In my last post, I chronicled the 2023 backpacking trip to Deception Pass, Marmot Lake, Jade Lake, and Tuck Lake with my twin sister and OG hiking buddy, Heather. Now I get to nerd out about all the rocks.

As we hiked, a few questions came to my mind…

• Was the granite-like rock at Tuck Lake the same as other granite-like rocks I had hiked on in the Alpine Lakes Wilderness?

• What was up with Cathedral Rock? It dominated the skyline on the flat section of the hike.

• Why did the cliffs look so different on either side of Jade Lake?

• What was the weird orange rock blob by Jade Lake?

First I’ll present a map, a cast of characters, and a timeline. Then I’ll drag you into the research to answer my questions.

View 1 is from a perch along the scramble from Tuck Lake to Robin Lake, and View 2 is on the north end of Jade Lake. Gray colors indicate either water, ice, or loose sediment deposited by landslide and streams.

Our route bridged two giant blobs of intrusive volcanic rock (pink on the map above), known to nerds (and readers of this blog) as “batholiths”. To the east – the Mt. Stuart batholith mentioned in my Thunder Mountain (https://bluemarbleearth.wordpress.com/2020/08/30/thunder-mountain-lakes-blew-my-mind/) and Lake Ingalls (https://bluemarbleearth.wordpress.com/2023/01/14/serpentinite-stories-beverly-creek-to-ingalls-lake/) posts, accompanied by that exotic Ingalls Ophiolite Complex. To the west – the Snoqualmie batholith. The Mt. Stuart batholith formed while dinosaurs still walked the earth, while the younger Snoqualmie batholith crystallized beneath a sadly dinosaur-free world. We also hiked on a few other rock types – lava flows from ancient volcanoes near the current Mt. Daniels, and sedimentary rocks called the Swauk Formation.

Tuck Lake – Mount Stuart Intrusive Rocks : 96 to 91 million years old. This formation is also referred to as the Mount Stuart Batholith, referring to its shape (giant blob) and manner of emplacement (cooled underground). Tonalite (a flavor of granite), diorite, and granodiorite with medium-sized grains. This rock is predominantly made of the mineral plagioclase feldspar, with minor quartz (pale gray crystals), biotite (dark crystals that flash in the sunlight), and amphibole (dark greenish black crystals). The rising plume of magma that would become the Mount Stuart Batholith punched its way through the Ingalls Tectonic Complex and contains small pieces of that formation that it gobbled up on the way. Note the age – this batholith is 96 to 91 million years old, while the Cascade Subduction Zone only fired up around 40 million years ago. The Mt. Stuart Batholith tells the accreted terrane story (along with the Ingalls Tectonic Complex), not the familiar subduction story. The Mt. Stuart batholith intruded into rocks far, far away and was transported to Washington by plate tectonics.

Bonus: Baby golden-mantled ground squirrels with big hopes for dropped cracker crumbs at Tuck Lake. fun fact – ground squirrels species have existed for 40 million years (https://www.welcomewildlife.com/ground-squirrels/#:~:text=Ground%20squirrels%20are%20small%20to,years%20to%20the%20Eocene%20Epoch.) – less than half as long as the rock they run on has existed.

Deception Pass and Marmot Lake – Swauk Formation: Between 54 and 42 million years old. This formation is a sandstone made of approximately 35-40% quartz grains, 15-20% fragments of rock, and 65% eroded rock of volcanic origin. It was cemented into rock by felspar and carbonate minerals. This unit is made up of material that was eroded from the Mt. Stuart Batholith, Ingalls Tectonic Complex, and other local metamorphic rocks. Streams and rivers deposited this material in a low-lying regional basin which was later split by the Straight Creek Fault. Related rocks can be found as far north as Bellingham, where they are called the Chuckanut formation. The Swauk formation tends to form rounded hills and ridges and the rocks weather into decent soil; exposures are mostly hidden under plentiful trees. Terrace Mountain, visible from Marmot Lake, is an exception and shows the layer-cake nature of this formation beautifully.

