As noted earlier, I had the good fortune to spend last week in Yosemite National Park, celebrating the wedding of my friends Jason and Lindsay, and in general poking around in one of the coolest places around. Below, a summary of the three-day trip:

Friday:
Lily and I flew to Modesto, California, and rented a car. It took about two hours to drive up to Evergreen Lodge, where we checked in and then headed out for a short hike in the Hetch Hetchy area. Hetch Hetchy was dubbed "Yosemite's sister valley" by John Muir in an attempt to keep it from being dammed. But the city of San Francisco had been destroyed in 1906 by earthquake-induced fire, and the call for a reliable water source was an important force in overpowering Muir's conservationist ideals. Ken Burns apparently explores this saga, the first instance of "development vs. conservation," in the second episode of his new National Parks series. (I saw the first episode, but haven't caught up on the rest of it yet.) The valley was dammed in the 1920's, creating the Hetch Hetchy Reservoir:


Here's the O'Shaughnessy Dam, named after the chief engineer of the project:


I didn't find it as spectacular as Yosemite, but it was sure a pretty place. Walking along the north side of the reservoir, I reaquanited myself with some fine Sierran granites and granodiorites. Here's a sweet little xenolith (or maybe an MME; how can you tell an MME from a mafic xenolith?):

Back to the Evergreen for the rehearsal dinner (Oktoberfest theme!) and then bed.

Saturday:
Up early, got some coffee, drove an hour to reach the Yosemite Valley. I liked how quiet things were compared to the throbbing pulse of summer. This view of El Capitan, for instance, is typically mobbed with tourists. This day, we had it to ourselves for five minutes or so, then shared it with one other car:


Time to stretch the legs! We decided to hike up to Vernal Falls. On our way up, the base of the falls was still in shadow, with low-angle morning sunlight dramatically illuminating the upper reaches of the falls:


Looking back down the valley we had climbed up... I like the dark shadow of the cliff merging with the dark shadows of the trees below:


But if we set the camera's F-stop a bit differently, we can see what's going on in all that shadow. There's the trail we climbed up, with fellow hikers for scale:


Up top, photographing the waterfall:


On our way back down, with more of the falls illuminated as the sun rises in the sky:


Looking north across the valley from where we parked our car, marvelling at the huge exfoliation joints there: rounding these exposed plutons into granite 'domes.'


... or Half Domes, as the case may be:


A view from further out, again with Half Dome the most striking landform:


Then, we headed back to clean up before the wedding. Great ceremony, amazing meal. Drinks, dancing, rhubarb jam, bluegrass, reminiscing with old friends and new. Ahhh.

Sunday:
Breakfast and coffee with the wedding party, then off to check out some big trees. We drove to the Tuolumne Grove of giant sequoias. It started snowing on the way there, but we didn't let that deter us. On the hike down from the parking area (where, by the way, they had closed the Tioga Road), we found this nice example of spheroidal weathering in an outcrop of granite:


But the real attraction was the enormous sequoia trees. Here's one:


And a dead one, with a car-sized hole cut through it:


I found these trees very impressive: they were just stunning in their grandeur and immense age. Snow continued to fall as we left. We had to get going to make our flight home. Somewhere on the way down the mountain, Garry Hayes and his wife passed us going up the mountain. Ships passing in the night -- sorry I missed you, Garry! We made a couple of roadside outcrop stops, then got back to Modesto and traded in the car for an airplane. Our "redeye" route back to DC took us through San Francisco and Los Angeles, and I ran into Thomas Friedman in the airport. Got back to BWI at 6am, and headed off to work...

Labels: california, glacial landforms, national parks, plants, travel, tv, water resources, xenoliths

One of the highlights of this past summer's Northern Rockies field course was an afternoon set aside as a "choose your own adventure" hike in Teton National Park. Some students opted for Cascade Canyon; others climbed Blacktail Butte. Four of us wanted something really challenging, so we chose Hanging Canyon at the recommendation of my friend Amy Manhart, who lives in Jackson and knows the Tetons like the back of her hand.

