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main index © Jeff Matthews entry July 2015
Getting a Rise out of Concrete
- natural concrete & Pozzuoli
Now high and dry, the so-called Temple of Serapis inIn the year 1982 a series of small but violent earthquakes struck the port town of Pozzuoli near Naples. Some of them were strong enough to cause damage to buildings and were so psychologically unnerving that many residents of the town slept outside on the beach. (I was teaching school about one mile from Pozzuoli at the time, so I know about the "unnerving" part.) After all, this was only a short while after the great Irpinia quake of 1980 that had killed 3,000 persons in the mountains near Avellino just 85 km to the east. Were these phenomena connected? Are we due for another "big one"? Those questions remained unanswered while the city administration of Pozzuoli simply decided to evacuate 40,000 residents from the town, building very quickly a New Pozzuoli a short distance away. Some years later, things calmed down, and residents moved back to their beloved homes on the seafront, relatively pacified in the standard explanation that, no, these local movements of the earth had nothing to do with a “big one”—past or present. They were just “bradyseisms”, which just means 'small earthquakes in Greek, but if it's Greek, it's impressive, must be right, and you feel better.
Pozzuoli, until the 1980s, was submerged to a point
about one-third of the way up the tall columns in front.
Bradyseisms are examples of “phreatic activity”—the violent reaction of magma (called 'lava' if it is flowing on the surface) and cold water. That creates steam, the steam creates pressure and tends to move pretty much anything in its path—a steam locomotive, the cover on your pot of boiling water, or the ground itself if that ground happens to be atop a magma chamber that has just come into contact with sea water, either by invading it or being invaded by it. Everyone knows that Pozzuoli is a volcanic area. It is the center of the Campi Flegrei (Fiery Fields), site of the great Campanian Ignimbrite eruption of 40,000 years ago, which produced the Archiflegrean caldera collapse (image, below, right), as well as the site of numerous secondary eruptions in the subsequent millennia. The whole area is still sitting on a chamber—or chambers—of volcanic magma in close proximity to seawater. That is steam waiting to happen.
Is that what happened? Did the upward thrust from “phreactic activity” simply push the surface of the town of Pozzuoli upward by as much as 2 meters/6 feet in some places—in two years?! (The port became so shallow that it could no longer handle large vessels and had to be rebuilt.) Yes, indirectly, that is what happened; that is, without magma and seawater at the root of it all, none of this would have happened. (It remains important to realize that we are talking about what happened to the ground beneath Pozzuoli—it moved upward. This has nothing to with the real sea level in the bay.) But there is more to the story.
An article by Ker Than entitled “Volcanic rocks resembling Roman concrete help solve a mystery, Stanford scientists say” in the July 10, 2015 on-line edition of the Stanford Report (available here) details the work of Tiziana Vanorio, an experimental geophysicist at the Stanford School of Earth, Energy & Environmental Sciences. She was herself a young resident of Pozzuoli in the 1980s, driven from her home by the micro-quakes and then driven into her profession to find out the geophysical secrets of her home town. She speaks of a “...a natural process in the subsurface of Campi Flegrei that is similar to the one that is used to produce concrete [...] Ground swelling occurs at other calderas such as Yellowstone or Long Valley in the United States, but never to this degree, and it usually requires far less uplift to trigger earthquakes at other places...at Campi Flegrei, the micro-earthquakes were delayed by months despite really large ground deformations."
A caldera is a collapsed volcano. The town of Pozzuoli rests at the top of a caldera, an ancient volcanic crater, a crater now filled with various kinds of pyroclastic material—that is, remnants of past eruptions going back to the local Mother of All Calderas, the Campanian Ignimbrite (the area encircled by red lines in the image, right); that gigantic collapse is still 20 km in diameter! (Today, you can stand on one rim and see the other side, quite peaceful now. The entire Campi Flegei is in the middle.) The Pozzuoli caldera was a secondary event from a later, smaller eruption. The nature of the material beneath the town is crucial to understanding why the sides and the bottom of the “bowl” of the Pozzuoli basin (caldera) underlying the modern town and port of Pozzuoli were able to withstand incredible strain from below without suddenly cracking. When it did crack, however, it did so very suddenly and violently.
Oceanographic and geological research beginning in the 1980s revealed data that are the focal point of Vanorio's considerable publications on the subject. For example, the cap rock in the Campi Flegrei is rich in pozzolana, or volcanic ash, from the region. The cap rock is a hard rock layer located near the caldera's surface, it is on top of other pyroclastic material filling the caldera. It is the lid. Campi Flegrei cap rock contains fibrous minerals that are also found in man-made concrete, making it ductile and explaining why the ground beneath Pozzuoli was able to withstand significant bending before breaking and shearing. Those minerals came to be under Pozzuoli and, indeed, all of the Campi Flegrei, through—in very simplified terms—"decarbonation": the action of heat (magma) from below plus the steam pressure (from the seawater) produce several chemical compounds at the cap rock, such things as calcium hydroxide, also known as portlandite or hydrated lime, one of the two key ingredients in man-made concrete, including the famous Roman concrete. (Seneca noted that the "dust at Puteoli [Pozzuoli] becomes stone if it touches water." The cap rock is turned into a very solid barrier of natural concrete, potentially very beneficial because it can withstand great stress. Unfortunately, an excess of sea water injected into the mix produces an excess of carbon dioxide, methane and steam, which then becomes the irresistable force. The immoveable object, the cap rock, moves after all. It shears, cracks and is forced up, pushing the surface and the town with it. And then people have to move.
(My thanks to Jeff Miller for calling this recent research to my attention.)