Carrara Marble: from the sea to Michelangelo's workshop

24/10/2020

Carrara marble was first discovered by the Romans, who found it high in the Apuan Alps of northern Italy. Centuries later, during the Renaissance, exploitation of Carrara marble was at its peak. Michelangelo transformed the valuable stone in the world's most astonishing statues. But the stone has existed for almost two hundred million years. At that time, southern Europe resembled a large puzzle of small islands. Let's look at the geology of Carrara marble.

Author 2020 Kathelijne Bonne.

The word Carrara evokes images of beautiful white sculptures by Michelangelo Buonarroti. Carrara is actually the name of a small town in northern Italy. It means quarry. The town rose to fame thanks to the marble. The word marble itself comes from the Greek word marmaros, shiny stone. Quarrying started during the Roman Empire, and exploitation continues today. You can order a slab of Carrara marble anytime. 

The white marble quarries can easily be spotted from an airplane, and even from space. The quarries have considerably changed, or mutilated according to some, the landscape. Hundreds of quarries lay scattered across the area. In them many varieties of marble are produced. Some varieties are exhausted or have become priceless.

Why is this rock so popular? Marble not only shines, as its name suggests, but it allows the light to penetrate a few millimetres below the surface, creating a soft glow, not unlike sunlight shining through a human hand.

This article leaves the cultural aspects aside and focusses on the natural history of this monumental rock type.

Window in time

The Apuan Alps, where Carrara marble is mined, is a mountain range located at the northern end of the Apennines, Italy's backbone. The Apuan Alps are a 'classic' example of Italian geology, because many different types of rock lay within relatively short distances from each other. Moreover, the ages of these rocks span more than 500 million years, from Cambrian to Quaternary times. During this long timespan, our planet changed from a world with single-celled and other organisms in the sea, to a green world full of plants and animals. Such an uninterrupted series of rocks is quite rare, it is difficult to find elsewhere in Italy. That is why many Italian earth science students go on excursions to study the Carrara geology.

Why are there so many different rocks in this location? Because the rock layers are arranged in a very large arch, vertically seen. At the core of this arch are the oldest rocks. As you move further away from the core, you will see increasingly younger rocks. You literary look through a window in time. Geologists refer to such a structure as a tectonic window. But what is so interesting about it? Each rock bears witness to a particular stage in geological history. This makes it possible to reconstruct the entire history of the Apennines and of the Italian peninsula. It also shows the evolution of life and climate during a prolonged period.

A sea teeming with life

Before marble can be found on the earth's surface, as in the Apuan Alps, a lot has already happened. The origin of marble begins in the sea. Imagine a sea teeming with life, large and small creatures and plants, plankton, and single-celled organisms. Many of them have a skeleton or shell that consists of lime (calcite), for example, sea lilies, starfish, ammonites, bivalves, snail shells and foraminifers (a single cell organism). Coral reefs, built by organisms, also consist of lime. All these life forms are part of an ecosystem. When the organisms die, they sink to the bottom of the sea or break down, at least if they are not eaten by a predator. Layer after layer, dead organisms accumulate. When they die, the soft tissue decays, but the lime will stand the test of time.

Crinoids, sea lilies of Jurassic times. (Kevin Walsh / CC BY 2.0)
Crinoids, sea lilies of Jurassic times. (Kevin Walsh / CC BY 2.0)

White, translucent crystals

Over time, the layers of lime 'petrify' to limestone. The limestone does not necessarily remain under the seabed forever. The tectonic plates are always on the move. After millions of years, the seabed can be thrusted up and become part of the land. Especially if it is located in a zone of mountain building. Mountains form where two tectonic plates collide. Then the rocks are not only broken and folded, but also exposed to enormous pressure and temperature. And here we come back to the marble.

The limestone succumbs under these stresses and dissolves, becoming liquid. The skeletons are deformed beyond recognition and the limestone crystallizes into new, large crystals. These crystals form a compact, perfectly-fitted pattern of carbonate crystals. We now have marble, a metamorphic rock, which is formed under high pressure and temperature. Each crystal is translucent, hence the special glow of marble.

Small tectonic plates: Apulia and Adria

The Carrara marble organisms lived and died in the Tethys Ocean, during the Jurassic Period. Then, Europe was a puzzle of small islands in the big Tethys Ocean. Some of these islands would become part of southern Europe. The small tectonic plates of Adria and Apulia moved slowly towards each other and towards the Eurasian continent until they collided.

As a result, the Italian peninsula rose from the sea. The collision area had the shape of a long boot. At the border with Europe, the Alps emerged.

Cloudy, veined and speckled

The marble of Carrara is highly variable and occurs in different colours, shades, and patterns. Some are more valuable than others. This great variety is a result of impurities in the rock, i.e. the presence of minerals such as pyrite, iron oxide (rust) and muscovite. Traders worldwide lure customers with the buzzing names of precious Carrara varieties. Carrara marble can show stains, streaks, speckles, clouds and veins. Veins are strings of another material, which disfigures the stone, although it can also be appreciated, especially if the veins make dramatic patterns. The mineral pyrite, or iron sulfide, is responsible for the dark veins. Least glamorous is the marmo ordinario. The marmo venato, venatino and venato forte exhibit many veins. The marmo nuvolato is cloudy, and the marmo arabescato and the opulent marmo calacata are rather dramatic. Macchia d'oro refers to golden spots.

Carrara quarry. (Julian Nyča / CC BY-SA 3.0)
Carrara quarry. (Julian Nyča / CC BY-SA 3.0)

Michelangelo's favourite marble

Then we come to the most coveted marble: the very pure marmo statuario. Pure means that it consists almost entirely of calcite crystals, with few impurities. The colour is ivory to pearl or milky white, and can sometimes tending slightly towards pale yellow to beige. The yellowish colour is due to the presence of the potassium-bearing mineral muscovite, which is very uniformly distributed throughout the stone, producing a soft beige glow.

This was Michelangelo's favourite marble. His most admired statue, David, was made from a block of marmo statuario.

The Carrara marble is predominantly white due to its purity, making it an unforgiving medium. Only the greatest artists can accurately mold it into a human body in all its detail. 

In general, due to the twists and turns of nature, marble worldwide appears in all imaginable colours, from red to pink, violet, black, brown, green, yellow, and more.

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Sources:

Stampfli and Hochard, 2009, Plate tectonics of the Alpine realm. In: Murphy, J. B., Keppie, J. D. & Hynes, A. J. (eds), Ancient Orogens and Modern Analogues. Geological Society, London, Special Publications, 327, 89-111. DOI: 10.1144/SP327.6 0305-8719/09/$15.00 # The Geological Society of London 2009.

Meccheri et al., 2007, The Carrara Marbles (Alpi Apuane, Italy): a geological and economical updated review. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 158(4):719-736, DOI: 10.1127/1860-1804/2007/0158-0719.

Stampfli, 2005, Plate tectonic of the Apulia-Adria microcontinents. Chapter: CROP PROJECT deep seismic exploration of the Mediterranean and Italy, Publisher: Elsevier, Editors: I. R. Finetti.

Alpi Apuane UNESCO Global Geopark (Italy).