Naukluft Mountains

We're 225kg richer in Namibian fault rock and we have a lot of stories to tell... now it's time to write.



Thanks Bandile, Zach and Åke for a great field campaign.

new frontiers in rock packing

hello from windhoek. We have just packed our rocks (wrapped in plastic bags) in steel 20-L paint buckets and filled the gaps with expanding insulation foam. any bets on whether this will work? not sure if it will kick off properly inside the bucket. not sure if it will withstand the motion of the rocks in the truck south. if it works as well as it appears to - we have revolutionized the preservation of structural rock samples which so often I have lost to mechanical weathering along the way home...

Geological terminology I hate.

Geological terminology is always evolving. The meanings of old terms can change through time. Early suggested terms may turn out to be inaccurate and may be replaced, or may become more or less specific in their usage. Nevertheless, old habits die hard. Certainly the use of certain terms help us establish or test identity or affinities - scientific jargon at large plays important roles in social relationships.

4. Basement
"Basement" is the ultimate relativistic term. It describes the rocks one is not interested in talking about. If you write a paper about soils, the recent sediments underneath may be the "basement". If you're writing about those sediments, the granite underneath those may be the basement. Basically it means whatever is older or under or around or near the rocks one is actually interested in. My friend Mike, who loves ice and lichen and moss and squirrels and everything else more than he loves rocks, gave me another variation on "basement" the other day when I accidentally picked him up on the Golden Gate Bridge: "underburden". Nice.

3. Subduction channel
Apologies to my good friend Åke, but this one has to die. The "subduction channel" refers to the area between the subducting lithosphere and the overriding plate in a subduction plate boundary, where sediments are subducted and variably deformed and metamorphosed and de-watered. It is generally seen as a tabular region - not linear - so it isn't the shape of a channel. The term also causes confusion due to some people suggesting models where material flows only downward through this "channel", while others invoke backflow within the "channel" to help uplift metamorphic rocks which record very deep conditions. Both camps use the term "channel" and neither one is describing a 1D feature. I'm just confused by this. Finally, in a channel, the flowing media flows in one direction relative to the walls of the channel, right? But in a "subduction channel" the flowing stuff (subducting sediments) moves in a direction and rate intermediate between the upper and lower walls. It's a shear zone. Not a channel.

2. Pseudotachylyte (or pseudotachylite)
Aside from having two spellings (-yte is older and therefore preferred although it is counter-intuitive most of the time and looks weird with 2 "y"'s so close to each other), the term "pseudotachylyte" is an example of defining something by what it is not, instead of what it is. Pseudotachylyte is a glassy rock formed by either seismic or impact-related melting of any rock (but in practice is restricted to silicates). It does not therefore include a whole suite of other glassy rocks (igneous, melt cortices on meteorites, etc), so it is not a good descriptive term, but requires an interpretation of how the thing formed. Finally, tachylyte is an igneous glassy rock similar to obsidian. So pseudotachylyte is something that could be mistaken for tachylyte but is not. Oh yah, and there are other very fine grained, dark coloured fault rocks (e.g. ultramylonite, ultracataclasite) that can't be distinguished from pseudotachylyte without some serious microscopy. So it's not useful as a field term either. Fail.

1. Pan-African
As far as I can devise from the literature, the term Pan-African refers to nearly any geological event (mostly magmatic but also metamorphic, deformational, etc) occurring during a period of approximately 250 million years (roughly 750-500Ma) anywhere in Africa or continents formerly associated with Africa. My dear colleagues who advocate the use of this term tell me the exact meaning can be deduced from the context of the specific location or events being discussed which makes this term actually less useful than not using any term at all. I can think of no good reason to use "Pan-African" at all unless one is trying to obscure the problem of massive dating errors or giant uncertainty about tectonic events. THIS ACTUALLY HAPPENS.

ok just to make me sound a little less cranky, here's a comment left by somebody called NJ on Kim's blog a while ago that makes me totally happy:

"You'd better wait. My desk is totally Franciscan right now and I have no idea where to start looking.

Or:

"He completely Franciscaned his first draft and his advisor wouldn't even read it."

Hollister - the Creeping Calaveras Fault

Every region has its particular strengths and weaknesses with regards to the type of geology which is easily accessible for student field trips. In the areas surrounding UCT, we have some seriously awesome geology but there are at least two things my students have to accept without seeing any really clear direct evidence in the field:

1. plates really move
2. plates really subduct.

Since I have a student from UCT with me in San Francisco this week for the AGU meeting (who gave a badass poster presentation by the way), it's a good opportunity to fill some of these gaps. We took an afternoon drive down to Hollister and San Juan Bautista to see some evidence for recent fault creep offsetting sidewalks and walls.

Hollister is positioned just north of the split where the Calaveras Fault branches from the main strand of the San Andreas Fault. The Calaveras Fault is creeping through Hollister, but rates vary along the fault in space and time from 3-18mm/yr (http://funnel.sfsu.edu/creep/SiteTable.htm).
(Map from http://quake.usgs.gov/recenteqs/)


My student is clearly excited by this right-lateral bulge in a garage wall.



Tension gashes where the fault crosses the street at a high angle and disappears straight under the middle of a house.
wonky sidewalk
another wonky sidewalk
seriously wonky sidewalk, and the steep small hill on the left of the photo is a pressure ridge
wonky sidewalk
more tension gashes in the street
carly will be creeping to your right as you look at this photo.

