22 February 2012
Mudcracks worth writing home about
Some sweet “columnar jointing” style mudcracks from an abandoned quarry in about the farthest west corner of Texas that you can get to, or maybe the southeasternmost corner of New Mexico. One of them, anyhow…
Did you spot the imposter?
21 February 2012
New GigaPans: source and sediment
Here’s two new GigaPan images that the M.A.G.I.C.* team has produced (my student Robin R. is responsible for both of these images) of samples I collected a few years ago in Hawai’i. Part of my vision for this collection is to have thematically-connected images that instructors could use to put together online geologic assignments for their students. Consider these two images, for instance. One is of a cobble of porphyritic, vesicular basalt, and the second, of sand collected in a small cove a few feet away.
It seems to me that students could use images like these in an introductory lab assignment on igneous rocks (crystal nucleation and growth, lava eruption and degassing), or in a sedimentary lab, thinking about the relationship between sedimentary characteristics and provenance. The professor could even assign students to make a rough quantification of how much basalt had to be weathered to generate a given volume of this sand. The professor could ask about other sources of particles in the sand, like orange/white shell fragments.
Other themes of connected GigaPans that occur to me:
Appalachian geology or Rocky Mountain geology (location is the theme)
Subduction zone geology, clastic sedimentation geology, karst geology (process is the theme)
Hand-samples, sediment samples, and outcrops from each physiographic province (scale is the theme)
An online sample set for Physical Geology, Historical Geology, Economic Geology, Structural Geology, etc. (an academic program is the theme)
What sorts of themes can you dream up that it would be useful to have gigapixel-resolution imagery for? How can we help you?
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* M.A.G.I.C. = Mid-Atlantic Geo-Image Collection
20 February 2012
Mystery rock
Here’s a mystery rock that’s just aching to have its identity be crowdsourced:
I got these photos from Rick Diecchio of George Mason Univerisity, who got them from a local fellow who dug it up in his yard in Dale City, Virginia. Rick says:
I was stumped at first, but the more I look at it, the more I think it’s a lightning strike into sand or more likely sandstone, maybe the Cretaceous Potomac Formation.
The host rock is all sandstone. The sand appears under hand lens to be fused, and on the surface there is a chert-like coating which I think may have been the result of melting. The sample is a lot heavier than I would expect, and very well indurated.
What do you think? Are there other options? Have you ever seen anything like this? If it is a fulgurite, how common or rare is a sample this size? Feel free to share these pics with others who may be interested or who may be able to provide information.
A silcrete concretion is possible. Then it becomes significant whether or not the sand grains are fused. maybe will have to get a thin-section. There is no sign if anything fozzilized. No bone or petrified wood as far as I can tell. The size is the biggest problem for me. Most fulgurites I have seen are an order of magnitude smaller.
Any ideas, wise readers?
Honey, I shrunk the grad students
On the airplane ride back from Texas, I bought a copy of Michael Crichton’s semi-posthumous final novel, Micro, which was co-authored by Richard Preston after Crichton’s death in 2008. Preston wrote a superb book about Ebola virus in the DC area, so I was intrigued to see his influence. Plus, and of utmost priority, I just wanted some light reading for the plane.
It was mediocre. If you’ve read Sphere, Congo, Timeline, Prey, Jurassic Park, or the Andromeda Strain, you’ve already pretty much read Micro. A band of diverse characters encounter adversity in a world where natural processes meet high technology, and only those who are both brave and lucky survive.
In this case, the adventurers are a team of graduate students from MIT, and they get tossed into a plot on Oahu, Hawaii. The big bad company (a standard Crichton trope) has developed shrinkage technology, and they are using it to shrink people and machines so that they can more efficiently access the microscopic world for the purpose of finding drugs and other useful biomolecules. Plus, they’ve got a side business going to weaponize the technology to create micro-assassins.
The head of the company is megalomaniacal and doesn’t mind killing people who get between him and his business plan. So the bodies start piling up, and when the grad student gang tries to intervene, he shrinks them all to about a few millimeters tall, and throws them into the jungle. There, they encounter the extraordinary diversity of very small organisms, and start getting killed by things like centipedes and wasps and ants. (As opposed to gorillas in Congo, jellyfish in Sphere, dinosaurs in Jurassic Park, microbes in Andromeda Strain, nanobots in Prey, or the British in Timeline). It’s always something!
Quibble: if you shrink someone to that tiny size, what happens to their mass? Do they retain their full complement of atoms, or are some of those atoms removed from the system? If so, to where? The tiny grad students are pretty much weightless (they “fall” through the air like ants would, and land without harm from “great” heights), so that doesn’t mesh very well with the idea that they retain their same mass. I guess this is just one of those details that a reader is supposed to suspend disbelief about, but it does undermine the entire basis for the plot.
