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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!

 

 

 

 

 

Oregon’s rocky headlands: geologic recycling through erosion and uplift and erosion…

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Crashing waves at Heceta Head, Oregon

You can’t avoid thinking about erosion while standing on one of Oregon’s rocky headlands. The waves keep coming, one after another, each crashing repeatedly against the same rock. Impossibly, the rock appears unmoved and unchanged. How can it not erode?

The answer, of course, is that headlands do erode, quickly, but on a geologic time scale. We just miss out because we live on the much shorter human time scale. And the erosion belongs to a cycle in which coastal uplift causes eroded and flattened headlands to rise and become headlands once again, all subject to more ongoing erosion and uplift.

Wave energy is most intense at headlands because the incoming wave typically feels the ocean bottom near the headland first, which causes the wave to refract. As shown in the aerial photo below, this refraction focuses the wave energy on the headland.

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Wave refraction causes wave energy to focus on the headland. Arrows are perpendicular to wave fronts.

As you can see in the next few images, headlands don’t erode evenly. They erode irregularly, as the waves exploit any kind of weakness in the rocks such as faults and fractures, or if they’re sedimentary, bedding surfaces. The products of this erosion are as beautiful as they are interesting: sea stacks, sea arches, sea caves… The list goes on and on.

Headland and lighthouse, Heceta Head, Oregon

Aerial view of Heceta Head, Oregon.

From the above photo, you can see that sea stacks are simply the leftover remains of a headland as it retreats from erosion. That’s a critical point, because some sea stacks, especially the one with the arch in the photo below, are a long way from today’s coastline.

Sea stacks and sea arch, southern Oregon

Sea stacks and sea arch, southern Oregon

Those rocks, 1/4 to a 1/2 mile away used to be a part of the coastline? The land used to be way out there? YES!!! For me, that’s one of the very coolest things about sea stacks –they so demonstrate the constant change taking place through erosion.

Taken to its extreme, erosion renders headlands into wave-cut platforms, such as the one below at Sunset Bay. Being in the intertidal zone, these platforms make great places for tide-pooling–and ironically, for people-watching too. Geologically, they form important markers because they’re both flat and form at sea level. When found at higher elevations, they indicate uplift.

Wave-cut bench, Sunset Bay, Oregon

Wave-cut bench at Sunset Bay, Oregon

In fact, looking carefully at the photo above, you can see a flat surface on the other side of the bay. It’s an uplifted wave-cut platform! Called a marine terrace, it’s covered by gravel and sand originally deposited in the intertidal zone. Those deposits rest on bedrock that, at an earlier time, was also flattened by the waves. The photo below shows a better view of this terrace from the other side.

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Breaking wave at Shore Acres State Park, Oregon. Tree-covered flat surface in the background is an uplifted marine terrace.

These uplifted marine terraces can be found up and down Oregon’s coastline. Researchers recognize several different levels, the oldest being those uplifted to highest elevations. The one in the photo above at Shore Acres State Park is called the Whiskey Run Terrace and formed about 80,000 years ago. You can see a similar-aged terrace below as the flat surface beneath the lighthouse at Cape Blanco, Oregon’s westernmost point. An older, higher terrace forms the grass-covered flat area on the right side of the photo.

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Cape Blanco, Oregon looking NE. The flat surface beneath the lighthouse is the ~80,000 year-old Cape Blanco Terrace, probably equivalent to the Whiskey Run Terrace at Shore Acres; the flat area on the right side of the photo is the higher Pioneer Terrace,  formed ~105,000 years ago.

Researchers take the approximate ages of the terraces and their elevations to calculate approximate rates of uplift. In this area, Kelsey (1990) estimated a rate of between 4-12 inches of uplift every 1000 years. That might seem slow, but over hundreds of thousands of years, it can accomplish a great deal.

And look! The uplifted terraces? They’re on headlands! Of course, because they’ve been uplifted! And the headlands are now eroding into sea stacks and then platforms –to be uplifted in the future and preserved as marine terraces that sit on top headlands. And on and on, as long as the coastline continues rising.

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Blowhole near Yachats, Oregon. Incoming wave funnels up a channel eroded along a fracture and explodes upwards on reaching the end.