We took a ferry across Jenny Lake along with the Cascade Canyon Crew, and then started climbing up. A thunderstorm rolled up Jackson Hole, with much ominous booming and lightning, but we didn't get hit with the storm directly. The climb was very steep, but we entertained ourselves along the way with a geological conundrum: We discussed how best to interpret a hypothetical piece of float that is half granite and half diorite: Is it more parsimonious to guess that the granite represents an intrusion or an inclusion? The implications for the relative dates of the two units are huge: if the diorite is an intrusion, it's younger than the granite. If the diorite is a xenolith (an inclusion) within the granite, then it's older than the granite. Consider the possibilities:



Ultimately, there's no answer to this question without finding an outcrop of the rock in situ, which is why it's entertaining to consider when you're slogging up a 2000 foot hillside. My co-instructor Pete Berquist and I upped the ante by each doggedly defending one of the two indefensible interpretations and sticking to it for the sake of argument. Pete was the xenolith man, whereas I came down fully on the side of the dikes. Our students Joel and Ken were "fortunate" enough to listen to Pete and I bicker about the relative merits of our favored interpretations. Rest breaks came whenever either Pete or I found a boulder along the hillside that showed evidence to support our position. We would stop to consider it, catch our breath, and the resume the uphill climb and the argument. The bad weather passed and the day was beautiful. We were unencumbered by the need to reach a conclusion or acknowledge the obvious: the best interpretation is that such half-&-half clasts "cannot be interpreted."

Here's Pete posing with an obvious dike (I forced him! Ha!):


Here's me posing with an obvious xenolith (Oh well, fair's fair...):


We had a similar ongoing "argument" on the trip about the merits of "Tertiary" versus "Paleogene." I think it keeps students amused to see their professors going back and forth over geologic ideas -- surely if these fellows spend this much energy and thought discussing some geologic question, it must be valid and important... ...right?

More on the Hanging Canyon hike tomorrow...

Labels: archean, geologic time, igneous, national parks, nova, structure, travel, wyoming, xenoliths

After we had collectively collected a hundred pounds of samples from Mineral Hill, the final stop on the University of Maryland petrology trip was in scenic Ellicott City, Maryland, where we visited the Ellicott City Granodiorite (map to outcrops).

Like everything else on this trip, the ECGD is intimately tied in with the Taconian Orogeny (late Ordovician; caused by the collision of ancestral North America with a volcanic island arc in the Iapetus Ocean basin). However, unlike the Port Deposit Tonalite we looked at early in the trip, this one crystalized from magma at 435 +/- 15 Ma (U/Pb in zircon). It is not only much younger than the PDT, but it's also pretty young even for the Taconian Orogeny, which reached its peak around 460 Ma.

It's more potassic than the Port Deposit Tonalite, as these K-spar 'megacrysts' show:


This potassium feldspar 'megacryst' shows internal growth laminations, as small mafic bits got caught up in the growing feldspar crystal, which consumed and included them:

Not only does this help us see how the feldspar crystal's habit is a reflection of its internal structure, but it's also an example of the principle of relative dating by inclusions, expressed in a single mineral crystal! Pretty cool.

As with the PDT, xenoliths may be seen in the ECGD:


Parts of it are equigranular, and parts of it are highly foliated:


And of course my eye is always drawn to the structures, like these small faults offsetting dikes of granite which cross-cut the ECGD:




The real prize with the Ellicott City Granodiorite is to view first-hand the magmatic epidote it bears:


Most epidote is metamorphic. However, as Zen and Hammerstrom (1984) showed that epidote could also crystalize from a late-phase magma as the melt interacted with hornblende at high pressures (8 kbar; roughly 30 km depth). You'll note in the photo above the intimate association between the epidote and the hornblende. (I'm not super-confident on my titanite identification, by the way; this rock also bears similar-looking allanite. Please correct me if I'm clearly wrong.) E-an Zen has guest-posted to this blog before, and once upon a time he tasked me with searching for magmatic epidote near Haines, Alaska, in 2006. I didn't find any, but it did pique my interest. So it felt good to be able to finally see some of this rare beast. I was surprised to find it locally, considering the the original magmatic epidote paper referred mainly to west coast plutons from California to Alaska. I was also suprised because of the tremendous depth of crystallization it implied: 30 kilometers down? Wild! I collected a sample for the NOVA lab.

