The Sierra Nevada of California is one of the most fascinating geologic provinces in North America (and I’m just not saying this because I’m a petrologist). It straddles the middle third of the western margin of the continent and it captures the magmatism, tectonism and geomorphic evolution of over 200 million years. So, it makes perfect sense that geologists would have a lot to say about the region. I recently attended a Geological Society of America Field Forum that took a close look at the magmatic, structural and tectonic processes at work across the Sierra Nevada from the Paleozoic through the Cenozoic, with a focus on what is the archetypal piece of the Sierra Nevada Batholith, the Tuolumne Intrusive Complex in Yosemite National Park.

One of the perks of flying in to San Francisco from the southeast (I transferred in Las Vegas) is they will route you over the Sierras – and for this flight, it was just to the north of where the Field Forum would be held. I could look down and see the transition from the Ritter Range wall rocks into the main part of the Sierra Nevada batholith (left to right in the image). It was definitely a nice way to start off the Forum.

Now, I’m not going to get into the many avenues of scientific discourse that was tackled during the forum – you can imagine what happens with over 50 faculty, graduate students and undergraduates get together in the field and talk shop. Instead, I’m hoping to give you a taste of the Field Forum and why studying the Sierra Nevada is so integral to our understanding of the evolution of North America.

I’m somewhat of a new kid to studying the Sierra Nevada – I was a faculty advisor for a student on a Keck Geology Consortium project at Sequoia National Park with Jade Star Lackey (Pomona College) and Staci Loewy (University of Texas). Now, normally I study crystals from young lavas, but for this project, my student looked at old rhyolites within a pendant (a piece of crust captured in the Sierra Nevada Batholith) that turned out to be intriguing pieces of the evolving Sierra Nevada arc from a time period that is not captured in many places. My curiosity was piqued – what can we learn about the volcanic rocks caught up in the Sierra Nevada plutons that can tell us about how arc magmatism changed along the western margin of North America. So, when the forum was announced, it seemed like the perfect way to wade into the Sierra Nevada community and learn as much as I could.

Once on the ground, we were greeted with vistas like this one looking into the Yosemite Valley. If you can believe it, this was my first trip to Yosemite National Park, so these views of El Capitan, Half Dome and more were all new to me. We would spend the next few days working our way through the valley (after already examining the older rocks of the western foothills), looking at how the Tuolumne Intrusive Complex was emplaced during the late Cretaceous.

The people on the forum were fascinating mix – some have been studying the Sierra Nevada for decades, some have never touched it. Some look at large-scale puzzles of the tectonics of North America, some look at microscopes inclusions of melt within crystals. Some spend most of their time interrogating the rocks in the field, while others examine the problems though intensive lab techniques. That was the point – what can bringing in all these different views do to help answer the outstanding questions in the Sierra Nevada … and identify where problems remain.

When thinking about a geologic province as big as the Sierra Nevada, sometimes you need to really look at the big picture. With Saddlebag Lake in the background, Forum leader Scott Paterson regales the group with tales of deformation within the Sierra Nevada batholith and the metamorphic pendants that are found intermingled with the plutons. Trying to parse out the source of these slivers of metamorphic rocks is tricky – some may have travelled hundreds of kilometers (not to mention tens of kilometers up and down) before ending up in the final location. The clues to this journey are all recorded in the rocks themselves.

So, what are the outstanding questions in the Sierra Nevada? Well, I could write a book on that subject, but our Forum leaders focused on two key lines of inquiry: (1) how did the Sierra Nevada batholith (and prior intrusion complexes) get emplaced (in what state and at what rates) and evolve petrologically, and; (2) How did the Sierra Nevada margin change from the Triassic to the present? These questions take a multipronged assault on the geology to even scrape the surface of the complex story told by the structures and magmatism of the province.

You don’t need to be looking at a whole mountain range to think about the processes that formed the Sierra Nevada either. At this outcrop in the Guadelupe Intrusive
Complex, Bob Wiebe (UC Davis) – an expert on interpreting magmatic fabrics – talks about what he seems in these mixing and mingled magmas to a group of onlookers from the Forum. These interactions are what helps make Field Forums such a great experience.

We spent much of the forum outside – actually looking at the rocks and thinking of them in the context of these key questions for the Sierra Nevada. Some of this required looking at the rocks on a regional scale, pondering where blocks of rocks many kilometers across came from (and this is usually a tricky question). Other times, we had to put our noses to the rocks to see fine-scale textures that might reveal clues to how different magmas interacted – are they solid or liquid (or both?) when they mingle? Sometimes, we got down to the crystal-scale (which can be easier than you think when you have megacrysts like you find in the Cathedral Peak Granite). Some of the places we visited were unlike anyplace I’ve even seen, places were you can envision magmas flowing, moving crystals throughout a magmatic system, producing sedimentary structures like crossbeds – but all happening in a magma. In other places, the intense metamorphic forces that some of the pendants have felt was apparent – betraying more of the complex history (dragged down to kilometers into the crust) they have felt as they were incorporated into the Sierra Nevada batholith.

