Friday, May 23, 2014

2014 Geologic Pilgrimage: Macellum of Pozzuoli

Some time ago I was at a small dinner party and guests were asked to describe spiritual pilgrimages they had made. Each guest began to take turns describing a pilgrimage they had made and planned to make. Fortunately I had a bit of time to think of something and invented past and future pilgrimages on the spot - geologic pilgrimages. I have since had at least one geologic pilgrimage a year. I have posted a few recent ones:   

Lisa and I recently returned from a vacation where I tacked on a geologic pilgrimage to a place west of Napoli that I missed on a previous trip. Like any pilgrimage, half the fun is getting there.

Commuter rail station in centro Napoli

The columns are a short walk from the train station in Pozzuoli. 
Marcellum of Pozzuoli

The Macellum is a former market that was built on the site in approximately 200 AD. It was initially thought to be a temple and is thus still locally called Serapeum (temple of Serapis) despite the site long having been determined not to be a temple.

The geologic aspect is the dark band across the lower third of the columns. The band was formed by borings of marine molluscs Lithophaga.

Band of borings in columns 

The columns had been partially buried in marine sediment until they were excavated in 1750. The borings were identified fairly early as the work of Lithophaga. The fact that a building constructed in 200 AD had subsided below sea-level and stayed partially below sea-level long enough to be partially buried and used as a substrate by molluscs and then rose out of the sea provided a stimulating point of debate in the early days of geology. 

Charles Lyell discussed the columns at length ( as evidence for subsidence and uplift in Principles of Geology (1830) and used a sketch of the columns as a front piece to the book and included a cross section of the column location.

Front piece to Principles of Geology 

Lyell's cross sections of Pozzuoli

Subsequent study and understanding of the area of Pozzuoli further explains the subsidence and uplift. Pozzuoli is located within a super volcano caldera on a scale not to dissimilar to the Yellowstone Caldera. Rather disconcerting that the floor of the caldera has been giving the columns and city a bit of a ride up and down as magma pushes upward under the area. The nearby more famous Vesuvius of Pompeii fame is hazardous enough, but is not the only volcanic threat in the area.

Vesuvius from Napoli

1 comment:

wynneforplants said...

very interesting post, Dan. Love the blog.