Thanks again to Rich Walker and Roberta Rudnick for graciously hosting me on this trip. I learned a lot, and I'm greatful for the opportunity to expand my local outcrop knowledge.

_________________________________________________________________

Reference:
E-an Zen and Jane M. Hammarstrom (1984). "Magmatic epidote and its petrologic significance." Geology, September 1984. Volume 12, no. 9, p. 515-518. DOI: 10.1130/0091-7613.

Labels: alaska, maryland, metamorphism, minerals, structure, xenoliths

Yesterday, I was fortunate enough to be able to tag along on the University of Maryland's petrology field trip, to five locations in Maryland showcasing a variety of igneous and metamorphic rocks. I'd like to thank Rich Walker and Roberta Rudnick for allowing me to come along on the excursion, and UMD graduate student Ryan Kerrigan for alerting me to the trip's interesting rocks in the first place. They have a crew of enthusiastic students, and some cool outcrops!

Our first stop was in northern Maryland's Cecil County. Along the banks of the Susquehanna River, just upstream from the I-95 bridge, is an abandoned quarry of the Port Deposit Tonalite.

Here's Rich and Roberta leading us into the quarry:


UMD students examine the semi-overgrown outcrops of the tonalite:


Tonalites are kind of like granites, except they have only very low amounts of potassium feldspar. This particular tonalite has a magmatic crystallization age of 515 Ma (U/Pb in zircon) and a metamorphic age of 490-480 Ma (Rb/Sr in biotite). Close-up of the rock's texture:


ADDITION: Kim notes in the comments that I didn't draw an explicit connection between the metamorphism and the metamorphic foliation that is so prominent in this photo. She's right: The wavy linear pattern you see in this photo is produced by minerals aligned by differential pressure. Squeeze the rock "top to bottom" and you produce a foliation that runs "left to right."

On the basis of isotopic evidence, the Port Deposit Tonalite is interpreted to have formed as an igneous pluton offshore of ancestral North America, underneath an island arc in the Iapetus Ocean. Later, subduction brought the island arc into contact with North America, triggering the Taconian phase of Appalachian mountain-building.

Here's a closer look at the texture and mineralogy. You can see some k-spar present here, though this was not a common mineral to see at the outcrop...


There were some nice xenoliths present, indicative of the host rock into which the PDT intruded:


Here's a quartz vein cutting through the tonalite. You'll notice that the vein is emplaced approximately perpendicular to foliation, suggesting the same maximum stress which imparted the foliation also extended the rock parallel to the foliation place, opening up fractures that when then fill with the most mobilizable minerals available (in this case, quartz):


If you look closely, you'll see that the fracture which opened up in the tonalite to allow this vein to be emplaced has a ragged edge (not a clean break):


Next up: the Setters Schist...

Labels: appalachians, cambrian, granite, igneous, maryland, metamorphism, ordovician, structure, teaching, xenoliths

What's the tallest mountain on Earth?

Everest, right? Well, yeah: if you're measuring from sea level. If you're measuring from the top of the crust the mountain rises from though, it's Mauna Kea, Hawai'i. It's about ~13,800 feet above sea level, but it rises ~33,500 feet from the oceanic crust to the peak (that's compared to Everest's mere ~29,000 feet from base to peak. So... you could say that Mauna Kea is the tallest mountain on our planet... (you could!)

On Thanksgiving day, my friend Lily and I took a drive up to the top of Mauna Kea, and did a little hike up there at high elevation. Today, I'd like to share some photographs of that excursion. We saw some pretty cool geology.

On the drive up the mountain, we saw an animal which was apropos, considering the day:

Gobble, gobble, gobble. Watch out turkeys, we'll be back after we work up an appetite...

