Geology Students Investigate Ancient Volcano System Near Yosemite

Summer Field Research Focused on Jackass Lakes Pluton in the Sierra National Forest

Cal State Fullerton News articleOpens in new window

Dr. Vali Memeti

September 19, 2022

Geology students and their faculty mentor spent a month this summer hiking to an elevation of 9,500 feet into the rugged and picturesque Ansel Adams Wilderness to investigate an inactive volcanic magma chamber.

The magma chamber — once filled with molten rock and crystals, called magma, used to be about 7 miles below the surface. This magma chamber fed ancient volcanic eruptions roughly 98 to 97 million years ago at the same time when dinosaurs lived. 

Unerupted magma remained in this magma chamber and resulted in intrusive rock, which is now the Jackass Lakes pluton in the northern Sierra National Forest, south of Yosemite National Park.

“Our research involved mapping a section of the Jackass Lakes pluton to examine the emplacement style and physical and chemical evolution of magmas over time. Our goal is to bring a greater understanding about magma systems underlying volcanoes,” said first-year graduate student Samantha Dunn.

Dunn was among six Cal State Fullerton students — two graduate students and four undergraduates — who participated in the summer research experience, led by Valbone “Vali” Memeti, associate professor of geological sciences.

Memeti studies the connection between volcanoes and the underlying magma plumbing system. She organized the field research trip last spring while on sabbatical to work on her research to better understand volcanic eruptions and the evolution of underlying magma plumbing systems.

Studying Magma Plumbing Systems

Magma travels upward to the volcano through a system of chambers, which together form a magma plumbing system. Magma chambers hold magma until a volcanic eruption occurs, which can be anywhere from hundreds of years to a few million years. 

While volcanic eruptions have fascinated people for as long as humans have lived, and most notably, have taken lives and destroyed livelihoods, magma plumbing systems play a large role in past and future volcanic activity. 

“This is why we geologically remapped the Jackass Lakes pluton and will conduct lab analyses to investigate what the rocks are telling us about the scales of magma chamber sizes and volumes, and types of magma processes that occurred here,” Memeti said. 

“We hope that this study gives us insight into how magmatic systems at depth grow and mature over time, the specific styles of eruption they feed, and how many eruptions might have taken place.” 

This academic year, Dunn and fellow students are working on analyzing the rock samples they collected for types of minerals and chemical analyses to determine the various magma processes that occurred millions of years ago. For instance, volcanic and plutonic rocks will be dated to determine exact ages and geochemical analyses will be used to understand how magmas of both erupted and intrusive rocks are related to one another, Dunn explained.

“Minerals record information about what was going on in the magma chambers,” Memeti noted. “A lot of this type of information has not been unlocked yet, so it is a very exciting time to be studying these kinds of systems.”

Training the Next Generation of Geologic Mappers

The students’ fieldwork at Jackass Lakes pluton is supported by a nearly $60,000 U.S. Geological Survey EDMAP grantOpens in new window for Memeti to train students in geologic field mapping. 

A geologic map shows the distribution of different rock types and structures, such as faults on a topographic map. The map identifies geological information of an area and how to find a certain rock type or a fault. 

From mid-July to mid-August, the researchers hiked 5 to 12 miles a day in the wilderness with  spectacular rock outcroppings and granite cliffs. They worked up to 10 hours in hot summer weather mapping the pluton and collecting rock samples. One week, heavy monsoon rains, thunder and lightning interrupted their fieldwork and forced them to huddle in their tents. 

In the field, students mapped on paper topographic maps. In the lab, they are now using cloud-based mapping and analysis software to digitize the maps. Graduates with geologic mapping and field skills are sought-after by employers hiring geologists, Memeti added. 

“Fieldwork is never easy. It includes long arduous days. It’s both physically and mentally draining some days, but it is also the most exhilarating, rewarding and wonderful experience,” Dunn relayed. 

“Geologic mapping is an important skill in geosciences,” Memeti said. “I incorporate it in my projects because my research requires detailed geologic maps. It is an excellent way to combine my research with my teaching.” 

Dunn, who earned a bachelor’s degree in geology from Cal Poly Humboldt in 2020, plans to pursue a doctorate and a teaching and research career at the university level. She shared that the fieldwork reinforced her passion for geology and learning more about volcanoes and volcanic systems. 

“Understanding magmatic systems by mapping and studying plutons gives us a glimpse into the processes that could be currently occurring underneath volcanoes worldwide,” Dunn said. “Our study can help scientists and people prepare for future volcanic eruptions.”