Helenite: can volcanoes really weep gems?

If you are on this blog, you know I’m a nerd. You’re probably a nerd too. Have you ever considered your nerd origin story? Mine began at age 7 or so in the basement of my grandparents’ house in Ottawa, Canada. Behind the washing machine and toy trains, the walls were lined with DECADES of National Geographic Magazines to pore over while the grown-ups wanted me out of their hair. When I was 9 they gave me a subscription so I could amass my very own giant yellow pile of nerdy heaven with priceless pull-out maps. I have an everlasting affection for this magazine. But it’s not the main content of the magazine I want to write about today.

It’s the ads.

In every edition, there’s at least one ad selling the reader some rare, shiny thing – a coin or a gem. The format hasn’t changed since at least 1975. The ad is full of small text raving about the product. There’s a close up with photoshopped sparkles, and a free gift with purchase. This month’s shiny trinket feature seemed tailor-made to my inner magpie. Gems? From a volcano? In my state? How can I be a geologist and never learn about a volcano spitting out gems before!

It, uh, turns out that volcanoes don’t actually spit out gems. More on that in a bit.

First off let me point out some of the positives from this ad, lest I seem too harsh. Helenite is shiny! very green! fairly cheap! 3/3 for Courtney’s inner magpie points. But there is a suspicious amount of purple prose in the ad. Let’s see what a quick google search will reveal.

The ad implies that helenite is a gem linked to the eruption. It weasels out of saying that the gem is naturally created or manufactured, but most people automatically link gems with being natural. The ad also hypes its rarity. Unfortunately they are false on both points.

Mount St. Helens erupted in 1980 in the southwestern corner of Washington, and blanketed the surrounding area in 540 million tons of ash. To give some context, 540 million tons is equivalent to 531.5 USS Nimitz nuclear powered aircraft carriers. Is the raw material rare? You be the judge.

USGS map of ash distribution

The suddenly ashen forest immediately surrounding the mountain was owned and logged by the Weyerhaeuser Company. The helenite legend goes that when Weyerhaeuser went in to salvage their equipment that had been caught in the crossfire, they found a strange effect. Their acetylene torches reacted strangely with the ash as they tried to free their machines. The heat from the torches fused the ash into a novel green glass.

Photo of uncut helenite, from geologyin.com

Of course anything shiny will draw people trying to make a buck, so now helenite is marketed and readily available as a cheap alternative to emerald, or as a souvenir. It’s softness gives it away – it’s only about 5.5 on the Mohr’s hardness scale that ranges from 1 (powdery) to 10 (diamond). An emerald scores 7.5 to 8 on that scale. Manufacturers also add cobalt or gold powder to add a blue or red tint to the finished product, so helenite is sold in a range of colors besides green.

And there have been some studies done that cast doubt on whether helenite is actually made of pure Mount St. Helens ash after all. A 1988 study published in Gems & Gemology melted known Mount St. Helens ash and compared it to a s purchased specimen of helenite. The study found that melted ash from the eruption looked like obsidian – dark gray, not green. The author used x-ray fluorescence and found out that the ash had much more iron and titanium in it which gave the dark gray melted ash its color, and higher quantities of aluminum which made the melted ash melt at a much higher temperature. The genuine Mount St. Helens’ ash melted at 1,300 degrees F, while the helenite melted at 800 degrees F. This variation is much more than you would expect, even given that ash composition within an eruption can vary. I made the dual pie charts from the information in the paper to compare the composition of genuine ash with the composition of green glass…

Even with some canny google searching and combing through the US Patent office, I couldn’t find out who actually manufactures helenite that is sold by jewelers, or how they obtain the ash. Maybe I just have to do an Indiana Jones style Hunt for the Helenite when this COVID-19 thing is over?

All this being said, I would absolutely buy helenite in a gift shop. Especially if it came in teal. After all, my inner 9 year old National Geographic-reading nerd is not that far from the surface.

Shiny!

References:

https://www.usgs.gov/faqs/how-much-ash-was-there-may-18-1980-eruption-mount-st-helens?qt-news_science_products=0#qt-news_science_products

https://geology.com/gemstones/helenite/

http://www.geologyin.com/2016/12/worlds-most-amazing-gemstone-found-in.html

https://www.gia.edu/gems-gemology/summer-1986-green-glass-nassau

https://pubs.usgs.gov/gip/msh/ash.html

https://www.pioneerjewelers.com/blue-radiant-bar-earrings-30h

Hometown aquifer, hometown beer!

Some things just go naturally together. Peanut butter and jelly. Christmas and presents. Geologists and beer. Which is why I got so excited when I went shopping for Christmas dinner fixings at the supermarket near my parents’ house and found that a local brewery had released an aquifer-themed beer!

The Memphis Sands aquifer is part of a larger aquifer system that stretches from the Tennessee/Kentucky border all the way to Gulf of Mexico. It contains layers of sand that transmit water and finer-grained sediments that slow its flow, all laid down by versions of the Mississippi River as the ice ages came and went. I was raised on this aquifer’s abundant and tasty contents. So let’s get down to some serious science. How accurate is the can art? And most importantly, how delicious is the beer?

Descriptive text: 7/10

“From deep within the Earth’s crust, the finest drinking water on the planet springs forth to Memphis. This unique aquifer supplies WISEACRE with the most necessary of ingredients for the production of crisp, light-colored lager. The rains that fell to earth 3000 years ago are filtered very slowly through hundreds of feet of fine grain sand, culminating in a huge underground lake filled with 57 trillion gallons of virtually mineral-free water. From this prestigious water reserve, we supply Memphis with Sands, our one-of-a-kind Lager. Beauteous in its simplicity, it is very low in bitterness and full of delicious flavors of bread and crackery malt.”

First and foremost, the description of an aquifer as ” a huge underground lake” is enough to drive hydrologists to grab another beer to drown their sorrows. It messes up people’s mental image of aquifers and leads them to ask me how the fish survive down there. In contrast the “57 trillion gallons” figure for aquifer volume beneath Shelby County is widely accepted, and the 3,000 year age of the water is reasonable. It’s calculated using the slope of units in the aquifer, information we know about the size of grains of sand in the aquifer, and the distance between recharge areas and Memphis wells. (source) This text got 2/3 scientific concepts correct, so I’ll round up and give it a 7/10. Generosity is part of the holiday spirit.

Can Art: 6/10

The artist had fun with this one – the gold “fossil” background and the blue contrast nicely. However, they made the baffling design choice to assign the blue color with watery bubbles to the aquitards (geologic layers that don’t transmit water) instead of the aquifers! This makes no sense! Why not assign the watery blue color to the units that actually transmit water? I could be a Scrooge and critique the fossil background because we don’t really find dinosaur skulls around here, but find the print too cute to complain about it. It gives the can a kind of easily recognized “hey, science!” vibe which appeals to me.

The artist also shrunk down the scale to fit with their placement of the info blurb, rendering it inaccurate. 3,000 feet below the ground surface should really correspond to the bottom of the units layered “Other Cretaceous Units”. An easy fix to make the scale bar fit above the blurb and stay accurate would be to just have it cover less ground – stop at 1,500 feet or so. At least the artist got the names of the units correct. I think the artist based the design on a widely republished diagram from the local chamber of commerce, exhibited below. It also assigns a blue color to the aquitards and a sandy tan color to the sands that make up the aquifers, which may have been what threw the artist for a loop with the color scheme.

I think it would be really cool if Wiseacre could tweak this can design to be a little more scientifically accurate. After all the beer-drinking public is much larger than the portion of the public that reads aquifer articles in the local paper. Why not spread some sneaky science exposure on Memphis’ great underground asset to those who crack open an ice-cold can at the end of the day?

Taste: 8/10

This is a great basic lager. I would guess that Wiseacre is aiming this at beer drinkers who would otherwise drink Budweiser and Miller High Life? My dad, my sister, and I split a can as part of an impromptu beer flight. I noticed that the fizziness is a bit unique – it looks flat when I pour it, but the fizz comes to life in my mouth. My sister says it has a bit of a floral and toasty note in the aftertaste. It’s a little bitter, mostly malty, and quite smooth. We didn’t taste a strong hop flavor. We all agree that it’s very drinkable: the kind of six-pack that would be popular at a cookout or camping trip.