Some links and references:
Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces of the Cascadia Subduction Zone near Cape Blanco, Oregon: Tectonics, v. 9, p. 983-1014. (Detailed study of Cape Blanco, including uplift rates).

Miller, M., 2014, Roadside Geology of Oregon, Mountain Press, Missoula, 386p. (General reference which details the concepts and includes several of the photos used here).

Earth Science Photographs–free downloads for Instructors or anybody: my webpage!

Scientists, Science, Icicles, and Faith

In January, I started teaching the Introductory Geology course “Environmental Geology and Landform Development” –with two lecture sections of about 200 students each. And this course, populated largely by folks who are fulfilling a science requirement and  otherwise try to avoid science like it was the plague, needed some general statement about science. After all, it’s science that may someday save them from the plague!

So science… what is it? Seems like scientists themselves have a zillion different definitions, so I started with “Scientist. –What’s a scientist?” If you google “scientist” and then look at the images, you see this. As this image is a screenshot of photos that aren’t mine, I intentionally blurred it, but you should get the idea of what’s there.

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Really??? these are the most popular images of scientists and in every picture–save the tiny one in the lower right– is some person in a WHITE LAB COAT and a microscope or a beaker. Ironically, it shows about 50% of the scientists as women. Go figure there too.

Looks like we’ve been fed a misrepresentation of what scientists are. We actually do a wide variety of things. In geology, we do a wide wide range of things. We spend time in the field (see picture below), we write, we draw maps and cross-sections, we look down microscopes (maybe in jeans and t-shirts), we write computer models, we do experiments, and we sometimes wear white lab coats.

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Geologist inspecting a fault zone between the dark-colored Beck Spring Dolomite and the overlying light-brown Noonday Dolomite. Death Valley, California.

All the time, we’re trying to understand something about our world. Our universe. We’re collecting information (data). We’re testing ideas. We’re adding detail to somebody else’s ideas. We’re building a framework of knowledge that’s grounded in our observations and testable ideas. Replace the word “ideas” with “hypotheses” in this paragraph –and you get science.

Ideally, most scientists approach their work using the “scientific method” –which is a highfalutin way of saying they see something they don’t understand (an observation), which causes them to ask a question (like how did this happen?); they come up with ideas (hypotheses) that may explain it, and then they test those hypotheses.

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Icicles?

Which is what we did in class with icicles! The month before–in mid-December–Eugene had this incredible ice storm, which covered everything in ice to make it look like a scene from the movie Frozen. It was beautiful and destructive. And we can all pretty much guess how icicles form: water starts to drip off the branch but freezes before it falls off. Icicles grow straight downward off the branch because water, like everything else, falls vertically with gravity.

As it turned out, some of the icicles seemed to grow straight out from the branches. Look at the photo below! How could this be? We know icicles should grow straight downwards! So as a group, we came up with some hypotheses, shown below next to the picture. I was the proud sponsor of hypothesis #4 and #5.

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Alternate hypotheses to explain near horizontal growth of icicles

As a group (all 200 of us), we could rule out hypothesis #3, that the picture was rotated. I shot the image and promised I didn’t rotate it! We could also rule out hypothesis #4, that the ice somehow grew horizontally towards the branch, because that idea conflicted with all previous observations we’d made on icicles, that they grow away from the branch as ice progressively freezes.

That left hypotheses #1, #2, #5. We figured ways we could test #1, #2. If it were the wind, for example, we’d expect all the icicles to go in one direction in a given place, regardless of the limb angle. If it were #2, we might expect to see some icicles show a curve to indicate progressive tilting of the branch–which you can actually see in the photo above!

Hypothesis #5, that “Some magical force caused it to grow sideways”isn’t testable. It’s NOT TESTABLE. We can’t come up with ways to support it or rule it out. You can believe it if you want to, but it’s not science.

That’s the point. To be scientific, a hypothesis must be testable. Most of us hold various non-scientific beliefs in our hearts that we know to be true –for us. I think that’s a good thing. For many of us, those beliefs lend us qualities like strength or courage or compassion when we need them the most. They’re still not scientific.

