So I dropped a lot of hints in my post about my Lake Ingalls hiking adventure about how I made incorrect assumptions about the rock. Every geologist is biased by the rocks they studied first. For me, I’m biased by the brown and orange sandstones of the Tennessee’s and Kentucky’s Cumberland Plateau. Ergo orange rock = sandstone. When I dragged my butt over Turnpike pass and saw the undulating orange cliffs with blocky fractures, my mind immediately went back to the Red River Gorge in Kentucky. “Sandstone! We’re finally back on familiar ground!”
(orange slopes to the left, granite (or is it?) peaks to the right. Seen from “Valley View” on the next map)
Well I was faked out. Not only was I on unfamiliar ground, I was on unfamiliar oceanic crust. It turns out that central Washington is part of the same accreted terrane story that I’ve investigated in Oregon and in the San Juan Islands. If you need background on accreted terranes before I dive into the world of the oceanic crust and why it’s hanging out at 6,00 feet, check out my blog post Accreted terranes: a slow-motion pileup on the Pacific Coast.
The vast majority of Washington’s foundation – all of it except for a sliver by Idaho – never belonged to the ancient North American craton to begin with. The breakup of Pangaea 200 million years ago activated a subduction zone here that gobbled up oceanic crust as North America crept westward. Bits of island arcs and seamounts were plastered to North America like icing around the mouth of a carefree toddler eating her birthday cake, as the dense ocean crust subducted below the lighter continent. But sometimes chunks of the oceanic crust itself would snap off of the subducting plate with the inexorable force of the collision. These piece of ocean crust beached on the continent are called ophiolites. Sometimes rocks from even deeper in the earth break free with the ophiolite – “ultra-mafic” rocks from the mantle, which underlies both the oceanic and continental crusts. As these almost- subducted rocks rise to the surface they absorb water from overlying rocks and change their mineral structure dramatically.
Oceanic crust sinks because its minerals contain more heavy elements than the minerals that make up the rocks of the continental crust. These minerals include magnesium and iron, whose names combine to form “mafic”, the common term that describes oceanic crustal rocks. “Ma” from magnesium, “f” from Fe, iron’s abbreviation, and an “ic” on the end to give the word the ability to impersonate an adjective. Mafic rocks are exclusively igneous in origin. Whether the mafic magma cools deep within the crust or erupt on the seafloor, they cool in environments low in oxygen. It comes as a rude shock to their chemistry then, when they’re doused in water and scraped onto the continent. The greenish black minerals containing ferric iron oxidizes to rusty ferrous iron as the rocks are exposed to air.
Which brings me to my point. These orange rocks? They’re actually green inside.
Here’s the TL:DR on the rocks I met on the way to Lake Ingalls, with gratuitous details below the map.
- My hike started in sandstone deposited in a river delta tens of millions of years ago.
- I started to get out of breath as I hiked onto the quaternary alluvial deposits – landslides from the tall peaks above my head.
- Those peaks are made of Lake Ingalls serpentinite – metamorphosed chunks of the mantle three times as old as the sandstone.
- After lunch I hiked up into Lake Ingalls diabase and gabbro at Turnpike Pass – ocean crust rocks that have the same composition as basalt but cooled without erupting.
- I climbed back down into more landslide and stream deposits before crossing the landscape’s youngest sediment at the base of Turnpike Valley – glacial till deposited only 15,000 years ago. A baby!
- After another jaunt along the stream deposits, I reached the campsite built on more serpentinite. So many black-speckled boulders from the Mt. Stuart granodiorite had tumbled down here that the serpentinite is barely perceivable. The granodiorite is only twice as old as the sandstone at the trailhead.
- The next day, I hiked on more serpentinite all the way to Lake Ingalls.
If you’re on a phone or small, you’ll definitely want to open this image and zoom around. The map below outlines these rocks I met in a visual form, and the the next image introduces you to the geology of the view of Mt. Stuart from the approach to Lake Ingalls.
Congratulations, you made it past the map! Time to translate what I learned from the Wenatchee Quadrangle USGS bulletin out of hyper-jargon into only moderate jargon and learn what stories these rocks tell.
Swauk Formation (Ec(1s)): Between 54 and 42 millions 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 of the Easton unit that I’m not going into in this post. 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 section of the Swauk formation that I hiked through was heavily forested and I didn’t meet any good outcrops.
Ingalls Tectonic Complex – Jurassic intrusive basic (mafic) rocks – Jib(i): ~140 million years old. A unit of the Ingalls Tectonic Complex that is made of predominantly diabase and pyroxene gabbro, with anorthite and weakly foliated amphibolite. Diabase and gabbro the same mineral composition as basalt, but unlike basalt which is cooled lava (erupted onto the seafloor), diabase and gabbro are cooled magma (cooled within the oceanic plate without erupting). Diabase cooled quickly and has very small grains that are barely visible to the naked eye, while gabbro cooled more slowly and so is made of larger mineral crystals. Anorthite is a rare kind of white feldspar mineral common in mafic (ocean crust) igneous rocks. The white rocks I encountered at the top of Beverly Creek were anorthite, and the shiny green rocks that distracted me were amphibolite. “Weakly foliated” means that the rock was under enough pressure to realign the mineral grains perpendicular to the direction of force, but only to a minor degree.
Jurassic Ingalls Serpentinite and Peridotite (Ju(i)): ~140 million years old. A unit of the Ingalls Tectonic Complex made of foliated and massive serpentinite, serpentinized peridotite, and metamorphosed versions of these two rocks call metaserpentite and metaperidotite. Serpentinite and its associated process “serpentinization” are named after the root word for “snake”, as the process transforms the original rocks with a green color and often a slick scaly texture. This happens when minerals rich in iron and magnesium that are abundant in rocks from the oceanic plate, such as olivine, chromite, and pyroxene are forced to absorb large amounts of water during subduction. This hydration causes the rock to swell 30% to 40% from its original size and releases large amounts of heat – enough to raise the temperature of the rock by up to 500 degrees F. This increase in volume makes the rock less dense and more likely to slowly bob to the surface of the subduction zone.