In an ideal, non-pandemic world Wiseacre’s tap room on Broad Avenue would be on my must-visit list for my trip back home. I suppose there’s always next year!

P.S.

The aquifer was recently in the news because of a groundbreaking legal case where Mississippi sued Tennessee for “stealing” groundwater. Mississippi argued that the cone of depression caused by Tennessee’s groundwater pumping caused Mississippi’s water to slip away into Tennessee, and wanted $615 million in damages and a halt to pumping. The Supreme Court appointed a Special Master to determine the case, Judge Eugene Siler. Last month he decided that groundwater is a flowing resource, not a static one. Therefore, the water should be distributed as surface water is distributed in the eastern states, by equitable apportionment. Judge Siler’s conclusion is the legal equivalent of locking two misbehaving kids in a room until they sort themselves out. He rejected Mississippi’s appeal for damages and ruled that if the two states find themselves so affected by groundwater pumping, they can apportion the aquifer to determine fair allotments. It only took 14 years – the case was first filed in 2006!

Useful article from the Memphis Flyer: https://www.memphisflyer.com/NewsBlog/archives/2020/11/06/federal-judge-sides-with-tennessee-in-water-rights-case

Shout-out to my thesis advisor’s blog, where he posts an interesting opinion from his colleague on the case: https://www.waterwired.org/2020/11/jjr-ms-v-tn.html

Further Resources:

The defining 2016 article in the local paper that brought awareness of the aquifer to the general public: https://www.commercialappeal.com/story/news/environment/2016/12/16/memphis-sand-aquifer-buried-treasure/93814278/

Dense 1983 US Geological Survey paper on the aquifer: https://pubs.usgs.gov/wri/wrir88-4182/pdf/wrir_88-4182_a.pdf

Wiseacre Brewery! https://wiseacrebrew.com/

Thunder Mountain Lakes Blew My Mind

Note to the reader – this blog post describes a trip to fragile ecosystem via a risky trail. If you decide to follow me here, please use caution and turn up your leave-no-trace skills to 11. 🙂

The third time really is the charm. In June I tried to make it here but the road was still snowed in. In early July I started from a different lower-elevation trailhead, but the pass was…. still snowed in. The snow finally melted in early August – success!

I finally completed this truly magical backpacking trip.

I started from the Hope Lake Trailhead up Tunnel Creek Road off of Highway 2, just a few miles west of Steven’s pass. The Hope Lake trail heads uphill to connect with the Pacific Crest Trail, where I turned south. There’s a rockfall just south of Hope Lake that has a booming and adorable small mammal population – I saw 2 families of marmots and a family of pikas! After the steep first 2.5 miles of the hike the trail flattens out, the trees open up, and all of a sudden you expect Julie Andrews to pop out and start singing the Sound of Music.

The hike from here to Trap Lake is a delightful ramble through wildflower meadows and groves of pine trees.

I got to Trap Lake around noon, claimed a campsite, and made ramen for lunch. And then I started to climb. To get to Thunder Mountain Lakes you head up the PCT to Trap Pass, and then turn south onto a seriously sketchy trail that follows the county line south and up to Thunder Mountain Ridge. Sketchy as in if you slip, you fall 500 feet. Not for the faint of heart, and it could be risky for dogs.

The trail takes you up above the snow line, and starts to be marked with cairns to guide hikers through the snow field and bare granite. There are a few islands of vegetation, including stunted pine trees, pink heather, and blue dwarf lupine. The bees were going wild on the heather – I guess they only have a short window for foraging up here.

At this point I was 6.5 miles from the trailhead and 3,300 feet higher up in elevation. Just as I was having to start counting my paces to keep myself going, I turned a corner and my jaw dropped.

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I had made it to Thunder Mountain Lakes! I could see all the way to Mt. Daniel and Hinman Glacier in the distance.

I hiked down to the lake and sprawled on the nice warm granite to eat a chocolate bar. As I got up to look down at the lower lake, I heard a rustle….

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… and there was a mountain goat with her fuzzy baby! They’re such lucky creatures, getting to live here full-time. I hung out with them until they made their way along the ridge. The baby kept trying to sprint up onto rocks only to fall – luckily straight into its mom. It takes true love to be your offspring’s bouldering mat.

I lingered up by the lakes until the sun started to sink close to the ridgetop. The nice thing about hiking downhill is that it leaves me with more energy to spare to admire the rocks. The downside of admiring the rocks is that I get distracted and loose the trail. But these rocks were great!

They’re all granite, but conceal a subtle story. Textbook illustrations often depict magma bodies as single lumps that rise and cool neatly independently. Reality, as always, is more messy. Magma bodies (or batholiths, as they’re referred to once they cool down) in reality can merge, or pick up bits of other batholiths, or be reworked once they’ve cooled.

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For example, this photo (above) shows a light gray batholith that cracked under pressure once it had cooled. A different kind of magma forced its way through the cracks as it rose, forming the dark grey stripe. Later the line of weakness was reopened and filled with hydrothermal quartz, creating the white streak. It’s kind of like a turducken of igneous rock.

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And in this photo, the dark grey magma body ripped up chunks of a paler batholith as it rose and incorporated them into its mix, Pac Man style.

As I headed north back to Trap Pass I got phenomenal views of Glacier Peak as well as glimpses of Mt. Baker to the northwest.

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Heading downhill back to my camp I met several fat marmots. Two of them were engaged in a clumsy but effective high-speed chase and high-volume screaming match. Their counterpart above Trap Lake obviously don’t think much of humans and screamed at me to let me know it. (lower left photo)

Trap lake was an idyllic place to spend the night, especially on a Sunday night when most of the weekend backpackers were long gone. I really got a prime campsite (above right) and enjoyed the luxurious packable chair that my sister (the one featured in all the Twin Trek posts on this blog) gave me for our birthday. Glamping all the way!

I spent a leisurely Monday in camp and then headed back to the trailhead at an equally leisurely pace. I really didn’t want to leave. Between snack, scenery, lunch, and pika -appreciation breaks I managed to stretch the 5 miles into 6.5 hours.

I may have had to drive back to civilization that day, but I’m definitely coming back here!

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Rampart Ridge Rocks!

At Vanderbilt University’s Wilderness Skills club we classified adventure into two types of fun. Type 1 fun was fun to experience and fun to remember. Type 2, the slightly more common type, was miserable while it happened but either has a great reward or created a story that got you attention at parties.

The hike up to Rachel Lake after work with a 30-pound pack on my back was decidedly Type 2.

RampartRidgeZoom

Where in the world was I? Trailhead marked in blue, trail in dark green. map created in ArcMap by C. van Stolk using trail shapefiles created by the US Forest Service and ESRI’s Streets basemap.

Rampart Ridge trail map

Trail in bright green – map created in ArcMap by C. van Stolk using trail shapefiles created by the US Forest Service and ESRI’s terrain basemap.

I got off work early on that Friday, headed east on I-90, turned off at the exit for Kachess Lake, bounced up potholed gravel roads to the trailhead, and set out for adventure. I knew the hike went from 2,800 to 4,800 feet in four and a half miles. What I had foolishly overlooked is that it gains 1,400 feet in the last 1.2 miles. A significant distance of that 1.2 miles is literally in a creek bed. It had me questioning my life choices. I had planned to go all the way up to Rampart Lakes at 5,100 feet but I was absolutely done by the time I got to Rachel Lake. I was too cranky to eat my ramen noodles. I set up my tent at the outskirts of an inordinately crowded back country campground just as it got dark, made a cup of tea, and turned in for the night.

The next day was 100% heavenly Type 1 Fun.

I left the burden of my camping stuff where it lay and headed uphill into a clear blue day. My goal was Alta Peak at the northern end of Rampart Ridge, elevation 6,152 feet. I climbed up the ridge past fields of glacier lilies and heather and creeping phlox. It took me about two hours to make it to the top, with views of Mt. Rainier, Mt. Adams, and Glacier Peak. I settled in for an hour with my chair and my map to plan further adventures while I could see so much of the landscape!

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Alta Peak, looking south at Mt. Rainier (center) and the Summit at Snoqualmie ski routes (center right).

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Alta Peak, looking north (the white tip of Glacier Peak is peeking out from behind the Four Brothers on the center right)

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Looking down past the snowdrifts to the basin below Alta Peak

I then rambled downhill to have lunch at Lila Lake. It was hopping with backpackers but I found a nice spot to eat my snack assortment and get yelled at by a marmot.

