Hey what’s that rock? Your go-to guide to the geology of the PCT

Hello hikers!  How goes it?  My name is Alan and I am a geologist who will be thru-hiking the PCT (NoBo) this summer, starting on May 10th, 2025. As a geologist, it will be impossible for me to ignore the beautiful rocks, fault lines, and volcanoes along the trail.  For this reason I plan to highlight the geology of the trail on this blog, in addition to creating geological waypoints through the FarOut app along the way.  So if you’re a fellow rock nerd dirtbag please follow along on this journey!

Deep time on a long trail

The PCT is an 18-inch wide strip of dirt (i.e. broken up rock) that runs 2,650 miles from Mexico to Canada, traversing California, Oregon, and Washington.  Along the way, the trail traverses the incredibly diverse geology of North America’s dynamic edge.  With each step along the trail, hikers move not only through space, but also through hundreds of millions of years of time.  Yeah, that’s right – we are freaking time travelers!

The geology of the trail consists of five distinct sections: Southern California, the Sierra Nevada, Northern California, Oregon, and Washington.  It’s not a mistake that hikers have long subdivided the trail into these sections – the distinct landscapes, environments, and vibes of each portion are each controlled at the most basic level by the underlying geology! This blog will provide a deep dive into each section of the trail, with lots of photos, as I complete them.  For now let’s paint some broad brush strokes on each section to temporarily scratch your rock itch!

Southern California: Plate Tectonics, Deserts, and Sky Islands 

For northbounders, the trail begins in sparkling granitic rock of the Peninsular Ranges, the eroded remains of a ~900-mile-long chain of volcanoes that went extinct about 80 million years ago. Granitic and volcanic rock are each examples of igneous rock – rock that cooled from liquid hot magma. 

Me atop the southern terminus (2024), a monument that itself sits atop weathered granitic rock.

The route proceeds through tectonically shattered Southern California, often crossing active fault lines capable of producing sizable earthquakes.  One notable example that hikers cross three times is the San Andreas fault, which marks the boundary between the Pacific and North American tectonic plates. In between the fault lines of Southern California, rocks are tectonically forced upward into “sky islands” such as the San Jacinto Mountains. The Mojave Desert, a geological “dog’s breakfast” torn apart some 70 million years ago, presents Southern California’s final geological obstacle before entering the Sierra.

Sierra Nevada: Granite, Glaciers, and the Ancestral Cascades

While hikers place the transition to the Sierras at Kennedy Meadows (mile 703), geologists mark the boundary at the Garlock fault – the second longest fault in California – which routes through Tehachapi Pass (mile 558).  

The Sierras, like the Peninsular Ranges, represent the plumbing system of an ancient chain of volcanoes.  What evidence do you have of this, you might ask? Most rocks exposed in the Sierras are granitic (not true granite – more on that in a later blog post), consisting of light-colored feldspars, grayish quartz, and dark minerals.  These sparkling salt and pepper rocks come about by cooling and crystallization of magma many miles beneath the surface.  Before granitic magmas invaded this neighborhood, it mostly consisted of layered sedimentary rocks like limestone, sandstone, and shale.  Guess what happens when liquid hot magma meets sedimentary rock, effectively putting it in the oven?  That’s right – it gets baked! This baking process (a.k.a. metamorphism) leads to the growth of new crystals and the development of amazing rock textures.  At this point in the trail you’ll for sure have seen igneous, sedimentary, and metamorphic rocks – the three types!

But wait – if the Sierras were a chain of volcanoes, where are the dang things?  Nice question!  With the exception of the Ritter Range northwest of Mammoth Lakes, they have been completely removed by erosion.  A significant amount of this erosion, and the overall  jaggedness of Sierran topography, owes itself to glaciers that scoured the landscape over the last few tens of thousands of years.

The first geological hints of the approaching Northern California section come in the vicinity of Sonora Pass, where darker volcanic rocks lie perched atop the lighter granitic rocks that we’re used to seeing in the Sierra.  The volcanic rocks erupted about 10 million years ago from a now dead volcano of the “Ancestral Cascades” – relatives of the active Cascade volcanoes to the north.

Northern California: Cascades Volcanoes and the Rock Conveyor Belt

Entering Northern California gets many hikers down but the geology will keep you stoked!  Slowly the granitic and metamorphic rocks of the Sierra yield to volcanic rocks of the Cascades.  This transition becomes complete as you enter the Lassen National Forest between Quincy and Chester, where Mount Lassen, the southernmost active volcano of the Cascades, rises prominently from the surrounding volcanic highland.

