Google+ for geologists

I am not certain if this is a good idea or not, but when a friend sent me a Google+ invite to my personal email address, I used that address to send an invite to the address attached to this blog. After all, this is the address that I keep logged in and have my GoogleReader geology feeds set up. Unlike the account associated with my personal email address, which had no suggestions of people I may know, this address came with a bunch of suggestions, several of whom I recognize as fellow geobloggers. Therefore I created a circle called "geologists" and added them to it. I have no idea if it tells you that you are now in my circle, but if it does you now know why…

It let me set up "sparks" for "metamorphic petrology" and "orogenesis", but they don't look like they have much information available at this time.

orogenesis

I have been far too busy enjoying the fact that I moved to Scandinavia for love to post very often, but I feel compelled to play in this month's Accretionary Wedge. My favourite geology word would have to be orogenesis. I love mountains better than any other land form, and the process of their formation is endlessly fascinating. The microscopic changes which happen in the metamorphic rocks at depth during mountain building are rich with information about the processes that built the peaks.

Metaphorical rocks

I have been taking a language class since moving to Scandinavia so that I can learn to understand the local language. Last week we had a homework assignment to write about our new city, and there was an essay in the textbook that we could use as an example/inspiration for our own writing.

Being a geologist with little interest in cities my essay didn't look much like the one in the book, which focused on various tourist destinations in a major Scandinavian city. Instead I spoke about the fact that January was a good time to arrive here because then it is cold enough it doesn't rain, mentioning that I do not care for rain in the winter, but I love snow, and find the ice-crystal covered trees much prettier than bare trees. From there I went on to discuss the interesting people I have met, and ending with a paragraph explaining that I am looking forward to summer because then I will be able to see the rocks; I have been told that the local rocks are ______, and that is the prettiest rock type.

The word I had in that blank was meant to be "metamorphic". However, when I asked my favourite on-line dictionary for the local word for "metamorphic" it didn't know the word, so I instead called upon Google Translate, which helpfully provided me the word "metaforiska", so I used it. However, when the teacher returned my essay she wrote on the page "metaphorical? Is that really what you meant?" Oops! Checking Wikipedia the local language it turns out that the word I should have used was metamorfa.

However, when I think of it further, it occurs to me that, perhaps, I did choose the correct word after all. Metamorphic rocks, can, indeed, serve as a metaphor for life. In this world we are born as soft, yielding creatures (babies = ocean floor sediments). Over time we are subject to heat and pressure ( = rules from parents & society) which gradually transform us into first older children (phyllite) and then teenagers (low grade schist) and finally mature adults (high grade schist, mylonite, or even migmatite) with the record of our experiences both moulding who we are as people and leaving a permanent record in our souls (zoning in porphyroblasts, inclusion trails, flow banding, etc…) The more difficult the life, the more character we develop, and the stronger we need to be (which is why the prettiest rocks are the ones which are most deformed).

links to other blogs

I just read a wonderful series of posts by Life in a Plane Light describing how metamorphic reactions turn boring old mud into beautiful garnet schist. If anyone missed it, part one talks about the factors that make up metamorphism, the second talks about the first changes that happen to the mud when heat is applied, and the third adds deformation into the equation and gets us to the garnet. If you haven't read them yet they are so worth a look.

slickensides vs slickenlines

In my current job I am learning any number of interesting things as I look things up in the course of editing so as to be certain that terms are being used correctly. Today's noteworthy subject to look up is the difference between the terms "slickensides" and "slickenlines". I only recall the former from my undergraduate structure courses, and neither term has come up in my own research since then. There was a time when looking up such geologic terms meant reaching for a geologic dictionary. These days it can sometimes be more effective to reach for the internet. I just found an interesting article which nicely clarifies the difference between the two words, and why there are two such closely related words in the first place.

Slickensides refers to the planar, polished, surface of a fault plane (the polishing occurring when the rocks rub past one another. Slickenlines, on the other hand, refers to the liner scratches which occur on such a surface, as a projection on one side of the fault scratches a grove onto the other. Most of the slickensides I have seen in the field contained slickenlines. No wonder my teachers and classmates simply used the term "slicks" when discussing them informally in the field.

What an effective way to learn!

I recently replied to an advertisement looking for people to do editing of geoscience journal articles. They first sent me a one page test document to edit to see if they were happy with my ability as an editor, and when I passed that exam they sent me an actual article to do. What a fun assignment! In order to properly edit a sentence one must understand what the writer intended to convey, and in order to do that one must understand the context—what procedures did the author carry out, what were the results, and what do they think it all means? I have written before about "active" reading, and the advantages that can give one in understanding the content of a journal article. It turns out that for me I am able to get even more information out of an article when I edit it, one sentence at a time. I have sent the first article back; I really hope they like my work with it and decide to send me more. I have so enjoyed learning the topic of this paper that I wish to do it again with other papers.

what goes into making a thin section before we can do optical mineralogy?

