What is the difference between contact and regional metamorphism?

In the comments on a recent post I was asked about the difference between contact and regional metamorphism, and thought that the answer deserved to be a post of its very own.

First of all I should remind you that “metamorphism” is about change—specifically the changes that take place in a rock due to changes in temperature and pressure. Each mineral has a range of temperatures and pressures at which it is “stable”. When in that range it exists happily because the pattern of the atoms within its crystal structure is appropriate for the conditions. If there is a large enough change in the temperature and/or pressure the minerals become unstable. If that change takes place slowly enough and the new conditions are held for long enough the unstable minerals will undergo chemical reactions to grow new minerals out of the component elements of the old minerals.

However, if the change is too fast the “unstable” minerals will continue to just sit in their rock, doing nothing. This latter feature is a good thing—without it we would never see high-pressure rocks at the surface. Diamonds are a high-pressure form of carbon, and graphite is a low pressure from of carbon. Their ingredients (only the element C) are the same, but their crystal structure is very different. The carbon atoms in the high-pressure version are packed much closer together than the low pressure version. If the diamonds didn’t come to the surface so quickly they would have been replaced with graphite (or the carbon would have been used to create some other mineral).

So today’s post is about those rocks that experienced new conditions in temperature and/or pressure long enough that chemical reactions happened and new minerals grew in response to the new conditions. There are a couple of ways this can happen: Contact Metamorphism and Regional Metamorphism.

Contact Metamorphism is what happens when something really hot (like a body of magma (molten rock) comes in contact with colder rock. The rocks get cooked, just like when one pours pancake batter onto a hot frying pan. Anyone who has ever made pancakes knows that over the course of the cooking process there is a range of textures to the batter before one flips it to cook the second side. The part against the pan got the hottest. As a result the liquid in that layer is completely cooked away and the solid turns a golden brown (or even darkens to black if left unattended too long), the next layer out from there also becomes firm, but with a different texture than the crust, and the far side of the pancake, which is exposed to the air, is still cool enough in temperature that it hasn’t solidified at all yet.

This is similar to what happens to (relatively) cold rocks that have been intruded by molten rock. The layer of rock closest to the intrusion heats up the most, and any fluids escaping from the rock as a result of the heat participates in the chemical reactions of the “cooking” process, so a suite of minerals grow in the layer closest to the heat source that are indicative of high temperature. The next layer out doesn’t get quite as hot, so a slightly different suite of minerals grows there, and even further away from the intrusion the temperature is lower still, so lower temperature minerals grow.

These sorts of patterns were very helpful in formulating theories of metamorphism in the early days of the development of our science, as geologists could easily find places where the pattern can be traced from completely unaltered “country rock” through a series of gradually more and more metamorphosed rocks right up to the contact with the intrusion. In places where there is good outcrop one can see that the layers form concentrically around the intrusion.

Regional metamorphism, on the other hand tends to involve much larger packages of rocks. It is often associated with mountain building (orogenesis), and happens when an entire region gets buried deep enough so that both pressure and temperature increase. This increase causes new minerals to grow. Regional metamorphism is, often, something that happens over a much larger body of rock than contact metamorphism (which goes a long way to explain why “regional” was chosen as the name). It also, often, takes place at higher pressure than contact metamorphism, since it takes a certain amount of depth to achieve the sorts of temperatures at which the metamorphic reactions take place, and that depth comes with corresponding pressure increase as well as temperature increase. Regional metamorphism takes place at a variety of temperatures and pressures, which have been divided into “facies”. The below diagram is figure 2-2 in Spear’s 1993 book Metamorphic Phase Equilibria and Pressure-Temperature Time Paths. It shows some of the more common “facies” of metamorphic rock. Each facies is defined by a group of minerals that coexist in stable equilibrium in that range for a specific rock type. Therefore we metamorphic petrologists can tell just by looking at which minerals co-exist in a single rock roughly what sort of pressure and temperature the rock was at when the metamorphic minerals grew.

timing is everything

I really should sit down and ready my 1000 words a day of geologic literature earlier in the day. Often what I read is inspiring and makes me want to go and do things for my own research. But, when I read it late at night, like tonight (and, I confess, many other nights as well), I’m too tired to follow through with the inspiration. Sadly, by the time I’ve slept and started a new day I’ve often forgotten the feeling of inspiration and nothing comes of it. Perhaps by stating this publicly I’ll revise my habit accordingly and gain the benefit thereby.