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Once I had my fill of exploring the bouldery outcrops at Lila Lake, I headed south to Rampart Lakes. I spent the rest of the afternoon basking on a rock with a view at the Rampart Lake furthest along the trail. I braved the water for a swim and dried off in the sun while reading a mystery novel and enjoying my sippy flask of rosé. I did not join the hikers who were penguin-sliding down the snowdrift straight into the lake, though.

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Rampart Lake, looking up at Rampart Ridge.

Around 6pm I headed back down to Rachel Lake to cook dinner and explore the campsite possibilities in the main campground for next time. I went to sleep in an exponentially better mood than I had the previous night.

Sunday morning I hoped to make it down the horribly steep creek bed part of the trail before the day hikers would be heading up it. Mission successful! I took loads of photos of wildflowers. I set up to finish my novel and eat lunch at a rocky waterfall overlook about a mile from the trailhead.

By this point Friday’s uphill slog was completely a thing of the past. All things considered, the trip had been a delight.

But wait! I couldn’t ignore the cool rocks. You know me.

The rocks exposed up at Rampart Ridge were gray with white clusters of larger elongated crystals. I thought they were really distinctive, but didn’t know their name.

It turns out these rocks have the epic moniker of “glomeroporphyritic basalt”. Glomeroporphyritic translates out of science Latin into “collected-together larger crystals”. In geology-ese, “porphyritic” refers to an igneous rock texture where larger crystals are set in a matrix of rock crystals with a much finer texture, like blueberries in a muffin.

Porphyritic igneous rocks form in two stages – the first one at deep in the earth’s crust, and the second in a shallower, cooler zone at or near the earth’s surface. The large white crystals in Rampart Ridge’s basalt formed when the magma was deep underground. They had plenty of time to slowly cool into large crystals in the hot environment at depth. However, some igneous or tectonic process suddenly shoved the magma body up towards the surface. This made the rest of the magma cool suddenly. Because these newer crystals did not have time to grow, they stayed very small.

But why did this one white mineral form crystals at depth, and not the others?  I turn to a familiar chart from my geology textbooks for the answer. It’s called Bowen’s Reaction Series, and describes the order in which minerals crystallize out of molten rock. This series springs from painstaking experiments involving pulverized minerals, a very very hot oven, and more patience than I possess. They revealed that minerals form into crystals at the different temperatures along a gradient.

The elemental mix of magma that becomes basalt creates the white mineral calcium plagioclase and the dark gray/black mineral pyroxene, with only trace amounts of other minerals. Calcium feldspar has a higher melting temperature, and so solidifies at a higher temperature while pyroxene has not yet formed into crystals. An important caveat is that not all magma contains all the elements necessary to make every rock in the series, so several minerals may be “skipped” in a certain magma body.

For example, quartz has the lowest melting temperature of all the common minerals, which is why it often forms decorative crystals or veins in the voids left when other minerals have already crystallized.

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Quartz veins in volcanic rock higher up on Alta Peak.

The two kinds of rock I saw on this hike date mainly from the Eocene and Oligocene time periods between 55.8 and 23 million years ago. Washington was roughly at it’s current location on the globe back then and the volcanoes of the Cascades were starting to rev up. Since then, these rocks have been folded by tectonic forces, broken by faults, and eroded until they cropped out in the patchwork patterns that geologists map today.

rampart ridge edited map

Summarized in MS Paint from the original Snoqualmie Quadrangle Geologic Map by Tabor, Frizzell, Booth, and Waitt of the USGS: https://pubs.usgs.gov/imap/i2538/

The glomeroporphyritic basalt dates from the late Eocene period. It’s colored medium green and marked as Tnbg on the map above.

Tv and the light pink color stands for Oligocene volcanic rocks – an igneous jumble that’s a few million years younger than the glomeroporphyritic basalt. The rocks on Alta Peak are describe in the USGS pamphlet for the Snoqualmie quadrangle as “coarse volcanic breccia and tuff with minor ash flow tuff). They look almost like concrete made with blocky, angular aggregate. Breccia describes rocks created when magma shattered and engulfed surrounding rock as it erupted. Tuff forms when ash becomes cemented by its own heat, like how I described in the Smith Rocks post from 2018. Breccia makes up the ridgeline of the photo below – you can really see how this one rock classification encompasses a bunch of different kinds of rocks that erode differently to create a mix of straight ridge lines and messy talus slopes.

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It’s hard to get a sense of the scale of the waterfall in the center from the photo. I could hear it roaring down the rocks from a quarter mile away!

I’m still doing research about how these rock types ended up juxtaposed. Western Washington’s rocks tell a complex story of bits of foreign continents (called accreted terranes) that were stuck onto the rest of North America by subducting plates, then covered with volcanic rocks and shuffled around by faults. It’s the northern relative to the process in Southern Oregon that I wrote about in my accreted terranes post. Up here, the terranes were even more altered by volcanism and faulting.

It definitely created a fantastic landscape!

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Volcanic breccia on Alta Peak

Resources:

Definition of glomeroporphyritic basalt: https://blogs.agu.org/georneys/2011/07/14/geology-word-of-the-week-g-is-for-glomeroporphyritic/

USGS map and pamphlet for the Snoqualmie Quadrangle: https://pubs.usgs.gov/imap/i2538/

Info on volcanic breccia: https://en.wikipedia.org/wiki/Breccia#Volcanic

Information on Bowen’s reaction series: https://courses.lumenlearning.com/physicalgeology/chapter/3-3-crystallization-of-magma/

 

Ireland: Rocks of Newgrange

This post is the last in a series that covers last summer’s van Stolk family trip to Ireland. Here’s the highlight map to get you oriented:

irelandtripmap

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We don’t know much about the people who built the massive passage tomb at Newgrange over 5,000 years ago. They left no written record, and we can only guess at how they used the ritual passages aligned precisely to the winter solstice. However one thing is obvious – they had a particular eye for beauty and detail.

How do we know this? The story is all in the rocks. Black and white, rough and smooth, plain and intricately carved. The massive “kerb stones” along the bottom of the monument were not shaped by human tools except for detailed swirling patterns created with small chisels. The builders must have scavenged all over the countryside for boulders of the appropriate size to decorate. And many of the rocks come from dozens of miles away!

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Swirling patterns on a kerb stone

Bright white quartz cobbles from the Wicklow Mountains (near Glendalough, which I wrote about in the last post), dark speckled granodiorite cobbles from the Mourne Mountains, and dark gabbro cobbles from the Cooley Mountains form designs on the outer walls of the monument. Smooth greywacke from Clogherhead in County Louth forms the inner passageway and outer kerb stones. Finally, the interior of the huge mound was built up from local gravel from the banks of the Boyne River.

NewgrangeStoneSource2

Keep in mind, Newgrange was built before the invention of the wheel made it to Ireland. Archaeologists think that the rocks were carried as far as possible by boat before being loaded onto log rollers or sledges to get to their final destination.The 97 kerb stones each weigh at least a ton, and it’s estimated that Newgrange contains 200,000 tons of stone. These ancient people committed themselves to a serious labor of love!

Archaeologists debate the period of time that people used Newgrange as a spiritual and cultural site. However they agree that it was “lost” around the 5th century and then rediscovered at the turn of the 18th century when a local landowner went looking for stones to expand his buildings. Amateur “antiquarians” from around the British Isles visited the site from its rediscovery in 1699 to the 1920s. A formal archaeological survey started in 1962 and was complete in 1973. During this process, the tomb was “restored” to the way that we see it today by educated (and controversial) guesswork. The researchers found the cobbles piled down by the base of the mound and posited that they had originally been pressed into the side of the mound as a kind of retaining wall.

newgrange rock types

So who make sup this cast of characters? Distances measured in ArcMap, from a generalized geologic unit to Newgrange assuming that the builders used boats as much as possible to transport the rocks.

Quartz: Distance traveled: about 65 miles. These are the same quartz veins that I wrote about in my Glendalough post!

Gabbro: Distance traveled: about 35 miles. This is a dark coarse-grained intrusive igneous rock.  It’s the most plentiful rock in the ocean’s crust, and when found on land is associated with rift zones where continents have torn apart from each other. It has the same mineral composition as basalt (like at Giant’s Causeway) but cooled slowly underground instead of solidifying quickly above the surface. It’s mostly made of the black mineral pyroxene, with smaller amounts of white plagioclase feldspar and green olivine. This gabbro even shares the same age and source as the Giant’s Causeway – about 60 million years old, and formed when Pangaea ripped apart to form the modern Atlantic Ocean.

