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Archive for the tag “Waterfalls”

Washington’s waterfalls–behind each one is a rock!

Of all the many reasons why waterfalls are great, here’s another: they expose bedrock! And that bedrock tells a story extending back in time long long before the waterfall. This posting describes 9 waterfalls that together paint a partial picture of Washington’s geologic history. The photos and diagrams will all appear in my forthcoming book Roadside Geology of Washington (Mountain Press) that I wrote with Darrel Cowan of the University of Washington.

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Rainbow falls along WA 6 in the Coast Range

And waterfalls in heavily forested areas are especially great because they may give the only view of bedrock for miles around! Take Rainbow Falls, for example–the small waterfall on the left. It’s in Washington’s Coast Range along State Highway 6–a place where a roadside geologist could otherwise fall into total despair for lack of good rock exposure. But this beautiful waterfall exposes a lava flow of the Grande Ronde Basalt, which belongs to the Columbia River Basalt Group. Significant? Yes!

This lava erupted in southeastern Washington and northeastern Oregon between about 16 and 15.6 million years ago and completely flooded the landscape of northern Oregon and southern Washington. We know how extensive these flows are because we can see them–and they cover the whole region. The photo below shows them at Palouse Falls in the eastern part of Washington. Take a look at my earlier blog post about the Columbia River Basalt Group? (includes 15 photos and a map)

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Palouse Falls in eastern Washington drops more than 180 feet over lava flows of the Grande Ronde and Wanapum members of the Columbia River Basalt Group.

You might also notice in the photo above that the waterfall is actually pretty small compared to its amphitheatre. That’s because Palouse Falls is part of another flood story –of the Ice Age Floods, described in rich detail on the Ice Age Floods Institute website. Basically, some 40 or 50 gigantic floods coursed through the area towards the end of the Ice Age, between about 15-18,000 years ago. and among other things, carved this canyon. Lobes of the continental ice sheet repeatedly dammed the Clark Fork River in northern Montana and then failed, repeatedly, after forming Glacial Lake Missoula. Imagine the flow volume in the above photo multiplied more than 100,000 times!

Mount Rainier and the Cascade Volcanoes
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At 14,410 feet above sea level, Mount Rainier is the highest volcano in the Cascade Range –and one of the highest spots in the conterminous United States. The volcano itself consists mostly of andesite flows that date back nearly a half million years.

Beneath those lava flows are older rocks that speak to a history of volcanic activity reaching back 70 times that of Rainier’s oldest lavas –to about 35 million years ago. At Christine Falls, you can inspect granitic rock of the Tatoosh Pluton, which is a crystallized magma chamber that formed beneath some early Cascade volcanoes. It was probably active at different times between 26-14 million years ago. At Narada Falls, you can see where Rainier andesite actually flowed over the top of the granite–which tells us that the granite was exposed at the surface 40,000 years ago when that flow erupted. Both these waterfalls are right along the road that winds its way from Longmire up towards Paradise Meadows.

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Christine Falls (left) cuts through granitic rock of Tatoosh Pluton; Narada Falls (right) flows over Rainier Andesite that itself flowed over Tatoosh granodiorite, exposed on the rocky hillside.

If you go to the south entrance of the national park, you can walk a quarter mile from the highway to Silver Falls and exposures of Rainier’s oldest rocks. The Ohanapecosh Formation, made mostly of tuffs and re-deposited volcanic particles, formed by explosive volcanic activity that stretches back 35 million years. The Ohanapecosh Formation forms cliffs throughout much of the national park –and shows up northward as far as Interstate 90.

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Silver Falls in Mount Rainier National Park, spills over outcrops of Ohanapecosh Formation, the park’s oldest rock.

Finding the oldest volcanic rock in the Cascade Volcanoes is important because this incredibly active volcanic chain is fueled by magma generated through the sinking of oceanic lithosphere at the Cascadia subduction zone –and the oldest rocks allow us to estimate when this process started. They get even older at Snoqualmie Falls, just north of I-90. There, rocks of the Mount Persis Volcanics reach ages of 38 million years. Most geologists agree that for Washington, these rocks mark the first volcanic activity after the formation of the Cascadia subduction zone.

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Snoqualmie Falls drops more than 250′ into a gorge of Mt. Persis Volcanics –rocks that mark the onset of volcanic acitity related to today’s Cascadia subduction zone.


Early Volcanic Roots and Continental Accretion

Here it gets a little complicated, because subduction also drove much of Washington’s geologic history before the Cascade volcanoes started to form. This older subduction also formed volcanic chains and through the process of continental accretion, caused Washington to grow westward.