some pretty building stone in an unexpected location

I have just returned from a four day visit to see friends who live in Ireland. I didn’t bring my computer along this trip, so I’ve returned to a fair bit of reading of e-mail/livejournal/blogs/facebook to catch up on. So I’ll just share this pretty building stone from a counter top in the Dublin airport. Lovely stuff, with plenty of garnet, definite foliation, some huge feldspar phenocrysts. No idea from whence it came, but it sure did brighten up the airport part of my adventure to see it.

Why Garnet?

A friend of mine recently asked me “Why garnet?”, and it occurred to me that others might also like to hear one petrologist’s thoughts as to why it is such a well-studied metamorphic mineral.

1. They are pretty!

2. They are very common in a wide rang

e of metamorphic rock types.

3. They are stable across a reasonably broad range of pressures and temperatures of relevance for metamorphism.

4. The often form “porphyroblasts” (metamorphic crystals that are noticeably larger than those which surround them).

5. They are easy to identify in hand-samples—their nice “garnet-red” colour often contrasts with the other minerals in the sample (though some of the less common varieties come in other colours, including green and yellow).

6. They are easy to identify in thin-section (a slice of rock only ~3 microns thick, which means that light transmits through most of the minerals so that one can look at it in an optical microscope): they have a high “relief” (they look like they are taller than the things next to them, even though they aren’t) and they are isotropic (they are solid black when the polarizing filters are crossed, no matter how the stage is turned, making them stand out against the changing bright blues, pinks, and yellows that the other minerals become when the polarizing filters are crossed).

7. They have a rather broad range of possible chemical compositions, with iron, magnesium, manganese, and calcium all able to slot into the same position in the crystal structure (this is part of what gives it a broad range of stable temperatures and pressures) and aluminum and silica can also do a certain amount of swapping one for the other. There are a handful of other, less common elements which can also substitute for others in its crystal structure.

8. They have very slow diffusion, which means that once they reach a certain size the center of the grains no longer get involved in chemical reactions. As a result it is normal for the composition of garnets to be “zoned”, with the center containing more Mn than the rims, and the rims containing more Mg than the core (each of the other major elements also typically change their concentration from core to rim).

We metamorphic petrologists talk about the garnet cores being “armored” by the rims. The rims are, in theory, in equilibrium (or trying to achieve equilibrium) with the matrix minerals at any given time—this means that the minerals present will be participating in the chemical reactions that are causing the growth of some minerals and the dissolution of others. For many minerals the normal grain size is small enough that the reactions involve the entire grains, but garnets often grow large enough that only the outermost shell is involved in the reactions, with the inner portion “freezing” in whatever composition was stable when it was the outer portion.

So, just as an Everlasting Gobstopper (do they still make those candies?) changes colors as you suck on it, so garnets show a range of compositions from core to rim. Part of the changes in garnet composition are due to rare ingredients having been used up making garnet (plus or minus any other zoned minerals present). So Mn, which tends to prefer garnet to any other mineral in metamorphic rocks, starts out “high” in garnet, but there is usually so little of it available in any given metamorphic rock it is soon used up and the garnets have gradually less and less Mn as they grow, until eventually the outer portions have no measurable Mn at all. The other reasons garnets change their composition is due to changes in pressure or temperature. Different recipes of garnet are stable at different pressures and temperatures. So if the conditions change different types of garnet grow on the outside of the pre-existing garnet. These features all combine to make it a very well-studied mineral because of all of the inform

ation one can extract about the history of the rock.

A large garnet in the wild (southwest coast of Tasmania, photo taken by Andrew McNiel):

Garnets (2mm) in thin-section from Collingwood River, Tasmania (also shown are biotite (brown), muscovite (pale but wavy lines), quartz (colorless and without lines + dots in the garnet):

Same garnets as above, but in crossed-polarized light:

Achievements VS Tasks; writing for your audience when applying for a job

Since my current contract ends in December, I’ve been watching the various geology email lists and job boards for interesting sounding jobs. Recently a friend of mine sent me a link to a position that isn’t in the academic world, but sounds like it would be fun, and I meet all of their “essential” requirements, though not necessarily all of their “desirable” ones, unless you take into consideration things I’ve done that are similar to what they are looking for.