And that’s what really gripes me about the “scientific creationists” –as well as today’s Republican Party. The “scientific creationists” say they use science to demonstrate the existence of God, or that Earth is young –when believing either requires a suspension of science and an act of Faith. By claiming they’re being scientific, the “scientific creationists” hamstring their own belief system. They take the wonder out of religion and render it baseless and sterile.

And the Republicans? They’re now all about “alternative facts”. Maybe it’s unfair to group “all Republicans” together –but I see very few standing up to this reckless leader we have. Maybe they just lack integrity.

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this photo was rotated

Landscape and Rock–4 favorite photos from 2015

Landscape and bedrock… seems we seldom connect the two. We all like beautiful landscapes, but most of us don’t ask how they formed –and even fewer of us think about the story told by the rocks that lie beneath it all. Those make two time scales, the faster one of landscape evolution and the much slower one of the rock record. Considering that we live in our present-day human time scale, it’s no wonder there’s a disconnect!

Take this photo of Mt. Shuksan in northern Washington. My daughter Meg and I drove up to the parking lot at Heather Meadows and went for a quick hike to stretch our legs and take some pictures just before sunset.We had about a half hour before the light faded –and all I could think about was taking a photo of this amazing mountain. But the geology? What??

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1. Mt. Shuksan and moonrise, northern Washington Cascades.

Thankfully, I’d been there in September scoping out a possible field project with a new grad student, and had the time to reflect… on time. From the ridge we hiked, shown as the dark area in the lower left corner of the left-hand photo below, we could almost feel Shuksan’s glaciers sculpting the mountain into its present shape. Certainly, that process is imperceptibly slow by human standards.

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Mt. Shuksan: its glaciated NW side, summit, and outcrop of the Bell Pass Melange.

But the glaciers are sculpting bedrock –and that bedrock reveals its own story, grounded in a much longer time scale.

It turns out that the rock of Mt. Shuksan formed over tens of millions of years on three separate fragments of Earth’s lithosphere, called terranes. These terranes came together along faults that were then accreted to North America sometime during the Cretaceous. At the top of the peak you can find rock of the Easton Terrane. The Easton Terrane contains blueschist, a metamorphic rock that forms under conditions of high pressures and relatively low temperatures, such as deep in a subduction zone. Below that lies the Bell Pass Melange (right photo) –unmetamorphosed rock that is wonderfully messed up. And below that lies volcanic and sedimentary rock of the Chilliwack Group.

Here’s another of my favorites from 2015: the Keystone Thrust! It’s an easy picture to take –you just need to fly into the Las Vegas airport from the north or south, and you fly right over it. It’s the contact between the gray ledgey (ledgy? ledgeee?) rock on the left and the tan cliffs that go up the middle of the photo.

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2. Keystone Thrust fault, Nevada–gray Cambrian ridges over tan Jurassic cliffs.

The gray rock is part of the Cambrian Bonanza King Formation, which is mostly limestone, and the tan cliffs consist of  Jurassic Aztec Sandstone. Cambrian, being the time period from about 540-485 million years, is a lot older than the Jurassic, which spanned the time 200-145 million years ago. Older rock over younger rock like that requires a thrust fault.

Talk about geologic history… the thrust fault formed during a period of mountain building during the Cretaceous Period, some 100-70 million years ago, long before the present mountains. And the rocks? The limestone formed in a shallow marine environment and the sandstone in a sand “sea” of the same scale as today’s Sahara Desert. We know it was that large because the Aztec Sandstone is the same rock as the Navajo Sandstone in Zion and Arches national parks.

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left: Limestone of the Cambrian Bonanza King Formation near Death Valley; right: Cross-bedded sandstone of the Jurassic Navajo Sandstone in Zion NP, Utah.

So… the photo shows cliffs and ledges made of rocks that tell a story of different landscapes that spans 100s of millions of years. But today’s cliffs and ledges are young, having formed by erosion of the much older rock.  Then I flew over it in about 30 seconds.

At Beach 2 near Shi Shi Beach in Washington State are some incredible sea stacks, left standing (temporarily) as the sea erodes the headlands. The sea stack and arch in the photo below illustrates the continuous nature of this erosion. Once the arch fails, the seaward side of the headland will be isolated as another sea stack, larger, but really no different than the sea stack to its left. And so it goes.