Here's Lily's jeep in the "saddle" between Mauna Kea and Mauna Loa, looking north (with Mauna Kea in the background and basaltic lava flows from Mauna Loa in the foreground):


Some cider cones (the Hawai'ian word for cinder cone is pu'u) in the saddle:


Turning the other way (looking south), you can see the bulky form of "the long mountain," Mauna Loa. What a classic shield volcano shape! I love the fact that it's so dang wide it makes a lousy photograph. You just can't capture its spread-out bulk in a photo; it's too massive:


This was the spot where I pretended to have my toes overrun by a pahoehoe flow:


As we drove up the road to the top of the mountain, I was amazed at the raw volcanic landscape, decorated with cinder cones like this one:


At one point, we passed a neat little angular unconformity on the roadside. Here it is, with a nickel (white dot left of center) for scale:


Here's a closer-shot of this small angular unconformity. Earlier layers of ash and lapilli were deposited at a steep angle, and then eroded (perhaps by glaciation? pure speculation there) before more ash and lapilli were deposited atop it, at a lower angle. There's not likely to be much time missing here, and so perhaps it's better to think of this as the top of a cross-bed, an advancing front of pyroclastic deposition moving down the mountainside, overrun by later eruptions, which may have scoured off the upper few inches (??? pure speculation) or so before deposition.

Really, the truncated tops of cross-beds are mini-angular-unconformities, when you think about it; just not with the same amount of time missing at a "real" angular unconformity (with millions of years missing) due to mountain building like the one at Siccar Point. (Video of cross-beds forming)

Here's something else which the clueless geologist might mistake for a sign of mountain building:
No, those aren't originally-horizontal strata that have later been folded. They're layers (again of ash and lapilli) deposited on the originally-rough topography of the mountainside, covering small ridges and filling small valleys. Where a given layer is exposed at higher elevation, I interpret to be a paleo-topographic high; where that same stratum is exposed at lower elevation, that's a paleo-topographic low. The roadcut reveals these layers have undulating shapes, but this is unlikely to be folding that results from tectonic compression: instead, I think it's showing us the lay of the ancient land surface.

Looking south, we could see past Mauna Loa to the actively erupting steam vent coming out of Halemaumau Crater at Kilauea Caldera (source of the vog!):


Near the summit of Mauna Kea, there are a bunch of astronomical observatories:






On the summit is where you find those examples I mentioned the other day of hawaiite, a rock of basaltic composition that is very dense (ostensibly due to erupting beneath the extra pressures of a Pleistocene ice cap):


Here's me on the summit:


View to the north from the summit: More cinder cones...


Here's a YouTube video of me pointing stuff out from the summit (Kilauea, Hualalai, Mauna Loa, observatories, hikers, etc.). Unfortunately the wind makes it all but unintelligable, but I filmed it, doggone it, so I'm going to post it:



I found a beautiful example of a volcanic bomb up there:


After the visit to the summit, we went for a hike to a small supposedly-glacially-gouged-out lake below the summit (Lake Waiau):


Here's a Google Map, showing the lake's location:


I was surprised to see a thick biofilm on the bottom of the lake:


Encrusting the pebbles and cobbles there, it reminded me of Nora Noffke's modern and Archean biofilm photos in the recent GSA Today, as well as my "Life in Extreme Environments" class this past summer at Montana State University.


We saw some nice examples of structural geology on this hike. Previously, I've mentioned plumose structure, a branching pattern on the topography of fracture surfaces in fine-grained rocks. We saw some of that on blocks of basalt atop Mauna Kea, as in this example (again a repeat photo, but the other day I showed it to you for the vesicle; today I'm showing it to you for the plumose structure.)


A similar feature are arrest lines, which again are minute variations in the surface of a fracture. Like plumose structure, which branches from a source point (where the fracture initiated) and branches out in the direction of propagation, arrest lines tell us about the development of a joint. Unlike plumose structure, though, they are not parallel to the propagating fracture front. Instead, they form perpendicular to it, and record how the fracture propagates in small "steps." Each of these arrest lines is interpreted as being a spot where the fracture grew a little bit, then stopped ("arrested") and then grew some more. In this case, the fracture face we're looking at started at the bottom of the picture and grew towards the top of the photo. You can even see some less-discernible plumose structure backing this up:

Similar arrest lines can be seen in basalt images here and here...