Granodiorite: Distance traveled: about 40 miles. This rock is very much like granite but has a different balance of the two feldspar minerals. It has more plagioclase feldspar that granite does, and so appears whiter. This contrasts strikingly with the black amphibole and pyroxene minerals in the rock, so granodiorite puts me in mind of a dalmatian’s color scheme. The other kind of feldspar, orthoclase, can add a pinker tinge to granite. The Newry Granodiorite complex crystallized in the Caledonian period – about 400 million years ago – as a byproduct of the closure  of the ancient Iapetus Sea.

Graywacke: Distance traveled: about 15 miles. This particular kind of sandstone forms in deep marine environments when undersea avalanches, called turbidity currents,  mix up all sizes of particles – silt, clay, sand, and gravel. This rock was formed during the Silurian period between 433 and 419 million years ago, and got shuffled to the surface since then.

I can only wonder what these rocks meant to Newgrange’s builders. Some thoughtful ancient travelers noted their favorite rocks around the island, and decided that they ought to be a part of their community’s treasure. I think we might have gotten along.

Resources:

https://en.wikipedia.org/wiki/Newgrange

https://www.ancient.eu/Newgrange/

Granodiorite at the Mourne Mountains: http://www.mournelive.com/how-the-mournes-were-formed.htm

Gabbro on the Cooley Peninsula: http://www.3roc.net/assets/CooleyPeninsulageology.pdf

 

Ireland: The Giant’s Causeway and Carrick-A-Rede

There are some geologic features that are just way too cool for humans NOT to write a myth about them. The long-standing explanation of the columns of the Giants Causeway was that the giant Finn MacCool wanted to build a bridge to attack his rival in Scotland and that they destroyed the bridge in their fighting, leaving only the Causeway in Ireland and Fingal’s Cave in Scotland remaining. Then that long-standing story gets to rub shoulders with whatever geologic explanation comes along a few centuries later.

On this vacation, we got to road-trip to one of the most stunning geologic sites in Ireland – the Giant’s Causeway.  While we were up there anyways we decided to check out the Carrick-A-Rede rope bridge too, where I got an unexpected extra dose of cool geology.

Ireland trip

Giant’s Causeway is a phenomenal exposure of basalt. Sure, the rock type itself is nothing new. I wrote my thesis about basalt in northeastern Oregon. This particular basalt became famous for the huge field of well-exposed basalt columns, a feature that forms when lava gets the chance to cool slowly. And not only do you get to see the sides of the columns like you can in Oregon, but you can clamber over the tops of them too. It’s well worth the money for the audio tour. There are two versions for two different audiences- normal person and geology nerd – and both are entertaining and informative.

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This basalt flowed out of cracks in the local limestone around 55 to 60 million years ago, when the Atlantic Ocean was opening up. It took two stages to create the landscape here. First, about 6 basalt flows spread evenly across the landscape. Then volcanism stopped for a while; there was enough time for a the top part of the basalt to be weathered a rusty brown color as chemical reaction changed the basalt to laterite, lithomarge and bauxite. This zone is called the Interbasaltic Formation here. There was also time for water to carve a valley into the landscape. Later, volcanism restarted and a huge volume of lava poured into the river valley! At the time of formation, this deposit would have been a lava lake 90 meters (295 feet) deep! We know it had to have happened all in one event, because there are no weathered layers within it.You can see a basalt cliff at the level of my head in the background of the photo to my right – that basalt was deposited on top of the plateau beside the river valley. The thickness of the lava in the river valley meant that it cooled quite slowly, allowing the formation of the regular 4 to 7 sided columns.

You can see what I just described in the photo below. Note that you can see all three layers behind my mom, but that she’s standing on the Giant’s Causeway basalt that flowed into the valley that was cut into the Lower Basalt Group.

Giant's Causeway basalt annotated photo

My mom Lise, always the best tour guide, is here to introduce you to basalt geology! We didn’t hike up to the Organ Pipes, but they’re a beautiful exposure of both the colonnade and entablature layers of the Giant’s Causeway Basalt formation. Finn MacCool’s Chimneys are precarious free-standing basalt columns whose companions were eroded away on the headland.

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Checking out the lower basalt groups – they were physically weathered into rounded shapes as groundwater percolated down into the formation. The fancy name for this is “spheroidal weathering”. You can see a little bit of the Interbasaltic group in the top left corner of the photo, where the dark gray basalt has been chemically weathered into orange/brown minerals.

Whether they’re the ones I studied in Oregon or the Irish ones in this post, basalt flows have a certain internal structure to them. It’s determined by the fact that the molten rock cools mostly from the top down, with a little bit of cooling driven by the cooler ground underneath the flow. The quicker a region of the lava cools, the smaller the size of the cooling features. Air or contact with underlying rock rapidly cooled the lava at the very top or bottom of the flow, giving it a crumbly texture. In the diagram below, these areas are the vesicular top and bottom. Just below the vesicular top, the rock was cooling more slowly. Small cracks propagated from the top of the flow downwards as the rock cooled and shrank. The rock was cooling just quickly enough that the pattern of cracks was somewhat chaotic, creating blocky shapes or the curved and twisting “fanning columns” in this “entablature” zone.

basalt flow interior

Illustration of a basalt flow interior with vesicular zones, entablature, and colonnade from: https://jgs.lyellcollection.org/content/157/4/715

At Giant’s Causeway you can see huge boulders from the entablature zone that have fallen down among the columns. They have kind of a “giant meatball” rubbly texture that contrasts with the smooth elegance of the columns but makes them easy to climb.

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Below that is the sweet spot of cooling, whose features dominate the Giant’s Causeway. In the “colonnade” zone of the basalt flow the cracking pattern propagates neatly downwards. A hexagonal pattern develops when cooling contraction occurs at centers of mass that are evenly spaced in a homogeneous body of lava. If there are variations in the thickness or composition of the lava then other geometries of fracture may occur, with anywhere from 4 to 8 sides.

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Giant’s Causeway’s famous colonnade layer, with geologist for scale.

You can see in the picture above that the columns are broken by joints perpendicular to the ones that define the columns. These are subtle ball-and-socket joints that formed when the area was subjected to horizontal stresses after the rock had cooled and the columns had formed. These ball-and-sockets allow the formation to bend a bit.

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concave “socket” column top on the left, convex “ball” column top on the upper right.

Later that day the sun came out and we decided to check out the Carrick-A-Rede rope bridge. It’s absolutely touristy but worth it on a beautiful day like that. The bridge and island are only accessible by paying for a group tour that departs at regular intervals. You end up parking in a nearby abandoned limestone quarry called Larrybane Quarry.  It has beautiful seaside views, and a fight scene from Game of Thrones was filmed here.

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Unfenced drops? Cliff edges? Count me in. Spoiler alert, our mom did not keep us close…

How can you ask a geologist to stay on the path when there are cool chert nodules to investigate in sea caves? In the right-hand photo, the black rock is chert. In the photo with my sister and me in it on the left, you can see how these chert nodules are distributed in horizontal bands in the limestone. These nodules are formed after the remains of microscopic organisms like coccolithophores, radiolarians, and diatoms are laid down and start to solidify into rock. Water in the formation dissolves some of the silica-rich remains and redeposits them in layers around impurities in the sediment, forming blobs of chert that are often called “flint” when they’re found in chalk beds like these.

In the top right corner of the photo you can see stalactites – this is the only known location in the Ulster Limestone (a.k.a. Upper Cretaceous Limestone) along the Causeway Coast with these distinctive formations.

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When the line of tourists waiting to get across the bridge is longer than the bridge, it might be a tourist trap.

This was just a detour to the main attraction – the Carrick-A-Rede rope bridge. It was constructed in 1755 to allow fishermen access to fruitful salmon fishing groups, and has been developed as a somewhat oversold tourist attraction. It has a neat geologic history though. The little island stands tall because it’s the eroded neck of a volcano! It formed 62 million years ago, just a little bit before the basalt at the Giant’s Causeway. The island is formed of dolerite, a kind of intrusive volcanic rock. When you’re on the island and looking back to the mainland, you can see large blocks of basalt and limestone suspended in a matrix of volcanic ash.

