February 2007

Economic Geology

The Basalt Controversy I (Part 15)

By R. J. Cathro

The Vesuvius volcano; the higher cone formed within a large caldera of the MonteSomma volcano | Photo courtesy of the Global Volcanism Project


"In 1700, all major Western scholars believed that the earth had been created just a few thousand years ago. By 1800, nearly all scientists accepted a great antiquity of unknown duration, and a sequential history expressed in strata of the earth’s crust. … By 1820, detailed geological maps had been published for parts of England and France … and the subsequent resolution of historical sequences by geological mapping must be ranked among the sweetest triumphs of human understanding" (Gould, 2000).

Information on the history of volcanology is surprisingly hard to find. The following general references were particularly helpful in the preparation of this chapter and are strongly recommended: Ashworth (2004), Sigurdsson (1999), Simkin and Siebert (1994), and the Smithsonian Institution ‘s Global Volcanism Program.

Repchek (2003) has pointed out that the term ‘scientist’ was not coined until the 1830s, and that individuals who studied science before then called themselves ‘natural philosophers.’ Starting as early as 1644, René Descartes, Nils Steenson (Nicolaus Steno), and others chose geological topics such as minerals, fossils, volcanoes, and sedimentary layering for study. However, the word geologist did not appear until the 1780s (Vaccari, 1998). In this series, the first scientists who studied the earth are called geologists.

Between about 1750 and 1820, geologists were divided into three camps. Those who supported the theory that all rocks were deposited or precipitated from a global ocean were known as Neptunists, after the Roman god of the sea. Those who believed that volcanic rocks were produced by fire were called Vulcanists, after the Roman god of fire. The third group, who believed that igneous rocks were produced by heat related to deep burial of sediments, were called Plutonists, after the Roman god of the lower world. Resolving the differences between these theories was only achieved through pioneering field studies and mapping. Unlikely as it seems now, basalt was the key to unlocking the puzzle because it revealed the link between magma, igneous rocks, and metals. Resolving the basalt controversy was an important step in the history of economic geology.

The oldest recorded use of the word basalt is credited to Pliny the Elder (23–79 AD), who died of asphyxiation while observing the eruption of Vesuvius. There is uncertainty about the etymology of the word and whether or not the rock that Pliny referred to was even basalt. He believed that the name was derived from an African word, perhaps bauhan (slate) from Egypt, where basalt is common. It is now thought to be derived from a Latin word, basaltes, which is a corrupted form of either an older Latin word basanites (after Basan, which is either a region in Palestine or a town in Syria), or of the Greek word basanos, meaning a touchstone (a stone on which gold leaves a yellow streak or, alternatively, a piece of slate used to test gold; Eyma, 2005; www.etymonline.com). Others say the word comes from Ethiopia.

As pointed out in Part 11 (CIM Magazine, June/July 2006, p. 98), study of the origin of metallic lodes, the naturally forming concentrations near the earth’s surface, was delayed for more than 200 years after Georges Agricola, Johann Joachim Becher, and others suggested that there was a link between mineral deposits, springs, vapours, and heat. None of their successors knew the source of the heat, and the conventional view was that it resulted from the subterranean burning of combustible matter such as coal. Volcanic activity was not suspected. One of the reasons for such slow progress was geographical, since the leading mineralogists and mining engineers of the time lived in the Central European Mineral Belt, where there were no active volcanoes, and it appears that none of them visited or studied the classic volcanoes in Italy. Timing was another factor, since Vesuvius didn’t have a strong eruption between 79 and 1631 AD.

Half of the 52 volcanoes in the Mediterranean and western Asia region have erupted since humans began keeping records, and 61 separate eruptions had been documented in the region by 500 AD (Simpkins and Siebert, 1994). This region was not only the ‘Cradle of Western Civilization,’ it was also the cradle of volcanology. The best-documented and most accessible volcanoes, all located in Italy, display wide variety, including the classic domes of Vesuvius and Etna, as well as Campi Flegrei (Phlegraean Fields). They are classified as a complex volcano, a shield volcano, and a collapsed caldera, respectively. In hindsight, it is hard to understand how the link between volcanic rocks, magma, heat, and metals was recognized so slowly.