We also saw some pretty spectacular xenoliths. Here's one of gabbro in basalt:


Here's one of peridotite in basalt:


And a few more:



My boots, with another volcanic bomb:


Driving back down the mountain afterwards, we got this nice view of the cinder cones (pu'us!) in the eastern part of the "saddle" between Maunas Kea and Loa:


This Mauna Kea excursion was one of my favorite things that I did on my all-too-brief trip to Hawaii. It was great to get up in the high country, where the air is thin (and vog free!) and the skies are deep blue, and the geology is surprisingly varied (at least it was surprising to me, and pleasantly so). The hike let us work up a good appetite, so we headed back down the mountain and straight to Thanksgiving dinner!

Labels: basalt, birdies, geology, glacial landforms, hawaii, igneous, mountains, msse, primary structures, structure, travel, unconformities, volcano, xenoliths

Contrary to what you may have heard, it's not all basalt. Even the basalt is astonishingly varied: the extrusive rock of a thousand faces... Here I'll share some pictures I took of rocks in Hawai'i:

There's pahoehoe:


...and there's a'a:


Here's a pahoehoe flow oozing over my boot (just kidding; it was cold when I did this):


Pahoehoe lobes can drain out, leaving only the outer skin as rock, but with a hollow center. These are lava tubes (nickel for scale):


Another one (nickel for scale):


Cool texture on the inside of this lava tube (nickel for scale):

...and zooming in a bit closer (it looks like wrinkled cellophane!):


A stack of cross-sectioned pahoehoe flows, showing their tubular (totally tubular, dude) shape:


Some Hawai'i basalt is massive, like this cobble...


...or like this cobble of hawaiite, a dense form of basalt found atop Mauna Kea (where it apparently erupted beneath Pleistocene ice caps):


But the majority of Hawai'i's basalts are vesicular, meaning they contain "Swiss Cheese" type holes that result from gas bubbles. When the lava erupts, it experiences less pressure at the Earth's surface than it was subjected to at depth. As a result, many gases (steam, CO2, sulfur dioxide, chlorine, argon, others) exsolve from the lava solution and make bubbles. If these bubbles don't get a chance to pop before the lava sets up into igneous rock, then they are preserved as vesicles. Sometimes the vesicles are small:


...and sometimes they are big:


Sometimes, they are really big. Here's one I could fit my entire Nalgene water bottle into:


When vesicles later get filled in with mineral deposits, we call them amygdules. Here's some vesicles that have gotten a light coat of a white mineral on their interiors: the first step to converting a vesicle into an amygdule:


Some of the vesicles show strain (almost certainly due to late-stage flow in the increasingly-viscous lava, getting stretched out like air bubbles in pouring honey). Surface tension on the bubble wants to make it spherical, and the lower the lava's viscosity, the easier it will be to attain that perfect spherical shape, minimizing the surface-area-to-volume ratio. So when we find them in cigar-shapes or pancake-shapes instead, that's a clue that they've been deformed. Deformed not by tectonic forces (ductile flow at depth in an orogen), but ductile flow as a result of their formation, in a sluggishly oozing blob of lava:


Another example of stretched-out vesicles:


A lonely vesicle in an otherwise massive basalt:


Not sure what's going on here, but it looks cool (popped vesicles in sticky lava?):


Another thing you see a lot of in these Hawai'ian basalts are phenocrysts of certain minerals. Here, for instance, is a cobble showing nice olivine phenocrysts:


...and another:


Here's one I showed you last week when we discussed Green Sands Beach:


Here's an outcrop which shows phenocrysts of plagioclase feldspar instead:


And a river cobble (also vesicular) bearing a healthy population of feldspar phenocrysts:


Holy feldspar, Batman! This rock has a huge proportion of feldspars (you'll note that it's still vesicular, though: in spite of the overwhelming volume of macroscopic crystals, this is still an extrusive rock):


Here's something else caught up in a finer grained (and yes, vesicular) basaltic matrix: another piece of basalt!

This is a xenolith of slightly-older basalt showing flow banding in its own trains of vesicles, that after solidification got broken off and included in younger flows of basalt. I'll post some additional xenolith photos later this week.