 

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The short hike to the bridge takes you down rocky staircase and along gravel paths with lovely views out to sea. The tour guides don’t encourage the group to spend too much time on the island itself, but we got to take twenty minutes or so to spy on the seagull nests, eat some snacks, and admire the view out to Rathlin Island.

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Here’s a map of an overview of the geology around Giant’s Causeway and Carrick-a-Rede that I put together in ArcMap. This is from the 1:500,000km shapefile available from the Geological Survey of Ireland. The more detailed 1:100,00K map excludes Northern Ireland… I guess political animosity somehow spilled over into data collection and sharing here. That’s a shame. So Carrick-A-Rede itself isn’t mapped here.

Northern Ireland Geology

Stay tuned for a geology/archaeology crossover event! My family headed to the Newgrange Stone Age Passage Tomb later that week.

Resources:

https://www.geolsoc.org.uk/GeositesGiantsCauseway

booklet on the geology of Northern Ireland’s Coast: http://ccght.org/wp-content/uploads/2012/05/geology_booklet.pdf

A field trip at the Sand Atlas blog whose author took some better photos of the formations: https://www.sandatlas.org/giants-causeway/

Debbie Hanneman over at GeoPostings did this trip back in 2017 and shared some great photos: https://www.geopostings.com/category/giants-causeway/

AGU resource on columnar jointing: https://blogs.agu.org/georneys/2012/11/18/geology-word-of-the-week-c-is-for-columnar-jointing/

Information about the geology at Carrick-A-Rede: http://www.habitas.org.uk/escr/site.asp?item=1145

Article about the formation of chert nodules in carbonate beds:

Maliva, Robert G., and Raymond Siever. “Nodular Chert Formation in Carbonate Rocks.” The Journal of Geology, vol. 97, no. 4, 1989, pp. 421–433. JSTOR, http://www.jstor.org/stable/30078348. Accessed 14 May 2020.

Cool podcast about the tiny creatures that become chalk, courtesy of Radiolab: https://www.wnycstudios.org/podcasts/radiolab/articles/190284-war-we-need

Ireland: Mining’s legacy in Glendalough

After Heather and I explored Brittany we headed north to join 35 other van Stolks and their partners in Ireland for a family reunion. No, we aren’t Irish, but the Dutch family wanted to vacation outside of the Netherlands and the American part of the family wanted to spend time in a scenic part of northern Europe. Ireland was a delightful compromise. We converged on a holiday cottage complex just north of Dublin where we spent a convivial time moving from porch to porch catching up on years of news. The whole bunch of us set out in a rather unruly convoy to highlights like Newgrange, Slane Castle, and a sheep herding demonstration further afield in Glendalough.

Ireland trip

Glendalough is a jewel of a lake in the mountains south of Dublin in County Wicklow, a rugged contrast to the gently rolling green hills usually associated with Ireland. We all oohed and aahed at the sheepdogs and their  puppies, and then a smaller group of cousins set out on a hike to work off the cabin fever.We did the “white trail” around the upper and lower lakes at Glendalough. It’s a stunning 7.8 mile loop! We did it counter-clockwise, which results in a gentler upward climb. If you hike this clockwise you have to climb up the hundreds of wooden stairs on the eastern side of the lake… not my idea of a great time. Going counter-clockwise also results in beautiful views of the monastery site from the top of the cliff!

Glendalough white trail

Trail map from Alltrails.com

It turned out to be a lesson in the importance of looking at the scale of the contour lines on the topographic map at the visitor’s center. We thought “oh, we only cross one topo line on the map, the trail must stay close to the lake.” Well it turned out that the distance between topo lines on that map was 0.4 kilometers – about 1,500 feet. My cousins with a strong aversion to heights were absolute troopers. The views from the top were amazing!

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Hanging out in a textbook-perfect glacial valley, with the old mine buildings in the background. Halfway to the top!

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At the top! Two cousins aren’t in this picture because they didn’t fancy spending more time at the top of this cliff than absolutely necessary and they also are much fitter than the rest of us.

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The southern cliff is topped with blanket bogs. They’re an extremely soggy and sensitive landscape, so the park put in a couple miles of boardwalk to minimize human impact. I felt like I was somewhere in Tolkien’s Middle Earth!

Glendalough seems like a valley outside time, once you step away from the tourist shops. The paths take you by an old monastery, streams in strange mossy landscapes, and the lake itself surround by hills and forests. It came as a surprise to me when we found the remnants of an abandoned mine at the eastern end of the lake.

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It turns out that between the 1790s and the 1920s this area a hive of mining activity. You can easily tell what the miners were looking for in the landscape – the scars of white quartz rubble are the giveaway (see the slope on the right side of the photo above). The miners were looking for lead, silver, and zinc. Specifically, they found it in the minerals galena and sphalerite.

Image of galena (dark gray) and sphalerite (orange-ish brown) in quartz (white) from the Glendalough mine from the National Museum of Ireland.

Galena is a mineral composed of equal amounts of lead and sulphur (its formula is PbS). In this area silver substitutes for lead in the crystal structure around 5% of the time, making it a valuable ore for silver as well as lead. Sphalerite is made of equal amounts of zinc and sulphur (formula is ZnS). But how did they get here, and why are they only found in the quartz?

Let’s take a step back an look at the history of the local landscape over the past few million years, courtesy of an interpretive sign at Glendalough’s ranger center. The information is great so I didn’t bother re-typing it, but you may have to click on the image and zoom to read it if you’re reading this on your phone.

 

Photo of an interpretive sign of Glendalough's geology at the ranger center.

Photo of an interpretive sign of Glendalough’s geology at the ranger center.

Two types of rocks form the foundation of this landscape: a metamorphosed version of mudstone or shale called schist, and the granite which muscled its way up into those rocks during the Caledonian orogeny. Remember that from a few blog posts back? This Irish granite is a cousin of sorts to the granite that became the Mont Saint Michel. It too was formed as the heat created by the collision of Laurentia, Baltica, and Avalonia created magma that rose up into overlying rocks and cooled into huge lumps of granite (called batholiths in geologist jargon). In the map below, the granite is shown in red. It also shows just how many mines were once active in this area!

Glendalough mining

A map of mining activities near Glendalough – we hiked past #7 and #8, Glendalough Valley mine and Van Diemen’s land mine. This map also shows the geologic contrast in the region between the schist (light pink) to the east and the granite (red) to the west of the lake. This map is from https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW065_Glendalough_Glendasan_Glenmalure_District_Overview.pdf

This particular batholith is called the Leinster Granite batholith and underlies much of County Wicklow. It’s harder than the surrounding schist and creates more rugged cliffs when assaulted with millions of years of wind, rain, and glaciers. In the Wicklow mountains the granite slopes tend to be covered in boulder fields, and the schist slopes are covered in heather and other creeping low bushes. Neither type of rock weathers into particularly inviting soil for plants, at least not in the geologically short period since the last Ice Age.

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Rock specimens that I couldn’t resist at Glendalough: Schist with neat protruding flexible sheet of mica (left), granite (center), bits of waste quartz from the mining operation (right)

Here’s a map of the mine site that is #7 on the map above, and the first area we came to on our hike. This map was put together by the educational group “Glens of Lead”. This group put up some great historical signs in along the park about how the old mining operations worked.

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And here’s a map of the second area on the hike, #8 on the map.

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Very little of the original infrastructure remains today, except for the stone buildings at the Glendalough mine site and the bright white quartz of the tailings rubble from the mines. The shafts and tunnels have been blocked off and the old tramways completely dismantled. The site seems very wild again.

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Standing on the schist side of the valley, looking over to the steep granite cliffs and the piles of quartz tailings below the exits of the old mine shafts 1,000 feet below.

Above right: granite with vein of hydrothermal mineralization (foot for scale) in the mining area, compared with schist exposed at the top of the cliff on the south side of the lake.

But how does lead ore get into quartz veins? I’ve written about continental collisions and granite before in this blog, but not really about smaller processes of metamorphism. It’s time to fire up MS Paint again.

subduction

Magma bodies (red) rise off of the subducting oceanic crust and cool into intrusive igneous rocks (pink). Water (blue speckles) in the oceanic crust allows the crust to melt at lower temperatures than the surrounding rock, and travels upwards as a part of the magma. Diagram by C. van Stolk.