Highlight: bugs are fascinating, and they appear quite alien when we view them through a macro lens. They do amazing, freaky things, both biochemical and behavioral, and it’s worth indulging in an implausible plot if only to reveal some of those things to a wider audience. My favorite scene was when the most unlikable character (another Crichton standard, who previously appeared as Dennis Nedry in Jurassic Park) gets his arm parasitized by a wasp. She lays eggs in there, and the eggs hatch into larvae and consume his arm, bulging against the skin while he is still alive. This horrific occurrence is standard fare in the natural world. As you might have heard, it’s red in tooth and claw. Parasitism as a way of life for such a large proportion of the Earth’s species is a beautiful, brutal distillation of evolution’s “anything goes” attitude to differential reproduction. It’s a good reminder for us humans, I think, to spend some time contemplating that. Bugs are extraordinary and amazing and they remind us of the fundamentals of natural selection. This would be my #1 reason to recommend the book for your reading.
Spoiler alert: A major flaw in the book is that the death of one character about 2/3 of the way through the book feels very much like the point where Crichton stopped writing and Preston took over. Up to that point, it really felt like Crichton was setting up the whiny environmentalist for a gruesome death (Crichton was a global warming denialist) and the even-tempered leader for the “last man standing” role. But then their roles pretty much switch most of the way through the book, with the good guy getting killed, and the blowhard greenie making it to the finish line (with the girl, of course: gotta have the token romance thrown in!).
All in all, I’d give it a 5 out of 10 possible.
19 February 2012
Plane views
A few scenes out the left side of the airplane from when I flew from El Paso to Houston a week and a half ago…
Sand dune field overprinting desert vegetation and human roadways:
Outcrop pattern of horizontal strata (tracing out the contours of this hill), and the weird geometry of human road systems:
More contour-hugging outcrops of horizontal strata, and a vertical joint set:
Same thing:
Bajada (apron of sediment, made from multiple alluvial fans coalescing) on the flank of fault-block mountain range, playa (salt flat) in the foreground.
Annotated version:
Meandering river with cut-off loops (dried-out oxbow lakes):
I love sitting in a window seat on a clear day!
18 February 2012
The xenobomb
One of the samples I was fortunate to acquire in Texas was this extraordinary thing:
Rotated 90° to the right:
And then rotated another 90° to the right, so now you’re looking at the opposite side relative to the first shot:
This was not a sample I found personally, but one that was collected by Josh Villalobos of El Paso Community College. He sawed it in half, gave it a coat of shellac, and then gave half to me, and half to Elizabeth Nagy-Shadman on our field trip the week before last. The sample was collected out at Kilbourne Hole, a maar volcanic complex west of El Paso, in southern New Mexico. Maars are big explosive craters that form when hot magma encounters groundwater, and steam explosions ensue. Chunks of rock and lava get flung through the air to land like geologic shrapnel around the hole.
Apparently what happened with this sample is that a xenolith of peridotite* (olivine-rich intrusive igneous rock) was tumbling along in the conduit of basaltic magma when the explosion occurred. It was flung through the air as a solid chunk surrounded by a coating of liquid mafic lava, which congealed and solidified as it traveled through the cold air, like a volcanic bomb. It landed, and eventually Josh found it, sliced it, and shared it.
Wild, right? It’s both a xenolith and a volcanic bomb. I hereby propose a new term for such a thing: I call it a xenobomb.
Similar xenobombs can be found at Kilbourne Hole of many other xenolith rock types, including migmatite.
It reminds me of a chocolate-covered almond.
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* Several sites online refer to these inclusions not as xenoliths but “giant crystals” of olivine. Others say they’re xenoliths. It’s clearly not a single coherent crystal – you can see the central portion of the xenobomb includes hundreds of 1mm – 2mm sized olivine grains. But are those actually separate crystals with independent nucleations and growth histories? (i.e., crystallization of an ultramafic magma, which then solidified into peridotite and had a chunk of that solid rock stoped off to be included in a later mafic magma?) Or are they clasts still in situ relative to the larger crystal they used to be part of? (In other words, does this represent a case of positive dilation and ensuing fracturing of a single original crystal into many fragments upon depressurization?) Is this just a case of people on the internet confusing minerals with rocks? That’s my bet.
17 February 2012
Friday fold, from in front of UTEP
Another sample from the collection on display, both indoors and out, at the University of Texas at El Paso.
Don’t know anything about it beyond its lovely differential weathering.
Happy Friday.
