Geology Escapades

Last week, I took my only geology field trip of the semester. That's pretty sad for me, because I haven't taken less than three geology field trips in a semester since I've been in college. In fact, by my count, I've been on 14 field trips in just four semesters at Pomona, including 7 overnight trips. That's pretty damn good, especially for such a small department, but that makes just one day trip in a semester a little disappointing.

Of course, I did really enjoy this particular trip, for my 'Metamorphics' class. It's about metamorphic petrology, which is basically looking at rocks which have been deformed over time, and how they became deformed. What's nice is by now, I've taken basically the equivalent of a full semester of metamorphic petrology, including about a half semester back at Pomona, and just over a half semester here at Otago.

For the trip, we drove south from Dunedin as far as Nugget Point, which is at the northern end of the Catlins, a beautiful stretch of coast on the south end of the South Island. You can see why this would be a geologist's paradise: So yeah, that would be layers of rock, but turned so they are completely vertical. Crazy, huh? In fact, they're quite popular among different types of geologists, even. For the paleontologist, there are Jurassic-age bivalve (think clams or scallops) fossils. For the structural geologist, analyzing how this block was rotated is the thing to do. For the metamorphic petrologist, however, this represents the beginning of a type locality for burial metamorphism - a type of metamorphism that often involves only the low pressure and temperatures associated with thick piles of sediments or other rocks. This is something you may see in huge river deltas (think Ganges or Amazon), for instance.

For the slightly more geology-inclined, what we saw at Nugget Point specifically was the very start of metamorphism. Light enough to preserve fossils, but just enough to begin the process of zeolitization - the transition from diagenesis (cementing of sedimentary rocks) to the zeolite facies. The rocks - in this case composed of volcanigenic sediments - appear essentially unchanged; we even found some of those fossils in it. As the day moved on, we moved 'up sequence,' with metamorphic grade increasing more and more, until at our last stop we reached greenschist facies (intermediate grade) rock. I've highlighted the rough temperature-pressure conditions that we saw on the trip on the facies diagram above. What made this interesting, besides the pretty consistent increase in grade, was that all of these rocks all had the same mineralogical parent - that is to say, if you melted all of these rocks and analyzed each melt's composition, it would be the same for all the rocks. Pretty cool.