It's cool to see how different sidewalks and houses of different ages have accumulated different amounts of offset. We also couldn't help noticing new skirting and lots of concrete repairs which presumably addressed the larger offsets. Also - in some places the total offset was accommodated by narrow strands (usually ~ 1m wide) but in others, the deforming zone seemed to be much wider (10m). Seems like this depends on local soil conditions as well as the rigidity of the surface features. Sometimes it is wide under a sidewalk and sometimes all the strain seems to accumulate on one joint between sidewalk panels, as in the last photo.

Today we are off to Ring Mountain with Åke to see some evidence for #2.

Stromatolites in the Naukluft Nappe Complex

Finally back in CT for a while. Lots to post but more to catch up on. Here are some cool stromatolites in dolomites of the Naukluft Nappe complex. There's a thin layer of sand over the top of the carbonate bed. I wonder what this represents. Was it a wave that washed sand between the bioherms? Did it kill them? I didn't see the beds above. Isn't it incredible how the sedimentary record is a stack of discrete moments - not a continuous record. Just snapshots.




I love being a geologist because I can hike up a cliff on a dry hot windy day in southern Africa, watch a meerkat shading himself with his tail, scare a herd of Hartman's Mountain Zebra up the slope ahead of me, then sit on this 550-million year old warm shallow sea and imagine a tropical, tectonically active world owned completely by algae and possibly some ediacaran fauna - no shells, no teeth, no fish, no birds. Must have been a quiet and peaceful world.

First Fold 2009

The traditional "First Fold" picture - see 2008 and 2007). I just can't get enough of this cool folded bed in the lower Prince Albert Formation.
This class was fun - and every one of them was pretty keen in the field! Also did their chores without nagging! Truly a first on both counts. It was a nice one to go out on - my last trip to Laingsburg, at least as a lecturer at UCT.

It was a big year for transitions - as Dr. John Rogers, our sedimentologist with whom I have taught this part of the field course for the last 4 years, will be retiring at the end of this year.
We were lucky to have the two new guys (our replacements) accompanying us for the trip, in a kind of hand-over. It was great to be in the field with them and see all the energy and interest and excitement they will bring to the department. They both saw a lot of research potential in the area too - I hope some of their plans will turn into future Honours projects for these students!

As you can see, they worked well together in the field. All the groups did. I haven't seen their final maps yet but I have a feeling they will be good.
Every year I change it up a little bit - we usually do a "structure training day" and a "sedimentology training day" before they start mapping on their own. This year it went particularly well. I decided to focus directly on field methods instead of rehashing the structure topics we discussed in the classroom. We practiced sketching from afar and ground-truthing the sketch, and talked a lot about scale and planning where to go. Here's an example of a student with his field sketch of a faulted anticline thrusted over a faulted anticline. That peaky Prince Albert Formation sure does take up a lot of the strain in this part of the fold belt.

Here we are on the last day - the last day, for me, of formal teaching at UCT. Pretty sad about that but also looking forward to the next phase of my life.

The Cake Practical

The theme of this month's Accretionary Wedge blog carnival is "Time to Think Out of the Box", in terms of approaches to teaching. I thought I'd contribute an activity I've been running in my classes which comes directly out of a box - a cake mix box.

We spend a lot of time in my Honours tectonics class (that's equivalent to a senior seminar in Tectonics in the states) talking about the lithosphere.

We discuss and in some cases, calculate:

• the flexural thickness (relatively thin, <10km>
• thermal definitions (thickness varies, but temperature is really a proxy for rheology),
  
• definitions based on a sheared layer (high seismic anisotropy) separating the lithosphere from the asthenosphere,
  
• more abstract definitions based on calculated rheologic transitions (using more complex proxies than just temperature)
  
• discussion expanded this year to include looking at sub-continental lithospheric mantle which has been a stable geochemical reservoir for a very long time.. therefore not likely communicating with the convecting mantle (shout out to my colleague Steve), a distinction which is obviously important and in some places observable but was previously not on my radar.

However, for the students to really grok* this they have to have a good grasp on the concept that the same material can behave both as either an elastic or ductile solid under different conditions of pressure and temperature. There begins the search for an analog material.

The Cake Practical

As it turns out, cake deforms elastically at low stress and non-recoverably (let's call it viscous) at higher stress. This is a cheap tolerable proxy for Maxwell behavior. I bake some thin sheet cakes - from an ordinary cake mix - each pair of students gets about 100cm² piece of 3cm thick cake. The technical staff of the department was kind enough to provide me with a few "core samplers" = pieces of 1" pvc, about 2" long, with a nice bevel cut around one end to sharpen it for cutting into the cake. They also need an ordinary metric ruler and a watch or cell phone with a stopwatch.

The students cut as many sample cores from the cake as they can. This leads to a bit of waste, invariably eaten, thereby increasing the general level of blood sugar excitement in the room.

Fernando shows his glee at performing rheological experiments
on rock analog materials. Photo: William Cheng.

I give a bit of a talk about elastic at low stress, viscous at higher stress rheologic models and remind them that elastic deformation is linearly related to stress by the Young's modulus:

σ = E * e
where σ is stress in Pascals, E is the Young's Modulus in Pascals, and e is the linear strain (change in length / original length).