However, this position has a much more formal application process than I underwent to obtain my current job. Their application packet makes it clear that a CV will not be considered, but instead we are instructed to fill in “details of your previous employment, starting with the most recent. Please include details of any time not accounted for (including unemployment)”. I had never had to compile a complete job history before, and it took a fair bit of time to remember everything I’ve ever done and fill in the boxes on the form (Date of employment, Name and address of Employer, Position held, description of main duties and responsibilities, achievements and reason/s for leaving). Once I’d completed that task and written my “Statement in Support of Application (Outline your reasons for applying and how your experience, knowledge and skills meet the job description, person specification and key competencies for the post. All the essential criteria must be covered)” I sent my completed form to my sister for comment.

While I have spent my life in Academia, my sister has been in the business world. She completed a Master’s in Education degree, but wound up going into corporate training rather than teaching at a school. She is also a writer (although I keep a blog, I don’t consider myself to be a writer; I’m a reader, who is willing to share thoughts via writing now and again). The feedback she gave me on my application packet was amazing—in just a few short paragraphs she captured the essence of one of the biggest problems I have in writing.

She had commented to me that I should delete some of the detail from my job list—that there is no need to show more than one job that happened at the same time, and no employer will care that I did modeling for a life drawing class while I was an undergraduate. I pointed out that I tend to err on the side of caution when deciding what information to include, and my brutally honest tendencies mean that I want to write down “everything”. To which she replied:

“Resumes and job apps are no different than any other type of writing: you must consider your audience and what they will be looking for, as well as your topic, angle, and goals for writing. That is, my guess is when you worked on a geology paper or presentation, you have more data points available to use than you actually include in your paper. That does not mean that you have been less than honest, or incomplete, it just means that you have likely chosen a specific idea to focus on, and analyzed, synthesized, and used the data that best informs that topic. Similarly, the level of depth and detail will vary if you're presenting to geologists in the same area of specialty to you, vs a broader field of geologist, vs a group of multi-disciplinary scientists, vs a room full of lay-people. In these situations, you would naturally think about who your audience is, the starting point they're at, what they would understand, and what might engage their interest.”

I laughed when I read this, thinking of the complaints I’ve heard people make about papers submitted for publication by recent PhD’s—that we tend to write papers that “read like a thesis”. In short we include too much information, wanting to share *everything* we learned in the course of our PhD projects. My first attempts at writing up my results certainly went that direction, and my advisor told me that it read too much like a thesis, and that I needed to go back and change it to the style of a paper. He even gave me papers I might use as a model. I’ve been putting off doing just that (it has been a year and a month since I submitted my thesis—my how time flies!), but reading my sister’s comments above, I think I may have a better handle on how to accomplish that goal.

My sister then went on to say:

“A resume or job application is only different in that it is a marketing document as much as it is a historical document. Its entire purpose is to get you to an interview. Anything that does not advance that purpose should not be included. Now, in the case of "list your entire work history and account for any gaps" listing all your experience does have a clear purpose. Still, you do get to decide how you present each item to make it relevant, or downplay it if it is not, as well as whether or not to include multiple items over the same time span. Also, you get to decide the "starting point" of your work history. Going back to a paper route when you were 9 years old is probably more information than the hiring manager needs to know (actually that was me with the paper route, but you get the idea).”

While I have long known the rule “write for your audience”, I don’t think I had *really* thought about what this meant in terms of job applications before. No wonder some of my applications never made it to the interview stage!

Her next paragraph included the gem “First, frame everything in terms of what you have achieved versus what you have done. (It's the difference between "managed the sales department" and "Improved sales by 40%" - or in your case between, "researched " to "published over xx papers, and conducted xx presentations on "” Re-writing my job history using this piece of advice has actually been quite fun—breaking up the information into the “duties” vs “achievements” has given me a whole new way of looking at it. Even if I don’t get the job for which I currently applying, the process of learning how better to apply for it has been so helpful I will still feel like I’ve won.

