March 7, 2012 | 3
Few places on Earth are so full of geological mayhem as a subduction zone. Life in the interior of a continent in no way prepares you for the chaos you’ll encounter when seafloor dives under continent. Where I grew up on the Colorado Plateau, the geology’s like a lovely layer cake: nice horizontal slabs of schist and sandstone and sediments from ancient seas stacked neatly one after the other, with a volcano on top. Washington state is also like a layer cake: one that had a tiramisu jammed in with it, and some mystery dessert stuffed into the last empty space on the table, and then the whole table got caught in an argument between a steamroller and a bulldozer, leaving a jumbled mass only just barely recognizable as bakery products – with a volcano on top.
USGS geologists describe the rocks in the Chilliwack Terrane as “Highly folded and commonly upside down.” That’s one of the better behaved bits, mind. Some of the rocks in the Northern Cascades are so messed up that geologists can only describe them as a mélange – a mixture.
It’s madness. And in less than a hundred miles, I’ll show you some of the wildest crustal contortions you ever did see. We’ll go from a beach that has got deep ocean floor stuck atop it to a place 2,000 feet up where plutons of contentedly crystallizing magma endured the twice-baked potato experience. On our way, we’ll cross something on the order of eight different terranes, bits of crust that belonged elsewhere before they found themselves emigrating to America.
Ready for a wild ride?
Rosario Head, Fidalgo Island
We’re in medias res, here at this idyllic island beach. To the west, the Juan de Fuca Plate is busy piling up the Olympic Mountains. The Juan de Fuca’s a remnant of a remnant: two other plates met their demise in the Pacific Northwest before it. One of those plates, rammed against and beneath the North American Plate, plastered the lands we stand on to the edge of the continent. That plate, the Kula Plate, was responsible for most of what we’re about to see.
Have a walk along this tiny little beach on Fidalgo Island, and you’re walking across terranes. When I took the above photo, I was standing on the Decatur Terrane. The head is a knob of the Lopez Terrane. Just beyond Rosario Head, a short walk, you’re back on the Decatur again. Faults and folds have sliced, diced and twisted these rocks into a remarkable jumble. You’re looking at an anticline, actually, when you view Rosario Head. Think of it as the crest of a land wave: synclines are the troughs. The land around the Pacific Northwest is wavier than the ocean.
Let’s go stand on an anticline, shall we?
If you want to get an idea of the power of the forces at a plate boundary, note how contorted these rocks are. They’re ribbon chert, formed in a deep ocean environment from the silica skeletons of radiolarians. For ages, they lived their lives and died their deaths, drifting silently down to the seafloor, where time and pressure turned them to stone. You’ll find ribbon chert all over here, the quartz layers interleaved with slate. These layers were once happily horizontal. Now, they’re standing on end, folded and twisted.
It’s gorgeous rock.
A geologist could get lost up there for hours, and I did. Don’t be afraid to take a biologist along if you visit: there’s tide pools in the ribbon chert at the bottom of the head that will make them scream for joy.
If you tear yourself away from the ribbon chert, you can investigate pillow basalts, greywackes, and a nice icing of glacial till, all within a few hundred yards. You’re walking on the remnants of a volcanic island arc that’s 160 million years old if it’s a day, acquired by the North American continent, and then planed by an ice sheet that left behind a jumble of debris only a shade over ten thousand years old.
And if you’ve ever wanted to visit an ophiolite, congratulations! You’ve done just that.
Tear yourself away from the ribbon chert and tide pools. Up SR 20, across the Skagit basin, and into the mountains, we shall visit another island.
Limestone Quarry, Concrete, WA
We’ve gone back in time. Here, a coral reef flourished sometime in the Carboniferous, roughly 330 million years ago. We’re in another island arc, where volcanoes erupted and islands with their reefs formed, although this islands’s quite a lot older than the one we just visited. And it wasn’t really here: these rocks have been rafted in on a plate and scooted along faults to reach their present location, arriving sometime in the Cretaceous. This is the Chilliwack Terrane, that exotic block of land that inspired the phrase “Highly folded and commonly upside down.”
Those poor crinoids and corals had a rough time of it after they died. Their calcium-rich skeletons got mooshed into limestone so thoroughly that it’s hard to find a good fossil here, and then squashed into folds, which are easily visible in the quarry walls. And, insult to injury, their destiny was to become concrete. Hence, the name of the town. These little limestone lenses litter the Chilliwack Terrane, and in a land full of volcanics and metavolcanics, topped with yet more volcanics, they’re a delightful interlude.
You can find any number of calcite veins running through the rocks here, where water dissolved the calcite from the limestone and redeposited it. Some of it has turned some rather spectacular oranges and reds, which indicates there’s an iron-rich source of something around. But as gorgeous as some of those samples are, it’s a bit hard to concentrate on them. You see, there’s this view:
I told you there’s a volcano on top. From here, you can see it. Mount Baker is a result of the subduction of the Juan de Fuca Plate. To its right, you can see Mount Shuskan. Young mountain and old, volcano and uplift, all in one glance. Mount Baker is only about 100,000 years old, and, despite its peaceful appearance, is the second most active volcano in Washington. It would have been further to the east had the angle at which the oceanic Juan de Fuca Plate is subducting under North America hadn’t steepened.