It's not all basalt, though. Here's a breccia made of basaltic cobbles (penny for scale):


And a closer shot of the same outcrop (penny for scale):


Finally, a rock I was surprised to see: an intermediate-composition extrusive igneous rock called benmoreite (nickel for scale, and note the rock hammer impact marks):


Benmoreite is way more felsic that anything else on the island. According to my volcanic advisor Jess, it's the result of late-stage partial melting of basaltic source rocks in the island's oldest volcano, Kohala. In other words, it's a distillation of basalt: concentrating the most felsic components in this decidedly-lighter-complected rock (nickel for scale):

Labels: basalt, geology, hawaii, igneous, national parks, pleistocene, primary structures, volcano, xenoliths

Last week was field trip week for me. I led trips to the Billy Goat Trail on Tuesday and Thursday, and to Washington, DC, on Saturday.

On the Physical Geology field trip to the Billy Goat Trail, we saw rocks like amphibolite, metagraywacke, and migmatite:







Hope and Ana checking out the migmatite:


The group poses with the migmatite, to show how close anatexis is to their hearts...


Jane examines lamprophyre in a weathered-out dike:


Noting the characteristics of metagraywacke:




Traversing 'Pothole Alley'... Joel looks chilly...


Our lunch spot... Alex pretends to dive into the Potomac River...


Traversing 'The Traverse':


On the Historical Geology field trip to DC on Saturday, we were amused to find a jack-o-lantern that had facial hair resembling mine...



But that's not all! We also saw some geology. While you can get a more complete picture at my "DC Rocks" webpage, I'll post a few new photos of new outcrops here...

Here's a nice slab of granite (very angular) set in metagraywacke matrix (metamorphosed accretionary wedge complex)...


Here's two members of the Georgetown Intrusive Suite, showing the (earlier) gabbro stoping xenoliths into the (later) granite:


I love field trips. I love seeing my students light up at being outside, at getting a handle on the stuff we talk about all semester in class. I think field trips are super duper important.

Labels: dc, field trips, granite, igneous, maryland, metamorphism, nova, piedmont, teaching, xenoliths

I led my "History Before History: The Geologic Saga of Washington, DC" tour twice this weekend as part of the twice-annual "Walkingtown, DC" weekend. The folks attending the tour both days were really cool, and were full of good questions. We covered the sedimentary origins of DC's rocks at the bottom of the Iapetus Ocean, their metamorphosis during Taconian mountain building, the intrusion of plutonic rocks, the erosion of those ancient mountains, the deposition of river gravels during the Cretaceous (together producing an unconformity), and the faulting of that unconformity sometime post-Cretaceous (probably Miocene). I'm kind of tired after all that geologic history, especially repeated twice in two days!



The photo above is of a boulder in Rock Creek Park showing all three members of the Georgetown Intrusive Suite, a series of igneous plutons that were intruded into the crust during late-Ordovician mountain-building. I like this boulder because it illustrates well two of the principles of relative dating: the gabbro must be older than the diorite, because there are xenoliths of the gabbro in the diorite (inclusions). You can't break off a piece of gabbro unless it already exists. The granite dike must be younger than the diorite, because it cuts across the diorite (cross-cutting relationships). You can't crack open diorite unless it already exists.

Just thought I'd share an informative little outcrop like this. Please ignore the white graffiti that mars the central part of the exposure. A pen at the top is circled to give a sense of scale.

I hope everyone had a relaxing weekend!

Labels: dc, granite, igneous, plutons, tourism, xenoliths

Today: I offer some photos I took in Boulder Canyon, Colorado, in June. These are all igneous rocks exposed in the Precambrian 'basement' rocks, brought to the surface by the Laramide Orogeny.

Directions: Drive to Boulder; go west up the main canyon into the Rocky Mountain Front Range.

Location map:


Granite pegmatite:


Contact! Granite pegmatite meets granodiorite:


Contact! Granite dike cutting across granodiorite (with one small mafic xenolith):


Contact! Mafic xenoliths afloat in granodiorite:


Put the previous two pictures together, and what do you get? My favorite outcrop of the whole excursion... Contact contact! A granite dike cutting across mafic-xenolith-bearing granodiorite. This would be a good practice photo for introductory level students to establish relative ages of the three different rocks shown:


Contact! More prosaic, but high-contrast... Granite meets basalt:


Epidote vein (Without any good reason, I love the color of epidote):


My Prius parked on the side of Boulder Canyon Drive:

Labels: basalt, colorado, granite, primary structures, prius, travel, xenoliths