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Five Awesome Minerals –for rocks and landscapes

All told, we humans have discovered more than 5000 mineral types. I challenged myself to list the five most important minerals when it comes to the formation of rocks and their subsequent weathering and erosion into landscape.

Rock, which makes up our planet, consists of different minerals. This granite is made of, Q=quartz, O=orthoclase feldspar, P=plagioclase feldspar, B=biotite mica (IP18-1012ce)

It’s an impossible task –and subjective, of course –and I fudged a bit by grouping some minerals together. But here they are: the silicate minerals quartz, mica, feldspar, and mafic minerals, and the carbonate mineral, calcite. I invite you to take me to task in the comments section. Even so, these five minerals are easy to identify and are critical to any discussion about rocks –which makes them important to understanding weathering and erosion. And, I’m adding them to my already packed curriculum for my introductory class on surface processes next term, so five seems to be a good upper limit on the numbers.

Oh! Click on any image to see it enlarged on a separate page.

The five awesomest minerals

Weathering, by the way, is the in-place physical and/or chemical breakdown of rock; erosion is its removal –so they go hand-in-hand. A lot goes into how susceptible a rock is to these processes—not just the mineral content—and when it comes to chemical breakdown, the main factors always involve water. If a rock is accessible to water, it will break down more quickly; if it’s not accessible to water, it will be more resistant.

Fractures in granite collect water and host vegetation, focusing weathering along the fracture. Acadia National Park, Maine. (100608-59)

Because some rocks will weather and erode more quickly than others, landscapes form with cliffs and valleys and steep slopes, gentle slopes –all sorts of variation you can imagine—depending in part on the bedrock. We call the phenomenon “differential erosion”. You can find more detail on the processes in one of my earlier posts, called “Shaping of Landscape“.

Differential weathering and erosion: Mexican Hat, Utah (9OtR-049)

Ok… the minerals:

Quartz  –most people have come to know quartz because of its pretty crystals –especially the purple variety called amethyst—but they might not realize that it’s a hugely important component of many igneous, metamorphic, and sedimentary rocks, and it forms the cement of many sedimentary rocks.  When it comes to weathering, quartz is practically inert chemically, so a strongly quartz-cemented sandstone will be more resistant to weathering and erosion than most other sedimentary rocks. The sandstone will stand out in relief, likely as cliffs or even overhangs. By contrast, weakly cemented, often finer-grained rocks tend to form slopes or even valleys.

Differential erosion along the Front Range of Colorado: resistant, inclined sandstone beds form ridges while intervening shale forms a valley. (131025-52)

If you look closely at a sandstone like the one below, chance are you’ll see mostly quartz grains. Quartz grains might break up into little pieces as they’re being transported by rivers, for example, but they won’t turn into clay like most other minerals. The result? Sediment gets increasingly rich in quartz the farther it’s transported from its source –and so if it travels a great distance, quartz is just about the only thing left!

Close-up views of quartz sandstone and cement. Photo on right is enlarged approx 3x. (click on it to see it even larger) Nearly all those little sand-sized particles are quartz! (201120-10i)

Micas are those shiny sheet-like minerals we see catching the light in a whole host of rocks. The best known micas are muscovite (white mica) and biotite (black mica). In metamorphism, they grow larger with increases in temperature, providing a handy way to distinguish between slate, phyllite, and schist — a nice thing to know in a geology class. The diagram below illustrates the process.

With increasing temperature, the clay within shale will recrystallize to microscopic mica to make slate a hard, metamorphic rock with a dull luster. Even hotter and the micas grow larger to produce phyllite, which has a reflective sheen, and then become easily visible in schist. Hotter still and coarse minerals segregate into layers to make gneiss. (201119-12e)

When it comes to weathering and erosion, those little mica sheets allow infiltration of water. Metamorphic rocks overall are pretty resistant because of their crystallinity –but slates or schist, for example, degrade much more quickly than gneisses because of their prevalence of mica minerals, which together create a fine-scale layering in the rock. Gneisses are coarse-grained, and their layering tends to be thicker and less permeable.

Collectively, feldspars are the most common minerals of the Earth’s crust –and there are numerous varieties, the most common being orthoclase and plagioclase, which often occur together. For the purposes of landscapes, I grouped them together as one: they’re all kind of chunky looking and are generally light-colored. Feldspars are the main components of igneous rocks, making up 60-70% (or even more) of most granitic rocks, which tend to form distinctive landscapes marked by cliffs and large rounded boulders.