Intro-8. Accretion series-CS4This diagram, modified from my book Roadside Geology of Oregon, illustrates the process of accretion. Basically, some element of the subducting seafloor is unable to fully sink beneath the continent, probably because it’s topographically high– such as with a series of seamounts. This material jams up the subduction zone and causes the sinking to stop temporarily. Eventually, a new subduction zone forms farther offshore and the thing that jammed up the zone in the first place gets added, or accreted, to the edge of the continent. In Washington and Oregon, the younger Cascadia subduction zone is the one that formed the Cascade Volcanoes and the stuff that jammed the zone was a huge fragment of oceanic lithosphere called “Siletzia”. Siletzia now makes up the bottom of Washington and Oregon’s Coast Range. The older subduction zone that got jammed up is the one that’s responsible for the rocks described below.

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Gorge Creek cuts a slot through orthogneiss (inset) of the Skagit Gneiss Complex along State Highway 20 in Washington’s North Cascades.

Gorge Falls along State Highway 20 in the North Cascades cuts this narrow slot through rocks formed because of that older subduction zone. These rocks started as the granitic roots to volcanoes, much in the same way as the Tatoosh Pluton formed the roots to some Cascade volcanoes. Those roots then got squeezed and reheated to make a metamorphic rock called gneiss. In some places it even partially re-melted.

The inset gives a close-up view of the rock. It’s called “orthogneiss” because it started out as an igneous rock. It forms a big part of the Skagit Gneiss Complex, which makes up the core of the North Cascades.

It’s hard to say if the Skagit Gneiss Complex was actually added to the edge of North America from somewhere else, but a lot of other rocks in Washington were–and those episodes of accretion are what caused much of the metamorphism in the North Cascades.

 

For accreted rock, here’s probably my favorite waterfall: Nooksack Falls, along State Highway 542 between Bellingham and the Mt. Baker ski area. It’s made of conglomerate of the Nooksack Group, which accumulated in a submarine fan somewhere off the coast of North America during the Jurassic and Cretaceous Periods, maybe 140 million years ago.

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Nooksack Falls in the North Cascades. the horizontal lines across the falls mark traces of bedding in the rock that’s inclined directly upstream.

Ancient North America

If you go eastward towards Spokane, you eventually find yourself on the North America that existed before all this accretion. Of course, much of the area is now covered by the Columbia River Basalt, but in the northeast corner of the state, you encounter Paleozoic sedimentary rocks that formed along the continental margin of that older continent. Sweet Creek Falls is one place to see these rocks, right off State Highway 31. There, the beautiful stream spills over ledges of Ledbetter Slate, deposited as shale during the Ordovician Period. In the foreground are cobbles of Addy Quartzite, formed as beach-deposited sandstone in the Cambrian.

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Sweet Creek Falls spills over Ledbetter Slate. Cobbles of Addy Quartzite lie in the foreground.

 

Washington’s Geologic Timeline

The timeline below shows Washington’s main geologic events –and you can see where these 9 waterfalls fit. The red text and red-colored bars represent geologic events represented by individual waterfalls, shown in blue.  Kind of amazing… these 9 waterfalls show many of Washington’s most important elements: the Cascade Volcanoes, the Columbia River Basalt Group, continental accretion, and the old continental margin.

And they’re nice places to hang out!

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Timeline of Washington’s geology. Red text signifies events described in this post and represented by various waterfalls (in blue).

 


For more geology photos, please check out my website–it contains a searchable database of more than 2000 geology photos for free download.

Roadside Geology of Washington should be out and available in August, 2017.

Thanks for reading!

 

 

 

 

 

Columbia River Basalt Group–outrageous!

I can’t stop thinking about the Columbia River Basalt Group–the series of basalt flows that blanketed so much of my state of Oregon about 15 million years ago. Abbreviated as “CRBG”, it covers a lot of Washington too, as well as parts of western Idaho and northern Nevada. If you’re driving across those parts, you’ll likely travel miles and miles and miles over basalt basalt basalt –and that causes some people to say (mistakenly) that it’s boring. Some geologists even get grumpy about it because it covers up all the older rock.  Outrageous!

Lava flows of the CRBG in northern Oregon and Mt. Adams of southern Washington.  With views like this, how can you say the CRBG is boring? (Location "F" on map below.)

Lava flows of the CRBG in northern Oregon and Mt. Adams of southern Washington. With views like this, how can you say the CRBG is boring? (Photo “F” on map below.)