They are also reminded that viscosity is the relationship between stress and strain rate, not linear strain, by the relation:

σ = η * ε

Where η is the viscosity in Pa*s and ε is the strain rate in s^-1.

We assume a Maxwell rheology wherein:

σ = η ε + E e

and also assume that if we do experiments at very low stress, viscous strain negligible (all strain is elastic) and therefore set η ε = 0 at very low stress. This enables the students to isolate the Young's modulus (E).

Load is applied by placing other food items (of labeled mass, e.g. small cans and jars) on top of the cake cores. The students measure the surface area of contact and calculate applied stress.**

The students are asked to design and execute experiments to determine:

• the Young's Modulus

Cake sample after elastic rebound (there's a bit of a delay there - hysteresis loop?)
Photo: William Cheng

This turns out to be amazingly reproducible. For my cake this year, everybody seemed to get a result between 5-8 kPa. Or, since the concept of significant figures doesn't seem to have taken hold, 6449.33352 Pa.... that's another issue.

• the elastic limit

100g jar of capers produces about 1.2 kPa load on the cake core.
Amazingly, at 50% shortening the cake core still rebounds elastically.
Must be all those eggs I put in.
Photo: William Cheng

This parameter is bracketed by increasing the load on the sample until the sample no longer rebounds elastically to its original height after the load is removed. For the cans and jars I brought, all groups bounded the elastic limit at between 100g capers and 400g organic kidney beans. With our cake core samples this is between about 1-6 kPa. Next time I would get more intermediate weights.

• the viscosity (for deformation above the elastic limit)
  

Under the 400g can o' beans, the cake has gone viscous and very little rebound is
observed. No conch shell or painted stick reported either...
(Photo: William Cheng)

The viscosity is the most unreliable part of this experiment, mostly because the students have to estimate the timescale of deformation and that timescale is very short. Perhaps larger pieces of cake would address this....

Anyway this prac gets good reviews. The discussions of the lithosphere seem to go well afterwards, although I will let you know for sure after the exams in October. I have used the same concept for a more open-ended inquiry and had students investigating the shear modulus by sheathing the cores in plastic wrap and shearing them, investigating the temperature effects by freezing them and microwaving them [don't tell the boss I broke into his lab for a little N₂ (liq)]. Obviously no amount of temperature increase is going to allow dislocation glide in chocolate so the metaphor rapidly breaks down. However - I never have to clean anything up with this lab... except wiping a few crumbs from the table tops.

Hoping someone out there will improve on this or suggest improvements... leave me a comment if you have any ideas!

*I was introduced to the Grok concept by my algebra II teacher in high school who made us chant (daily) the quadratic formula "so that if any of you two reproduce, your children will be born knowing it". Well, Mr. Steiger, x equals the opposite of B plus or minus the square root of B squared minus four-A-C over two-A.

** Yes I'm aware that the labeled weight on the can only refers to the food inside and does not include the weight of the container. Trust me this is not the largest source of error here and anyway, saves time and hassle when the students have only a short time to complete their experiments.

I should be more aquatic myself

You know what? I know I'm going to be eating crow about this for the foreseeable future but I've changed my position about the Aquatic Ape Hypothesis. I just watched the Elaine Morgan
TED talk.

In a nutshell, Elaine Morgan has been arguing for decades (from outside the academic mainstream, which may be important in this case) that the major phenotypic differences between humans and chimpanzees are the characteristics we humans share with aquatic mammals. These include hairlessness and the subcutaneous fat layer which no other primate possesses. She also notes that apes, which are all capable of walking upright when they feel like it, always walk upright when entering the water.

She makes other points which I can't independently verify, that all hairless terrestrial mammals (e.g. elephants and rhinos) have aquatic forebearers (save the naked mole rat, a freak by anyone's measure) and that breath and diaphram control is common to aquatic animals but otherwise unknown in apes. This control gives us the power of speach.

I've read her arguments before and there wasn't anything new in the TED talk that I hadn't heard already. So I googled around a bit to see if she was under-representing the strength of arguments made by her detractors. Most of the arguments I found rely either on the lack of fossil evidence to support the theory, or on logical arguments which don't seem to me to be significantly stronger than those in favor of the aquatic ape.

Evolutionary change can occur when a fortuitous coincidence of environmental pressures with the occurence of a mutation in some population which directly affects the likelyhood of survival relative to those pressures. It also occurs when random mutations which don't affect the likelyhood of survival are also prevalent in the selected population - so it is not reliable to look at a single characteristic and deduce a past environmental pressure. In the long term, traits which advance survival or reproductive success are most likely to survive. However, the rate of mutations is such that this model is never run out to its conclusion. No organism exists, or is likely to ever exist, which is perfectly adapted to its environment at the time we observe it. The disadvantage to survival caused by the occasional appendicitis is not sufficient to cause the next generation to be born without an appendix. The reproductive struggles introduced by the upright human pelvis are likely a more significant challenge to survival than the appendix. But these are accomodated by cultural means, or technology, and have clearly not affected the survivability of the species. In short: the logical arguments made on both sides of the aquatic ape hypothesis so far fail to produce a unique conclusion, only explore what could have been possible.

