I lived through two revolutions of geological thought. In fact, I was educated right through both of them. In 1965 when I started college, I was not a Geology major. I took an Earth Science course or two just before changing majors to Geology in 1969 (I worked my way through, so it took a few years extra). The earlier courses were based on the old "vertical motion" paradigm, in which mountains were thought to have been raised up on the surface of an Earth that was shrinking as it cooled. I remember the old Geosyncline model, and how unsatisfactory it seemed.
When I finally started taking Junior-level courses in 1970, we began to learn a new paradigm: sea-floor spreading plus continental drift, which now we call Plate Tectonics. The vertical motions are seen to be caused by much larger horizontal motions. For example, at one time India was a separate continent. Plate motions have caused it to collide with Asia, driving up the Himalayan and Tien Shan mountain ranges, which are still rising. The Alps, the Andes, the Rockies, and indeed all mountain ranges mark continental collisions of various ages. Even the lowly Appalachians, which were once at least as high as the Andes, mark an old collision between America and Africa. The Atlantic has opened up in the time since, and the mountains have been eroding away.
In 1978 I returned for graduate school. Plate tectonics was firmly established, but we were still in thrall to an old idea: Uniformitarianism, stated as "The present is the key to the past". While this is usually true, it is short-sighted. More things can happen in a million years than in a hundred, or a thousand. The profession of Geology is less than five hundred years old. It can fairly be said to originate with Nicolaus Steno in the mid-1600s. The Uniformitarian principle was established by Charles Lyell in the early 1800s. Evidence for events larger than those experienced personally by living geologists was forced into the mold of "gradualism".
Then in 1980 a classic, seminal document was published, and the extraordinary evidence that an asteroid had hit the earth and caused a catastrophic level of extinction—and wiped out the dinosaurs—changed geology forever. In a time period thousands of times longer than all of human civilization, things can happen that are thousands of times more severe than history records. As it turned out, a few small dinosaurs survived, to become today's birds. Another record of scarce survival emphasizes just how catastrophic the end-Cretaceous extinction really was.
On a hillside near Gubbio, Italy, there is a road cut where you can walk up to the sedimentary rocks that were laid down, at a very steady rate, right through the "Cretaceous-Tertiary boundary", or K-T boundary as it is called, 65 million years ago. The fossils in those fine-grained sediments tell quite a story. The critical "boundary" is marked by a thin, darker layer that happens to contain lots of extra heavy metals, particularly Iridium. Iridium means "extraterrestrial". It rode in on the asteroid. But the fossils tell of the effect on the living creatures of the time.
You won't find big shells there, just tiny "forams", less than a millimeter across. Magnified, they look like tiny snails, groups of soap bubbles, and decorated Christmas-tree ornaments. They are very distinctive. There are thousands of different kinds. They make up a large proportion of the sediment older than the K-T boundary. In the younger sediment, however, just adjacent to that iridium-rich, darker layer, there are just a few kinds of forams. Nearly all of the different kinds, perhaps 99 percent or more, simply don't exist in the younger sediments. The "few that made it through" are the ancestors of all later forams, including the ones living in today's oceans. This sudden disappearance of most of these tiny creatures makes it clear that catastrophes do sometimes occur.
The man who is most responsible for bringing about this second revolution is Walter Alvarez of UC Berkeley. He has spent so much time working in Italy over the past generation, that you could call Berkeley his vacation home. He wrote of the K-T extinction in 1997 in T. Rex and the Crater of Doom. In the years since, he has become involved in a third revolution. These three revolutions form the basis, more than the subject, of his newest book, The Mountains of Saint Francis: Discovering the Geologic Events that Shaped our Earth. The early chapters of the book bring out what I hadn't known, that Dr. Alvarez and his co-workers in Italy provided some of the crucial evidence that led to the Plate Tectonic revolution that occurred just about the time I became a Geology major. The later chapters bring us to a third revolution, which I'll get to anon.