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3. Sea arch and headland at Beach 2, Olympic Coast, Washington.

And of course, the headland’s made of rock that tells its own story –of  deposition offshore and getting scrunched up while getting added to the edge of the continent.

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Bedrock at Beach 2 consists mostly of sandstone and breccia. The white fragment is limestone mixed with sandstone fragments.

And finally, my last “favorite”. It’s of an unnamed glacial valley in SE Alaska. My daughter and I flew by it in a small plane en route to Haines, Alaska to visit my cousin and his wife. More amazing landscape–carved by glaciers a long time ago. But as you can expect, the rock that makes it up is even older and tells it’s own story.

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4. Glacial Valley cutting into Chilkat Mountains, SE Alaska.

Of course, this message of three time scales, the human, the landscape, and the rock-record time scale applies everywhere we go. Ironically, we’re usually in a hurry. I wish I kept it in mind more often, as it might slow me down a little.

Here’s to 2015 –and to 2016.

To see or download these four images at higher resolutions, please visit my webpage: favorite 10 geology photos of 2015.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conglomerate!

A trip to Death Valley over Thanksgiving two weeks ago reignited all sorts of things in my brain, one of which being my love of conglomerate. Honestly, conglomerate HAS to be the coolest rock!

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Tilted conglomerate in Furnace Creek Wash, Death Valley.

Just look at this stuff! Just like any good clastic sedimentary rock, it consists of particles of older rock–but with conglomerate, you can easily see those particles. Each of those particles opens a different door to experiencing deep geologic time.

As an example, look at the conglomerate below, from the Kootenai Formation of SW Montana. It contains many different cobbles of light gray and dark gray quartzite and pebbles of black chert. The quartzite came the Quadrant Formation and chert from the Phosphoria Formation. So just at first glance, you can see that this conglomerate in the Kootenai contains actual pieces of two other older rock units.

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Conglomerate of the Kootenai Formation, SW Montana.

But consider this: The Quadrant formed as coastal sand dunes during the Pennsylvanian Period, between about 320-300 million years ago and the Phosphoria chert accumulated in a deep marine environment during the Permian, from about 300-250 million years ago. The Kootenai formed as river deposits during the early part of the Cretaceous Period, about 120 million years ago. All those are now together as one.

Similar to the modern river below (except for the glaciers), the Kootenai rivers transported gravel away from highlands –the highlands being made of much older rock that was uplifted and exposed to erosion. That older rock speaks to long gone periods of Earth history while the gravel speaks to the day it’s deposited.

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Athabasca River in Jasper National Park, Alberta

But this is where my head starts to spin: the modern gravel is made of rounded fragments of old rock –so when you look at a conglomerate, you glimpse at least two time periods at once: you see the conglomerate, which reflects a river or alluvial fan –or any environment near a bedrock source– and you also see the particles, which formed in even older environments.

And it gets worse –or better. What happens when you see a conglomerate eroding? The conglomerate is breaking up into modern sediment, which consists of pieces of older sediment –that at one time was modern sediment that used to be older sediment?  Look at the pebbles below. I keep them in a rusty metal camping cup on a table in my office.

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“Recycled” pebbles of the Kootenai Formation.

These stream pebbles eroded out of the Kootenai conglomerate. So… they’re simultaneously modern stream pebbles and ancient ones –AND… they originated as the Quadrant and Phosphoria Formations. Four periods of time, spanning 300 million years, all come together at once.

And if that’s not enough, those conglomerates in Death Valley? They  contain particles of… conglomerate! Look! The arrow in the left photo points to the boulder of conglomerate on the right. If you click on the photos, you can see them enlarged.

All those particles, which are now eroding and becoming modern sediment, were yesterday’s sediment. And the conglomerate boulder? It too is becoming “modern sediment” and it too was “yesterday’s sediment” when it was deposited on an alluvial fan with the rest of the material. However, it goes a step further: its pebbles and cobbles were both “modern” and “yesterday’s” sediment at a still older time. And before that? Those pebbles and cobbles eroded from even older rock units, some of which date from the Cambrian, about 500 million years ago.