When we see serpentinized rock at the surface, it means that a chunk of the ocean crust was partially subducted but somehow was spat back up to the surface where it ended up beached on the continental crust. Metaserpentinite and metaperidotite occur when that rock stayed within the subduction zone for a longer amount of time and was metamorphosed by heat and pressure as well as chemically metamorphosed by serpentinization. In the wild, this rock unit presents as strong outcrops with a blocky shape that have been weathered to an orangish tan color as the high iron content of the rock reacts to precipitation and oxidizes into rust. If a curious geologist were to take a hammer to these rocks, the insides would be light to moderately dark green. Around Lake Ingalls, fresh exposures of this unit are light brownish green with dark speckles of the mineral chromite.
Fun fact – the minerals in serpentinite, most notably chromium and magnesium, are significantly poisonous to vegetation, and it is the reason that serpentinite landscapes are most often barren of trees or only inhabited by scraggly struggling trees.
The two photos above shows two small scale features in the serpentinite – pressure fractures with remineralization, and also a close-up of the rock as it weathers. The right-hand photo shows just how orange the chromite crystal are, compared with how black/dark green they look in the fresh face of the first photo. The paler mineralization in the cracks is very brittle and broke off in my hand. Talc and tremolite are associated with hydrothermal deposition in serpentinite, so I think they’re the primary suspects.
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). 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. In some areas near the Mt. Stuart batholith, the heat of the intruding granodiorite literally cooked the surrounding serpentinite minerals into the soft blue-gray mineral called talc. This very durable rock forms the setting for the Enchantments and also for the Thunder Mountain Lakes which I’ve written about previously. On this map, it is separated into three concentric zones of felsic (continent-derived) intrusive rock with slightly different mineral content.
- Kit(sc) – Cretaceous intrusive tonalite – >20% quartz, significant plagioclase feldspar, some amphibole and biotite. Mostly pale, more gray than white, scattered dark speckles. Occurs around the southern and western edges of the batholith.
- Kiq(s) – Cretaceous intrusive quartz diorite – 5% to 20% quartz, significant plagioclase feldspar, some amphibole and biotite. Mostly pale, whitish, scattered dark speckles. Makes up the bulk of Mt. Stuart, with the exception of the top hundred feet, as well as much of the Enchantments.
- Kigd(s) – Cretaceous intrusive granodiorite – >20% quartz, both plagioclase and potassium feldspar, ~ 25% amphibole and biotite. Mostly pale, with gray quartz, white plagioclase feldspar, and pinkish potassium feldspar crystals as well as around 25% dark crystals of amphibole and biotite. Occurs in the center of the batholith, where it makes up the summit of Mt. Stuart and the southern bulwark of the Enchantments including Little Annapurna.
Quaternary Lakedale Hyak Till (Qlht): ~15,000 years old. What a cute little geologic baby, someday it will get buried properly and become real rock. This unconsolidated sediment was deposited by glaciers during the Pleistocene era. It’s jumbled mixture of all kinds of sediment grain sizes from clay to boulders that the glacier scraped off the rock and deposit as it moved. I passed this unit on the southern side of the junctions of Turnpike Creek and Fourth Creek with Ingalls Creek.
Time to zoom out onto the big map! Let’s put all these rocks into order based on their history – oldest on the bottom of the legend. Lake Ingalls is a little light blue dot just left of and below center on this map.
Even the oldest unit on this map is still only 4.5% the age of the earth. Deep geologic time messes with my mind. But more obviously, the representation of geologic time looks like a MS Paint scribble doodle. Talking about MS Paint, I drew an extremely rough cross section from north to south through the map to give the scribbles some 3D context.
I’m not going to get into the whole story of the many, many terranes that built this area in this particular blog post. I will defer to Professor Nick Zentner (Exotic P – Easton & Ingalls video) if you want to learn more about this phenomenon as it pertains to Central Washington, as I would only be (at best) giving a Spark Notes of one of his lectures. But the order of events in this region is thus:
- The Easton terrane containing the Chiwaukum Schist docks onto the North American Craton. Schist is a metamorphic rock made of sedimentary rocks that were subjected to intense heat and pressure until their mineral grains stretched and warped into new shapes.
- The Lake Ingalls Ophiolite docks onto North America, and,
- the Windy Pass Thrust Fault carries it over the Chiwaukum Schist,
- The Mt. Stuart Batholith punches through both the Easton and Lake Ingalls terranes,
- The sandstones of the Swauk formation are formed from sediment eroding off of all of the above rocks.
Thanks for reading my post, and I hope you can eventually hike out here too! I didn’t take as many photos as I should have on this hike. If I make it out here again next summer I will add a follow up to this post.
- Nick Zentner on Ingalls Ophiolite: https://www.youtube.com/watch?v=mfFqEmEYvOI
- The big primary source: Wenatcheee Quad Geologic Map: https://pubs.usgs.gov/imap/i1311/
- Nick Zentner Ingalls and Easton terrane story https://www.youtube.com/watch?v=URtNdXMS6ww
- Info on the local glacial timeline: http://faculty.washington.edu/tswanson/ESS/302/ESS%20Readings/Porter1976.pdf
- Info on how serpentinite affects vegetation: https://www.fs.usda.gov/wildflowers/beauty/serpentines/adaptations.shtml