Tiny Crystals, Giant Story: Cal Poly geology students set to take part in summer Yosemite National Park research

The towering granite walls and grand cliffs of Yosemite draw millions of visitors each year. Hikers marvel at the sprawling landscape’s beauty and rock climbers scale up daring routes in search of ways to connect to the Valley’s magic. But beneath the iconic stone lies a story still unfolding –– one that dates back over 90 million years ago to the geological events that formed the massive granite bodies.

Digging deeper into this very history in Yosemite are six Cal Poly earth and soil science undergraduates, seeking to uncover during their upcoming summer research the origin story of the area’s landscape. The group’s work will be focused on the Tuolumne Intrusive Suite (TIS) in California's Sierra Nevada, which is made up of five main rock layers that range in age and were formed during the Mesozoic Era from magma that cooled and hardened underground.

“When people go to Yosemite they see granite, but don’t necessarily think about how that granite forms,” Cal Poly geology professor Scott Johnston said, who is head of the research. In the months leading up to the summer, Johnston and his students are preparing for whatever they find.

“No one has ever collected data like this on these types of rocks before, so we don’t know what we’re going to see,” he said. “Interpreting that data is really hard, and I don’t understand all the data we have. But nobody in the world does, which is exciting.”

The group’s samples will be collected in Tuolumne Meadows and surrounding high-elevation regions such as Cathedral Peak, Rafferty Creek and Tenaya Canyon that make up the TIS. Through geochronology and geochemistry to test models for arc magma evolution and emplacement, the team will use the mineral zircon as a tool to rewind the clock and uncover the history and processes that formed Yosemite’s iconic granite landscape.

According to Johnston, analyzing zircon concentrations is fundamental because the mineral contains uranium, which can be dated through radioactive decay and thus allow geologists to accurately pinpoint the age of certain rocks. The zircon crystals, which are accessory minerals in granite rocks, meaning they are present in small quantities, are smaller than a grain of sand –– but act as time capsules useful to geologists studying their compositions.

Zircon crystals can help trace magma temperature and uncover the exact time they formed, according to Johnston. This research unlocks those secrets, showing how science can use something microscopic to tell us about massive geological events.

There are competing hypotheses that Johnston and his team of undergraduates are seeking to better understand and apply to the formation of the TIS in Yosemite.

The first hypothesis theorizes a magma system deep in the Earth’s crust that periodically sends larger batches of magma to the upper crust, melting the rocks that had previously cooled and leading to mixing. The other speculates the TIS was formed when a small, constant stream of magma in the lower crustal area was sent into the upper crust, hardening and leading to no mixing.

The answer affects how geologists understand the formation of continental crust, as well as volcanic behavior and magma storage beneath Earth’s surface.

“This is science at its best,” Johnston said. “There’s different ideas about how a specific process may occur.”

Prep work

Preparation has taken many forms for the research students, leading up to the mid-July backpacking trip the team will embark on in Tuolumne Meadows. Among the six undergraduates working on the research are earth and soil science juniors Marissa Levally and Marie Clare Reali, who meet with Johnston and the rest of the group every Monday for a special class that lays the groundwork for the research to come.

The lecture-style course feels very involved and hands-on, Reali said –– the only student out of the group who will be a full-time researcher over the summer. The beginning weeks consisted of reading a constant stream of academic papers and taking in Johnston’s knowledge.

“We were just getting our feet underneath us. We had no idea what [Johnston] was talking about at first, and because he had done a ton of research on it, he was 10 steps ahead of us,” Levally said.

But with so few students in the class, the discussions have been larger and more comprehensive than some of the students’ typical major courses. Johnston provides maps, diagrams, and information; sometimes the group is listening, taking notes, and looking through a microscope simultaneously.

Reali says Johnston has ingrained a helpful process into the students of examining and disseminating the research they embark on collecting.

Levally and Reali recently went to Lake Isabella in Kern County to examine sections of rock for a metamorphic petrology class with Johnston, unrelated to the small research class. Back at the on-campus lab, they looked at the samples as thin sections prepared to be analyzed under the microscope –– much like the 20 bedrock samples they will collect this summer from two areas across the TIS, and later examine through thin sections in the lab.

“You can make all the inferences you want in the field, but the only way you’re actually going to find out what’s in a rock is by looking at a thin section,” Reali said. Geologists determine different minerals in a rock thin section by analyzing the light that passes through them and their optical properties, including color and refraction. 

Students look at the big picture first: The Sierra Nevada Batholith, the lake and the tectonics. Then, they dial in their focus and goal, pinpointing what they are looking for, before zooming out to understand how this relates to everything else.

And in May, the research group drove south to UCSB to use the school’s laser ablation equipment. The laser, thinner than a human hair, blasts microscopic spots on minerals in the rock and vaporizes them through an argon plasma.

Reali said they picked 500 spots on five different thin sections of granite –– samples that Johnston had collected from the TIS on a prior research trip to Yosemite. Finding the concentrations of the elements in the granite’s minerals is a way to date the rock.

From this work, the group was tasked with parsing, sorting, charting and interpreting the large amount of data amassed from this process –– a weeks-long process they will soon embark on again, after they return from their summer Yosemite trip with more data.

Having spent weeks viewing slideshows and photos relating to the TIS in Yosemite, and occasionally getting out into the field –– though just around San Luis Obispo –– the students are fervently awaiting their mid-July excursion to Tuolumne Meadows.

“It’s going to be so nice to backpack and see [the granite], touch it, and learn about it,” Levally said, who has always loved being outdoors. “All kids grow up playing in the dirt, and I never grew out of it.”

All on equal footing

As a child, Johnston grew up backpacking, climbing, and skiing in the Sierra Nevada; the time spent in the mountains ignited a developing curiosity in him about how the towering peaks formed. Prior to landing at Cal Poly, Johnston –– having received a Ph.D from UCSB –– studied the faults of the Eastern Sierras, and the formation and evolution of Norway’s and Greenland’s grand mountains.

Since arriving at Cal Poly in 2008, he has worked with over 50 students on varying research projects.

“It’s really refreshing to see,” Reali said of Johnston’s research, “because we’re not supposed to know it all, just as [our professor] is not supposed to either. We’re reminded often that he does not have all the answers, and that we are working as a team to figure them all out.”

On equal footing, students work with Johnston as a peer, and in this environment, can easily build off each other's knowledge, curiosities and findings.

Reali said research should be a fundamental part of completing one’s degree, because it is where students can apply learned concepts to real world problems, and Cal Poly’s ‘Learn by Doing’ slogan truly comes alive.

Science extending outwards

Beyond helping undergraduates navigate the research process, Johnston hopes the TIS research can be a way to mend the disconnect between everyday people and the work of scientists.

“I think that in our current political environment, science is not appreciated as well as it could be,” he said. “I think to some extent it is because people don’t understand science.”

To help connect this cutting-edge research with the public, Johnston will work with Yosemite National Park geologist Greg Stock to develop annual field trips for the Interpretive Rangers. These trips will teach rangers how to identify different rock types within the TIS and give them tools to communicate the science behind the research in a way that resonates with everyday park visitors.

By making the research more accessible to Yosemite’s millions of annual visitors, Johnston hopes to foster in the public a deeper appreciation for and stronger belief in the scientific process.

He believes that anyone –– once shown what to look for –– can begin to recognize the subtle differences in Yosemite’s granite. That spark of realization can kickstart a line of other curiosities, and suddenly, you’re seeing the magic of Yosemite through a geologist’s eye.