So. Is it possible that a semi-aquatic hominid existed in our lineage, which could have perservered long enough in the lakes of the East African Rift to adapt some characteristics common to aquatic mammals, although not our nearest ancestors? Logical arguments could be resolved by adequate data from the fossil record.

The hominid fossil record is painfully scarce (relative to other more numerous, longer lasting species). In order to be fossilized, an animal has to die in the right place at the right time. On the savannah, animals are dismembered, desicated, and the bones dry and crack in the sun. There is almost no potential for preservation. Fossil beds from which we understand the record of terrestrial animals come from environments where rapid burial in sediment can take place - lakes and rivers - where we know that terrestrial and aquatic animals gather together. Hominid fossils are also found primarily in lake sediments but this does not really address where they lived, only where they died. Even so, it is not possible from the fossil remains that have been found to determine soft tissue characteristics such as identifying the emergence of a subcutaneous fat layer in our hominid ancestors.

So... Dad... although I am unwavering in my opinion that when discussing scientific theories, "interesting" and "likely to be true" are mutually exclusive - I am no longer hostile to the Aquatic Ape Theory. This has mostly to do with a recently developed appreciation for the scarcity of the hominid fossil record, and therefore the higher degree of uncertainty, than anything else. Now I'm going to the pool.

Some advice if you are looking for a grad school program

Greetings all. As the application season is coming around soon, I've been getting a lot of questions lately about how to find the right graduate school or project. So here are some comments about the criteria I think are important... and how to go about looking for the right match. This is by no means a comprehensive set of instructions, more like an incomplete list of FAQ...

APPROPRIATE PROJECT
You best find something that turns you on right from the beginning. You're probably going to hate it at some point, sooner or later, and you'll need some serious motivation to help you get through the low spots. You should be interested in both the BIG QUESTIONS and the PIDDLING DETAILS of your project.

The project ought to be the right scale for the degree you are seeking (MS or PhD) or have the potential to scale up or down if needed. It ought to afford you the opportunity to learn some specific skills or get experience which will make nice bullets on your CV.

THE RIGHT GROUP FOR THE PROJECT AND FOR YOU
The adviser, and members of the research group, should be experts in the background of your intended research projects. Someone should have those specific skills that you intend to learn. It is common for a student project to require a skill which is not currently in the group repertoire - something the student can learn and then contribute to the group. This depends on the past experience of the student.

Adviser personal skills and attitude toward advising can affect the quality of life for a newby grad student to a great extent - take a look at the rest of the group and ask current students privately about the group dynamics. However, keep in mind that graduate students are nearly always disgruntled. It's a point of pride. After a campus visit or two, you will easily pick out happy/functional groups and... other groups. There are sometimes sad mismatches of adviser and student style/personality. Learning about the group dynamics can help you avoid these.

Think about how you enjoy spending long hours. If you are fascinated by the outer core, but hate math, you may be out of luck because most people who study the inner core are seismologists and modelers. Even if you don't know exactly what topic you want to study, but have an idea of what you enjoy/are good at, you will have a better chance of finding the right project for you.

STARTING THE SEARCH
Use a search engine to find journal articles that interest you. Google the authors. Nearly everybody teaching at a university has a website that includes a summary of their research, publication list, and past student projects. I googled about 50 people and narrowed down 12 to email directly. I have no idea if these are typical numbers. Ask your professors for advice. If you are able to, go to conferences! GSA and AGU are great places to shop programs/advisors/projects and distribute your CV. Watch the ads in EOS and GSAToday

When emailing potential advisors:

Learn what you can about the advisor/group and write a personalized email showing that you have done your homework. I immediately delete emails of inquiry which make clear that the student has not looked at my webpage and is not interested in anything I do. Ironically, anything that says "Dear Sir" will get a response - A strongly worded response.

Keep it short. One paragraph is plenty. Be focused and organized. Include a sentence or two summarizing your background and offer a CV on request or a link to your CV download (do not attach it to the first email as you might end up in the spam folder, and it's presumptuous). Clearly state why you are interested in this particular person's research. If your interests are broad, you may write to more than one member of a department. It is expected that you are shopping around.

Don't kiss ass. Don't alter your email so strongly for different targets that you look like you're kissing ass or being fake - people do talk, especially people in related fields. Keep your CV short (1 page is usually plenty) and if you have job experience which is not related to your application, just summarize it briefly (e.g. no list of babysitting references here!)

Edited to add: read FSP's post before you send that email!

FINDING FUNDING

In the USA - finding funding is usually a joint effort between advisor and student.

Students typically support their education by some combination of grants, teaching assistantships, research assistantships, and loans.

Grants/Fellowships: can come from the Univ, Advisors' grants, or any number of private institutions. Potential advisors will have knowledge of appropriate opportunities in your field. These could be for direct support (living, tuition) and/or research expenses.

-- NSF Graduate Research Fellowship. This money follows the student even if they switch universities, advisors, projects for 3 years. quite competitive and lucrative. Obama just tripled the number of Fellowships!!! applications open in August 2009.

Most of the advisors' grant money comes directly or indirectly from the National Science Foundation, or in some fields, DOE or other agencies. Many of these grants are written with some student support in the form of TA or RA positions. Expect to be asked to work ~20hrs/wk and receive adequate compensation for tuition, health insurance, and a small living allowance. FYI that 20hrs is a completely made up number and has almost nothing to do with your workload in practice.... (see above reference to graduate student disgruntlement).