Mount Shuskan is over a hundred times older. It formed when yet another terrane slammed into North America in the Cretaceous, about 120 million years ago, and is composed of metamorphosed oceanic basalt. This is what happens to quiet ocean floors when subduction grabs them: they can end up stuffed miles below the Earth’s surface, where they get cooked, then popped all those miles back up to an elevation of nearly 10,000 feet, high and dry.
And we’re about to see what happens to rocks that get deeply buried and then raised up as high peaks, although we won’t be personally encountering any more ocean floor.
Diablo and Ross Lakes
SR 20 takes you up and up, from what we in the West call hills (although in some regions, they’re mountains in their own right) and into a wonderland of sharp, towering peaks, with chains of lakes that don’t look like they were created by simple human beings damming the Skagit River.
We’re in the Metamorphic Core Domain now, standing on the Chelan Mountains Terrane. This area has been mountains more than once. As the Kula Plate collided with North America and terrane after terrane was plastered to the continent, great slabs of rock were thrust up. Magma stitched everything together, forming plutons more than once, which were then deeply buried and metamorphosed. The results are all around us here. Colonial and Pyramid Peaks are made of the beautiful Skagit Gneiss. If you’re a fan of banded and orthogneiss, this is your bliss.
Photos can’t capture how gorgeous these 60-90 million year old rocks are. The snowy white feldspar and quartz set off the glittering crystals of hornblende and biotite wonderfully. Everything glitters.
Those rocks testify to great masses of magma, which cooled underground and stitched newly-accreted terranes to the resident rocks. It might have been the weight of those masses that drove everything 20-30 kilometers (12-18 miles) down, where those granitic rocks got metamorphosed to orthogneiss. But that wasn’t the end of the story. About 45 million years ago, another episode of magmatism sent dikes of granitic pegmatite through the orthogneiss. You can see a magnificent example of this Challis Episode activity just across the road:
Things get rather hot in subduction zones.
Just up the road, you can also see evidence of how broken up things get when plates ram each other. Through all of these collisions, the crust has been folded, faulted, and fractured. As terranes got shoved over and under one another, bits broke off and went along for the ride. Things have been so thoroughly smashed up and metamorphosed that geologists find it difficult to figure out just what things used to be. Some of the gneiss and schist in these areas may have begun life as sedimentary rocks, but they’ve been so transformed it’s hard to know for sure. One thing’s definite, though: there were calcite-rich critters living in shallow seas, probably around volcanic island arcs, and their remains can be found down by Ross Lake, turned to marble.
You may have to get your feet wet to get a good look at it, but the cold dip is well worth it. There’s something about a rare bit of locally-grown marble that’s irresistible. This isn’t Michelangelo’s marble: it’s not that glowing creamy white, and it’s full of folds and fractures and evidence of a very rough life. But it’s ours, and we love it.
Coming down the trail to the lake, you’ve passed a fabric of orthogneiss and schist, cut with faults. We’re down in the Ross Lake Fault Zone, which marks the border between the highly-altered Metamorphic Core Domain, and the lightly-touched rocks of the Methow Basin to the east. Rocks have been scooted along this fault zone for tens of millions of years as movement along faults continued. Stand in the center of the dam, looking northeast toward Jack Mountain, and you’re staring at a stack of terranes piled up like so many bagels in a bag.
A thin slice on top, rising from left to right, is a bit of the Methow Domain’s Hozameen Terrane. Beneath it is the Little Jack Terrane, and then we’re back down on the Metamorphic Core Domain. Three terranes in one glance. One ginormous fault zone. Some truly outstanding geology, and we haven’t even gotten to the story of the modern Cascades yet.
The Kula Plate, followed by its sibling Farallon Plate, disappeared millions of years ago, swallowed up beneath North America. The Farallon Plate is still with us, in fragments: the Juan de Fuca Plate is one of its last gasps. As the Farallon broke up, and the angle of the subducting slabs changed, the edge of the continent warped. And with that warping came the mountains. They were born a mere five million years ago, and they’re still growing.
In a mere hundred miles, we’ve crossed hundreds of millions of years, at least five terranes, and barely seen a native North American rock. We’ve passed active volcanoes and walked on extinct island arcs. We’ve tramped ocean basins, reefs and plutons. The scenery here is dramatic: the geology even more so. We’ve only just begun to unravel the tangled history of the mélange that makes up one of the greatest geological stories on Earth.
Quite a trip, isn’t it?
Brown, E.H. et al (2005). “Revised ages of blueschist metamorphism and the youngest pre-thrusting rocks in the San Juan Islands, Washington.” Canadian Journal of Earth Sciences, v. 42, p. 1389–1400.
Brown, E.H. et al (2007). “Tectonic evolution of the San Juan Islands thrust system, Washington.” The Geological Society of America Field Guide 9.
Carson, Bob and Babcock, Scott (2000). Hiking Guide to Washington Geology. Sandpoint, ID: Keokee Books.
Figge, John (2009). Evolution of the Pacific Northwest. Seattle: Northwest Geological Institute.
Haugerud, R.A. and Tabor, R.W. (2009). “Geologic Map of the North Cascade Range, Washington.”
Tabor, Rowland and Haugerud, Ralph (1999). Geology of the North Cascades: A Mountain Mosaic. Seattle: The Mountaineers.
Tucker, Dave (2010): “Baker River limestone and the town of Concrete, Washington.” Northwest Geology Field Trips.
Washington State Department of Natural Resources: Geology of Washington.
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