16 February 2012
Wyler Aerial Tramway and the Franklin Mountains of West Texas
One of the last things I got to do on my recent visit to the Lone Star State was to ride the tramway up the east side of the Franklin Mountains, a north-south-trending fault-block mountain range north of El Paso. I’ll be exploring other aspects of the Franklins’ history in subsequent posts, but if you can’t wait, here’s an overview from the good folks at UTEP.
Here’s a map to get you situated:
Here is a fisheye portrait of our team (Elizabeth Nagy-Shadman of Pasadena City College, our host Josh Villalobos of El Paso Community College, and me, of Northern Virginia Community College) waiting to ride the tramway up the mountainside. On the right, you can see the orange tramcars coming and going:
Once we had boarded, the first item of geological interest that we saw was the contact between the Mesoproterozoic basement granite, and the overlying Fort Bliss Sandstone, a Cambrian-Ordovician unit. This view is looking towards the south:
This is a nonconformity: We’re missing about half a billion years of geologic history right at this contact.
Now let’s swivel around and look north. You can see the lower tramway station, and Fort Bliss Sandstone in the foreground. In the middle distance is a limestone quarry — El Paso Group limestones of Ordovician age.
These limestones overlie the Fort Bliss Sandstone, so given the obvious westward dip to the strata, you might be wondering how on Earth those younger limestones got way down “below” the older sandstones… Hold that thought.
Further up the hill, you can see a sill of granitoid igneous rock (termed a “felsite”) that has intruded parallel to the bedding in the Fort Bliss Sandstone. View remains to the north, so these strata continue to dip to the west:
This sill is likely to be of Eocene age, same as the Campus Andesite and the Cristo Rey laccolith beyond that. When it was dated, using the K/Ar method, it yielded a date of only 28 Ma (Oligocene), but that’s probably too “young” due to argon loss. Someone should retry it with the Ar/Ar method!
Here’s a Google Earth screenshot (view to the west/southwest) that shows the sill prominently, and also the course of the tramway up the Franklins:
Above that, we can see the transition to the limestones of the El Paso Group. View remains to the north, so these strata continue to dip to the west:
So how can the same limestone strata be both above the Fort Bliss (and its basal nonconformity) and also below it? If you’re thinking “fault,” you’re on the right track. The limestone to the east (in the quarry) is part of a big block that slid down the east face of the Franklins during an extensional episode called the Rio Grande Rift. Here’s a diagram that Josh drew (note his choice of ‘whiteboard’!) to show the regional structure:
The Rio Grande rift is a big, widely-distributed structure, with many subsidiary faults. These faults bound grabens and half-grabens, which have accumulated a thick stack of sediments. There’s something like 11,000 feet of sediment beneath Ciudad Juarez, for instance, with the same bedrock beneath that as we see at the top of the Wyler Aerial Tramway in the Franklin Mountains. There is a lot of offset along these “bookshelfed” fault-block mountains due to that rifting episode.
The fault-bound blocks occur on multiple scales: at the scale of mountain ranges, we get the Palomas Range, the Franklins, and the Waco Range, but on either side of the Franklins there are medium-scale fault blocks that have slid down and out under gravity’s influence. And within those blocks, there are human-sized offsets to be observed, too.
The view from the top was lovely, but we had to turn around immediately and head back down so that I could catch my flight back to DC. Amazingly, from the top of the mountain to the ticket counters at the airport only took about 15 minutes. I was astonished how quickly we managed it. I caught my flight with 15 minutes to spare!
15 February 2012
Colluvium-choked channel in cross-section
After seeing the contact of the Campus Andesite with Western Interior Seaway sedimentary rocks (Cretaceous in age), we moved a bit on down the line, and saw this disconformable contact between the Cretaceous shales below, and a bouldery sedimentary breccia above. Note the concave-up shape of the contact to the left of Elizabeth Nagy-Shadman (of Pasadena City College, California): that’s probably an old river channel scoured into the shale bedrock, then filled with what appears to be colluvium…
Another perspective on the same outcrop:
14 February 2012
Contact of the Campus Andesite with host rocks
First thing we saw on the post-InTeGrate field trip to the rocks of El Paso, Texas, was this contact between the aforementioned Campus Andesite, and the Cretaceous sedimentary rocks into which it intruded (contact metamorphosed in the area of this photo):
I decided to try switching up my annotation fonts. Whaddya think?
Callan Bentley is an assistant professor of geology at Northern Virginia Community College in Annandale, Virginia. He is particularly interested in structural geology and the evolution of the Appalachian mountain belt. Callan draws cartoons and writes for EARTH magazine. He lives in Washington, D.C.
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