By Dan May
On Our Land
Dan May
I enjoy a glass of chilled white wine when grilling and eating outside. One challenge is keeping the wine cool during summer. So I purchased a wine bottle chiller, and being a geologist picked one made of stone, specifically one labeled “green marble” that was imported from Taiwan.
I knew it was actually not carved from marble, but instead is made of serpentinite. This rock forms when bits of oceanic crust are carried deep into the Earth by plate tectonic movement and recrystallized into a metamorphic rock made of soft green and white minerals that is easily carved and takes a lustrous polish. The texture is often swirling as well; hence the ‘serpentine’ name.
Since ancient times serpentinite has been used as a decorative stone. Many statues, sarcophagi, ornamental columns and other monuments from the Greek, Roman, Byzantine and Ottoman empires were carved from this rock. Historically, and commercially today, it is known globally by its Italian name, verde antico, or ‘ancient green.’ The common name in domestic US usage is verde antique.
Quarries harvesting this ornamental stone are common around the eastern Mediterranean, as that sea is the last vestiges of a much larger ancient ocean that has been recycled back into the Earth’s interior through the plate tectonic process called subduction. Most old oceanic crust is resorbed into Earth’s mantle, but some is caught up in a broad zone of complex deformation and can make its way to the surface through uplift and erosion. The island of Taiwan is an uplifted part of a complex subduction margin that extends from Alaska, past Japan and south beyond the Philippines. So it is not surprising that serpentinite is found there as well.
However, it was really not necessary to import this bottle chiller from Taiwan, as verde antique was mined locally at small quarries in West Haven and Milford in the 19th century and could have been crafted locally. Many distinctive ‘green marble’ fireplace mantels in historic homes in the New Haven region were carved from this rock, and one even made its way to adorn the fireplace in the White House East Room. The Milford quarry was located approximately where the Barnes & Noble bookstore is currently sited. The West Haven quarry is now overgrown and is located just north of Maltby Lakes.
The dark green rocks exposed along Route 34 crossing from West Haven into Orange are part of a geologic formation called the Allingtown Metavolcanics. Rare verde antique material can be found within this formation. This metavolcanic unit is part of a larger geologic assemblage known as the Orange–Milford belt, which in turn is a distinctive part of the main bedrock Iapetos Terrane that underlies most of central Connecticut.
About 450 million years ago, these rocks were forming in a geologic setting similar to that of the present-day Philippines. The more than 7,000 islands that make up that nation are part of a volcanic island arc developing above a subduction zone as the Pacific Ocean crust is being recycled back into the mantle below by plate tectonics. All of the rocks of the Iapetos Terrane have oceanic affiliations and became part of North America when an ancient proto-Atlantic Ocean was consumed by subduction and an ancient island arc collided with the continent and was accreted to its edge.
The original features of that island arc were mostly obliterated by deformation and metamorphism during that collisional event, but their chemistry remains. The dark green rocks are rich in iron and magnesium, and the bands of white in serpentinite are often a fibrous mineral called chrysotile. This is one of the most common of the minerals known collectively as asbestos. It is probably best that verde antique is no longer mined, cut or polished locally, as asbestos minerals when ground to a very fine powder and inhaled can yield a number of lung diseases.
Serpentinite forms when water reacts with oceanic crust at high temperature and pressure. That process is called hydrothermal metamorphism, and one byproduct in the formation of serpentinite minerals is hydrogen gas. Hydrogen is the cleanest of all fuels, as it does not produce any carbon dioxide or hazardous organic pollutants in combustion. One current proposal for an alternative energy source is to pump seawater into hot oceanic crust and capture the released hydrogen using technology similar to fracking for natural gas on land. It would also synthetically produce lots of verde antico in place. There’s no easy way, though, to mine that rock from beneath the deep ocean floor.
Dan May is a geologist and professor of environmental science at the University of New Haven. He can be contacted at dmay@newhaven.edu.