In South Africa...
If you are South African, you can apply for an NRF bursary. This is something like R35 000/yr for MSc students and R60 000/yr for PhD. Not enough to get by in Cape Town but maybe in some other cities it's enough? I'm not sure. Many students get support from an employer and nearly everyone supplements that with demonstrating (TA) jobs. There are other pools of money available specifically for scarce skills development (including geology) and for previously disadvantaged populations (everybody but white males who had citizenship or permanent residency before 1994, varies whether white females are still considered disadvantaged. But that's another topic for another post.)

In any case, your supervisor will have more insight on funding opportunities and again, it should be a joint effort between student and supervisor to get the money together.

AND NOW SOME UNSOLICITED ADVICE...
Don't be afraid to apply overseas for your graduate studies. It's a great stage of life to try out living in a foreign country, as your life is relatively institutionalized so it's not as lonesome as moving somewhere entirely on your own. There are additional challenges - funding is often earmarked for citizens of that country, there are issues with translating transcripts and qualifications... but it's an excellent chance to spend a few years building relationships/support networks in the international community. In particular, for South African students it's a chance to get a world view in your field in a short period of time, during which somebody (your supervisor) is officially committed to supporting you and your development as a scientist. You can bring those relationships back to your home country, but it's far more difficult to develop them without investing the time overseas. When so much research funding is available through international collaborations with Europe or the US, those connections can help support you through subsequent decades of your career.

Alright i'm off the soapbox... did I forget anything?

Corona Heights Fault

Emily found this fault by searching the internet for local slickenline photos and landing at Andrew's site, geology.about.com. What did we do before the internet? The fault outcrops in the Castro Area (Corona Heights) where a post-1906 aggregate quarry has been repurposed for a playground and pet cemetery. The relatively recent exposure of the fault allows excellent preservation of the slip surface itself.

The fault is a thin breccia zone (less than a meter) with an anastomosing network of highly polished, slickenlined surfaces within the breccia zone.

Nils admiring his reflection in the fault surface. Seriously it is so so shiny. I've never seen anything like it. Twice now I have gone out to the Marin Headlands Terrane (same cherts) to look for fault surfaces and see if I can find anything similar. Can't. Went the second time because it was too foggy the first time and I worried I might have missed it.
Emily and Amir worked on a big normal fault in southern Oregon which had a similar fault rock structure - thin, superfine-grained polished slicked core, thin breccia zone with pinch-swell structures, rapidly dropping off to undeformed bedrock on either side of the fault. That thin breccia zone probably takes up a lot of deformation and accomodates the roughness on the polished slip surface. (See Sagy, Brodsky and Axen (2007) in Geology; it's available here.)

Here's Emily getting a grip on "Layer II", that granular layer which must break or flow in order for slip on these uneven, anastomozing polished surfaces to occur. Her hand is on that ~ 40cm layer of breccia in between two slip surfaces.

The texture of the polished surface is truly remarkable. You can see some rounded fragments of chert - which are green, while the local rock is all red chert. They are embedded in a translucent super fine-grained silica matrix. You can see through it. It is positively glassy (in a descriptive sense; I have not examined it for molecular structure). It has beautiful tensile cracks which are generally perpendicular to local slickenlines - and rotate as the slicks rotate and the fault surface undulates. Truly amazing.

OK what you can almost see in this photo are tiny round white spots in the superfine silica layer - they are lensoid fractures in the silica. I have some theories about what this stuff is and what the tiny fractures mean. But I have apparently been too far out on a limb lately (at least for some reviewers) so I will just ask you, on the off chance that anybody is still reading this blog...
1. What is the shiny, transluscent silica and how did it form?
2. Why does it have tiny lens-shaped fractures inside?

Probably the way I worded those questions leads toward my ideas about the answers... but ... anyway...

Flying over Greenland

On the plane from Heathrow to San Francisco, Nils and I sat across the aisle from an Icelandic glacial geologist on his way to vacation to Hawaii. When we flew over the east coast of Greenland I had to wake Nils up and make him look out the window! So amazing.

Medial moraines where two glaciers flow together:


A beautiful long fjord:

Tidewater glaciers calving

Field of bergs:

Winners

The Art of Subduction took 3rd place at the annual Geological Society of South Africa Western Cape Branch Quiz night.

Nice job team.

Fun with Folds

BrianR at Clastic Detritus posted a great photo of some disharmonic folds in bedded strata. Here are some more!


I particularly like the chocolate-tablet layers visible on the top of this outcrop.

Rock of the Week #10

1. What are the sedimentary structures in this rock?
2. What can they tell you about the depositional environment?
3. Why is this rock reddish in colour?




Solution to Rock of the Week #9:

This is a SKARN – that is, a contact metamorphic rock produced when magma intruded low-grade country rock. This sample comes from the Birch Creek Pluton in the White-Inyo Mountains of California. The older rock here was a Cambrian (530Ma) carbonate and shale marine platform sequence. The intrusion of granitic plutons (80Ma) transferred heat and Si-rich fluids into the sedimentary rocks, causing metasomatism and metamorphism. This was a moderate-sized pluton and the zone of contact metamorphism around it is about 600m thick. This sample was taken ~200m from the pluton where temperature reached about 500°C. It contains garnet, dolomite, calcite, fluorite, epidote, and quartz and is classified as a Ca-Fe-Si skarn.