But of course, the CRBG is outrageous for a whole host of other reasons. For one thing, it really is huge: it covers an area of more than 77,000 square miles with a volume of more than 52,000 cubic miles –that’s more than 50x the volume of air between the north and south rims of the Grand Canyon! Really—the National Park Service estimated the volume of “Grand Canyon Air” to be about 1,000 cubic miles. It also erupted over a fairly short period of time: from about 17 million years ago to 6 million –but 96% of it erupted between 17 and 14.5 million years ago.

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And… most of it erupted from fissures in eastern Oregon and Washington –the roots of which are now preserved as dikes. And… many of the lavas made it all the way to the Pacific Ocean. And… (here’s the outrageous part), on reaching the Pacific, many of the flows re-intruded into the existing sediments and sedimentary rocks along the coastline to form their own magma chambers, some of which were thousands of feet thick! AND… some basaltic magma from those chambers then re-intruded the country rock to form dikes –and some even re-erupted on the seafloor!

All these outrageous details. Now think about them for a moment. They really happened. That’s what I find so wonderful and amazing about geology. We learn all these things and we put them in some part of our consciousness that doesn’t really let them soak in –but once in awhile they do.

Finally, the CRBG is beautiful and forms beautiful landscapes! Below are some photos to illustrate it, from feeder dikes in eastern Oregon to sea stacks eroded from a giant sill on the coast.

And I’ll save my snarky comments about young earth creationism for another post.

–and at the bottom, I’m adding a short glossary to explain some of the terms.

Ok… the photos!

 

Photo A. Steens Mountain and Alvord Desert.  The CRBG started with eruption of the Steens Basalt about 16.7 million years ago, which makes up the upper 3000′ or so of Steens Mountain, shown here.  Steens Mountain is one of our state treasures –it’s a fault-block mountain, uplifted by Basin-Range extension along a normal fault along its eastern side.

Fault-bounded east front of Steens Mountain and Alvord Desert.

Fault-bounded east front of Steens Mountain; mudcracked playa of Alvord Desert in foreground.

 

Photo B. Steens Basalt at Abert Rim. Like most of the CRBG, The Steens basalt covered outrageously huge areas.  It also makes up the cliffs above Lake Abert about 75 miles to the east.  Called Abert Rim, the cliffs are also uplifted by a big normal fault.  Lake Abert occupies the downdropped basin.  And much of the Steens basalt consists of this really distinctive porphyry with outrageously big plagioclase crystals!

Photo B.  Steens Basalt at Lake Abert; Abert Rim in background.

Photo B. Steens Basalt at Lake Abert; Abert Rim in background.

 

Photos C-1, C-2. CRBG dikes.  One reason we know that the CRBG erupted from fissures is that we can see their roots, as dikes cutting through older rock.  C-1 shows a dike cutting through previously erupted basalt flows in Grande Ronde Canyon, Washington; C-2 shows some narrow little dikes cutting accreted rock of the Triassic Martin Bridge Limestone. Photos 5 and 6 of my last post shows some aerial photos and describes this area in more detail.C. Feeder dikes

 

Photo D. Imnaha Canyon. The next major unit of the CRBG is the Imnaha Basalt, followed by the Grande Ronde Basalt.  Both these units erupted from sites in northeastern Oregon and southeastern Washington.  This view of Imnaha Canyon in Oregon shows the Imnaha Basalt near the bottom and the Grande Ronde Basalt at the top.

Photo D.  Imnaha Canyon, Oregon.

Photo D. Imnaha Canyon, Oregon.

 

Photo E. Picture Gorge Basalt at the Painted Hills.  And the next youngest unit of the CRBG was the Picture Gorge Basalt, shown capping the ridge in the background. Unlike most of the CRBG, the Picture Gorge Basalt originated in central Oregon, not too far from here–there’s a whole swarm of dikes near the town of Monument, Oregon.

The colorful hills in the foreground make up the Painted Hills of John Day Fossil Beds National Monument, another of our state treasures.  I like this photo because it gives a sense of what lies beneath the CRBG –and the John Day Fossil Beds are outrageous in their own way–but save that for another time.

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Photo F. Lava Flows of the CRBG and Mt. Adams, a modern volcano of the High Cascades in Washington.  See the first picture at the top of the post!

 

Photo G. Wanapum Basalt near The Dalles. This exposure of the Wanapum Basalt, which overlies the Picture Gorge Basalt, tells the story of the CRBG as it flowed into and filled a lake along the Columbia River some 15 million years ago. At the bottom of the flow, pillow basalt formed as the lava poured into the lake, while the upper part of the flow shows the columnar jointing typical of basalt that flows across land.  What’s more, this exposure lies less than a mile off I-84 in The Dalles, Oregon.  See page 251 of the new Roadside Geology of Oregon for another photo and more description!