Jade Lake – Mt. Daniels Volcanics: between approximately 26 and 25 million years old. Primarily composed of dacite, with minor andesite and rhyolite in flows, flow breccia, welded tuff, dikes, sills, domes, and plugs. Dacite forms when viscous felsic lava rapidly cools at or near the surface, creating small mineral crystals. It is composed of feldspar and quartz (~85%, with >20% being quartz) and small amounts of mafic minerals such as mica, pyroxene, and hornblende (<15%). It has a higher silica content than andesite, which leads to more explosive eruptions. The rocks are intruded by the 25-MYA phase of the Snoqualmie Batholith- the volcanic rocks are older than the 25-MYA phase of the batholith.

Jade Lake – Snoqualmie Batholith: 28 to 22 million years old. This batholith is composed of granodiorite, tonalite, and granite that rose as blobs of magma off of plates subducted below North America within the period of Cascade Volcanism. These rocks were emplaced in at least eight phases and cooled at depths originally between 4,000 and 8,000 feet deep. Given that I was hiking on it at an elevation of 5,600 feet, this rock has risen an impressive 2 to 2.5 miles in its lifetime as overlying rocks lost the battle with erosion. Its modern equivalents lurk miles below Glacier Peak, Mt. Rainer, and the other Cascade volcanos.

(A timeline of our rocks of interest)

Let’s look at some photos to put rock faces to these formation names. Here’s the landscape from “View 1′ on the map – looking west from the hike above Tuck Lake. Here, we were sitting on the ancient tonalite of the Mt. Stuart batholith and looking across the valley to the Swauk Formation and the volcanic rocks that punched through it.

Cathedral Rock rises abruptly out of the ridgeline like a spire and is a destination for ambitious climbers. It has a geologic reason for its popularity. It is what’s called a volcanic plug (https://en.wikipedia.org/wiki/Volcanic_plug), and is made out of a similar mineral composition as the adjacent dacite flows (but with a bit less silica). However, while dacite and andesite are both technically extrusive rocks, the andesite of Cathedral Rock rose up into a volcanic vent and cooled more slowly underground. Because the magma cooled comparatively slowly, the minerals formed larger crystals with fewer gaps and bubbles to allow weathering to get a foothold. This structure makes it more resistant than the adjacent dacite and older sedimentary rocks.

Moving to the next viewpoint, Jade Lake lies at the contact where two rock formations meet – the Snoqualmie batholith muscled its way through the Mt. Daniels Dacite approximately 25 million years ago. The location of Dip Top Gap above the lake directly at the contact between granodiorite and dacite indicates that this intrusion weakened the dacite, enabled erosion when the altered rocks were once again exposed to the elements. The dacite forms angular, blocky formations and cliffs that collapse into talus piles on the eastern side of the lake, and the granodiorite forms smoothly sloping formations on the western side.

View 2 – Jade Lake, looking south from the trail

Dip Top Peak is made of the granodiorite, while Lynch Peak is made of the dacite. You can really see the difference in how the rocks weather in this panorama photo that Heather took, looking from the other end of the lake:

Looking north from the south side of Jade Lake- big chunky blocks of Snoqualmie batholith granodiorite on the left, rubbly cliffs of Mt. Daniels dacite on the right.

Much of this difference is based on how these rocks handle pressure here at Earth’s surface. The dacite is an extrusive volcanic rock – it was deposited at or near the surface, at standard atmospheric pressure. It’s currently in its pressure happy place. The granodiorite is an intrusive volcanic rock – it crystallized under the enormous pressure of 4,000 to 8,000 feet of overlying rock. Being at the surface is a strange new experience for it. In the absence of the crushing forces that it was used to, the rock depressurizes and pops apart in layers like an onion skin – this is called exfoliation, or “onion skin weathering”. This is shown in the cartoon below.

cartoon from https://planetgeogblog.wordpress.com/2014/09/19/rock-exfoliation-caught-on-film/ (https://planetgeogblog.wordpress.com/2014/09/19/rock-exfoliation-caught-on-film/)

The most famous example of exfoliation weathering of intrusive volcanic rock is the granite of Half Dome in Yosemite National Park. This phenomenon creates the smooth terraces of granodiorite west of Jade Lake.

