Deskcrop #4

Embarrassingly I did not post to The Accretionary Wedge centered on Deskcrops. I have some lame-o excuse, but that's all it is. So, belatedly, here is the next installment of my deskcrops.


As an undergraduate I was the beneficiary of an amazing alignment of geologic planets. My first year as a major (sophomore year) was also the first year a new professor was teaching at TMLAC. He happened to be a co-PI on a large NSF funded project, and had included funding for undergraduate research in his portion of the proposal. I turned out to be a good student, and for two summers (after sophomore and junior year) was able to do field work with him, and an excellent cast of high-falootin' geologists in northwest British Columbia. It was really an amazing experience for me. The first summer I went from school ending, to a 2-week geology field trip in the northeast, to 6 weeks of field camp, and then straight to British Columbia. Incidentally, that summer I spent all but 2 weeks total living out of tents and sleeping bags. It was awesome.

My main project, what later became my senior thesis, focused on a large tonalitic pluton near the town of Prince Rupert. I was mapping the contact, and looking at variations in the magmatic fabric of the pluton as it related to a major bounding structure. The thesis is embarrassing to read now, but it did end up as a talk at GSA my senior year (Salt Lake City, 1997.) Anyways, even though I was mainly looking and sampling the tonalites, we did venture into the country rock every now and then. The sample above is one of my favorites. This is one of many chunks I brought back, although I've never seen it in place. This particular chunk was whacked from a big piece of float while waiting for the helicopter to come and pick us up. Yes, perhaps the coolest thing about the field work was that we got to use helicopters, not typically for day trips, but we'd get flown in and left for a few weeks of field work, then picked up and given a few days to shower and get away from the black flies (and hit Cowpuccino's). I digress, back to the deskcrop.

This is a piece of the country rock, a garnet-sillimanite gneiss. How much sillimanite you ask? Well, roughly 70%. I basically see this rock as a metamorphosed beer can, the amount of aluminum is really astounding. I was on a field trip to the Funeral Mountains in Death Valley where one of the big stops was where we could see sillimanite (you can, in Monarch Canyon, walk from a brittle detachment fault into sillimanite grade schists and gneisses.) When we got to the outcrop, I was a tad unimpressed. The sillimanite we spent the afternoon hiking to occurred as fuzzy growths on biotite, something you needed a hand lens to see. Previous to this my entire field experience with sillimanite was where it was the dominant mineral in the rock. I have since learned to appreciate all forms of sillimanite, but this is still my favorite. I have yet to find many reference for this rock, although it is kind of famous among the people who work up there. There was one outcrop that was mapped and sampled just prior to being blasted away to make a road, but other than that it has only been found as float. As such, I cannot provide much insight into it's geologic significance, but I have about 15 pounds of it in my office.

Deskcrop #3 - Ventifacts

Today's deskcrop is a ventifact. A ventifact is a rock that has been abraded by the windblown particles: sandblasted if you will. They typically have a heavily grooved or polished surface. Large ventifacts can have the grooves aligned with the prevailing wind direction. Small ventifacts, like mine, appear to not remain stationary, and therefore can develop a "brainy" texture. Ventifacts are not all that common, reflecting the relatively small importance of wind erosion. Water (both solid and liquid) is far and away the most powerful erosive agent around; wind is much less significant. In order to see well developed ventifacts, it helps to look in places where the wind is absurdly strong, there are abundant loose and abrasive particles (i.e. sand), erosion from water is relatively minor, and you don't have to worry about plants or soil covering things up. My samples are from Ventifact Ridge, in Death Valley National Park. (legal note, these were collected by someone who had the proper permit to take a few chunks). The rock itself is a basalt, Pliocene in age I believe (but cannot confirm right now).


Besides general interest, ventifacts and other eolian-erosion related features are the best earth analogs we have for many of the images sent back from the different martian landers. Below are images taken by the Viking and Pathfinder landers thought to represent Martian ventifacts (from Greeley et al., 2002.) Greeley et al., is an interesting paper that compares images sent back from the Martian landers to eolian features in the Mojave Desert and in Iceland.


In addition to dry environments, ventifacts need time to form. I haven't found any references for the average amounts of time needed to create well-developed ventifacts, but I imagine it is strongly dependent upon wind speed, lithology, and the type of particulate matter being thrown through the air. References would of course be appreciated.

Greeley, R., Bridges, N.T., Kuzmin, R.O, and Laity, J.E., 2002, Terrestrial analogs to wind-related features at the Viking and Pathfinder landing sites on Mars. Journal of Geophysical Research, v. 107, n. E1, 10.1029/2000JE001481.

Deskcrop (?) #2 - Mantle Xenoliths

As pointed out by Ron and Kim, the next samples from my rock collection for posting are my mantle xenoliths. Xenoliths are pieces of a pre-existing rock that get incorporated into a magma but for any variety of reasons, do not melt. When the magma crystallizes the xenoliths appear as distinct bodies and are usually pretty recognizable. You find xenoliths in all sorts of igneous rocks: basalts, granites, andesites....really almost anything. Somtimes the xenoliths are pretty local in origin. This summer while sampling granites, for example, we found xenoliths a few kilometers from the pluton-country rock contact that were easily identifiable as pieces of one of the wall rock units.

Magmas that rise through the crust relatively quickly can incorporate pieces of all of the rocks they pass through. Magmas with deep origins, therefore, can bring up pieces of the lower crust, or in some cases, even the mantle.

