of cooler granitic magmas. These granitic magmas focused over time and collected in the region around Yosemite ca. 90 million years ago to form large volumes of magma within just a few hundreds of thousands of years. The magma focusing coincided with eastward migration of the arc, the voluminous Cretaceous arc flare-up (where magma addition rates to the arc increased by 1-3 orders of magnitude),  crustal thickening and thermal maturation of the arc column, suggesting that these processes all helped to prime the arc to form such superplutons. The rocks that formed from this magma show evidence for melt escape and likely eruption. They also indicate that once the system matured, i.e., a large volume magma storage site formed, magmas mixed and minerals were recycled between different batches of magma resulting in characteristic isotopic and elemental geochemistry signatures. The Tuolumne pluton collected enough large magma volumes that it culminated in super-volcanic eruptions.

Outside the Yosemite magma focusing center, smaller plutons, such as the Jack Main Canyon pluton, did not contribute to the regional magma focusing. It migrated in the opposite direction of the magma focusing. The study found that this phenomenon is more common than previously believed. However, even this much smaller system built its own local focusing pathway at the end of its formation when the system finally matured.

The implications of this study are that magma focusing occurs at multiple levels and scales in the crust, potentially throughout the entire crust (30 km or more), and contributes to the formation of centrally located large magma storage sites (super-plutons) that may have fed corresponding large volcanic eruptions (super-eruptions).

This RUI project, a collaborative study with colleagues at the University of Southern California (USC), was led by female principal investigator Memeti who is now an associate professor at the Hispanic-serving institution and received tenure during the completion of the project. Cal State Fullerton (CSUF) largely focuses on undergraduate education with a large population of first-generation college students. The study involved over a dozen undergraduate students completing senior and Honors theses, four Masters graduate students who completed their MS projects, and one PhD student at USC who worked closely with Memeti. A majority of these students are women, first-generation college students, and belong to minority groups. The students were involved in all aspects of the study, ranging from field and lab work to the publication of results in ten peer-reviewed publications, three of which were first-authored by MS students and two were led by the USC PhD student. Memeti with students and collaborators published over 30 conference abstracts and presented the research results at national and international conferences. Memeti gave talks to local interest groups, at public outreach events, and for university seminars. In collaboration with colleagues at USC and CSUF, and the involvement of undergraduate students, Memeti helped create a Travelstorys GPS-triggered audiotour with the title Yosemite National Park, A Story of Fire and Ice, which includes some of the major results of this study. This audiotour aims to educate the general public about ancient magmatism in Yosemite National Park and how glaciers carved the rock out to produce the rock cliffs we see in Yosemite today. It has been used and downloaded hundreds of times since its publication. Memeti also regularly interacted with Yosemite staff to update park personnel on research progress. She also led field trips to Yosemite to share research results with other U.S. and international experts.

Last Modified: 12/29/2022
Modified by: Valbone Memeti