Back around 300 million years ago, the ocean Iapetus was closing as the old continents Laurentia, Gondwana, and Avalonia moved towards each other. The oceanic crust under Iapetus had to go somewhere; it subducted under the continents. After a few million years of being underwater that oceanic crust was pretty soggy as rocks go. The conveyor belt of plate tectonics drove the heavy oceanic crust down under the lighter continental crust.  It began to melt as it sank beneath the continent and into the upper layer of Earth’s mantle called the asthenosphere.

It turns out that this water trapped in the crust is kind of the “secret sauce” of metamorphism. The presence of water allows rocks to melt at lower temperatures than they would otherwise. Metamorphism boils down to two variables – heat and pressure. Both increase vertically with depth in the earth’s crust. Pressure also increases horizontally in collision zones. In the presence of equal amounts of heat and pressure, wet rock will melt to a greater degree than dry rock.

Anyone who has taken a ride in a hot air balloon learns that heat rises – the hot air in the balloon keeps the passengers aloft in the cooler surrounding air. The blobs of magma rising from the subducting wet oceanic crust are much like extremely dense, slow-motion hot air balloons – they rise through any weakness they can find in the surrounding cooler and drier rock. The blobs of magma become batholiths of intrusive rock when they cool, like the granite here. As the granite cooled, the heat had to go somewhere, just as the oceanic plate had to go somewhere as the ocean closed. The magma “cooked” the shales that surrounded it into the metamorphosed version – schist (see purple “contact metamorphism” on the diagram below). However the story of the water that magma contained isn’t over.

subduction with contact metamorphism

If water can’t fit into the crystal structure of the magma as it cools into intrusive igneous rocks, it is released from the melt. It takes along ions that can be dissolved in it and travels into cracks in the surrounding rock. Often these “cracks” are caused by faults or by joints caused by horizontal pressure. One of the most common elements carried along this way is silica, which in combination with oxygen forms quartz veins as it cools. This mineral-rich hot water is called a “hydrothermal solution”.

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Diagram of hydrothermal alteration from https://opentextbc.ca/geology. (a) Shows a magma body that has risen into cooler rock and is “cooking” it shown by the purple aureole. (b) Shows magmatic water being released from the magma body through veins. (c) Shows how groundwater moving past the magma body can also carry dissolved minerals away from it to other locations.

As the hydrothermal solution rises and cools, minerals form out of the solution like rock sugar forming out of hot sugar syrup as it cools down. Not every part of the solution is really well mixed – some parts of the solution are like oil and water and stay somewhat distinct as they travel together. Examples of this are silicate minerals (i.e. quartz SiO2, feldspar KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) and sulphide minerals (galena PbS, sphalerite ZnS). As the solution cools, these two types of minerals form adjacent but separate structures.

So here at Glendalough, you have granite and schist cut through with veins of hydrothermal rocks that contain chunks of sulphide ore minerals in the more abundant quartz. The miners followed these hydrothermal veins to find the valuable ore, and discarded the attractive but comparably worthless quartz in tailings piles at the site after the rock was put through a huge crusher to break out the softer sulphide ore minerals. I was sorry to notice that they were so thorough that a casual geologist really can’t find any of that ore nowadays.

I was happy enough to take away great memories, beautiful views, and a few new rocks in my pocket.

Resources:

A good brief history of the mines here: https://www.mindat.org/article.php/368/A+History+of+Glendalough

Brief intro to geology of Glendalough here: https://www.wicklowmountainsnationalpark.ie/nature/geology/

https://www.gsi.ie/en-ie/geoscience-topics/geology/Pages/Geology-of-Ireland.aspx

detailed geological survey of mining in the region: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW065_Glendalough_Glendasan_Glenmalure_District_Overview.pdf

detailed geological survey of mining at Glendalough specifically: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW025_Glendalough.pdf

geological map of Ireland: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Bedrock/Other/GSI_BedrockGeologyOfIreland_A4.pdf

Maps of lead mining in the area by the local education group “Glens of Lead”: https://www.avenzamaps.com/vendor/209/glens-of-lead

More about the Iapetus Suture, which connects the half of Ireland/Scotland that was once Laurentia (proto-North America) with the half that was once Avalonia (proto-Europe). https://en.wikipedia.org/wiki/Iapetus_Suture

Interesting map of the terranes that make up Ireland: http://www.askaboutireland.ie/reading-room/environment-geography/physical-landscape/Irelands-physical-landsca/the-formation-of-the-phys/

Ireland through geologic time: https://www.gsi.ie/en-ie/education/the-geology-of-ireland/Pages/Ireland-through-geological-time.aspx

Specimen of galena in quartz from the mine: https://www.museum.ie/The-Collections/Documentation-Discoveries/July-2016-(1)/Lead-Bearing-Minerals-from-Glendalough

More about contact metamorphism/hydrothermal alteration/sulfide ore bodies

Contact metamorphism with good diagrams: https://opentextbc.ca/geology/chapter/7-5-contact-metamorphism-and-hydrothermal-processes/

http://www.geologyin.com/2014/11/veins-and-hydrothermal-deposits.html

Overview of sulfide ores: https://uwaterloo.ca/earth-sciences-museum/resources/hydrothermal-minerals

Cliffsnotes version of hydrothermal metamorphism: https://www.cliffsnotes.com/study-guides/geology/metamorphic-rocks/hydrothermal-rocks

Detailed review of ore genesis, including immiscible solutions. https://courses.lumenlearning.com/wmopen-geology/chapter/outcome-ore-and-mineral-resources/

Mazes of mines and catacombs beneath Paris

The SacrĂ©-CĹ“ur Basilica was swarming with tourists on the first day that Elaine, Heather, and I set out to explore it. The line for entry stretched all the way across the plaza and we could hear the muffled din of the crowds within the sanctuary spilling out through the doors. We made a unanimous decision to avoid the chaos by heading to the quieter sanctuary of the nearby MusĂ©e de Monmartre. It’s dedicated to the artists and cabarets that gave the neighborhood its bohemian reputation at the end of the 1800s, and was the last place I though I’d find anything geological. But lo and behold, it had a small exhibit on the gypsum mines that used to be active on the Montmartre Butte, as sketched by the Impressionists.

These mines complicated the construction of the Basilica that we had considered visiting that day. In the 1870s the Parisian government committed to building a huge Catholic monument there as an unmistakable reminder of the power of church and state. Montmartre had been the birthplace of the radical socialist Commune movement that had unsuccessfully tried to overthrow the government in 1871, and the government wanted to remind the neighborhood of who really called the shots. However before the structure could rise above the surface, the foundation required 83 pillars sunk 130 feet deep into the rock layers below the gypsum mines.

I had something of an epiphany (or more accurately, and “oh, duh!” moment) – gypsum is the main ingredient in “Plaster of Paris”! So that’s where it came from! In particular, it came from the green areas on the map below:

Mines of paris translated

Montmartre is clearly visible as the ring-shaped cluster of gypsum mines and the center top of the figure. map translated from French by C. van Stolk. By Plan: Émile GĂ©rards (1859–1920) BnF Notice d’autoritĂ© personneDigital copy: ThePromenader – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=28798468

 

Most of the pale building stone that makes the City of Light so distinctive actually comes from within its limits! It gave me a sense of the huge scale of mining in the city that most of what we see on the surface came from underneath it. The gypsum was mined extensively beginning in the Middle Ages to create fire-resistant covering for wooden structures. You can see the distinctive cream or yellow-ish limestone in almost all of the buildings in Paris built before the 1770s, from the Saint-Germaine church to the grand mansions between the Marais and the Place des Vosges. The limestone is riddled with little cone and spiral shaped fossils too, if you look carefully.

These quarries have been active since the Romans occupied what they called Lutece, but the city didn’t get its wake-up call until 1777 when a gaping sinkhole swallowed an entire city block near what’s now the Place Denfert-Rochereau. That’s when King Louis XVI banned mining within the city limits. He also commissioned Paris’s first mine inspector, Charles-Axel Guillaumot, to map the warren beneath their feet and shore up the weak places to prevent a repeat of the “Place d’Enfer” disaster. M. Guillaumot earned the nickname “the savior of Paris” and was one of the few royal appointees of that era to survive the guillotine – the revolutionaries considered him too useful.

