Friday, July 24, 2015

Quick tour of the urban geology of Rome

I just returned from two, very warm, weeks in Italy. The group wanted to go somewhere with volcanoes and good food, so Italy was a natural choice. While Italian geology is fascinating in its own right, that will have to be saved for a different post. Today's post is a bit of urban geology.  I found that I spent most of my time in Rome looking at the building stone, and took mostly pictures of rocks and stone. The Romans collected the most beautiful stone from their empire and then used to make buildings and show off their wealth. These stones were then scavenged from Roman sites to build the next round of impressive buildings by the Catholic Church. So when you tour buildings in Rome it includes a geologic tour of the whole Mediterranean.
While most visitors expect Rome to made of endless white marble there are a range of stone types from around the far reaches of the Roman Empire.
One of the classic stone types used to impress visitors is this deep red porphyry from Egypt. Porphyry is a crystal-rich igneous rock and is where we get the term porphyritic (for rocks with some large and some small crystals).
This stone was reserved for the most important monuments and people because of its color. 
The stone that impressed me most was the many colored breccias. A breccia is broken rock that has been cemented back together naturally. The color of the clasts and the cement can be vibrant and contrasting and make for some impressive pieces.

Many of the most famous breccias originated in Turkey.


Some of the multicolored stones have a metamorphic origin. Rather than being separate pieces of stone that were stuck back together, this gneiss (pronounced nice) was a solid piece of rock that was put under great pressure and heat to deform it.

In metamorphic rocks some of the elements moved from their previous crystals to form new crystals (like the big pink one) and give it a whole new texture.

Other stones have a complicated history. This one may have been a breccia that was later heated and squished (metamorphosed).

These crystals have dissolution textures on their edges which suggest that they were in contact with hot fluids (or melt) that were different than the one they formed in.
There were also many pieces made out of agate. Agate forms by the growth of minerals (silica) inside some sort of crack or void in a volcanic rock.

There were also lots of different types of granite. Despite what your counter top may be made of, granite is an igneous rock that is made up of large crystals of similar sizes. The composition of granite is high in silica, so you get lots of quartz and feldspar.
The abundance of the pink mineral (potassium feldspar) means this column likely came from Egypt.

There was also lots of diorite. Diorite has a similar texture to granite, but has less silica, so different minerals.
My favorite rocks though, were all local. The cheapest building materials will always be what is nearby, so you will find the base of many of the Roman (and pre-Roman structures) were all made out of local volcanic rock.

Pompeii was made with a combination of basalt bricks and blocks, clay brick, and tuff.

A tuff is a rock made of pyroclastic material (ash, pumice, crystals and bits of broken rock) that has become lithified (i.e. is now a rock). The types of rocks I study are usually tuffs so I couldn't help but take photos of lots of them.

I found tuff at the bottom of the Colosseum, in the Roman Forum, Pompeii, Herculanium, old Greek theaters in Sicily and in modern banks and gelato shops.
Residents of Pompeii also used basalt (lava) to pave their streets. This image is of a cross walk in the streets of Pompeii. The modern cities of Sicily still use basalt blocks, but they use paint for the cross walks.

Any stone can be used to make a wall, and here are some pieces of scoria mixed with some chunks of coral.
The white squares are crystals, the brown clast is scoria and the black is a patina from ~2000 years of people and weather touching this piece of the Colosseum.
The tuffs are mostly the deposits from pyroclastic flows. They form during explosive eruptions where hot gas and debris (the ash, crystals and pumice) move down hill at high speeds. Flows like this are what buried Pompeii. This image is of some of the 79 AD deposit that covered Pompeii. This material is not as lithified (still unconsolidated) so it was possible to excavate the city.

 My visit to Rome was an excellent chance to see layers of human history exposed all in one place (Greek, Roman, Medieval, Modern). It reminded me very much of the layers of rock that we use to reconstruct the history of the Earth. Seeing the walls made out of pyroclastic material in Pompeii and the pyroclastic material that buried it was a stark reminder that we have a lot to learn still about this planet, and the clues are so often right in front of, or below us.

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