Unlike the well known history of Vesuvius and the burial of Pompeii, or the ongoing eruptive activity at Etna, Campi Flegrei is relatively unknown. About 12 to 14 kilometres in diameter and 458 metres high, it is situated 25 kilometres west of Vesuvius and 15 kilometres west-northwest of Naples, on the edge of the Campanian Plain. The caldera formed about 35,000 years ago and has not erupted since 1538, when a new cinder cone appeared practically overnight. It is noted for the Solfatara fumarolic area, widespread hydrothermal activity, and pyroclastic rocks. The top of the magma chamber is only four to five kilometres deep. The caldera has a long history of slow uplift and subsidence, and parts of the Roman ruins are now below sea level.

In addition to the geographic factor, another complication was that basalt only occurs in Central Europe as sills interbedded with sedimentary rocks and was mistakenly identified as part of the sedimentary sequence. That was not a difficult error to make since fine-grained basalt, arkose, or greywacke can closely resemble one another in hand specimens. Columnar jointing was likened to quartz crystals.

However, the principal reason why local basalt was misinterpreted was that scientific ability was overpowered by religious conviction. One of the principal tenets of the Book of Genesis was belief in the Great Deluge (Noah’s Flood), in which all rocks were interpreted as being deposited or precipitated from the sea as sediments in order of density, granite first. Gould (2000) called the philosophical conflict between scientific theory and religion ‘theological dogmatism’ or ‘ecumenical prejudice.’ In the case of evolution, this conflict is still continuing.

Credit for recognizing that basalt is a product of a volcanic eruption belongs to two remarkable French geologists. The basalt controversy began in 1751 (Toulmin and Goodfield, 1965; Sigurdsson, 1999) when Jean-Étienne Guettard (1715–1786) visited the Auvergne district of France on his return from a trip to southern Italy (other authors give the year as 1752 and some question if he had visited Italy). The Auvergne district is part of the Central Massif, the only mountainous region that occurs entirely within France. The other parts of the massif are called Cévennes, Velay, and Viverais. It is 93,000 km2 in area and the source of many of the most famous French rivers. The highest peak in the massif reaches an elevation of 1886 metres. The present Auvergne volcanoes began to form in the Miocene (13 to 5.5 Ma) although the youngest and best-preserved cones have been dated at between 33,000 and 8,000 BC. They are of the Strombolian-type except for the Puy de Dôme (puy=volcanic dome), which is a more viscous, spasmodic, and explosive Pelean-type volcano. Weakly radioactive hot springs in the district, such as Vichy, have become renowned as spa towns. The Auvergne is now a regional nature park known as the Parc de Volcans (Castle, 1992).

Guettard is variously described as a geologist, mineralogist, botanist, and naturalist, a member of the Faculty of Medicine of Paris, keeper of the natural history collection of the Duc d’Orleans, and a member of the French Academy of Sciences. When Orleans died, Guettard received a pension that made him financially independent and allowed him to continue his research into botany and geology. He became a prolific author, publishing more than 200 papers and six books.

During his visit to the Auvergne, he noticed some unusual black rock being used as mileposts along the roads, as paving stones, and in the walls of buildings. He traced the source to a quarry in a large lava field that overlooked the village of Volvic, located about 15 kilometres north of Clermont-Ferrand. He is said to have exclaimed “Volvic, volcani vicus” (volcanic village), which implies that the village name dated from Roman times. The lava was derived from the Puy de la Nugère (Sigurdsson, 1999). Volvic stone has been mined since the Middle Ages and used in the construction of major buildings such as the Gothic Cathedral in Clermont-Ferrand, built in the 13th century (Toulmin and Goodfield, 1965).

During his visit to Italy, Guettard had become familiar with volcanic rocks and he immediately suspected that he was looking at a type of lava. The next year (1752), he presented his ideas to the Royal Academy of Sciences in a paper titled On Some Volcanic Mountains in France. He still adhered to the conventional view that attributed volcanic eruptions to the combustion of flammable materials such as coal, and failed to see the link between columnar basalt and volcanic activity because he hadn’t seen any columns at Vesuvius (Sigurdsson, 1999). Guettard published another paper with those conclusions in 1770, titled On the Basalt of the Auvergne and Moderns, in which he also argued that basalt was precipitated from a fluid (Dean, 1998). That has detracted from his reputation as a great geologist who linked active volcanoes with the eroded craters in Auvergne. Ironically, he can be considered a father of both Vulcanism and Neptunism (Sigurdsson, 1999).