1. (1 pt) 2 minerals?
: 1 point if 2 of the above listed minerals are given.

2. (1 pt) Rock name?
: 1 point for skarn (only).

3. (2 pt) How did it form?
: 1 point for contact metamorphism or metasomatism.

A paradigm switch on the simple seds

Sedimentology, when I was an undergraduate student, was kind of seen as the easy course. Structural geology was the bottleneck course that you'd have to be insane to take before your junior year (See exhibit 1: insane undergraduate seen here in final throws of thesis preparation at which point I was sleeping under the poster printer in the computer lab to the chagrin of the academic staff who found this horribly embarrassing.)

Don't get me wrong, Sedimentology was a lot of work and rigorous work at that. In particular, I credit Sed with really helping me develop my field observation and writing skills. But I think we thought it was easy because it was accessible - it's geology's first line in a lot of ways. You can see an outcrop and look around you and see the depositional setting at the same time. I was reminded of this when my brother visited last year and we were on a beach watching each wave lay down a thin, well-sorted layer of course white sand. All around the beach were beautiful outcrops of Ordovician Peninsula Formation - the fluvial/marginal marine pile of ... massive and laminated well-sorted coarse white sand. I couldn't resist pointing this out and I saw the Principal of Uniformitarianism spontaneously pop into my brother's pointy social scientist head. Sedimentology, more than some other fields, includes opportunities for accessible discoveries like this for introductory geology students and anybody else who's not old and jaded about little obvious things like the Age of the Earth. In other words, seeing seds in the field can give people an easy entree into the paradigms of geology... and everything that logically follows from something as profound as Uniformitarianism. These moments do not require someone to be able to conceptualize Deep Time or visualize things way beyond human scales in order to see Geology in Action. Teaching structure and tectonics involves a lot of convincing students to suspend the tangible world for a minute and imagine things NOBODY HAS EVER SEEN and convincing them that some gnarly rotten outcrop of blueschist 1000's km from Cape Town is enough evidence to suggest that these things are REAL and HAPPENING RIGHT NOW far below the earth's surface on continents they have never visited.

So... Let's just say that for years I labored under the impression that field sedimentology was somehow light weight science. I see it now in some of my students as well. Most of the textbooks make it clear that if you can just identify some sedimentary structures that the depositional environment will be uniquely determined and then you just put together some transgression/regression model and you're done. There are obvious complications (e.g. the afore mentioned Peninsula Formation - if the structures are fluvial, why does it have the grain composition and size distribution characteristic of aeolian sandstones?) but it's always possible to work up a story of reworking during environmental changes to get around some of that.

So it turns out of course that South African geology has given me yet another paradigmal smack down (is that a word?). In yet another installment of the IBD but not really chronicles, there are mystery environments recorded in the Cape Supergroup rocks that surround us here in the Cape for which there is no conveniently adjacent depositional model with which to compare.

Take the Peninsula Formation for example. It's nearly all quartz. Like upwards of 98% SiO₂. It is bedded on a scale of on order 1-10m. It is regionally extensive over 100s of km (1000s if you include comparative formations in South America, which we should). It is incredibly well sorted with regards to grain size and rounding. The crossbedding shows bars and meanders and shoals and maybe some lagoons. It's been through the Permo-Triassic Cape Orogeny which involved a lot of (now extremely hard to detect) layer-parallel thrusting so aside from the section 400m from my house on Table Mountain, nobody can really be sure how thick it is - but it's really thick. (Exhibit 2: Sandstones so thick you have to take a cable car from the base of the section to the top).

So... moving on to the "easy" sedimentology part... How do you make a huge sand sea (or erg) of perfectly sorted and rounded quartz sand? On today's earth: wind. Dry, hot or cold, and plantless ergs occur on every continent in specific climate zones. So we have analogs - but they are not the only possibilities. Here is an exception to Uniformitarianism - in the Ordovician there were no land plants anywhere. So environments existed then for which we have no analogs on earth - places where sediment was unconstrained by vegetation which could have occurred everywhere and not just in places of extreme climate.

Issue 2: the sedimentary structures. We can imagine a case where a wind-sorted erg was re-sorted by rivers and shallow seas by sea level rise or continental subsidence. The Ordovician southern world was a very cold place, and Africa resided at more southerly latitudes than it does today. Or - a situation where a windy sandy desert provided sediment to a subsiding margin where the sand was sorted by rivers and surf as it accumulated along the coastline. Maybe the overlying units can provide more of a clue.

To be continued....

On the topic of scientific dissent

There is a buzz going around the blogosphere in the wake of press releases on a new paper coming out which challenges the asteroid impact correlation with the K-T extinction event.

This seems like an opportunity to point out that the whole search for a "smoking gun" in extinction events is probably off-target: All the largest extinction events in earth's history seem to be a result of positive interference between multiple factors which change the habitability of one or more environments on earth. In the case of the K-T event, the longer-term carbon cycle perturbation associated with the flood basalt eruption of the Deccan Traps in central India (68-60Ma) coincided with the Chixulub impact in Yucatan, Mexico (64.5Ma) as well as the increasing domination of the plant world by angiosperms (flowering plants) which may have been less edible to the terrestrial herbavores. Several other events, both fast and slow, have been suggested to correlate temporally with the end-Cretaceous extinctions.

