word of the day

While reading a journal article* today I encountered a word I’d not noticed before: Aulacogen. The first mention in the article was "Continental extension that fails to lead to ocean opening and subsequently undergoes compression can occur at failed arms of ocean basins (aulacogens) and at intracontinental settings isolated from plate margins".

A quick check shows that there are 220 articles in scopus with that term used. (For comparison—“continental extension” yields 4,780 articles in scopus.) This is a somewhat specialized term, so I don’t feel bad that I missed noticing it (if, indeed, it has even been mentioned in the articles and textbooks I’ve read hitherto). Thus far my research has focused on the results of compression, be it deformation or metamorphism, rather than extension, so there would be no reason for me to think about what happens when extension starts and then stops before an ocean basin forms.

*Cawood, P. A., Kröner, A., Collins, W. J., Kusky, T. M., Mooney, W. D. and Windley, B. F. (2009). Accretionary orogens through Earth history. Geological Society Special Publication: 1-36.

Word of the day

While reading my 1000 words of Geologic Literature today I encountered a word with which I was unfamiliar: palimpsest.
According to the on-line Oxford English Dictionary the orginal sense of the word (when used as a noun) is as "Paper, parchment, or other writing material designed to be reusable after any writing on it has been erased. Obs."—this form dates back to 1616. By 1825 the word had evolved to mean "A parchment or other writing surface on which the original text has been effaced or partially erased, and then overwritten by another; a manuscript in which later writing has been superimposed on earlier (effaced) writing." By 1845 the word had also expanded to apply to "a thing likened to such a writing surface, esp. in having been reused or altered while still retaining traces of its earlier form; a multilayered record."
It has also been used (as early as 1876) specifically to refer to "A monumental brass plate turned and re-engraved on the reverse side. Cf. A. 2. Obs.". However, the most interesting uses to my mind are the geological senses of the word: "A structure characterized by superimposed features produced at two or more distinct periods." (1914) and "Of a rock structure: partially preserving the texture that existed prior to metamorphism." (1912). There are also geographical senses of the word: "Of a landscape or landform, esp. a glaciated topography or a drainage pattern: exhibiting superimposed features produced at two or more distinct periods."(1922).
It isn’t often that I encounter a word I don’t already know, and I find I rather enjoy it. There is something nice about being able to open a good on-line dictionary, paste in the new word, and be given not only the meanings, but dates for the earliest use of the word in that context that they could find. In this case I particularly like the development of the word over time first being limited to something humans make/use and re-use and then expanding to other categories. The rock record is full of instances where information is written upon pre-existing information to a greater or lesser degree. Our challenge, as geologists, is to correctly interpret each layer of information and take care not to confuse different layers of information with one another. Particularly when the layers of information may be recorded in so many varied manners.

Geospeedometry

Today whilst reading my 1000 words from the geologic literature (61 days in a row this time, and counting) I encountered a term that either I have never seen before, or I somehow managed to overlook it on other occasions. “Geospeedometry” was coined by Lasaga (1983), who related the cooling rate calculated for minerals based upon the diffusion of atoms within minerals with the time it takes for the “exhumation” of the rocks within which the minerals grew. What does this mean?

Metamorphic rocks form when any preexisting rock is subjected to increased temperatures and/or pressures for sufficient time to grow new minerals which are stable at the new conditions. One very common way for this to occur is for the rock to be taken sufficiently deep below the surface of the earth that both the temperature and pressure are elevated. If it were to happen that a package of rocks were to be taken to such pressures and temperatures and held there until all new minerals grew to replace the original minerals, and then those rocks were to be very slowly brought to the surface so that new minerals continued to grow to replace older minerals during the changing conditions the ultimate result would be a rock which contains only minerals which are stable at surface conditions. However, it happens often that the metamorphic rocks containing minerals which grew at elevated pressure and temperatures are brought back to the surface too quickly for those minerals to be replaced by their lower pressure/temperature counterparts. As a result we have a record of the conditions at which the metamorphism happened. The process of bringing the rocks back to the surface is called “exhumation”, and it refers to great quantities of over-lying rock going away (often due to a combination of faults bring up underlying rocks, and erosion carrying away broken bits of overlying rocks).

Ever since geologists realized that each mineral has a specific range of temperatures and pressures at which it will grow people have been attempting to figure out how to relate the list of minerals present in a given rock with the temperature and pressure at which it formed. The next logical question after the conditions of formation have been determined is one of “how long”. How long did the minerals take to grow? How long (or how quickly) did it take to get this rock from where it formed to the surface of the earth? Those people who study compositional zoning in minerals and calculate the rate of diffusion of atoms within the minerals and who then relate those numbers to the time it took for the diffusion to occur describe what they are doing as “geospeedometry”. Since the term was coined in 1983 there have been 52 papers which list that term in their title, abstract, or key words that have been entered into the Scopus database. One each published in 1983 and 1984, and then a six year break before the next was published. Since 1990 there have been one to five papers on geospeedometry published a year, save for 1993, which didn’t have any.

It is interesting to me that even after completing a PhD and making a point to try to read papers from the geologic literature on a daily basis, I am still encountering terms that are new to me, though they have been around for decades.

Lasaga AC. 1983. Geospeedometry: an extension of geothermometry. In Kinetics and Equilibrium in Mineral Reactions, ed. SK Saxena, Adv. Phys. Geochem., 2:81–114. Berlin: Springer-Verlag