Wednesday, July 3, 2013

Mountains Of Saint Francis - Walter Alvarez

A certain type of travel book or TV show on Italy features the adventurer driving through sun dappled rolling hills and winding narrow roads to a picturesque village in search of that one undiscovered Trattoria not featured in similar other books or TV shows. Walter Alvarez though refreshingly keeps driving past these rustic eating places to an old quarry just beyond the village. There, he begins poking around in the rocks in an attempt to unravel their secrets.

Walter Alvarez is quite a famous geologist. He was one of the proponents of the theory that a meteorite impact precipitated a mass extinction 65 million years ago, an idea that is now amply supported by evidence.  He has written a story about that discovery in T Rex And The Crater Of Doom (dinosaurs were the most famous casualty of this event). He has had a long professional relationship with Italian geologists and he uses the Italian rock record to explain the methods and basic principals used by geologists in this enjoyable book The Mountains Of Saint Francis: Discovering The Geologic Events That Shaped Our Earth. 

The Mountains of Saint Francis (after Francis of Assisi) is Alvarez's name for the Apennine mountains of the Tuscany and Umbria region and this books explains step by step how they came to be. Throughout the Mesozoic until mid Cenozoic, what is now Italy, was a promontory of the African continent sticking out like a north pointing thumb into the sea of Tethys that separated Africa and Europe. An enormous pile of mostly limestone accumulated on this submerged promontory. These Jurassic to mid Cenozoic limestones form the building block of the Apennine mountains. They stand spectacularly exposed in road cuts and cliffs and have attracted the attention of geologists from all over the world.  As a result, the Apennine rock exposures along with younger Pleistocene deposits have become some of the best studied strata in the world. They not only tell us about local geological evolution, but have provided key insights to answer some broad geological questions.

1) Pinning down Pleistocene ice age intervals. Geologists understood that over the past few million years polar glaciers have waxed and waned but were not sure in terms of absolute years of the frequency of these glacial cycles. Pleistocene river valleys were cut in the areas around Rome at regular intervals during sea level lowstands i.e during glacial conditions. Fortuitously, there were volcanic eruptions during that time too. That gave geologists the right type of minerals that could be used to date rocks using radioactive clocks. The finding was that Pleistocene glaciations occurred in 100,000 year cycles.

2) The Jurassic through mid Cenozoic section of the Apennines is a deep water limestone containing iron minerals. That enabled Alvarez and his colleagues to measure the pattern of  paleomagnetic reversals preserved in these rocks. These limestones also contain fossils of the single celled protozoan foraminifera. Changes in foraminifera assemblages and species through the section allowed the geologists to place the magnetic intervals within a specific foraminifera defined subdivision of the section. Foraminifera subdivisions have been over the years also been dated using radioactive clocks from around the world thereby giving them an absolute age. By comparing the patterns of normal and reversed magnetic zones in Italian sections with the magnetic record of the ocean floor allowed dating those ocean floor intervals as well,  thus giving geologists one way to peg down the rates of sea floor spreading.

3) Deep water environments persisted in Cenozoic Italy but instead of limestone, there are sand deposits. Rivers erode continents and bring and deposit sand in coastal areas. But how does that sand reach the deep sea? It does so via sea floor hugging currents known as turbidity currents. Again, the splendid Italian rock exposures of deep sea sandstones helped Italian geologists conceptualize and understand these sediment transport processes.

All this and many more discoveries are explained using direct and simple language aimed at a broad readership. Neat diagrams illustrate the role of compressional and extentional tectonics in building the structure, topography and drainage of the Apennine region. I like Alvarez's use of language and analogy.

Here is an example of how new mountains are built on older crust:

The top of the Italian continental crust is a fundamental geological break. Geologists call the old crust "basement". It might have been called the "floor", because it acts like the floor of a house when a rug gets pushed across it, crumpling into folds - a good analogy to the way the Apennine anticlinal folds were built.

And another on the vastness of geological time:

These are fecal pellets, shaped by animals that lived on a shallow sea floor, ate the chalky white carbonate mud of the sea bottom, digested any organic matter it contained, and excreted the rest as pellets. If we needed yet another demonstration of the enormous extent of geologic time, we could look at the half-mile thickness of Massiccio, forming great cliffs, and deposited pellet by fecal pellet, by tiny organisms, in a small fraction of the Jurassic Period alone!

By the end of the book I learnt a lot about Italian geology and Alavarez's long and enduring friendship with Italian geologists. This book is partly a tribute to their hard work and often seminal but under-appreciated contributions to this field. Alvarez not only describes the important work of Italian geologists in recent times but also goes back to the 17th and 18th century and writes affectionately about the work of Nicolaus Steno on the order of superposition which preceded that of the more famous Scottish geologist James Hutton. And he gives equal credit to the monk Ambrogio Soldani who in the mid 1700's understood the concept of faunal succession and pioneered micropalaeontology before British geologists like William Smith.

At the beginning, I poked fun at our adventurous traveler in search of the unknown. I was a bit harsh for I have been that traveler too. And so is Walter Alvarez. It's just that for people like Alvarez and me that fine Umbrian wine and food goes down so much better after a hard day's field work. Alvarez describes the Italian countryside and quaint villages and inns as well. He draws connections between the Apennine and much younger volcanic landscapes with Italian civilization and gives brief lessons in Roman history. But at the heart of his narrative is a serious science book which abundantly displays his sense of curiosity and wonderment of geology:

"From high up on a peak called Monte Nerone, on clear, crisp autumn mornings, you can see far across the landscapes of Italy....And along the northern horizon stretches a wilderness of scrambled rock that has glided, mysteriously and imperceptibly, a hundred miles across Italy, hinting at the almost unthinkable wilderness of time that recedes back into our planet's past." 

The Mountains Of Saint Francis: Discovering The Geologic Events That Shaped Our Earth.

See also Andrew Alden's review of The Mountains of Saint Francis.

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