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