Tuesday, June 3, 2014

Of rocks and humans

If you love science, history, and words, it's not every day that you find a book that addresses all of those interests, and it's even rarer to find one that's also engagingly written and great fun to read. I was lucky enough to have just such a book recommended to me recently: The Mountains of Saint Francis: Discovering the Geologic Events That Shaped Our Earth, by Walter Alvarez (public library). And now I'll pay it forward by recommending the book to you.

You may be familiar with Alvarez (a geologist at the University of California, Berkeley) because of his key role in the discovery of the impact that ended the Cretaceous period and caused a mass extinction, which he wrote about in T. rex and the Crater of Doom  (public library). He has become interested in Big History, a new discipline that studies and presents history as an integrated whole, from the Big Bang to now. This interest is evident in The Mountains of St. Francis, which I enjoyed greatly. It's a beautifully constructed book, combining personal history, the history of geology, European history, and the history of the Earth in a gradually unfolding story that reaches further and further back into geological time. The story is focused on central Italy; it starts on one of the seven hills of Rome and ends up encompassing the fascinating and complex story of the building of the Apennines.

While it's not specifically about geological lingo, it inevitably contains a good bit of it; this includes the lovely Italian terms for some of the rocks of central Italy, for example, Scaglia rossa (a pinkish limestone tinted by iron oxides). Geologists call the limestone quarried near Assisi the Scaglia, meaning scale, because the rock can be shaped by flaking off small chips or scales; rossa means red. (Speaking of limestone, I live in limestone country on a different continent, so I was pleased to see the shout-out to limestone for its importance in making Earth habitable.) Each term is explained when it appears, and there's a glossary in the back (as well as a good index), so the reader is never lost in a sea of beautiful but unfamiliar words.

I was particular intrigued with the word ignimbrite, which is the name given to rocks that solidify out of a pyroclastic flow or ash flow. (They're also called ash-flow tuffs.) As you may recall, pyroclastic flows are among the most terrifying and deadly of volcanic phenomena; these clouds of vaporized rock and ash roll swiftly down the sides of an explosively erupting volcano, scorching all in their path. I've always kind of liked the French term for them, nuée ardente, which translates as glowing cloud or burning cloud. Thus, it seemed appropriate to me that the Latin roots of ignimbrite are ignis (for fire; we see this in ignite and igneous as well) and nimbus (as in cumulonimbus). These rocks originate in a cloud of fire.

Interestingly, however, other sources give the Latin root words as ignis and imber, which is a rain shower or rain cloud. (You may know the Latin word imber from its use in the name Mare Imbrium, which is generally translated as Sea of Rains or Sea of Showers.) I suppose it also makes sense that the rocks produced by a "cloud of fire" might be named for the shower that eventually comes out of the cloud. I looked up the 1935 paper in which Peter Marshall, a New Zealand geologist, evidently first proposed the term ignimbrite; luckily, the paper was available online, but in it he didn't explain his thinking when he coined the word. So I will leave it there. If there are any geologists, historians of science, or etymologists who are reading this and can share any stories about the origins of this word, please leave a comment.


  1. Tuffs are also often lovely bits of earth to stare at. We have some examples in southern New Mexico:




    The hoodoos at Chiricahua National Monument (which I somehow have yet to visit) are also tuff:


    1. Very nice, thanks! From the URL, I'd guess that third image is from City of Rocks State Park in New Mexico--is that right? I visited there in 1987, and it was lovely. (Sorry the URLs aren't links; I don't know why that is.)

  2. Yup, the third is City of Rocks State Park. The first two are near a little peak on the southwest side of the Organ Mountains called Peña Blana.