Reading Maps, or How I learned, yet again, that it is never safe to make assumptions

When I was an undergrad enrolled in a Field Methods course my teacher wouldn’t even talk to us about the map unless we first “oriented” it (aligning the north arrow of the map with the north arrow of our compass). This is a very sensible idea, as one is much less likely to confuse one’s location by thinking that this hill on the map is that hill in the real world, when, really, it is the other hill over there. However, when printed in a book or posted onto a sign maps are often oriented with a “north at the top” convention.

Before today I would have used the word “always”, rather than “often”, but today I found out that the maps in Glasgow, Scotland are not oriented with north to the top. I have just spent a week visiting friends in Edinburgh (and enjoying the fact that the temperature was nice and cool—unlike southern Europe was before I left), and my return flight was out of the RyanAir airport in Prestwick (which is not that far from Glasgow). To get there from Edinburgh one takes a train to Glasgow Queen Street Station, and walks from there to the Glasgow Central Station to board the train to the airport.

I had thought about printing a map from google of Glasgow city centre before traveling, but forgot to do so before shutting down my computer. However, I didn’t worry about not having a map of my own, since I’d seen a Glasgow map and I knew that the two stations were quite close to one another; my destination is about two blocks west and 4 or 5 blocks south of the station to which I would be arriving. In addition to that information, my friend also told me that there are maps of Glasgow posted on signs in quite a few locations in the city center, especially near the train station, so I knew I could look this information up again when I got there.

When I arrived in Glasgow I exited the train station, looked around, and saw a map just up the street. When I reached the map I looked at the street signs for the intersection upon which I was standing to learn their names, then looked at the map (marked with a “you are here” arrow and a large circle of everything that the mapmaker thought was within a five minute walk of that point). Sure enough, the “you are here” spot was located at the intersection of the two streets whose signs I’d just read.

However, the bottom of the map ended only a couple of blocks below the “you are here” mark, just outside of afore mentioned large circle. Central Station was not visible at the bottom of the map (where I expected it to be, since I was using the assumption that the map was oriented with the top = north convention). Therefore I reasoned that the station I wanted is located just off of the area covered by the map, and I started down the street in what I assumed to be the correct direction.

One block later I found another sign with a map posted on it, but much to my surprise, there was still no Central Station at the bottom of the map. However, looking closer at the map I discovered that the station is on the map, but instead of being where I expected to find it, it was located near the top of the map.

A more careful perusal of the map in front of me revealed an over-street foot bridge located just above the “you are here” marker on the map. Looking past the map I saw that there is, in fact, such a foot bridge over the street in front of me. Ah-Ha! This map is not oriented “north = top”, it is oriented “top” = “direction you are facing as you look at the map”.

This prompted me to search the map for any sort of north arrow. There wasn’t one. Therefore, in order to determine that “up on the map” = “direction you are looking” one would need to either expect it to be so, or take time to do a careful comparison of the map with the real world. Fortunately, I’d only walked one block out of my way, so no harm was done, and I now know to take just a bit longer when reading maps posted in strange towns to see which convention (if any) they are using for how the maps are oriented.

What could be more fun than extending the boundaries of the sum of human knowledge?

Not too long back I got into a conversation with a friend of mine, who was complaining about his job and how little he enjoys it. It is always difficult for me to be properly sympathetic in such conversations. Having learned as a child that “work” or “a job” meant "something unpleasant that people had to do but didn’t want to do", I have done my best to avoid ever having to do such a thing myself. Indeed, this was one of the largest motivating factors in my decision to be a life-long scholar—to stay happily in the academic world, learning interesting things.

When my academic career progressed to the point of undertaking a PhD project I was totally excited about it—I was about to learn something new, something that no one else ever knew before. This is heady stuff. Sure, there can be a fair bit of tedium in research, but that is more than offset by the ability to set one’s own hours and choose for oneself the topic and direction of the research. Having finished the PhD and moved on to a post-doc position doing experimental petrology I find that I am still expanding the bounds of human knowledge. The experiments I’m doing are ones that no one has done before—each run I do provides new information that needs to be analyzed—looked at, compared with the previous information, and understood.

I am aware that there are people out there who have the misfortune to spend their days doing things that don’t interest them, and I am grateful that I am not one of them—that I have the joy that comes from learning new things. Not just new to me, but new to everyone. What could be more fun than that?