Sweet Rocks

So, a lot of descriptions of rocks are pretty bland but rocks are actually really sweet! In fact, rocks do a lot of things that can be recreated with sugar.

The following are the sugar analogs of geologic features:

Lava tubes: super long roasted marshmallows

Aa lava: spattered melted marshmallow

Pahoehoe lava: frosting

Pumice: cotton candy
Obsidian: squashed cotton candy

Collapsed lava tube

This collapsed lava tube is similar to the outer shell of a roasted marshmallow with a melted inside, but the interior of this tube is sent through in pulses, leaving different layers that are identifiable by composition and superposition. When the melted inside is gone, the lava tube often collapses, providing openings for cavers to explore.

Scaly plates near Chimney crater

Imagine that a small volcano is a pot of boiling melted marshmallows. A lot of drops will splash up over the side and splatter in flat scales around the pot. Now imagine that when the pot is near empty, new marshmallows are added. This picture shows an area near a small crater that had bursts of activity, including lava tubes and this spattering process.

This lava is called aa because that's the sound you make if you step on it barefoot. It is sharp, with holes like swiss cheese (we call them vesicles) and is similar to what would result if you threw melted marshmallows at a pan and then froze them.

Giant Crater, with a lava tube exit on the far side in the middle.

Giant crater is a very impressive sight and worked with mechanisms similar to Chimney Crater.

Pahoehoe lava, likely chilled while erupting

This is Tim's favorite spot. It's the only pahoehoe lava in the area and he thinks that it was formed when there was a snow storm at the same time as an eruption from Giant Crater. It looks similar to piped icing on the surface but becomes vesicular less than 1 cm from the surface (see the upper right part of the photo).

Glass Mountain

Glass mountain is a huge mass of obsidian and pumice with evidence of magma mixing at all scales. If you're not sure what pumice is, it was probably used to exfoliate your feet at the spa. Glass mountain has been mined for pumice several times as different products have gone in and out of use. Obsidian is so sharp that we can't make anything sharper so we use it for surgical scalpels.

I prefer to think of these rocks as cotton candy and squished cotton candy. They have the same composition and all started out with vesicles but the obsidian got rid of their bubbles. In the same way that cotton candy changes color when compressed, obsidian is much darker than pumice.

I'll let the rest of the pictures speak for themselves.

Obsidian next to pumice

Statuesque obsidian chunk

Pink rocks are remnants of magma mixing

Color changes with concentration of vesicles (holes)

"Jig-saw" obsidian

Large vesicles vs. small vesicles

Yes, it's that big.

Blob of obsidian

Obsidian vs. pumice with brass rat for scale

We were all wearing gloves to protect ourselves if we slipped. Also, the pumice makes a grinding noise when you step on it and the obsidian clinks like glass (because it is glass) so walking around was very noisy. Walking around Giant Crater was also noisy but if you were careful then you could make less noise. There is a video of us walking on Glass Mountain but I don't have it, so look for it soon!


So I hope that you now see that rocks are sweet, and not just to geologists.

We saw these things on 8/27/15 in Northern California.

Of Tuffs and Tufas

On Tuesday, I (Reena) was feeling pretty darn great. This may have had something to do with two cups of unexpectedly good coffee at breakfast, or having gotten to take a bath in a river the night before. Anyway, in the morning we drove to Mono Lake to see its iconic tufa towers: tall, lumpy buildups of chemically precipitated calcium carbonate. All lakes are natural aqueous chemistry labs, each one with a unique mix of dissolved ions. At present, Mono Lake is very very alkaline, very very salty, and very very rich in dissolved inorganic carbon – much more so than sea water. The otherworldly-looking tufa towers grow where calcium-containing groundwater seeps in beneath the surface of the unusually carbonate-rich lake (Ca levels in the lake itself, meanwhile, are really low, as any entering calcium is quickly precipitated out with the tufa).

Mono Lake tufa. Photo by Kelly K. '15, with what is presumably her Brass Rat.

Once I'd adequately recalled how sad I am that I never had time to take Course 1's limnology class, a park ranger demonstrated the tufa-forming reaction for us by adding dissolved calcium chloride to a flask of lake water. Here's Madison D. '16 being excited about it:

We also learned that, since the tufa forms beneath the water surface, the exhumed towers provide a lower bound on the paleo-depth of the lake, and can be radioisotopically dated to help track how its hydrology has changed through time.

Then Kelly and I got distracted by the flies. Zillions of flies, for once not wanting anything to do with us, just sitting atop the salty sand and detritus along the shore. You see, when you run through them, they scatter, and unfortunately, terrorizing them like such is absurdly fun. Until I have our video of it, here's a random stranger's...