The mantle xenoliths I have in my office are from Kilbourne Hole, a maar in New Mexico that is part of the Pleistocene Potrillo Volcanic field. Maars are explosive volcanoes that form when magma flash heats groundwater. In the case of Kilbourne Hole, a basaltic magma carrying pieces of the lower crust and mantle erupted sometime between 80 and 17 thousand years ago. The xenoliths are medium to coarse grained peridotites, with P-T-ometery suggesting origin depths up to 67 km (Thompson et al., 2005). One of the things I always think about with chunks of the mantle is how odd our perspective is as geologists. I like to show these rocks off, even to non-geologists, because they are odd-looking and distinct. But, if you assume that some flavor of peridotite (or related olivine- and pyroxene-rich ultramafic rocks) makes up the entire mantle, then this is volumetrically the most abundant lithology on earth. Most of us make our living studying the dynamics of the outermost scum of the planet. The lithologies we regard as common, ones that I wouldn't even bother displaying on my window ledge, are really some of the rarest. It is just our limited surficial perspective that makes mantle rocks seem rare, and granites or shales seem abundant. There are of course good reasons for this, but it always sticks in my head.

First, a field map (taken from Thompson et al., 2005.) I collected these rocks on a field trip in 1996 while in college. We spent two weeks going up and down the Rio Grande Rift. Kilbourne Hole is on the west side of the rift, where the obvious rift features start to give way to the Basin and Range province. The xenoliths occur as volcanic bombs, often with thin basalt crusts. Collecting the xenoliths is pretty simple, you basically walk around on the rim of the crater, picking up bomb shaped items, and cracking them open. Well worth the drive if you are in the area.


And a google maps view, you can see Kilbourne Hole as the bluish splotch in the middle of the field of view. The Potrillo Volcanic field includes the big pockmarked region to the west of Kilbourne Hole. Las Cruces is the city in the northeast part of the view, at the intersection of the interstates. Las Cruces is the home of the Whole Enchilada Fiesta. Man, I miss mexican food.

Now the mantle peridotite xenoliths!

Thompson, R.N., Ottley, C.J., Smith, P.M, Pearson, D.G., Dickin, A.P, Morrison, M.A., Leat, P.T., and Gibson, S.A., 2005, Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle? Journal of Petrology, v. 46, n. 8, pp. 1603-1643; doi:10.1093/petrology/egi028. Available here.

The outcrop I've been carting around Part 1

For the past year I've been living in the academic equivalent of the penthouse apartment. I was the sole inhabitant of a two-room office; the outer room has two enormous windows looking out over the campus. It is a first floor office, but elevated enough that I have an excellent view. This past week, the postdoc population of my group, and consequently my office, doubled. This is an excellent development on a lot of different levels, one of which is that I actually tend to work better with someone else in the office (well, I am sure there are exceptions to that, but in general it is true.)

So when I moved into the office, I had way more space than I needed. Consequently my crap kind of exploded all over the place. My rock collection was no exception. I ended up having them all piled on the heating grate under the window. This served double duty, not only was it a place to pile the rocks but it also helped regulate the air, which in the winter is set at "Thermochronic total fusion", and in the summer "Cryogenic cold trap."

So I had to rearrange all of the crap in my office, including the rocks. I've decided to photograph and post about some of my favorites.

First, I want to draw a distinction between my rock collection and my sample collection. I've been collecting rocks since my first Introductory Geology field trip (heh, some of those first Wisconsin Ordovician dolomites and glacial erratics are still in a bucket in my parent's garage.) Rocks in my collection were all picked up on field trips, during field work, while hiking, plus a few gifts. I rarely took very good notes on the samples when I picked them up, or if I did the notes are long gone. This was especially a problem early in my career as a geologist, I've been trying to work on it, but the end result is I have a lot of rocks with vague pedigrees.

Samples, on the other hand, I could take back to their place of origin and more than likely put them exactly where they came from. Samples all have GPS coordinates, they are marked on maps, and I have pictures of the outcrops before and after sampling. Why I can't replicate that for random rocks I pick up I have no idea.

So I moved all of my collection over to my side of the window, it is a little more compact, but I think it still works. Notice how I am covering up about a third of the air vent.



Today's sample is one of my personal favorites. It is something I picked up during my first year in graduate school, when I was a field assistant for my office-mate. Incidentally, Clastic Detritus provided the original inspiration for putting this rock on a post. (And, as a general inspiration, Loose Baggy Monsters recent post about her desk also inspired me to both clean and post a general shot of my workspace. Unlike her post, however, you can actually see my desk in these pictures.) As far as I remember, it is an Early Cambrian shale that I collected along the Transcanadian Highway in eastern British Columbia, just east of Golden B.C., and about as close to the Burgess Shale as you can get on the highway. I remember my office mate telling me we were actually in a late Pre-Cambrian unit, but perusing as many geologic maps as I can find makes me think we were firmly in the Cambrian. I'll hunt some more, but we'll see. Anyways, these are burrow trails in a really old shale. A trace fossil! Our ancestors!



This has always been one of my favorites to show off. Whenever I've participated in people's practice qualifying exams I use this as one of the test rocks. It is amazing how many people want to call it a metamorphic rock, staurolite schist or something.

So more to come, some with better explanations. But I bet that most professional geologists reading this blog have similar piles in their office. Show them off! Let me know and I'll link in this post.

Also, there are outcrops and roadcrops.... so what are these called? Deskcrops? Workcrops? Any suggestions? I am trying to play with the word diamictite, but it is just not working right now.