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Could you please tell me what "bulk rock chemistry" is? It's driving me nuts!!! No where has an answer.

--- George; Newtown High School; Newtown, Wales


I'm not familiar with this term, but... I think they are talking about what I've always called "whole rock analysis." What you do is get a sample of the rock, grind it up, and run it through a VERY sophisticated machine called a "mass spectrometer." What this does is give a very accurate measurement of what elements are in the rock, and how much of each is present. This gives petrographers (guys who study rocks) valuable information on the chemical composition of the unknown sample, and therefore the ability to (sometimes) determine what type of rock it is.

There can be problems, however - it's not always as easy as spitting out the elemental composition and coming up with a name. I'm taking my college students to southern Oregon's "Table Rock" next week because of this. There is a sign which says that the lava flow which makes the cap rock is "Andesite" - an intermediate composition lava with a specific whole rock analysis. Several of my students went there last month, and came back sure that they knew all about it. However, from the local setting and eruptive history of the flow, the rock is actually "basalt" - a higher temperature, mafic composition lava which flows like water for long distances, makes cap rocks, has columnar jointing, and air bubbles (called vesicles) - all of which are present in the Table Rock flow. The problem is that the Table Rock flow happens to have a slightly higher silica/oxygen content than is "normal" for basalt. This makes it chemically an andesite, but, in several of the important ways which really matter to a field geologist, it's basalt. Anyway, I'm gonna walk them up there and let them convince themselves that whole rock analysis isn't always the best (or only) tool for rock identification.

The short version is: whole rock analysis is good stuff (when you can afford it - did I mention that it can get expensive?), but don't let it make you disregard what your eyes and training tell you.

Thanks for the question.


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