Anthony Philpotts

Since this is the only textbook on petrology I've read, I don't have much basis for comparison. But I enjoyed it, learned a lot from it, and will probably order my own copy for future reference.

So what did I learn? A lot about how igneous rocks are classified, and a lot about normative composition. You can analyze a rock, get the percentage of each kind of oxide in it (almost everything in an igneous rock is a combination of oxides; fluorite and pyrite are about the only significant exceptions). There are some rules for calculating a "normative" mineral composition from the oxide percentages, assuming rock formed at low pressure and relatively low temperature and was dry; water changes pyroxenes to amphiboles. (Unless there is also a lot of alumina, in which case you tend to get micas.) The normative composition is quite old but has held up quite well, closely reflecting what you get for a rigorous computation based on free energies.

But formulas for free energies are in the book, too, if you want to do that. I have enough fascination with equations of state (though heretofore mainly for plasmas and cosmic dust) that I may do just that.

A lot of theory of diffusion, cooling, flow, etc. that I knew something about from other contexts. I tended to skip.

A good introduction to the "room problem." How does rising magma make room for itself? It's not like you can politely tap the country rock on the side and cut in; you have to either melt it or shove it out of the way. Melting it does occur, but there's rarely enough heat to make enough room this way. More common, probably, is stoping, where bits crack off the ceiling of the magma chamber, sink to its bottom, and allow the magma to start pushing on the new higher ceiling.

Good discussions of various kinds of volcanoes and igneous intrusions. I did not know this (and the book didn't actually tell me; it just got me searching in the right places) that there are prominent diatremes, kimberlite, and other ultramafic eruption products throughout the Navajo reservation in the Four Corners area. Where, alas, you cannot collect rocks. But there are a few spots here and there where you can probably drive up and look at them. One of the largest diatremes in the world is located in northeastern Arizon just across the New Mexico border.

I enjoyed the photos and discussions of flood basalts. They cool differently from top down and bottom up; the bottom up cooling tends to produce the famous regular hexagonal column, but the top down ends to be a bit chaotic. You end up with a flow having two layers, the top called the entablature and the bottom called the colonnade. The Junction Basalt at Yellowstone shows this very well.

Good discussions of tectonic settings for various kinds of rocks: tholeiitic series at mid-ocean ridges and flood basalts, alkalic series at continental rifts and hot spots, and so on. The volcano in Africa that erupts molten sodium carbonate may be more common than has been appreciated, since sodium carbonate tends not to linger in the geological record. The calcite, dolomite, and siderite left behind by such carbonatite reactions are probably tough to distinguish from sedimentary rocks at first glance. Some nice discussion also of impact geology and the minerals that form at impact sites. I am finding myself starting to think that New Mexico is too small to limit my geological expeditions to; well, I was planning on coming up to Colorado and Wyoming for the 2017 eclipse, and looking at kimberlites along the way, already.

It's a textbook. Recommended if that doesn't put you off.… (altro)