Thanks YouTube.

To be honest, I'd been expecting Mono Lake would be my personal favorite stop on this trip. But by the end of the day, it was surpassd by the Bishop Tuff, the humungous mass of rhyolitic lava that erupted at the Long Valley Caldera ~760,000 years ago. We'd read about the Bishop Tuff in our spring seminar, and it had been pretty impressive then, but standing within canyons of it was another thing entirely. Complete mind explosion. Allegedly, as far away as Illinois, the tuff layer is still a foot thick.

The rock was surprisingly friable. So basically, the apocalypse happened here 760,000 years ago, and its remains are miles and miles of baby-pink powder?

EAPS undergrads and undergrad-alums in front of the Bishop Tuff.

Yosemite

During our stay at Yosemite, we explored the famed Tuolumne intrusive sequence. This is a more or less representative pluton in the Sierra Nevada that records the mixing and chemical processing of numerous pulses of magma in a single magma chamber. The sequence is a classic geologic field trip that runs right along the highway through the Tuolumne Meadows. It turns out magma mixing is a shockingly controversial subject in geology, but these outcrops show evidence that individual feldspar crystals were partially dissolved and re-precipitated as additional pulses of magma changed the local chemistry. As a result of various processes, such as melting the surrounding country rock and partially crystallizing out certain minerals, the later formations of the intrusive sequence are more felsic (higher SiO2 content and lower FeO and MgO2 contents) and have larger crystals. One famous formation in this sequence is the Half Dome granodiorite, which makes up Yosemite's iconic Half Dome. 

This culminates in the awesomely huge MEGACRYSTS of the Cathedral Peak granodiorite.

We also got some really great views of Tuolumne Meadows, a particularly pretty portion of Yosemite National Park.

Yosemite Sparkles

Originally posted by Kelly Kochanski in 'The Frontier Beneath our Feet' (Sept 2, 2015)


I open my eyes, slowly. The sun has begun to slide over my hammock and into the hood of my sleeping bag. I push the fabric away. The sky is already pastel blue, but the trees are still lit by dabs of early morning gold. The soft roar of a gas burner off to my left means that Prof. Grove has just started the first pot of coffee. My toes are warm in their down covers. I don’t swing them into their boots until the last brush of gold threatens to fall from the trees, and I can snap a photo of the now-bright Yosemite sky.


It feels like a sin to be this happy on a Monday morning. A line floats into my mind:

Down the long mountain-slopes the sunbeams pour, gilding the awakening pines, cheering every needle, filling every living thing with joy. – John Muir, My First Summer in the Sierra

I have read Muir’s books often, always smiling with wonder at the love and grace with which he describes the mountains he walks through. His words seem especially apt now, for without his beautiful Sierra morning I would likely never have had one of my own. Muir’s writing and activism drove the preservation movement which led to the creation of Yosemite National Park. Today, this exceptional landscape – when Muir wrote, a pasture for sheep or, as he once called them, “hoofed locusts” – is filled with family campgrounds, ambitious climbers, and (starting just a mile or so from the roads) a seemingly endless alpine wilderness. National Parks are, for me, a place of thanks: for the world that is beautiful, and for those before me who have shared and saved that beauty where they could.


I am in Yosemite as part of an MIT geology field trip – a bittersweet experience, as I have just started my Ph.D. at CU-Boulder and this will be my last week with the MIT department. The fifteen of us – undergrads, grad students, an outgoing postdoc and Associate Dept. Head Tim Grove – have nine days to look at the landscape and geology of Eastern and Northern California. The undergrads have described many of our educational adventures in ‘Getting High in the West’. Our August visit happened during peak wildfire season, and the valleys were filled with blue smoky haze which marred the distance and the colors of the peaks. Below, therefore, are some of the least impressive photos you will ever see of Yosemite in summer.

Yosemite was built from granite, and is being scraped away by glaciers. Both features make it bright and beautiful. The raw material, granite, forms when magma cools underground, where heat escapes only gradually to the surface, and its crystals have millions of warm years to grow and interlock. Silicon, oxygen, aluminum, sodium, calcium and titanium are arranged. Clear quartz and white feldspar grow alongside black hornblende, shiny biotite and sphene. The final rock varies slightly with the ratio of elements within it – most of Yosemite Valley is actually made of diorite, granite’s darker and less silica-rich cousin.† The diorites here are unfractured and exceptionally strong.