For fun, here’s a photo of another conglomerate boulder.

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Conglomerate boulder in conglomerate of the Furnace Creek Formation, Death Valley, CA.

 

I can’t help but wonder how Young Earth Creationists would deal with these rocks. Given their story of the Grand Canyon, in which the Paleozoic section was deposited during early stages of “The Flood” and the canyon was carved during the later stages (they really do say that too!), they’d probably roll out that same blanket answer: The Flood. End of discussion. No questioning, no wondering.

In my opinion, one of the beautiful things about geology is that we’re always questioning and wondering.

 

 

for more geology photos, please visit my website.

 

 

 

 

Rockin’ countertops–geologic time in our kitchens and bathrooms!

I stopped by a “granite” supplier yesterday –the kind of place that sells “granite” and “marble” slabs for countertops.  Besides the fact that almost none of the slabs were actually granite or marble, they were spectacular rocks that showed wonderful wonderful detail. I nearly gushed at the idea of taking a geology field trip there.  It’s local, and you seldom find exposures like this anywhere else!

slabs of polished rock at a "granite" warehouse --not sure if any of this is actually granite, but it all reflects geologic time.

slabs of polished rock at a “granite” warehouse –most of it’s not actually granite, but it all reflects geologic time.

Generally speaking, “granite” in countertop language means “igneous” or “metamorphic” –crystalline rocks that form miles beneath Earth’s surface and so require great lengths of time to reach the surface where they can be quarried.  When I first started this blog, geologic time with respect to igneous and metamorphic rocks were some of the first things I wrote about –it’s such pervasive and important stuff.

So the main point is that your friend’s kitchen with “granite” countertops surrounds you with geologic time every time you walk in there!

But check out that green polka-dotted rock on the right side of the photo.  Full of rounded cobbles –it’s a conglomerate, originating by sedimentary processes on Earth’s surface. Does it indicate great lengths of geologic time? A Young Earth Creationist might say it were a deposit of “the Flood” and end-of-story.

Here’s a closer look:

Polished conglomerate --individual cobbles are metamorphic rocks. The green color comes from the mineral chlorite.

Polished conglomerate –individual cobbles are metamorphic rocks. The green color of the background material comes from the mineral chlorite. That’s a penny (on the left) for scale.

The conglomerate is made of beautifully rounded cobbles and small boulders that are almost entirely metamorphic in origin.  Most of them are gneisses, which form at especially high grades of metamorphism, typical of depths greater than 8 or 10 miles!  After a (long) period of uplift and erosion, the rock was exposed to erosion, gradually breaking into fragments, which eventually became these rounded cobbles, and ended up in the bottom of a big stream channel or on a gravel bar somewhere.

But that’s not the end of the story, because this deposit of rounded cobbles itself became metamorphosed –so it had to get buried again. We know that because the rock is pervaded by the mineral chlorite, which gives the rock its green color.  Chlorite requires metamorphism to form.  Granted, the rock isn’t highly metamorphosed –there’s no metamorphic layering and chlorite forms at low metamorphic temperatures– but it’s metamorphic nonetheless, typical of depths of a few miles beneath the surface.

And if you look even closer, you can see some of the effects of the reburial pressures: the edges of some of the cobbles poke into some of the other ones. This impingement is a result of the stress concentrations that naturally occur along points of contact.  The high stress causes the less soluble rocks to slowly dissolve into the other, more soluble rock.

cobbles, impinging into each other. Stars on right photo show locations.

cobbles, impinging into each other. Stars on right photo show locations.

I’m already jealous of the person who’s going to buy this slab of rock. It tells a story that begins with 1) metamorphic rock forming deep in the crust, then 2) a long period of uplift and erosion to expose the rocks, then 3) erosion, rounding, and deposition of the metamorphic cobbles, 4) reburial to the somewhat shallow depths of a mile or two–maybe more, 5) more uplift and erosion to expose the meta-sedimentary deposit, 6) Erosion by human beings.

And me? Personally, I’d like to make a shower stall or a bathtub out of this rock –can you imagine???