WHY DO WE NEED A SMOKING GUN? Clearly this is a multiple-whammy event. Every K-T paper seems to have to take a side. From my reading it appears to me that a long (multiple-million-years) decline in biodiversity (caused by plate tectonics and climate effects such as volcanism and plate reorganization in latitude and coast-line length) was punctuated and probably accentuated by spikes in volcanism rates (CO₂ increases) and at least one big asteroid impact. Without the combined effects of ALL of these driving factors, the extinction event would have been less pronounced, or the biota would have had more leeway to recover. Maybe the "everybody's right" approach isn't as headline grabbing.

Anyway, the more interesting discussion growing out of all this is the topic of scientific mavericy: the role of a scientist or group of scientists who argue against the generally preferred interpretations.

The Lab Lemming and my perennial favorite, Female Science Professor have each written superior posts on the topic. FSP in particular highlights the intrinsic value of minority opinions to the overall debate, while acknowledging the erroneous effects of the popular media's tendancy to highlight "both sides" of a "debate" by digging out a quote from some dinosaur who thinks the world is flat.

There are ways for non-experts, including science journalists to tell the difference! And we ought to hold them accountable for this!

The paper which triggered these discussions (Press Release here) reports some new stratigraphic work in the Yucatan area which IS INCONSISTENT with one of the key attributes of the hypothesis that the Chixulub Impact ALONE was responsible for the mass extinctions of the K-T boundary. The stratigraphy records both the extinction event and the impact event and they are NOT PRESENT IN THE SAME STRATIGRAPHIC LEVEL, suggesting that they did not occur at the same time.

This is a key piece of data, which is important to the ongoing conversation on the relative role of different factors which caused the extinction. The community of scientists who promote the idea that this impact directly led to the extinctions will have to somehow explain or discredit this data or their case for their model will be weakened. Although there will be moments of drama (and for us, a "dramatic moment" involves somebody standing up in a dark conference room and telling somebody else they are wrong.... ooooo drama), people are right now in their offices reviewing their own data sets and hypothesizing new explanations or new narratives that incorporate the new data along with all the existing data. Science will go on. So will the debates about the dinosaurs' extinction.

This does not make the authors of this study some kind of gadfly harbingers of doom, shooting poison darts into the currently dominant hypothesis. I beg of you, journalists and general public -- do not mistake the give-and-take of ideas, data and hypotheses in the scientific sphere for intellectual cage wrestling. Disagreeing does not have to lead to drama between scientists... it's part of our daily lives and we enjoy the process of working out the details to find concensus or narrow the points of contention. If you are married to or dating a scientist, you may experience the great joy of this process as an integral part of your personal life.

Fully Equipped Field Geologist

Alright I'm a bit behind the curve here... but ever since The Lost Geologist posted a photo showing all the bells and whistles comprising the field geologist's kit, the world of geobloggers has been weighing in. (Also see... Geotripper, Hypocentre, Kim, the Ethical Palaeontologist, Johannes, Silver Fox, etc...)

Over the years I've developed some very climate and duration specific field kits. I hate to carry anything I don't need and I hate to be overloaded as to be uncomfortably hot. I also hate to run out of water and/or food.

Always have:
1. No Belt. I wear pants or shorts with many deep pockets. In those: Brunton Compass (I have one for S and one for N-hemisphere field work) Rite-In-The-Rain field book, at least 2 mechanical pencils, fatty eraser, many fine-tipped sharpies in multiple colours, a few big black sharpies for marking samples. Can't have enough sharpies. (often: cell phone, gps, whistle)

2. Handlens(s) on a chord around my neck.

3. At least twice the water I think I will need. Two or more pieces of fruit which are waste-free (I eat apple cores and orange peels rather than leave them in the field or carry them home. Thanks to Eric Thompson for long ago convincing me of the edibility of citrus peels). To keep this light as possible, I have knit some water bottle slings which I will use in place of carrying a pack if the water is all I'm bringing.

4. Map board - still using the Hilde Schwartz-style boards from UC Santa Cruz which are made from two pieces of plexiglass (one with a 1.5" bit cut off from one side), duct tape, and binder clips. Put the topo maps +/- aerial photos in here. I'm still looking for a replacement for standard binder clips which does not affect my compass, as I do end up taking measurements on the map board pretty often.

5. Peanut butter and jelly sandwiches in zip-lock baggies (also twice as many as I think I could possibly need; these roll over every day if I don't eat them. They keep just fine, even improve with age(?), they're high energy, and there's no need to wash out the ziplock between pb&js.

6. Camera. Currently rocking the Canon EOS 1000D. Always with spare battery and SD cards.

Hot Weather Kit: Photo by Taufeeq Dhansay, Near Monapo, Mozambique (2008)

----That's it for the "always" items. ---

7. Hammer and heavy plastic bags, duct tape, and super glue for sampling - The way I do field work often involves several days of structural measurements at one outcrop or small area - normally then I do all the data collection and then set aside a day for sampling at the end. That way I don't a) smash anything I should have measured or b) take unneccessary samples before I understand the full picture. This has been a pretty important adaptation to my field plan because as a structural geologist who works on brittle rocks I NEED REALLY BIG ROCK SAMPLES and when rocks cross borders in Africa, they often have to go through customs with a certified currier company. This means I pay by the kilo. I want fewer, bigger, better samples.

