Igneous Rock Compositions from Big Bend Ranch State Park as a Teaching Tool in Petrology
Linda Lee Davis Washington and Lee University
This is a partially developed activity description. It is included in the collection
because it contains ideas useful for teaching even though it is incomplete.
The project forces the students to evaluate a geochemical data base, to draw inferences from the data that they plot in various ways, and then to make inferences about tectonic changes.
GSA Poster (Acrobat (PDF) 34.9MB Nov14 03)
Higher Order Thinking Skills:
the students have to address the questions regarding why major element and trace element signatures from the Bofecillos Mountains tell different stories. The students have to wonder why rocks from completely different volcanic centers would plot on the same trends as the rocks that erupted from the Bofecillos Mountains. The students also examine why the chemistry of the volcanic rocks changes when the tectonic regime changes from compressional to tensional.
mastering plotting in Excel or IGPET,
upper level undergraduate and graduate
They have to have some understanding of differentiation and how one goes about distinguishing different processes using data plots. ONe can teach them to plot in Excel or IgPet quite quickly.
Role of Activity in a Course:
I have used this as a wrap up to a semester-long seminar-style course on the Petrology of the Big Bend Ranch State Park....which included a ten-day field trip.
Data, Tools and Logistics
literature on the Big Bend Ranch State Park rocks, and on regional tectonics
students just plotting the data and not truly evaluating it.
I think that the students understood differentiation from mafic to felsic rocks much better after this exercise. But it would be nice to have this validated.
Students are given a data set which includes geochemical analyses of a volcanic dump in the Bofecillos Mountains. The rocks are not all from the Bofecillos vents, but come in part from Mexico and from the north. So, there is no reason that the data all ought to plot along the same trends. But, in fact, with the major element data, all the rocks seem for the most part to be genetically related. It is not until trace element data or isotopic data are considered that differences are obvious. Students are asked to plot all data together, then to distinguish by source and by ages. I think that by plotting and creating the trends themselves, the students gain a firmer understanding of differentiation. Additionally, the students examine how the geochemistry changes when the tectonic stresses changed from compressional to tensional.