Spheroidal weathering in Cretaceous granitic rock, SE California. Mt. Whitney and the Sierra Nevada in background. (180314-91)

The “mafic minerals” are named so because they’re generally rich in iron and magnesium and poor in silica when compared to a lot of other common minerals –and they’re typically dark green to black in color, so “mafic rocks” like basaltic lavas also tend to be darker colored. The main mafic minerals are olivine, pyroxene, and amphibole. Olivine, as it turns out, is our planet’s most common mineral! It’s the main stuff of the mantle, which depending on where you are, lies some 10-40km below the surface.

Hikers on basalt flow on Mauna Loa Volcano in Hawaii; cinder cone (also basalt, but weathered red from oxidized iron) in background. Inset shows large olivine crystals in basalt surrounded by mostly pyroxene and calcium-rich plagioclase feldspar. (170917s-14) and 200517-4) Please click the link for a post on this amazing volcano!

The physical properties of these mafic minerals explain all sorts of things like why oceanic plates subduct beneath continental ones (being made of mafic minerals, they’re denser so they sink) to the shapes of volcanoes (mafic lavas, being less viscous than silicic ones, tend to form broad, low-relief shield volcanoes). For surface weathering and erosion, the mafic minerals tend to break down into clay more quickly than most other silicate minerals –which means that all else being equal, rocks with more mafic minerals will weather and erode faster than rocks with the other silicate minerals described here.

Mauna Kea Shield Volcano, Hawaii (left), made from basalt, which is more fluid (less viscous) than more silicic lavas like andesite, which make steeper cones such as Mt. Shasta in California. (170918s-83 and 140617-114)

Finally, calcite, the sole non-silicate on this list, is hugely important because it’s what makes up the very common sedimentary rock limestone. Limestones occupy a separate class of sedimentary rock than sandstones: they’re “biogenic”, having formed through the precipitation of calcite through biological processes, as opposed to “clastic”, which are just broken particles. As a result, they’re important to understanding sedimentary rocks, and because of the biology connection, Earth history and evolution. For landscapes, limestones are also really important because the calcite will dissolve in slightly acidic water to form caves and sinkholes–there’s a whole class of landscape called “Karst”, which results from the dissolution of limestone.

The “fizz test”: calcite crystal (left) and limestone (right) both react with a weak solution of hydrochloric acid to give off carbon dioxide gas –because they’re both made of the same material (201122-11 and 201111-17)

And re-precipitation of calcite within caves forms the beautiful speleothems we so love –stalactites, stalagmites, flowstone… the list goes on. That rock, called travertine, occupies a third class of sedimentary rock called “chemical sedimentary rocks”.

Stalagmite, Carlsbad Caverns NP, New Mexico. (100131-63)

Phew! My apologies to those mineralogy people who might read this and think, “but she missed that important idea! And that one too! And what about that mineral?” Maybe that’s the point though –this is just a start. There is so much these five minerals can teach us –and there are so many other wonderful minerals I didn’t even mention. So here’s to reading more info that somebody might’ve put into the comments section. Here also to all the geology majors out there who take upper level geology courses that delve into all the amazing detail and make connections that I wasn’t able to in this little space!

Olivine. I already know that I didn’t do olivine justice. (201119-20)

And if you want to download any of these photos for your own –just type in the photo id into the search function of my geology photo website.

Also, I posted a primer on rock types –if you’re interested, please have a look –and thanks for reading!

Touring the geologic map of the United States

Published in 1974 by the US Geological Survey, the geologic map of the United States beautifully lays out our country’s geology. If you’re stuck at home these days –as most of us are—you can gaze at this masterpiece and go anywhere! USAGeolMap-allllr

S Willamette Valley

Southern Willamette Valley. Eugene’s in the center of the map

At their simplest, geologic maps read like road maps: they tell you what rock unit or recent deposit forms the ground at a given place. So right here where I am in Eugene, Oregon, I can see that I live on “Q” –which stretches north up the Willamette Valley. “Q” stands for Quaternary-age material (2.5 million years to present), which is typically alluvial material –or sediment deposited by rivers and streams.  Just to the south of me lie a variety of older volcanic and sedimentary rocks. You can look them up in the map’s legend using their symbols. Here’s Rule #0: white areas, being mostly alluvium, mark low areas; colorful areas indicate bedrock, so are typically  higher in elevation.

The beauty of this map is that you can see the whole country at once, and using just a few rules, can immediately glean the underlying structure of a region.

For any of the maps, photos, and diagrams here, you can see a larger size if you click on the image.