Some links you might like:
a blog I like that’s about science and creationism
another blog about an ancient Earth and deep time
my original song “Don’t take it for Granite“. (adds some levity?)
Geology photos for free download.

 

 

 

San Andreas Fault

Here’s a view of the San Andreas fault and Pt. Reyes in northern California, looking northward.  The fault runs right up the narrow Tomales Bay–and in just a few miles, runs along the edge of San Francisco.

The San Andreas fault is amazingly well-studied –it’s probably the most-studied fault zone in the world.  After all, it is capable of generating huge earthquakes in heavily populated areas, so the more we know about it the better.

San Andreas fault and Tomales Bay

Aerial view of San Andreas fault and Pt. Reyes --just north of San Francisco. View is to the north. The fault runs down Tomales Bay, the narrow arm of the ocean that runs diagonally across the photo.

One thing we know about the San Andreas is that it generally moves in a side-by-side way (strike-slip) so that rock on the east side moves south relative to that on the west side.  And over time, the fault has moved the eastern rock more than 300km relative to the western rock.

Now, 300 km –that speaks to millions of years of geologic time.  We can measure the rate at which the Pacific Plate moves relative to the North American Plate –about 4.5 cm/year.  The San Andreas takes up most of that –but not all.  But if we assume it takes it all, we’re looking at a total of 300km at 4.5cm/year –so at least 6.6 million years.

Of course… if you think planet Earth is only 10,000 years old, that means the fault’s moved some 300 meters (3 football fields) every 10 years.  And considering that the displacement was about 6 meters during the M 8.3 1906 San Francisco Earthquake…that’s a lot of earthquakes in just a short period of time!

Or another way of putting it, if planet Earth were 10,000 years old AND the San Andreas fault formed at the very beginning, 10,000 years ago… then there must have been 50 of those San-Francisco-sized Earthquakes every ten years –or… 5 of those every year.  Yikes!

But of course… we know that the San Andreas isn’t as old as the planet.  It cuts that granite at Pt. Reyes… which is related to the Sierra Nevada granite –which is really pretty young –but older than 10,000 years by about 100 million.

click here if you want to see more photos of the San Andreas fault –with a map!

Great Unconformity in Montana –and rising seas during the Cambrian

Here’s yet another picture of the Great Unconformity –this time in southwestern Montana.  Once again, Cambrian sandstone overlies Precambrian gneiss.  You can see a thin intrusive body, called a dike, cutting through the gneiss on the right side.  You can also see that the bottom of the sandstone is actually a conglomerate –made of quartzite cobbles derived from some nearby outcrops during the Cambrian.

Great unconformity in SW Montana.

Photo of Cambrian Flathead Sandstone overlying Proterozoic gneiss in SW Montana.

 

And that’s me in the photo.  My left hand is on the sandstone –some 520 million years or so old; my right hand is on the gneiss, some 1.7 BILLION years old.  There’s more than a billion years of missing rock record between my two hands.  Considering that the entire Paleozoic section from the top of the Inner Gorge in the Grand Canyon to the top of the rim represents about 300 million years and is some 3500′ thick… yikes!

And… just like in the Grand Canyon and elsewhere, there is Cambrian age shale and limestone above the sandstone.  This rock sequence reflects rising sea levels during the Cambrian.  It’s called the “Cambrian Transgression”, when the sea moved up onto the continent, eventually inundating almost everywhere.  If you look at the diagram below, you can see how this sequence formed.

Marine transgression

Sequence of rock types expected during a transgression of the sea onto a continent.

If you look at time 1, you can see a coastline in cross-section, with sand being deposited closest to shore, mud a little farther out, and eventually carbonate material even farther out.  As sea levels rise (time 2), the sites of deposition for these materials migrates landward, putting mud deposition on top the earlier sand deposition and so on.  At time 3, the sequence moves even farther landward, resulting in carbonate over mud over sand.  If these materials become preserved and turned into rock, they form the sequence sandstone overlain by shale overlain by limestone –just what we see on top the Great Unconformity.