The next important geologist to study the Auvergne was Nicolas Desmarest (1725–1815), a senior official in the French government. He made four visits between 1763 and 1769 during which he mapped the area between Volvic and Mont d’Or in detail. In addition to showing that the lava had flowed downhill from the volcanic cones, he concluded that the hexagonal cracks surrounding the columns were the result of cooling, and he demonstrated the volcanic origin of the basalt. He was familiar with columnar basalt from a visit to the Giant’s Causeway in Northern Ireland. His findings were delivered to the Academy in 1771 and published as an important three-part paper with a geological map, between 1774 and 1777. In spite of these conclusions, Desmarest still believed that volcanoes resulted from local melting of primitive basalt of aqueous origin. He was a Neptunist at heart (Sigurdsson, 1999).

Desmarest was one of the pioneers of geological mapping. He presented a preliminary ‘mineralogical map’ of France to the Academy in 1746 and subsequently published similar maps of other regions, including parts of North America (Lessing, 1998). As a result, the Secretary of State in charge of mining commissioned him, in 1766, to conduct a geological survey and publish maps for all of France (230 in all). The first 16 were published in 1770 but only 45 were completed before the project was sidelined by the French Revolution. These were not geological maps in the modern sense since they were only intended to show the positions of mineral deposits and rock types (Gould, 2000). To view part of his map of the Auvergne, as well as sketches of columnar basalt there and in the Viverais and Volvic districts, see Ashworth (2004, p. 30-34).

In 1766, Desmarest enlisted the help of a young assistant, Antoine-Laurent Lavoisier (1743–1794), who is usually remembered as a great chemist who died on the guillotine during the French Revolution because he had once worked as a tax collector. Gould (2000) called him “the greatest chemist in human history” and pointed out that Lavoisier published a major geological paper with maps in 1789. His maps included a tabular key to rock symbols and a vertical section on the margins of the page, a more modern and useful type than anything produced before. Gould judged these maps to be “a stunning and remarkable work” that marked “the birth of geological mapping.” It should be noted that the geological maps published in France by Guettard and Lavoisier between 1746 and 1789 preceded those produced in 1815 by William Smith, in England, by many years (Winchester, 2001).


References

Ashworth, W.B. Jr. (2004). Vulcan’s Forge and Fingal’s Cave: Volcanoes, basalt, and the discovery of geological time. An exhibition of rare books from the collections of the Linda Hall Library of Science, Engineering and Technology, Kansas City, Missouri, 95 p. available at
www.lindahall.org/events_exhib/exhibit/exhibits/vulcan/about.shtml

Castle, A. (1992). Walks in volcano country: The Auvergne & Velay, France (pp. 9-23). Milnthorpe, Cumbria: Cicerone Press.

Dean, D.R. (1998). Plutonists, Neptunists, Vulcanists. In G.A. Good (Ed.), Sciences of the earth: An encyclopedia of events, people, and phenomena, 2 vols. (p. 692). New York: Garland Publishing, Inc.

Eyma, A. (2005). Egyptian loan-words in English Version 16.0 available at
www.geocities.com/TimesSquare/Alley/4482/AEloans.html

Gould, S.J. (2000). The Proof of Lavoisier’s Plates. In Penultimate reflections in natural history (pp. 91-114). New York: Harmony Books.

Lessing, P. (1998). Geological Maps. In G.A. Good (Ed.), Sciences of the earth: An encyclopedia of events, people, and phenomena, 2 vols. (pp. 301-305). New York: Garland Publishing, Inc.

Repchek, J. (2003). The man who found time: James Hutton and the discovery of the earth’s antiquity (p. 14). Perseus Publishing.

Sigurdsson, H. (1999). Melting the earth: The history of ideas on volcanic eruptions (pp. 131-139). New York: Oxford University Press.

Simkin, T. & Siebert, L. (1994). Volcanoes of the world (2nd ed.). Tucson, Arizona: Geoscience Press, Inc. in association with the Smithsonian Institution’s Global Volcanism Program, available at www.volcano.si.edu/world.

Toulmin, S. & Goodfield, J. (1965). The discovery of time (1977 ed., pp. 150-152). Chicago: University of Chicago.

Vaccari, E. (1998). Geology: Disciplinary history. In G.A. Good (Ed.), Sciences of the earth: An encyclopedia of events, people, and phenomena, 2 vols. (p. 329). New York: Garland Publishing, Inc.

Winchester, S. (2001). The Map that changed the world: William Smith and the birth of modern geology. New York: Harper Collins.

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