Being a petrologist interested in how magmas interact, some of the outcrops during the Forum were amazing. This example of magma mixing/mingling from the Fine Gold Intrusive Suite shows a dark, mafic magma interacting with a light, felsic magma. Some of the interactions clearly suggest the mafic magma was cool enough to form angular blocks (to the right of the hammer). However, other parts for very delicate crenulate (wavy) margins to the mafic blocks, suggesting that the magma was still molten during their interaction (labeled in image). Looking at these outcrops, you can begin to envision lobes of mafic magma flowing into a pile of mushy, felsic magma beneath a volcanic edifice.
Until you’ve been to the Tuolumne Intrusive Complex, it might be are to believe that you can find single crystals of feldspar that are upwards of 10 cm across – but sure enough, in the Cathedral Peak granite, you find these giant megacrysts. In a glacially polished surface like the one in the image, you get to look at cross-sections through the crystals and you can see the zoning of the crystals with the naked eye (labeled above). How exactly the magma was able to grow such large crystals (and the ultimate source of the megacrysts) is still up for debate.
At first glance, you might think that this is an outcrop of sedimentary rocks, with crossbeds and channels. However, this is all granite (and their close allies) within the Tuolumne Intrusive Complex. George Bergantz (University of Washington) provides the scale for this magnificent outcrop that shows separate of light and dark minerals (mostly feldspar and quartz juxtaposed with amphibole). If you look closely, you’ll see some megacrysts in the outcrop, all aligned as if they were flowing in the magmatic pile – and they very well could be – which begs the question: what was the state of the magmatic system when this was active? Every time I see this, I notice something new that makes me scratch my head.
We saw more than just magmatic features. The immense strain that many of the wallrocks and pendants felt were evident in the rocks as well. This outcrop is a Triassic conglomerate that is on the western margin of the Tuolumne Intrusive Suite – part of the host rock for the intrusive. On one surface, the pebbles look relatively normal (surface with lens cap), but slice through the rocks like this outcrop has nicely provided, as you can see how stretched out and smeared these pebbles has become. These rocks have felt a lot of stress during their lifetime, and the stretching of these pebbles is part of the way it has been accommodated.

Of course, the geology of the Sierra Nevada wasn’t the only exciting aspect. Sometimes, just seeing El Capitan right there in front of you is enough to take your breath away (if only for imaging being the climbers on the side of the granite face). The weather was great for much of the trip, although it didn’t stop the group from getting cozy underneath the only tree at one of our lunchstops. However, at other lunches, it was worth it to wander off a ways to get an unbridled view of the Sierra Nevada peaks around you and feel like you’re the only one for miles of this view.

El Capitan was one of the most stunning views of the trips. This was not only because of the beauty of such a shear face of granite, but also the climbers clinging to the side of the cliff. I’m not one for heights, but seeing these people, like tiny speaks, on the side of El Capitan made me pleased to have both feet firmly planted on the ground.
California, on the whole, is a very dry place – especially so this year. One of our lunchstops might have been greener (and swimmable) at other times of the year, but when we got there, it seemed that this tree was the only respite from the sun. Might explain why the whole group could be seen hiding under its branches (one of the few times people weren’t drawn to the rocks).

Hopefully, this gives you a taste of the Field Forum. It is hard to encapsulate all the geology, all the intellectual (and less intellectual) discussions, all the wonder of spending a week in the mountains. However, it was exactly the crash course in Sierra Nevada geology that I was hoping – and I know that almost everyone I spoke with found the Forum to be well worth their time. These Forums are what make being a geologists so captivating – taking the views of the natural world and trying to deduce how they came about through the clues that are left for us. It is a tricky endeavor, but one that keeps me coming back for more.

As great as the camaraderie of the Field Forum, sometimes you just need that chance to see it all by yourself. On our all day hike to the western margin of the Tuolumne Intrusive Complex, I snuck off at lunch and enjoyed an unparalleled view of the Sierran crest without a person in sight. Right there is one of the best reasons to become a geologist.

I wanted to finish with a special thanks to the organizers of the Forum – especially the lead organizer, Scott Paterson (USC), who put together a seamless Field Forum. The other organizers were also on top of their game, whether it be for their field day or across the week – Jade Star Lackey (Pomona College), Vali Memeti (USC/Durham), Jonathan Miller (San Jose State University), Robert Miller (San Jose State Univeristy), Roland Mundil (Berkeley Geochronology Center) and Keith Putirka (Frenso State University).

Erik Klemetti is an assistant professor of Geosciences at Denison University. He also writes the blog Eruptions for Wired Science.