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Rule 1: Rock bodies without internal contacts appear as color swatches (or “blobs”) as seen on the block diagram below. These rocks include intrusive igneous, undifferentiated metamorphic, and flat-lying sedimentary rock. (If you’re not sure what geologic contacts are, please see my recent post on the nature of geologic contacts.) Read more…

Aerial geology photos– favorites from commercial flights of 2019

I always try for window seats when flying and I always try to shoot photos out the window –with varying results! So often, the window’s badly scratched, there are clouds, it’s hazy, the sun angle’s wrong –there are myriad factors that can make good photography almost impossible from a commercial jet. Last year though, I had a few amazing flights with clear skies and a great window seat –and I’ve now loaded nearly 100 images onto my website for free download. Here are 10 of my favorites, in no particular order. You can click on them to see them at a larger size. They’re even bigger on my website.

Mt. Shasta at sunset. Volumetrically, the biggest of the Cascade Volcanoes, Mt. Shasta last erupted between 2-300 years ago –and it’s spawned over 70 mudflows in the past 1000 years. From the photo, you can see how the volcano’s actually a combination of at least 3 volcanoes, including Shastina, which erupted about 11,000 years ago.

Mt. Shasta at sunset, California

Aerial view of Mt. Shasta, a Cascades stratovolcano in northern California.

If you want to see more aerials of Mt. Shasta (shot during the day) –and from a small plane, go to the search page on my website and type in “Shasta”.

 

Meteor Crater, Arizona.  Wow –I’ve ALWAYS wanted to get a photo of Meteor Crater from the air –and suddenly, on a flight from Phoenix to Denver, there it was!

Meteor Crater, Arizona

Aerial view of Meteor Crater, Arizona

Meteor Crater, also called Barringer Crater, formed by the impact of a meteorite some 50,000 years ago. It measures 3900 feet in diameter and about 560 feet deep. The meteorite, called the Canyon Diablo Meteorite, was about 50 meters across.

 

Dakota Hogback and Colorado Front Range, near Morrison, Colorado. Same flight as Meteor Crater –and another photo I’d longed to take. It really isn’t the prettiest photo, BUT, it shows the Cretaceous Dakota Hogback angling from the bottom left of the photo northwards along the range and Red Rocks Amphitheater in the center –then everything behind Red Rocks, including the peaks of Rocky Mountain National Park in the background, consist of Proterozoic basement rock.

Hogback and Colorado Front Range

Aerial view of hogback of Cretaceous Dakota Formation and Colorado Front Range.

 

Distributary channels on delta, Texas Gulf Coast. I just thought this one was really pretty. Geologically, it shows how rivers divide into many distributary channels when they encounter the super low gradients of deltas. And whoever thought that flying into Houston could be so exciting!

Distributary channels on delta, Texas Gulf Coast

Distributary channels on delta, Texas Gulf Coast

 


Meander bends on the Mississippi River.
My mother lives in Florida, so I always fly over the Mississippi River when I go visit –but I was never able to take a decent photo until my return trip last October, when the air was clear, and our flight path passed just north of New Orleans. Those sweeping arms of each meander are about 5 miles long!

Meander bends on Mississippi River, Louisiana

Meander bends on the Mississippi River floodplain, Louisiana

 

Salt Evaporators, San Francisco Bay. Flying into San Francisco is always great because you get to see the incredible evaporation ponds near the south end of the bay. I always love the colors, caused by differing concentrations of algae –which respond to differences in salinity. And for some reason, salt deposits always spark my imagination. Salt covers the floor of Death Valley, a place where I do most of my research, and Permian salt deposits play a big role in the geology of much of southeastern Utah, another place I know and love.

Salt evaporators, San Francisco Bay, California

Salt evaporators, San Francisco Bay, California

 

Bonneville Salt Flats and Newfoundland Mountains, Utah. And then there are the Bonneville Salt Flats! They’re so vast –how I’d love the time to explore them. They formed by evaporation of Pleistocene Lake Bonneville, the ancestor of today’s Great Salt Lake. When the climate was wetter during the Ice Age, Lake Bonneville was practically an inland sea –and this photo shows just a small part of it.

Bonneville Salt Flats and Newfoundland Mtns, Utah

Aerial view of Bonneville Salt Flats and Newfoundland Mountains

 

Stranded meander loop on the Colorado River. I like this photo because it speaks to the evolution of this stretch of the Colorado River. Just left of center, you can see an old meander loop –and it’s at a much higher elevation than today’s channel. At one time, the Colorado River flowed around that loop, but after breaching the divide and stranding it as an oxbow, it proceeded to cut its channel deeper and left the oxbow at a higher elevation.