 

 

 

Great Unconformity –in the Teton Range, Wyoming

As it turns out, the “Great Unconformity”, the contact between Cambrian sedimentary rock and the underlying Precambrian basement rock, is a regional feature –it’s not only in the Grand Canyon, but found all over the Rocky Mountain West –and for that matter, it’s even in the midwest.  As an example, here are a couple photos from the Teton Range in Wyoming.

The yellow arrow points to the contact between the Cambrian Sandstone and underlying Precambrian metamorphic rock... the Great unconformity.

This top photo shows the Grand Teton (right) and Mt. Owen (left) in the background… in the foreground, you can see a flat bench, which is underlain by flat-lying Cambrian sandstone.  Below that are darker-colored cliffs of Precambrian metamorphic rock.  The unconformity is right at their contact (arrow).

Also notice that the Grand Teton and Mt. Owen are made of metamorphic (and igneous) rock –but they’re much much higher in elevation than the sandstone.  That’s because there’s a fault, called the “Buck Mountain fault” that lies in-between the two.  The Buck Mountain fault moved the rock of the high peaks over the ones in the foreground during a mountain-building event at the end of the Mesozoic Era.  Because the metamorphic and igneous rock is so much more resistant to erosion than the sandstone, it stands up a lot higher.

Precambrian metamorphic and igneous rock of the Teton Range and overlying sedimentary rock.

This lower photo shows the view of the Teton range from the top of the sandstone bench (appropriately called “Table Mountain”).  As you look eastward towards the range, you can pick out the Buck Mountain fault (between the metamorphic and igneous rock of the high peaks) and the Cambrian sedimentary rock (the layered rocks).  Significantly, the Cambrian rocks, just like in the Grand Canyon, consist of sandstone, overlain by shale, overlain by limestone.

And geologic time… remember… for the sandstone to be deposited on the metamorphic or igneous rock, the metamorphic and igneous rock had to get uplifted from miles beneath the surface and exposed at sea level.  And since then, it’s been uplifted to the elevation of The Grand Teton (13370′) and Mt. Owen (12, 928′) !

Click here to see more photos of unconformities.
or… click here to see a geologic map of Grand Teton National Park, Wyoming.

Great Unconformity –Grand Canyon, Arizona

So just like intrusive igneous rocks, metamorphic rocks require great lengths of time to accomplish the uplift and erosion in order to be exposed at Earth’s surface.

So what do we make of this photograph?  It shows a sequence of sandstone, shale, and limestone sitting on top metamorphic rock (called the “Vishnu Schist”) in the Grand Canyon.  The sandstone was deposited right on top the schist.

Great unconformity, Grand Canyon, Arizona

Sequence of Cambrian sandstone (the ledge across the middle of the photo), shale (the overlying slopes) and limestone (the upper cliffs) deposited on top the Vishnu Schist in the Grand Canyon.

 

Since sedimentary rocks, like sandstone, shale, or limestone, are deposited at Earth’s surface –and metamorphic rock forms beneath the surface, this photo shows that BEFORE the sedimentary rocks were deposited, the metamorphic rock (schist) had to have been uplifted and exposed.  So all the time required to bring the schist to the surface had to take place before the sandstone was even deposited.

The surface of contact between the sandstone and the schist is called an unconformity because it is here that we see evidence for a great deal of missing rock record.  The sandstone must be much younger than the schist –for the very reason that the schist first had to get uplifted and exposed at the surface before the sandstone was deposited on top of it.  So… because the sandstone is so much younger, but it was deposited right on top the schist, there must be a gap in the rock record between them … an unconformity.

And here is where we see evidence for even LONGER periods of time.  Overlying the sandstone?  Thousands and thousands of feet of more sedimentary rock.  And much of that sedimentary rock was marine… formed at sea level.  It is now over a mile above sea level.

And the schist itself?  The people who’ve studied it have determined that much of it was originally volcanic –which means that it originally formed at the Earth’s surface.  So… over geologic time, it must have been buried to the depths needed to turn it into a metamorphic rock BEFORE it was uplifted and exposed.

So… how old is Earth?  Some say 6 or 10,000 years… I think we’re looking at 10s of millions in this photo.  And if we consider the numerical ages for these rocks, 1.7 billion is the age of metamorphism of the schist –its original volcanic rock must have been older!

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