There has been a lively discussion on the MSA-Talk e-mail list the past few days on the subject of optical mineralogy. Life in a Plane Light has a very nice summary of the discussion and an update of other posts that came in after the first summary.

I have been following the conversation with interest because optical mineralogy is very important to metamorphic petrologists. For those of you who are not familiar with the process a brief description is in order. Step one is to obtain an interesting rock from the field—break off a hand-sample from the outcrop (a chunk of rock small enough to carry in your hand), record on your map the location from whence it was obtained, and write down any important observations you have made about the outcrop (how large is it, does it contain more than one rock type? What sort of contacts separate the rock types? What structures are present? If your sample has any structures which are measurable (foliation, lineation, strike & dip of the rock itself) measure them *before* obtaining your sample, and make marks on the rock with a waterproof marker showing what you measured and what the measurements were—then carefully break that portion of rock away from the outcrop without destroying the marks.

Why is step one so important? Because knowing the field relationships of your sample really helps your understanding of what you will see in the thin section once you have it.

Step two is the creation of the thin section. To do this we use a rock saw to cut the hand sample open. If there are structures of interest in the sample (foliations, lineations, etc.) it is important to make the cut at an angle of interest. There are times when one wishes to see the minerals parallel the structure, times when one wishes to the minerals perpendicular to the structure. If the structure is complex one will likely wish to make more than one thin section of the rock at more than one angle.

We use the saw to cut a small rectangular block of rock out of the hand sample, with one of the long sides of the rectangle oriented in the direction of interest. If your sample is an "oriented" sample (it has marks on it showing what was measured in the field) take care to transfer those marks onto the small block so that you will know which way is "up" or "east" or "north", or whatever orientation you have marked. Once the hand sample is a small block we polish one side of it (the one which is oriented in the direction of interest with respect to the structure) and when it is very smooth we "glue" that side to a glass microscope slide (using a rock appropriate epoxy). At this point we record the sample name on the glass, and also transfer any marks showing the orientation of the sample to the glass itself (this can be done low-tech by scratching on the glass with a nail, or high-tech by using a laser, or with a few other options in between of intermediate technology levels) After the epoxy has set we use a rock saw to cut away most of the rest of the block of rock (which we save for later comparison with the thin section—it can be helpful to also polish the face of this block that was closest to the glass before removing it).

The next step is to polish the thin layer of rock which is attached to the glass slide. This is done carefully because at the end of the process we wish the layer of rock remaining attached to the slide to be only 30 microns thick (remember that there are 1000 microns in every millimetre). The reason we wish it to be that thin is because at this thickness many of the rock forming minerals are translucent—light will pass through them.

This is where the "optical mineralogy" part really begins. By using a petrographic microscope we can see and identify the minerals in the sample and see their relationship to one another. How do we do this? By carefully comparing the various features we can see with an understanding of the features usually displayed by minerals. A course in optical mineralogy teaches us how to distinguish one mineral from another based on their colour or lack thereof, how high is their "relief" (does it appear to stand up above the plane of the glass slide, appear to be lower than that plane, or more or less lie in the plane?), what is its crystal habit of each of the minerals? All of these clues, taken together, permit us to recognize the minerals.

Once we know which minerals are present we can then make observations of how they relate to one another. Are they all aligned with their long axis parallel to the same direction (foliated)? Is that alignment a planar feature, or is it folded? Is there more than one foliation? Are some minerals present only totally surrounded by (included within) certain other minerals? When minerals are present both as inclusions and within the matrix is there any difference between them? (are the inclusions smaller or larger than the same mineral in the matrix? Are they oriented the same direction in both locations? Is there a colour change from one to the other?

These are just a few of the things we look for in a thin section to help unravel the story of the rock. When I was an undergraduate student we not only had a dedicated course in optical mineralogy, but it was a requirement of the course to make one thin section ourselves, starting from collecting the sample in the field and ending with a report about all of the minerals present in the sample and their relationship to one another and what we learned about the rock thereby. In those days we had to make a drawing of the thin section which showed these relationships.

Now days we have fancy, high-tech optical microscopes with motorized sample stages that are attached to powerful cameras and computers. As a result we can program the machines to take a series of photographs at very close scale of thin section, then "stitch" them together to make a single high-resolution image of a large portion of the thin section, which we can then annotate in a drawing program, labelling the minerals, tracing the structures, etc. For those of us who are not practiced artists this is a much better approach for illustrating what we see in the thin sections. However, in the old days a talented petrologist with a steady drawing hand was able to do drawings of thin sections which are works of art and which show an amazing amount of detail of what is in the sample. For a good example of this check out the book Metamorphic Textures by Alan Spry, published in 1969 (ISBN: 0080133169 / 0-08-013316-9), which appears to be available for purchase in pdf form these days, though I haven't tried to do so.