Around the same time, Parisians realized that the former quarries posed a solution to another pressing problem – unmanageable overcrowding at cemeteries above ground. Between 1785 and 1814 the bones of over 6 million people who had died since the founding of the city were disinterred and moved in nocturnal religious processions into the properly sanctified sections of the tunnels designated as the municipal ossuary or catacomb just outside what was then the southern boundary of the city. The catacombs were opened to the general public for tours in 1810.

119 years afterwards, Elaine, Heather, and I set out to wait in the long line to enter the Municipal Ossuary a.k.a. the Catacombs of Paris. It’s a rare attraction that appeals the Elaine’s love of spooky things, my love of rocks, Heather’s interest in history, and all of our pressing interest in getting out of the 97 degree heat. We just managed to get into the last tour group before the gates closed for the day! Because of the time crunch we had to fly through the geology exhibition in favor of getting to the creepy bits but I took photos of the placards, some of which I have translated below. For photos of the ensuing Super Spooky Aesthetic ™ check out my travelogue post.

The rocks below Paris are younger than the rocks we clambered on in Brittany – they’re from the Eocene era between 56 and 38 million years ago, when the Paris Basin held a shallow sea that left behind characteristic limestones and shell fossils. The sea went through periods when it very nearly dried up, leaving layers of evaporate minerals such as gypsum.

translated paris paleogeography

Image photographed by author at the catacombs, legend translated

North-south tectonic pressures slightly buckled the basin in the time since the rock was formed. This created the Meudon anticline (A-shaped fold) on the southern side of Paris, which is why limestone and gypsum from different ages are mined at similar elevations on either side of the Seine.

translated cross section of paris

Image photographed by author at the Catacombs, and subsequently translated

Left: labeled layers in the limestone, Right: teensy 5mm stalactites!

Paris has a thriving community of “cataphiles” who risk law enforcement action to explore the subterranean side of the city. National Geographic did a great special on them in 2011. Over the years since the quarries were abandoned in the 1950s, they’ve made the flipside of Paris into their playground and undertaken mapping efforts. Here’s an elegant map of the “Great Southern Network”, with notes and annotations. It’s the kind of treasure map that got me into cartography in the first place.

If you’d like to learn more, Dr. Jack Share at one of my favorite blogs “Written in stone, seen through my lens” wrote two fantastic, extremely detailed posts about the geology of Paris – one focusing on the gypsum quarries and the geologic origins of the Paris Basin, and a second one on the mines and catacombs. I highly recommend those posts, and the entire blog!

Postscript:

We did eventually visit the Sacre Coeur Basilica towards the end of our stay in Paris. It turns out that the basilica is quietest directly after services in the evening, and there is a lovely organ postlude. From the inside it’s a wonderfully peculiar building. Sacre Coeur was built between 1875 and 1919 and the stained glass windows weren’t added until after WWII, and so it encompasses huge changes in the French design aesthetic. The architecture is a mix of the stately Neo-classicical style like the Pantheon and the Byzantine revival style, it got a gloss of whimsical Art Nouveau statuary at the turn of the century, and ended up with weird abstract stained glass windows from the post-war period when artists felt that the world was broken beyond repair. The building is made of travertine from the Souppes-sur-Loing quarry, in the Seine-et-Marne department about 100 km south of the basilica. Travertine is an exceedingly hard, fine-grained stone that releases chalky white calcite when it rains. So basically, it’s self-cleaning and is able to stay gleaming without pressure-washing!

English-language resources:

https://en.wikipedia.org/wiki/Mines_of_Paris

Paris: From quarry to catacombs

http://catacombes.paris.fr/en/history/geology-and-quarries

http://www.sacre-coeur-montmartre.com/english/history-and-visit/article/architecture

A really excellent post about the mines of Montmartre: http://written-in-stone-seen-through-my-lens.blogspot.com/2014/04/geological-legacies-of-paris-basin-part.html

Another very thorough post from that same author on the rest of Paris’ quarries: http://written-in-stone-seen-through-my-lens.blogspot.com/2014/06/geological-legacies-of-paris-basin-part.html

reddit post hosting an AMAZING map of all the explored catacombs under Paris: https://www.reddit.com/r/MapPorn/comments/b50j1r/detailed_map_of_the_paris_catacombs_in_english/

French-Language resources:

detailed post on mines, mining techniques, and mine inspection in Paris: http://exploration.urban.free.fr/carrieres/index.htm#exploitation

Sea caves and wild cliffs at the Crozon peninsula

After the Cote de Granit Rose, Heather and I drove about as far west as you can get in France – to the Crozon Peninsula. We settled in at a sailing club hostel in little fishing port of Camaret-sur-Mer and then hiked out to the cliffs at the Pointe de Pen Hir to enjoy the French tradition of apero (pre-dinner wine and snacks). We held on tightly to our packets of cheese crackers – 70 meters would be a long way to drop them off of these cliffs.

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The cream-colored cliffs turned golden as the sun set and I was intensely curious about them. The coast and  sea stacks were made of pale layers that were tilted about 60 degrees from horizontal towards the east and eroded in jagged shapes. Luckily, we ran into a sign for the Regional Nature Reserve with an a few answers. These cliffs are made of the Armorican Sandstone, dated at 475 millions years old. The rock is tough, but not strong enough to resist the waves entirely. The southernmost part of the cliff has been broken up into a series of six sea stacks called the “Tas de Pois”, or “Pile of Peas”.

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These tilted layers are a thing of beauty! You can see here how the purer sandstones form ridges (center right) separated by the weaker and more eroded shale layers (center). This photo was taken looking south.

The Armorican Sandstone is at the bottom of a thick stack of tilted sedimentary rocks, and we met more of them as we hiked east along the southern side of the peninsula. The rest of this stratigraphic sequence forms the Veryac’h Cliffs, which have been designated a national geologic heritage site.

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A beautiful view, and a sign that was both helpful and disappointing. It had little icons forbidding both rock hammers and sample collection on the beach!

In previous posts about the region, I’ve brought up the two mountain-building events that created metamorphic and igneous rocks in northern France. The Cadomian orogeny 750 to 540 million years ago created the metamorphic and igneous rocks near Mont Saint-Michel, and the Variscan Orogeny 360 to 300 million years ago left its mark in the granites at the Cote de Granit Rose. What I haven’t covered yet on this blog is the time that elapsed between those two continental collisions. The Armorican sandstone and the rocks of the Verac’h Cliffs were deposited in a sedimentary basin that opened up between 500 and 360 millions years ago, as the result of tectonic extension in between those two mountain-building events. Sediments eroded from the nearby Cadomian mountains and were deposited in the extensional basin. The Variscan orogeny then squeezed these horizontal layers into folds – that’s how these layers ended up tilted on their side.

French geologists are immensely proud of this 1000 meter stretch of cliffs because they represent an unbroken 50 million year record! Unbroken is the key word here. It’s rare to find an area were sediment has been laid down continuously, without the sea level changing and eroding away layers to create an unconformity. The Veryac’h cliffs hold an encyclopedia of fossils and information about the environment from the Orovician, Silurian, and Devonian eras.

I didn’t get to explore them on the ground, though. Heather’s tolerance for staring at rocks has some limits. There are some great field guides online if you read French…

If you feel like diving into a full literature review in academic French, Vidal et al. published an exhaustive geologic history of the Crozon Peninsula in 2010. It’s in French again, but has excellent figures. I’ve modified and translated one of them in the figure below (it’s large, sorry, you may need to click on it to see it full-size and read the text):

 

Crozon peninsula geology translated

It’s a great mess of a geologic map, isn’t it? The rocks were deposited in a stratigraphic sequence that was orderly enough, but in the millions of years since then they’ve been folded, broken, and shuffled around.

Imagine stacking a dozen or so carpets on top of each other. Next, recruit a few friends to shove the carpet stack from each side until it’s a rumpled mess of folds. Once you’ve done that, attack the top of the pile with a chainsaw to level it out but remove the cut-off bits as you do this. Shove the pile around a bit more for good measure and make more passes with the chainsaw, and you’ve got a representation of what happen in this corner of Brittany during the Variscan mountain building event when the ancient continent of Avalonia ran into Gondwana.

deposition folding faulting

Extremely simplified sketch of the metaphor above… figure drawn by author.