California’s final stretch trades the volcanic landscape of the Cascades for metamorphic and granitic rocks of the Klamath Mountains (with great views of Mt. Shasta – the next active volcano in the Cascade chain).  The Klamaths represent the ancient edge of the North American tectonic plate.  This range formed through “accretion” over hundreds of millions of years, whereby the oldest rocks were stuck to the plate edge, then younger rocks stuck to those rocks, and so on. This process can be thought of like the conveyor belt at the grocery store – a bag of tortillas you put on first hits the checkout area, then some canned beans collide with the tortilla, then some cheese.  Before you know it, everything is smashed together into a quesadilla (in this analogy, the Klamaths are the quesadilla).

My students doing geological field work in the Klamaths in 2015, on the PCT north of Seiad Valley.

Oregon: The Land of Volcanoes

Congratulations – you’ve made it to Oregon – a volcanologist’s dream!  After exiting the Klamaths near Ashland, you will become immersed in the Cascade Range and its volcanic majesty.  Consider that this chain of volcanoes is what the Sierras looked like about 90 million years ago and that granitic rocks like we saw in the Sierra Nevada are actively forming some 5-20 miles below the surface.

You might consider charting your progress through Oregon on a volcano-by-volcano basis. After Shasta, you’ll encounter McLoughlin, which is classified as dormant and has not erupted in some 20,000 years.  Next is Crater Lake (a.k.a. Mt. Mazama) – the remains of a volcanic eruption so violent it literally blew its top off 7,700 years ago, leaving a huge crater.

From Crater Lake, you’ll pass several extinct volcanoes – the horned peak of Mt. Thielsen (the background photo for this post shows the author atop this peak in 2021), similarly spiny Three-Fingered Jack, and the spire atop Mt. Washington.

Next on the volcanic docket are the iconic Three Sisters.  While North and Middle Sister are probably extinct, South Sister last erupted 2,000 years ago and strong evidence exists for a chamber of magma four miles below the surface that could erupt at some point – stay tuned!  Each of the Three Sisters have deep, U-shaped, valleys flanked by piles of rock debris known as moraines, hallmarks of glacial activity over the past few ice ages.

My family hiking along the PCT near North Sister in 2021.

Two volcanoes remain on our tour of Oregon’s Cascade Range – the two tallest!  First is Mt. Jefferson, which erupted most recently about 1,000 years ago, and whose sides have been deeply scoured by glaciers.  Mt. Hood, voted by volcanologists as the most likely volcano in Oregon to erupt in the near future (it last erupted in 1866), follows.  Consider hiking a bit quicker through this stretch (just kidding, you’ll probably be ok.)

As you approach the Bridge of the Gods and the Washington border, the PCT enters the Columbia River Gorge, which was formed by cataclysmic flooding at the end of the last Ice Age (about 15,000 years ago).  Basically a 2,000 foot-high ice dam that was blocking a lake formed by glacial meltwater in western Montana ruptured, sending over 100 Crater Lakes worth of water down the Columbia River.

Washington: A Little Bit of Everything

The Cascade chain of volcanoes continues into Washington, with extinct volcanoes generally having a jagged appearance due to glacial erosion and active volcanoes appearing more conical. You’ll see for yourself as you contrast the active and relatively smooth-sided Mt. Adams with the long extinct and jagged terrain of Goat Rocks, which used to be a volcano comparable in size to Mt. Adams and Mt. Hood.

Mt. Rainier, the tallest peak of the entire Cascade chain, becomes visible from Goat Rocks.  Because of its towering elevation and large snow collecting area, Mt. Rainier is the most glaciated peak in the lower 48.  It is also the most dangerous, due to its potential for large eruptions and its proximity to large populated areas.

The geology underlying the PCT changes abruptly in the heart of the Alpine Lakes Wilderness, trading Cascade volcanoes for granitic and metamorphic spires.  In terms of the geology, we are now in the North Cascades and will be the rest of the way to the northern terminus. The complex geology of this region developed over the last few hundred million years, with accretion of exotic land masses, intrusion of granitic magmas, shuffling along fault lines, and glaciation.  This is one area along the PCT that I have little experience in and am very excited to learn more about!

Conclusion

This crash course on the geology of the PCT is just scratching the surface. I can’t wait to take a deeper dive into the amazing geology of the trail, publishing blogs from each trail section as I go, and to have an epic personal adventure in the process!  If you’d like to connect deeper with the PCT by immersing yourself in its wild geology please feel free to follow along!