First, I, following our Author, need to introduce a few of the tools of the working geologist. Not the rock-hammer, backpack, or portable gravity meter, but the diagrams that guide our thinking. I'll show these using scans from the book, illustrations seen on pages 22, 53, 214, and 237. I've included the captions, though you may need to click on an image to see a larger version in which the caption is easily readable. The first intellectual tool is the Column.
The column shown here depicts not geology, but archaeology. Sometimes a column records the layers (the stratigraphy) found at one location. Others, such as this one, summarize information found over a larger area. Archaeologists do not draw diagrams such as this, but geologists do. This is geological thinking applied to the archaeology of Rome for the past 3,000 years. The primary column records historical events and evidence, and secondary columns show the water supply and human population. The time-scale, with "today" at the top, it the typical arrangement.
Geological columns show the sequence of rocks plus other indicators such as, for example, abundances of certain fossils, or a geochemical indicator such as salt content. But the rocks are primary, and are shown with various shadings or patterns. A collection of columns that follow a line through an area of interest can be drawn as a Section. A stratigraphic section is a kind of cartoon depicting what would be seen if you sliced through the earth along a line, whether straight or wandering.
This section, using information from outcrops and wells in a dozen or so locations, shows the rock layers, the stratigraphy, along a line that wraps through the southern half of the Capitoline Hill of Rome. The pale gray layer at the top labeled Archeology indicates the combination of buildings and accumulated debris that are the human contribution to the stratigraphy of this hill. Though the section is shown flat, the actual evidence was collected over a wandering line that wraps through the hill in a U shape. From bottom to top it shows the following, beginning with an ancient flood plain:
- A layer of volcanic ash and "tuff" that filled the ancient Tiber valley
- A cap of mudflows from reworking of the ash
- Erosion that washed away material to the side more than the volcanics or mudflow remnants
- An "ignimbrite", or fiery ashfall that covered the whole area to a rather uniform thickness
- Erosion in some areas and lake sediments in others, followed by more erosion
- Human occupation and construction
The heavy black lines show thrust faults, which are one symptom of continental collision. If you push two blocky materials into one another, some stuff will ride up and over the top. But there are a few "normal" faults also shown, which came later and are in motion now. The indicate that things are being stretched instead of compressed. Now, we know that the Alps are still rising; Africa is still pushing Italy into the "belly" of Europe. Where could extension come from?
This map, which represents another great intellectual tool, is a summary of what is going on in and under Italy. During the first revolution in about 1970, the rocks and fossils of central Italy were used by Dr. Alvarez and his colleagues to pin down the timing of many crucial "magnetic reversals" that proved that the continents move. During the second, the scenario I outlined above, with iridium and forams, helped prove the asteroid impact at the K-T boundary. Now, the interesting configuration of the Apennine mountains themselves, the range that contains Dr. Alvarez's beloved "Mountains of St. Francis", is seen to be crossways to the trend of the Alps…with very good reason.
Not to spoil it all, I'll tell part of the story. When the Alps and early Apennines were thrust up, the crust thickened, not just above, but below. In fact, to push up a kilometer of mountains, one must push down, into Earth's viscous mantle, two or three kilometers of material. It is like an ice cube that floats only 20% above the surface, with the rest below. When deep crust is pushed more than forty kilometers or so into the mantle, its minerals are heated and squeezed into new minerals. Given the right composition, these new minerals are denser than the mantle is. In time, a big slab of these denser rocks will peel off the bottom of the crust and begin to sink toward the bottom of the mantle.
The lighter gray areas on this map show where such peeled-off slabs exist at depths as great as 600 km. They started out at a depth of less than 100 km. The sinking of such a slab causes horizontal motions in the mantle above it. This has led, in particular, to the extensional features that surround Rome and extend into Tuscany. These are the cause of the large normal (extensional) faults seen in the deep section above.
These ideas are a hint of what the book has to offer. The author takes us on several journeys, using his own career as one framework, and the geography and geology of Italy as another, to introduce all the great ideas that make up the profession of geology. Were I taking Geology now as a new student, I'd want this book to be the primary text for the course.
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