Sloping exfoliation planes create long shallow terraces north of Dip Top Peak (top of the image)

Both rock types can commiserate about the millennia of tectonic pressure as terranes crashed into this area from the west, oceanic plates subducted, and the Cascade Mountains rose higher. This stress cracked the rock at angles perpendicular to the direction of the force, creating aligned blocky fractures in both the granodiorite and in the dacite. Both types of rock have been exposed to frost-heave weathering. In this process, water infiltrates into cracks in the rock, freezes, expands, and wedges the rock apart. This, combined with gravity, is the source of the piles of rock talus seen on the slopes on both sides of the lake.

The lake itself is so blue because of how fine glacial flour reflects light, ground down as a glacier gnawed its way through Dip Top Gap above the lake. This glacier might be waning now, but its mark can still be seen on the landscape in the form of linear gouges on the granodiorite.

A boulder above Jade Lake with textbook-perfect glacial scour marks.

The intrusion of the Snoqualmie batholith into the Mt. Daniels dacite created some smaller features that caught my eye. An orange lump by the lake stood out amid a landscape with grey rock, turquoise water, and light blue sky. Of course I had to check it out. Heather claimed the bag of gummy bears and waited out my nerd moment on a sunny rock with her book. The orange feature was about 10 feet wide and 30 feet tall. It had a jumbled, crumbly texture and pockets filled with dull white crystals.

Find the geologist…

I was stumped, and later reached out to the geology team at the Washington Department of Natural Resources. They said that this orange feature is a hydrothermal vein with additional hydrothermal alteration occurring in the forms of white clays, calcite (https://www.britannica.com/science/calcite), and iron oxides. The iron oxide here specifically are likely goethite (https://www.britannica.com/science/goethite)and limonite (https://www.britannica.com/science/limonite)which gives the outcrop its orangey brown color. In other words, the orange color is likely from oxidation (rust) of iron that traveled with the hydrothermal fluids as iron sulfide(s). The exact source of the fluids may be from the magma itself or a later magma or a deeper semi-related source. I also ended up in contact with Jeff Tepper of the University of Puget Sound, who gave the feature a name – a “pegmatite dike”. He said that such dikes typically form from “left over magma” near the end stages of batholith solidification, and those late-stage magmas tend to be rich in water.  This leads to a rock with open cavities (initially filled with a hydrothermal fluid) and an abundance of quartz, feldspar, and pyrite.  The pyrite oxidized into orange goethite and limonite. He said that the crystals look like a mix of quartz (gray, glassy) and calcite (yellowish-white with rhombohedral cleavage).

very rusty rocks, with granodiorite for comparison

White clay lump in the iron oxides

Calcite crystals

Our hike showed us at least four beautiful examples of geologic contrast – intrusive volcanic rocks that look similar but were formed millions of years apart, extrusive volcanic rocks that burst explosively through older sandstones, dacite and granodiorite cradling a stunning glacial lake between them, and and orange dike standing out in among the gray like a sore thumb. I hope you can travel here some day!

references:

• geology of the Snoqualmie batholith: https://www.washingtonminerals.com/snoqbath.html (https://www.washingtonminerals.com/snoqbath.html)

• USGS Skykomish River Quadrangle pamphlet: https://pubs.usgs.gov/imap/i1963/skypampa.pdf (https://pubs.usgs.gov/imap/i1963/skypampa.pdf)

• USGS Skykomish River Quadrangle geologic map and data: https://pubs.usgs.gov/imap/i1963/ (https://pubs.usgs.gov/imap/i1963/)

• The Cenozoic era in the Pacific Northwest: https://commons.wvc.edu/rdawes/Lectures/lect8.html (https://commons.wvc.edu/rdawes/Lectures/lect8.html)