Photo G. Single flow of Wanapum Basalt near The Dalles, Oregon.

Photo G. Single flow of Wanapum Basalt near The Dalles, Oregon.

 

Photo H.  Upper North Falls, Silver Falls State Park, Oregon.  This wonderful state park hosts about a zillion waterfalls that spill over cliffs of CRBG, 14 of which lie in the main river channels of the north and south forks of Silver Creek.  The falls depicted in this photo are 136 feet high!

Upper North Falls at Silver Falls State Park, OR.  The roof of the alcove consists of Wanapum Basalt, the bedrock near the river channel consists of Grande Ronde Basalt.

Upper North Falls at Silver Falls State Park. The roof of the alcove consists of Wanapum Basalt, the bedrock near the river consists of Grande Ronde Basalt.

Notice that the picture’s taken from behind the water. The trail goes into a big alcove, so it’s easy and safe.  The alcove formed because this particular waterfall crosses the contact between the Wanapum Basalt and the underlying Grande Ronde Basalt –and there is a 10-20′ thick, easily eroded, sedimentary unit between the two.  Remember the Grande Ronde Basalt –from Photo D in northeastern Oregon? Here it is, just east of Salem!

 

Photo I.  Saddle Mountain, northern Coast Range.  Here starts the truly outrageous part of the CRBG story.  Saddle Mountain, the highest point in the northern Coast Ranges, consists almost entirely of the rock on the right: brecciated pillow basalt, full of the alteration mineral palagonite. Apparently, the basalt started to flow into the ocean at about here, formed pillows and fragmented like crazy in the water-lava explosions. I. Saddle Mtn

But!  These flows were likely confined too –such as in a submarine canyon–which allowed them to develop enough of a pressure gradient to intrude downward into bedding surfaces, faults, and fractures of the Astoria Formation.  The diagram below illustrates the process in cross-section.  The diagram also give a context for photos I-L.  intrusive CRBG diagram4

 

Photo J.  Sea stacks of intrusive Columbia River Basalt Group at Ecola State Park.  Some of the magma chambers were several thousand feet thick and are now exposed as gigantic sills along the coast.  One such sill is Tillamook Head, of which Ecola State Park is a part –and it’s eroding into the sea stacks you can see in the distance.J.130120-11lrs

 

Photo K.  Haystack Rock at Cannon Beach, OR.  Go figure, one of our iconic state landmarks is an undersea volcano?  You can actually walk out to this thing at low tide and see lots of pillow basalt and dikes intruding the Astoria Formation.  The smaller sea stacks are part of the same complex.K.130121-12

 

Photo L. Seal Rock, Oregon.  Seal Rock is the southernmost exposure of CRBG on the coast –and it too, is intrusive.  It’s a big dike that trends NNW for about a quarter mile out to sea.  And along its edges, there are smaller dikes that you can see intruding the Astoria Formation, such as in the smaller photo.  The arrow points to where you can see the small intrusion, at low to medium tides.

L. Seal Rock


Some Terms:
dike: a tabular-shaped intrusion that cuts across layering in the surrounding rock.  Imagine magma flowing along a crack and eventually cooling down and crystallizing.  That would form a dike.  A feeder dike is a dike that fed lava flows at the surface.

normal fault: a type of fault along which younger rocks from above slide down against older rocks below.  They typically form when the crust is being extended.

porphyry: an igneous rock with larger, easily visible crystals floating around in a matrix of much smaller ones.

sill: an intrusion that runs parallel to layering in the surrounding rock.


 

Some references:

Reidel, S.P., Camp, V.E., Tolan, T.L., Martin, B.S. 2013. The Columbia River flood basalt province: stratigraphy, areal extent, volume, and physical volcanology. In The Columbia River Basalt Province, Geological Society of America Special Paper 497, eds. S.P. Reidel, V.E. Camp, M.E. Ross, J.A. Wolff, B.S. Martin, T.L. Tolan, and R.E. Wells, p. 1-44.

Wells, R.E., Niem, A.R., Evarts, R.C., and Hagstrum, J.T. 2009. The Columbia River Basalt Group—From the gorge to the sea. In Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest, Geological Society of America Field Guide 15, eds. J.E. O’Connor, R.J. Dorsey, and I.P. Madin, p. 737-774.


Some links:

Roadside Geology of Oregon
Geology pictures for free download
Geologic map of Oregon

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