Architectural Geology of Pisa, Italy - Part 1: The Leaning Tower (begun 1173 AD; completed 1372 AD)

In my current neck of the woods, this building may not be as famous as the Leaning Tower of Niles, Illinois, its half-sized, latter-day replica largely made of "cast stone"—i.e., concrete-based faux rock. Nonetheless, I'd guess that the original Tower is one of the most photographed edifices in history. I dare to add my vintage version of it only because its architectural geology is just too wonderful to ignore.

While the whole Tower/Cathedral/Baptistery/Camposanto complex features a number of geologically fascinating rock types, the main building stone is the locally produced Monte Pisano Marble. That's mostly what is visible here.

The Monte Pisano, which was taken from the same geologic unit (but not the same locale) as the more famous Carrara Marble, is derived from a Upper Triassic to Lower Jurassic limestone protolith. It was metamorphosed into its present form in the Tertiary period.

For the Tower itself, modern geochemical analysis has determined that its Monte Pisano Marble was quarried specifically in the Casa Roncucci quarry, due south of town of San Giuliano Terme, and about 5.5 km (3.5 mi) northeast of the Tower. However, the same stone used in the Cathedral (only its apse section is visible here) came from another site a bit farther north.

Among the other stone types present are three cited in the scientific literature:

- A brecciated form of the Lower Jurassic Massiccio Limestone, also taken from the Monte Pisano area. Apparently it was used for some of the ashlar and column capitals.

- The Monte Capanne Granodiorite, quarried on the island of Elba (best known as the site of Napoleon's first exile, as in the palindromic "Able Was I Ere I Saw Elba"). This igneous intrusive rock reportedly was used for some of the column shafts at the Tower's base. It dates to the Upper Miocene, and is thought to have formed in a pluton during crustal extension following the compressional phase that created the Apennine Mountains.

- The Giglio Island Granite, from another isle off the coast of Tuscany. It, too, is supposedly found in some of the Tower's ground-level columns. Lower Pliocene in age, its origin was much the same as the Monte Capanne, though it's some 2 Ma younger.

Of course, the Tower's other geologic story lies in the leaning itself. This tall and narrow structure, like the incomparable Cathedral and Baptistery adjoining, is anchored in the oozy, shifting sediments of the Arno River floodplain. Supported by a shallow foundation only, the Tower began to lean long before it was completed.

In the years after my photo was taken, the Tower's tilt increased to about 5.5 degrees from vertical—even more than the vertigo-inducing angle I encountered when I climbed its slippery marble steps to the top. By the 1990s, the Tower had to be closed off to visitors.

But finally it was stabilized by clever geo-engineering. Some of the substrate under the lifted side was removed, which actually reversed the tilt to just about 4 degrees, and the sediments were also dewatered to lessen their shiftiness. The Tower was deemed safe again, and has been reopened for almost twenty years now.

Interestingly, the skyscraper architects of Chicago and Milwaukee, USA have had to grapple with the same sort of squashy sediments. Often, their solution has been to go deep, with caissons or piles driven all the way down to stiff hardpan or bedrock. But what sort of foundation the Leaning Tower of Niles has, I'm not sure. I stand ready to be enlightened.

To see the other photos and descriptions in this series, visit my Architectural Geology: Italy album.

Taken in: Italy / Tuscany / Pisa / Pisa (show map)

Taken on: July 23, 1977

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Camera: MAKER NAME 96660, f/1.8, 1/130 sec, 2.8mm, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
Aperture:	f/1.8
Exposure Time:	0.008 sec (1/130)
Focal Length:	2.8mm
ISO:	50
Exposure Bias:	0 EV
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
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