 

The rocks near Morgat on the western side of the Cap de la Chevre where we kayaked are also tilted layers of the Armorican Sandstone, but tilted to the opposite direction. The beds also strike roughly northeast-southwest but dip steeply to the northwest. I didn’t find a direct reference to this in my sources. One likely explanation is that the two outcrops of the Armorican sandstone are two limbs of a syncline ( U-shaped fold) that were broken apart in the chaotic faulting in the region during the Variscan orogeny.

Photos above: looking at the Armorican sandstone in the Cap de la Chevre from the south (left) and from the north (right).

Variations in the hardness in the sequential layers of sandstone are more or less resistant to the waves, which creates the wonderful arches and caves that we explored in our kayaks.

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Illustration of sea cave and sea stack formation, from The British Geographer http://thebritishgeographer.weebly.com/coasts-of-erosion-and-coasts-of-deposition.html

Next up on the blog: the creepy catacombs and ancient mines beneath Paris!

Sources:

https://www.presqu-ile-de-crozon.com/geologie/000-geologie-finistere-presqu-ile-de-crozon.php

Click to access Saga_312_Crozon.pdf

http://sigesbre.brgm.fr/Histoire-geologique-de-la-Bretagne-59.html

portal for geologic maps of Brittany from the BRGM: http://sigesbre.brgm.fr/Cartes-geologiques,147.html

https://sgmb.univ-rennes1.fr/vie-associative/excursions/12-excursions/51-crozon

Vidal, Muriel & Dabard, Marie Pierre & Gourvennec, R. & Hérissé, Alain & Loi, Alfredo & Paris, Florentin & Plusquellec, Y. & Racheboeuf, P.R.. (2011). The Paleozoic formations from the Crozon Peninsula (Brittany, France). Geologie de la France. 3-45.

Crazy pink rock formations at the Cote du Granit Rose

Part 2 of the geology of my summer vacation. For an idea of where this fit in our trip, check out the travelogue post. This post follows the first post on Mont Saint-Michel.

I had left all of the vacation planning in Heather’s able hands so I could focus on my thesis last spring. My only requirement (only halfway in jest) was that the vacation had to include eating pastries on rocks. And boy, did Heather deliver! Days 5 and 6 of the trip found us near Trebeurden and Ploumanac’h on the fabulous pink granite coast. The sun was shining, the pain au chocolat was as delicious as I had ever hoped for, and a giant granite playground awaited us.

croissant and rocks

my dreams came true!

There isn’t a shortage of granite on the Brittany coast – we met some in the last blog post too. Much of was grey and only visible in isolated outcrops. As we hiked east from the little port of Ploumanac’h along the coast, the grey granite gave way to crazy piles of unmistakably pink rock! I couldn’t help but start wondering what caused the change in color, not to mention the weird shapes!

It turns out that the explanations come in threes: the pink granite is made of three minerals, it belongs to one of three different igneous events in the region, and three different substances have sculpted the granite into the wild shapes at Ploumanac’h.

The pink granite gets its rosy hue from potassium feldspar, while the greyer granite has more creamy-colored plagioclase feldspar in its makeup. I illustrated their mineral composition in the figure below. The natural history museum in Ploumanac’h informed me that the pink granite is  approximately 50% potassium feldspar, 30% quartz, and 20% biotite. They didn’t give details about the less glamorous grey granite and I was too focused on getting to the pink stuff to even take a close-up of it, so I’ve only approximated its composition.

Pink Grey Granite Comparison

Both colors of granite at Ploumanac’h were put in place around 300 million years ago (mya) during the last gasps of a mountain-building event as the ancient continents of Gondwana and Laurussia crashed together to form Pangaea. I talked in depth about this massive game of continental bumper-cars in the previous post, so I’ll skip it here. Over time erosion unearthed the buried masses of granite, as shown in the figure below.

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Photo of a diagram in the exhibit at the Maison du littoral, text translated by me.

To get even more specific, the granite in the area was put in place in three physically distinct phases around 300 mya. In the first phase, two magmas with different compositions intruded the surrounding metamorphic rock at the same time. The first was rich in silicon and formed the coarse-grained pink granite and the second was poor in silicon and formed the dark gabbro visible near Tregastel. These two igneous rock types melted in the same event from two different types of source rocks, giving them their unique compositions.

During the second phase, another silica-rich magma forced its way into joints in the now-cool first pink granite. This magma had a similar composition  to the pink granite in the first event but cooled more quickly than its predecessor, forming smaller mineral crystals.

In the third phase, a magma with a more basic (as in pH) composition intruded into an dome-shaped weakness in the cooled granite from the first two phases. This magma cooled into the blue-gray granite near Ile-Grande.

The difference between the colors of the ~520 million year old granite at Mont Saint-Michel, the ~300 million year old grey granite at Trebeurden, and the ~300 million year old granite at Ploumanac’h isn’t merely ornamental. The rocks’ mineral compositions give geologists clues to the kinds of source rocks that melted into the granite. Feldspars and quartz have high silicon:oxygen ratios in their composition, and so indicate that abundant silica was present in the source rocks.

A whole host of different kinds of minerals are built from silica and oxygen, ranging from the densest minerals with 4 oxygen atoms  for every 1 silicon atom to the less dense minerals with only 2 oxygen atoms for every 1 silicon atom. In general, the less dense silicon-rich minerals are more represented in the continental crust, while the denser silicon-poor minerals are more common in the oceanic crust.

You can see these relationships between minerals’ properties and igneous rock types below in the igneous rock classification chart every mineralogy student learns by heart by the end of the term. It’s only a guideline – if a mineral was missing from the source rock, it will not show up in the igneous rock created from its melting. For example, amphibole and muscovite are missing from the pink granite.

This indicates that the pink granite was formed predominantly by the melting of low-density, high-silica rocks at low melting temperatures. The grey granite at Trebeurden is a little bit to the right of the pink granite on the classification chart – still a granite, but including more minerals with higher melting points and less potassium feldspar (a.k.a.  orthoclase feldspar). The gabbro at St. Anne is even further to the right, and likely formed from the melting of a chunk of oceanic crust. Sometimes rocks are completely off this chart. For example the magma that formed the pale granite that we saw at Mont Saint-Michel either melted at low temperatures (geologically speaking) of ~600 C or melted from source rock whose chemistry didn’t allow for the formation of dark mica or amphibole crystals.

So I figured out why the granite was pink instead of grey. But what created its otherworldly shapes? And where did all these boulders come from?

Usually boulders are created in steep landscapes where chunks of rock falling off the canyon walls are tumbled aggressively in mountain streams and carried long distances. In contrast, these boulders have barely moved relative to each other since the granite cooled! They were formed in place by erosion, shown in the diagram below. The technical French term for this formation is “un chaos”, which seems very appropriate.

granite chaos creation

The important factor here is a change in the rate of weathering and erosion. In this case, the erosion regime changed from slow dissolution of the rock by groundwater (shaping the granite into boulders underground) to more rapid erosion as the waves crash on the shore (exposing the boulders).

Once the boulders are exposed to the elements, two slower types of chemical erosion nibble them into even more convoluted shapes. Chemical reactions between salt spray and the the mica and feldspar crystals in the rock transform them into weaker clay minerals that wash away, creating divots and creases in the rock wherever salt collects.

As saltwater works on the rocks from the top, organic acids in soil eat away at the rocks at ground level over tens of thousands of years to create subtle mushroom shapes.

acidic soil erosion

The end result is an utter delight to explore!

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Heather points out a quartz vein in the pink granite. The boulder on the center left shows a distinct salt weathering divot on its top.

pink granite castle

Climbing to the top of a formation, I found a 2-foot deep crenelated “crow’s nest” formed by salt weathering!

Sources (all are in French):

Great summary from the local natural history museum, the Maison du littoral: http://ville.perros-guirec.com/fileadmin/user_upload/mediatheque/Ville/Maison_du_littoral/refonte_page_environnement/expo_origine_du_granit_roseBD.pdf

Less technical summary from the local tourist board: http://www.cotedegranitrose.net/la-cote-de-granit-rose/geologie-le-granit-rose/

Long and extremely thorough field trip guide published by the Geological and Mineralogical Society of Brittany: https://sgmb.univ-rennes1.fr/geotopes/decouvertes/23-decouvertes/67-ploumanac-h

Short summary/technical field trip guide: http://www.saga-geol.asso.fr/Geologie_page_conf_Ploumanach.html