Stranded meander loop, Colorado River, Colorado

Stranded meander loop (oxbow) on the Colorado River, eastern Utah

 

San Andreas fault zone and San Francisco. See those skinny lakes running diagonally through the center of the photo? They’re the Upper and Lower Crystal Springs Reservoirs –and they’re right on the San Andreas Fault. And you can see just how close San Francisco is to the fault.  As the boundary between the Pacific and North American Plates, its total displacement is about 200 miles. See this previous post for more photos of the San Andreas fault.

San Andreas fault zone and San Francisco

San Andreas fault zone and San Francisco

 

And my favorite: Aerial view of the Green River flowing through the Split Mountain Anticline –at Dinosaur National Monument, Utah-Colorado. Another photo I’ve so longed to shoot –but didn’t have the opportunity until last year.

The Green River cuts right across the anticline rather than flowing around it. It’s either an antecedent river, which cut down across the fold as it grew –or a superposed one, having established its channel in younger, more homogeneous rock before cutting down into the harder, folded rock. You can also see how the anticline plunges westward (left) because that’s the direction of its “nose” –or the direction the fold limbs come together. The quarry, for Dinosaur National Monument, which you can visit and see dinosaur bones in the original Jurassic bedrock, is in the hills at the far lower left corner of the photo.

Split Mountain Anticline, Utah-Colo

Split Mountain anticline and Green River, Utah-Colorado

 

So these are my ten favorites from 2019. Thanks for looking! There are 88 more on my website, at slightly higher resolutions and for free download. They include aerials of the Sierra Nevada and Owens Valley, the Colorado Rockies, including the San Juan Volcanic Field, incised rivers on the Colorado Plateau, and even the Book Cliffs in eastern Colorado. Just go to my geology photo website, and in the search function type “aerial, 2019” –and 98 photos will pop up. Boom!

 

 

 

 

 

 

 

Science got it right… Maybe we can now accept the reality of climate change?

Along with a zillion other people in the US, I witnessed the total solar eclipse today. Yes, it was amazing and yes, I feel somewhat addicted. The quality of light just before totality was something I’d never before experienced –and the sun’s flash just as it reappeared was something I’ll never forget.  Apparently the next one will be in South America on July 2, 2019–and the next one in the US will be April 8, 2024. Oooh!

Total Eclipse of the sun (170821-19)

Sun’s corona as seen during the total solar eclipse, August 21, 2017 from Salem, Oregon.

Amazing that us humans can accurately predict these phenomena –to the exact place and time –to the second. Seems like our predictions work! These predictions, of course, are grounded in the physical sciences.

At the same time, many people insist that scientists are mistaken or misguided when they predict global climate change.  I wonder if any of those people saw the eclipse. If so, they might want to reflect on their contradiction.

That’s all.

Glacier in retreat, Athabasca Glacier, Alberta, Canada (120713-65).

This monument marks the position of the front of the Athabasca Glacier of Alberta, Canada in the year 2000. Photo taken in 2012.

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).

Read more…

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??

151023-22

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.

Shuksan combo

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.

Cambrian-Jurassic

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.

150912-42F

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.

ShiShi

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.

Conglomerate clast in conglomerate

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.

 

 

 

Cambrian Limestone, Death Valley National Park, California.

Limestone’s a common sedimentary rock –it’s made from calcium carbonate.  The calcium carbonate is precipitated in shallow marine conditions with the help of biological activity, most commonly algae, but also by the many invertebrates that form shells.  This material then settles to the ocean bottom as a lime-rich mud and if the conditions are right, eventually becomes rock.

Compared with many other sedimentary rocks, limestone deposits can accumulate pretty rapidly –about 1 meter per thousand years in many cases –and even two or three times that under optimal circumstances.  These rates are for uncompacted sediment, and a great deal of compaction occurs as the sediment turns into rock.  Additionally, if the deposit is to accumulate to any significant thickness, the crust on which it is deposited must also subside.

Thousands of feet of limestone, deposited during the Cambrian Period, are exposed in the Death Valley region. Click here for a slideshow of Death Valley geology

 

So all this limestone in Death Valley was deposited as a bunch of horizontal layers in a shallow marine setting –not too deep, or light wouldn’t penetrate to the seafloor to allow photosynthesis –key to the ecosystem that produced the calcium carbonate in the first place.  And since it was deposited, it’s been uplifted and tilted and eroded.

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