----Matters of personal style---

When mapping in arid and semi-arid environments, I wear running shoes ("takkies") with short cotton socks. I hate being too hot more than I hate getting my legs all scratched up in the blasted fynbos. I wear SPF40 super waterproof sunscreen everywhere but somehow end up burned anyway.

When mapping in cold wet places, I wear NO COTTON WHATSOEVER not even underwear. Synthetics and wool only. In Alaska I often wear extra-tuffs while mapping in the field but I'm not sure this is the best way to go.

Hair: Always with the dual-braid configuration. Fits best under hats.

Hat: My SeaHawk Air hat has been my standard since 02. However, I lost it when it blew away in sub-gail force winds while I was sitting on top of a really fantastic sycline-axis koppie with a crinkly little bit of Prince Albert Formation in a sea of Dwyka diamictites. I got SeaHawk to send me another and it's almost as good. Finally, Sila talked me into getting a proper 360-degree brimmed floppy hat and it's ... alright. But I feel like such a dork.

Pants: Dork score increasing here: I wear zip-offs these days. Specifically, Convertable nylon pants from Cape Storm. They have kick-ass pockets with zippers so i don't lose keys. They look terrible because there is some bunchy elastic at the back for some odd reason. I don't care. They are light-weight and seemingly bulletproof, even in the face of elephant-skin weathering (also known as tareponts weathering to the Poleta crew).

Kit for Alaska field work: Photo: Asuka Yamaguchi, June 2006



---- Also, things that live in my backpack forever ---
knife
15m of good strong 3mm nylon line (good for clothesline if nothing else)
a powerbar or two of unknown antiquity
ziplock baggie of extra TP (also of unknown antiquity)


Ha ha I'm looking for pictures of myself in the field and I realize something that's present in nearly every photo but I completely forgot to add to the list:


Students. Not technically required for every field campaign but they sure do make it more fun. That's me in the green. Laingsburg field trip 2008. (not sure who took this picture.)

Fire on Devil's Peak

2am on 18 March - I got home from cutting rocks in the Stellenbosch geology department to find my neighborhood being evacuated.





It burned fast through the fynbos but as everything is built of cement, nothing else caught fire near my house. Now there are pink and white lillies growing in the ash. Rumour has it they only bloom after a fire.

Suck it Bobby Jindal

Lahars at the Drift River Tank Farm, Mt. Redoubt, Alaska. Photo from AVO.

The "IBD Effect" and a trip to Ring Mountain

I keep finding geologically interesting spots in South Africa. There is a never ending supply, actually. However, often times when I ask around to find out what kind of work has been done in one of these areas, I get an answer to the effect of, "It's been done". In the Saldania Belt, represented locally by the slate belt that underlies the Cape Town area, a lot of people told me "It's been done". Digging into the literature and archives of theses from the local universities, I found several pieces of really nice work. Some of them emphasized the structures in the area. But to my mind, there is actually a *giant hole* in the literature when a few theses, fewer than 10 journal articles, and a sprinkling of unpublished conference abstracts are considered "DONE" for an orogenic belt or accretionary complex (the literature is ambiguous as to which this is). After finding this phenomenon at several different localities around South Africa, my friend Jodie and I christened it the "It's Been Done Effect" or IBDE, which sounds like "Ibid". Get it?

In terms of the local slate belt, my friends at the survey agree it needs some more attention so I got a few students together and we're going for it. Peeling off the coastal belt of the Malmesbury Group for scrutiny. Hoping to get started asap with a mapping day starting at the Sea Point Contact. Starting where the last thesis left off... and importing an old friend with a global slate belt habit for help!

My beloved Franciscan Complex has suffered a bit of the IBDE. It was such a hot topic a few decades ago that people tend to think that It's Been Done. In fact, there are a lot of secrets still hidden in the Franciscan and a lot of unanswered questions. Only a small but hardcore loyal following still works on it (often as a pet project without substantive funding). As such, I took some visitors there during AGU - subduction people from New Zealand - California's geologic evil twin, in some ways - whom I thought would adequately appreciate it.

Ring Mountain has low grade sandstones and shales on the lower slopes. On top there is a plate of ultramafic rock. In between the two there is a complete mess. Sheared serpentinite melange in green, purple, white, yellow and black wraps around blocks and boulders of strange metamorphic rocks.

Here's a blueschist boulder with some intense folding of the internal foliation.
On its outside surface, the reaction rim between the silicate minerals of the boulder and the ultramafic serpentinite matrix breeds an actinolite and talc-rich reaction zone (green) wrapping the glaucophane-rich blueschist (violet).

Another boulder clast contains epidote-garnet zones where fluid has entered? or departed? along fractures in the blueschist rock. Sorry met friends, I don't know which. Can I use a lifeline? I'm going with departed.

Here's what reminded me to finally post these photos: One of my fellow members of the underground Ring Mountain club mentioned fucsite on Facebook. Is this it? I'm talking to you Naomi.



My camera took a crooked picture of me with VT and AF. Perhaps it's because it was distracted by the metamorphic block it sat on:
"Wakabayashi Block", in tribute to another die-hard Friend of the Franciscan.