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...

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 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.

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!


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.

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?