My flight to Budapest

I flew to Budapest this morning, to attend the upcoming IMA2010 Conference (Aug. 21-27, Budapest). When I finally turned on the computer this evening there was an e-mail from the conference organizers announcing that this event “includes in its programme 13 plenary lectures: seven IMA2010 Plenary talks and six Elements5 talks (the latter celebrating the 5th anniversary of our excellent journal). The topics cover all representative areas, and they are tailored for a broad audience, while the authors are well-recognized experts in their fields, but also attractive speakers.” They also announce that those of you who weren’t able to come out for the meeting can still follow along at home by listing to their live web casting (end public service announcement).

Because I had a morning flight it was necessary to leave my home at 04:00 to begin my journey to the airport. Therefore I decided to just stay up all night, thinking “I can sleep on the plane”. However, what I failed to consider was that the flight path went along the south side of the Alps, for a rather large portion of the range’s extent. Consequently, while I did nap in the bus on the way to the airport, and at the airport itself, I spent the flight with my eyes focused out the window, enjoying the lovely view. Some of the peaks in the second row in still have snow on them, which sight is balm to eyes that have had to endure summer’s heat. I have no idea where I’ll be going when my current contract ends, but I sure hope it is somewhere that I can see (and walk in!) mountains on a daily basis.

Article review: “Metamorphism: from Patterns to Processes”

As a member of the Mineralogical Society of America I receive a paper copy of the journal, Elements, each month. Some of there previous issues have focused upon a specific element or a specific mineral. The June 2010 issue (Vol 6, #3), on the other hand, focuses on Fluids in Metamorphism. Needless to say, as a metamorphic petrologist this caught my attention. I’ve just finished reading the article titled Metamorphism: From Patterns to Processes by Bjørn Jamtveit, who is located at the Center for Physics of Geological Processes at the University of Oslo and decided that it is worth mentioning here.

This article is a very good one for underscoring one of the reasons I am fond of metamorphic rocks: they are pretty! He includes a variety of figures to illustrate the types of changes that happen to rocks as a result of metamorphism (change), with a focus on how those changes are facilitated by the presence of fluids. As a teaser for the article, I have copied his figure 1 below. The first photo is of a dark, fine grained basalt (formed by the cooling of lava after it erupted from a volcano), which contains the minerals augite, plagioclase, and olivine, though the individual grains of each are too small to see at this scale. The second photo is of a lovely eclogite, which contains red garnets, green omphacite, and white clinozoisite, all of which are much coarser-grained than minerals in the basalt. Metamorphic processes are responsible for the transformation of basalts into eclogites.

The differences in the crystal structures of augite-plagioclase-olivine vs. garnet-omphacite-clinozoisite mean that the eclogite is a denser rock (~3.5 g/cm³) than is the basalt (2.9 g/cm³), which, the figure caption tells us, means that the transition from one to the other is important for large-scale geodynamic processes, including basin subsidence and subduction.

In addition to illustrating the beauty of metamorphic rocks, the article also touches upon the factors that cause the change, and how long these processes take. I highly recommend it to anyone curious about metamorphism.

My host’s great-grand father

When I was in Norway earlier this summer, rather than staying in hotels I chose to couch surf, for much of the trip. There is something quite nice about staying with local hosts; one gets to meet people one wouldn’t have otherwise met, and one gets to learn something about the local area and the people who live there. Or, in some cases, the people who have lived there long ago.

My host in Bergen, when she discovered that I’m a geologist told me about her great-grandfather, who was a geologist, so I looked him up. Tom Barth (1899-1971) published over 200 papers in his life in the fields of mineralogy, petrology, and geochemistry. When he was young he studied with Goldschmidt and Eskola, names that should be well known to anyone who has ever read a basic metamorphic geology text. He was one of the people involved in the early stages of developing an understanding of the way crystal structure works, and was the first to demonstrate that chemically different atoms can occupy crystallographically identical sites.

What fun serendipity that my host while I was traveling happens to be descended from a scientist whose life-work comprised an important part of the framework that was a necessary prelude to my own research.

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: