Stories in Stone (36 page)

Fire often plagued ancient Rome.
When the Gauls conquered it in 390 BCE, they burned Rome.
Major and minor conflagrations
hit once or twice a decade for the last two hundred years of the millennium.
To combat fire at the Forum of Augustus, builders
used an olive gray tuff called
Lapis Gabinus
, for a one-hundred-foot-high boundary wall.
The wall still stands although Rome burned in 64 CE (Nero’s great inferno), followed
by five large fires over the next two centuries.

Recent geological work by Jackson and her colleagues has confirmed that Vitruvius made astute observations of the strengths
and weaknesses of Rome’s local building stone.
She compared the durability of tuff and travertine in dry, humid (foggy), and
wet (rainy) conditions, as well as the two stones’ ability to withstand carrying a compressive load.
The tuffs acted more
like sponges and absorbed varying amounts of water, some to the point of crumbling, whereas travertine was more or less impermeable
and retained its strength in wet conditions.
Because of these qualities, travertine played a central role to Roman builders
both as a structural reinforcement to tuff construction and as a decorative and protective facing on tuff walls.

Nowhere are travertine’s attributes better seen than in arguably the most impressive building of ancient Rome, the Amphitheater
of Flavium, or the Colosseum, as it came to be known in medieval times.
Started in 70 CE by Vespasian and finished a decade
later by his son Titus, the Colosseum contains 3.5 million cubic feet of travertine, or eighteen times the amount of Salem
Limestone that covers the Empire State Building.
Archeologist Janet DeLaine has estimated that in order for that much travertine
to be transported the twenty miles by oxcart along the Via Tiburtina from Tivoli to Rome, one cart carrying half a ton of
rock had to leave every four minutes.
She concluded, “I leave it to the reader to work out the implications for keeping the
roads clean.”
⁴

Despite offering such potentially pungent numbers, DeLaine suggested that most travertine reached Rome by the Anio River (now
known as the Aniene).
Barges would have been able to carry large quantities of stone down the calm, meandering river from
the quarries at Tivoli without the need to acquire, feed, and take care of hundreds of oxen.

Few, if any, barges would be able to descend the Aniene now.
For a recent event focusing on river pollution, an Italian environmental
organization highlighted a green space on the Aniene about ten miles from Rome.
The group had brought in a backhoe to remove
trash, mostly plastic debris, but also refrigerators, wood paneling, and metal file cabinets, that covered the Aniene in a
fifty-foot-long raft from bank to bank.
Not that the modern Via Tiburtina is much more appealing; the road runs for miles
past car dealerships, fast-food restaurants, and stolid apartment buildings.

Modern travelers can speed from the Tivoli quarries to the Colosseum via bus and subway, which drops you on a low rise with
a fine view of the long and more intact north side of the amphitheater.
From the vantage point on the hill, you can see how
the massive stones play a central role in providing structural support.
Three levels of travertine arches rise to support
a final, unarched story of squared blocks of travertine.
There are no false walls here; the building’s exterior skeleton stands
because of the interplay of stone and arch.
You can also observe how traffic has blackened the stone with pollution, a problem
found in travertine-clad structures around the world.

To reach the travertine, descend the hill and cross Via dei Fori Imperiali.
Once across you can touch the stone.
The walls
feel pitted and eroded from centuries of weathering; the steps and walkways feel smooth from millions of feet.
For nearly
two thousand years, this awesome building has dominated and graced Rome.

A clockwise circumnavigation takes you around to the entrance and a quick security check.
Travertine building blocks make
up most of the outer corridors.
In the open amphitheater,however, travertine gives way to brick and tuff, particularly in
the walls of the substructure and the
cavea
, or seating area.
The brick and tuff look refined and well chosen, but the travertine arches of the outer walls are the glory
of the Colosseum.
Masons cut each block on site so that keystones, voussoirs, and imposts fit together as an intricate puzzle,
where each piece balances another.
In many blocks, you can still see the holes where builders used forceps and levers to lift
the blocks into position.
(Other holes in blocks indicate where thieves excavated iron dowels and clamps that once held blocks
together.)
⁵
On many arches the builders placed the blocks with their bedding planes aligned with the curve of the arch so that the beds
look like rays shooting outward.
Each arch is a small-scale illustration of the wonderful marriage of Roman geology, engineering,
and art.

Not everyone has appreciated the Colosseum.
Starting in the 1300s and continuing for nearly five hundred years, the amphitheater
served Rome as a quarry.
One Giovanni Foglia of Como received permission in 1542 to remove 2,522 cartloads of travertine.
Particularly egregious was the stealing of stone by the great families of Rome, who used the travertine for the Barbarini,
Venezia, and Farnese palaces, each of which still stands.
In 1540 Cardinal Farnese’s uncle, Pope Paul III, gave his nephew
permission to ransack the Colosseum for twelve hours.
Farnese brought four thousand men to help him.
Although no direct evidence
exists, one who most likely benefited from the cardinal’s travertine transgression was Michelangelo, who took over the design
of the Farnese palace in 1546 and completed the building in travertine.

Arches at the Colosseum, Rome.

No other building may be better known in Rome for its travertine than the Colosseum, but you cannot travel more than a few
minutes without stubbing your toe on the stone.
You can walk up the travertine Spanish Steps, stand with hundreds of tourists
at the travertine Trevi Fountain, wait to cross streets on travertine sidewalks and curbs, admire Bernini’s oval, travertine
Sant' Andrea al Quirinale, and drink water out of a splendid travertine fountain carved in the shape of stacked books.
Travertine
is the most common building stone in Rome.
After two thousand years of use, travertine has more than lived up to Vitruvius’s
observations of its durability and suitability for those who want to avoid mistakes when building.

Robert Folk’s fellow geologists describe him as controversial, eccentric, a publicity hound, and a hyperactive pseudo-elf,
but they also agree he is a brilliant observer and a wonderful teacher, and that he draws original scientific connections.
He also loves Italy—the food, the scenery, the arts, the music, the architecture.
In 1979 he was able to combine his interests,
energy, and passions in what would become a seminal study of travertine.

“Well, if you really want to know, I was working in Italy and looking for another excuse to stay,” said Folk.
⁶
He was in Rome in the piazza of St.
Peter’s looking at Bernini’s colonnade of Doric columns when he realized he couldn’t
place the rock.
He knew it was travertine, but the stone didn’t fit into the best-known classification system for carbonate
rocks, which was odd because Folk developed that classification in a landmark 1959 paper.
“This gave me an excuse to go look
at travertine,” he said.

To study the travertine, Folk teamed up with his former student Henry Chafetz, now a professor at the University of Houston.
They traveled to Bagni di Tivoli (the Baths of Tivoli), the most famous travertine deposits in Italy and the source of Bernini’s
columns, the Colosseum, and most of Rome’s travertine.
The open pits are concentrated in a flat plain about three miles east
of Tivoli and just north of the Aniene River.
Cavatori
during Roman times removed rock from an area next to the river still known as the Barco, a corruption of
barga
, Latin for barge, in reference to how travertine traveled to Rome.

Between fifty and sixty quarries operate in Tivoli at present.
At the one owned by the Mariotti family, who have been quarrying
the site for four generations,
cavatori
cut rock in a manner similar to what occurs in Indiana, using diamond wire saws, gargantuan front-end loaders and trucks,
and gallons and gallons of water.

Travertine deposition at Tivoli occurred and still occurs in lakes, ponds, and swamps on a flat, volcanic plain pierced by
thermal springs.
Stone in the primary quarries is younger than two hundred thousand years with a concentration of deposition
around eighty thousand years ago.
Geothermal activity heats water deep underground and it rises to the surface via faults.
Along the way the water passes through and dissolves Mesozoic-age, calcite-rich limestone and transports the calcite in solution
to the surface.

Similar to what happens when you open a soda bottle, the pressure drops when the heated water emerges at the surface, and
the carbon dioxide escapes from the fluid into the air.
The release in pressure increases the saturation state of the water
with respect to the carbonate minerals aragonite and calcite, and after the water degasses, the minerals settle out of the
water column.
(To simplify the story, I will just refer to calcite.) Depending upon the environment, vent mouth or quiet pond,
the calcite accumulates as a fine-grained mud, gloms onto other solid particles in the water, or fills voids.

Folk said of hot springs, “They can be very fickle.
Some days they have an enormous flow when there’s been a lot of rain and
you get a very rapid precipitation rate of calcite.
On drier days, you get a very slow precipitation rate, perhaps on the
order of a tenth of a millimeter a day.” He has recorded calcite deposition of one-sixth inch per day, or a million times
faster than calcite accumulates in the ocean.
Carbonate accumulation can occur so rapidly in travertine that Folk once described
the ghastly scene of algae and mosses “being calcified while still alive.” I don’t recommend letting children see this shocking
geologic process.

Hydrogen sulfide also escapes into the air above the hot springs, and in such great quantities that Folk and Chafetz saw many
dead birds littering the pools at Tivoli.
Chafetz remembers hearing that three boys died breathing the toxic fumes, a year
or so after the geologists finished their study.
The fumes smell like rotten eggs and are a classic sign of hot springs, such
as at Yellowstone National Park, where travertine forms the falls at Mammoth Hot Springs.

Geologists refer to this degassing of carbon dioxide and precipitation of carbonates as an inorganic, or abiotic, process.
For the last hundred-plus years, geologists accepted that this abiotic process fully explained how most travertine formed.
But then in 1984 Folk and Chafetz published their study of Tivoli.
Titled
Travertines: Depositional Morphology
and the Bacterially Constructed Constituents
, it proposed the radical idea that bacteria bore primary responsibility for travertine deposition.
⁷

They were not the first to recognize the connection between bacteria and calcite deposition.
As far back as 1914, British
marine biologist George Harold Drew proposed that bacteria helped generate calcite in the Bahamas, but not until Folk and
Chafetz did anyone present such a thorough study linking significant carbonate accumulations, identifiable structures, and
bacteria.
“We made this dumb luck discovery.
It was exciting,” said Folk.
“We didn’t go there with any idea of finding bacteria
at all.
A lot of scientific studies are just plain dumb luck.” Luck or not, the paper has become the most cited scientific
study ever written about travertine.

Bacteria influence travertine formation by altering the microenvironment around themselves.
First, they excrete carbon dioxide
during photosynthesis, which provides carbon and oxygen, the main building blocks of calcite.
Second, bacteria can process
inorganic nitrogen and generate ammonia gas, which raises the water pH, makes the water supersaturated in calcite, and generates
calcite deposition.
The microbes must achieve a balancing act because if they induce mineral precipitation too quickly, they
get entombed, “to the dismay of the bacteria,” as a sentimental pair of later geologists wrote.