Yosemite Valley is famous for its cliffs. Half Dome and El Capitan both boast thousands of vertical feet of sheer rock, making them pilgrimage sites for climbers around the world. Again in Muir’s words, which after a century still ring fresher than my own:

“Marvelous cliffs, marvelous in sheer dizzy depth and sculpture, types of endurance. Thousands of years have they stood in the sky exposed to rain, snow, frost, earthquake and avalanche, yet they still wear the bloom of youth.” – John Muir, My First Summer in the Sierra, ch. 5

No photograph can adequately capture the cliffs’ scale (but National Geographic’s photo gallery is a great attempt). Most rocks would crumble from the strain. Yosemite’s diorite stands tall, except for the top edges of its rocky domes, where the stress of curved slopes and surface temperature changes cause it to fracture and fall off in layers like the skin of an onion.


A cliff, however, is only half rock. The other half is empty space. To make a mountain, you must take a valley away.

The high Sierra has a mirror finish. The ripples on Tenaya Lake shine in the August sun, and so do the sides of the mountain above. I waded into the lake up to my hips. It was the coolest and most calming water I have ever stood in, and I was sorry to leave it though I longed to clamber up the slopes above. When we did climb, Prof. Grove encouraged me to train my eyes on the black slivers of hornblende in the rock under my toes to look for changes in the rock’s composition. I didn’t see the rest of the valley’s story until I slipped on it.


Granite, in most places I’ve wandered on it, weathers into crystal ridges that easily holds up my hiking boots. Here, my feet spin like I’m in dancing shoes. Glacial polish – the reason the rocks shine on the hills – is best found by the fingers and the feet. It’s not a climber’s friend. It’s smooth, like a granite counter-top sanded flat right there on the mountain. It feels a little like rubbing your fingers on coarse paper. Once you’ve seen glacial polish, and felt it, you find it everywhere in Yosemite.

Look at the rock slab under our feet in the bottom-right of the photo above. It’s patchy. The dark patches are rough places, mottled by shadows and by lichen growing in the cracks. In the light, polished patches, there’s hardly a crack. Water doesn’t stay on their surfaces, and plants can’t grip them. They’ve stayed smooth and fresh for over ten thousand years, since the last time that glaciers descended from the High Sierra into these valleys, where their icy sides scraped and strained against the sides, excavating loose and fractured rocks and rubbing the walls smooth.


Glaciers’ traces are visible everywhere in the Sierras. Many scientists have painstakingly cataloged them: glacial polish; deep U-shaped valleys; erratic boulders and moraines (the last two are are carried miles by the ice and dropped on seemingly random plains and hilltops). The first of these was John Muir (c1865), who spent years walking the mountains and demonstrating that Yosemite Valley was once covered in ice. Today, with the aid of modern techniques, we know that the Sierras have been glaciated, revealed, and re-glaciated many times. The names of these glaciations – such as Tioga, Tahoe, and Sherwin – roll out over hundreds of thousands of years. Their remnants today are beautiful.

Here’s one last photo of our group wandering between glacially-dropped erratic boulders, a few hundred feet above Tuolumne Meadows. The sun glints on the distant peaks like on water. To our north stretches the wilderness of the Sierras. Pine forests lie between sheer slopes, and their depths surely sparkle with glacier-fed lakes. Everything is brilliant. One day I would love to come back here and wander for days or weeks through these high forests, taking time enough to soak my feet in every stream and run my fingers over every polished stone along my path.


John Muir chose to worship at a spectacular natural cathedral.


†For more geochemical details about granitic rocks in Yosemite, see Reid, Evans & Fates, 1982. For a related field guide, see ‘Magmatic Evolution of the Tuolumne Intrusive Complex’ by Memeti, Paterson & Mundil, 2014 (fieldguides.gsapubs.org).


‡ A (now refuted) hypothesis, that Yosemite Valley was formed in an earthquake, was put forward by Josiah Whitney c1865. Many books and articles have been published on the subject since. Mary Hill’s ‘Geology of the Sierra Nevada’ is a particularly accessible overview.

Beautiful Manteca...

     Despite some mishaps with luggage and credit cards, and having only a brief but highly acclaimed cameo from Taylor Perron, the first day of the trip was a great start! We had a short drive to Manteca, the home of nearly every fast food chain you can name, and got $600+ dollars worth of groceries for 15 people. We finished off the day by eating at an awesome taqueria with absurd portion sizes. Super stars who finished their meals: Ben Mandler, Madonna Yoder, and Max Collinet. Good showing from Kelly Kochanski and Paul Richardson (who also has the honor of naming this blog), but ultimately they were defeated by their "Wet Burritos."

Epic taqueria in Manteca

Onwards and upwards to Yosemite!