Sunday, March 16, 2008

Go and See

Today, it seems an evident fact that volcanoes are formed from molten rock pouring out from deep inside the earth. We know from documented examples that volcanoes form little by little, starting as small upwelling mounds and steadily growing upwards into a cone shape by accretion of successive lava flows. Most of us have seen film of red hot lava flows on the television and the explosive dance of lava and ash on top of volcanoes. It follows from these observations that rock inside the earth is molten, and this is now a known fact: that descending inside the earth, temperatures increase gradually (around 200 deg C at 6000m for example) until, at around 35 km depth, rock begins to become fluid and we reach the base of the Earth's outer crust.

This first hand view of the action of volcanoes was something that most 18th Century geologists lacked. Consequently, their explanations of explosive volcanic action leant heavily on known mechanisms of explosion and involved the action of 'wind' as a medium for blowing material out of the earth and the mixing of petroleum or coal and sulphur inside the earth as an incendiary and explosive mixture. The idea of "wind pent up within the earth" went all the way back to the writings of Aristotle, Strabo and Pliny the Elder who lost his own life investigating Pompeii's eruption of A.D. 79 which overwhelmed Herculaneum and Pompeii.

According to Geikie's 'The Founders of Geology', the first significant insights into volcanic geology came from Jean Etienne Guettard (see 2 posts ago), who, during his extensive travels, first came to realise that the hills of the Auvergne are in fact a series of extinct volcanoes. His ideas concerning the origin of volcanos were the accepted ones of his day. He wrote: "For the production of volcanoes, it is enough that there should be within these mountains substances that can burn, such as petroleum, coal or bitumen, and that from some causes these materials should take fire. Thereupon the mountains will become a furnace, and the fire, raging furiously within, will be able to melt and vitrify the most intractable substances." In recognising the presence of volcanoes as an important generator of rock masses, Guettard became the originator of the Vulcanist party in the famous debate (Geikie calls it 'warfare') with the Neptunist party at the end of the 18th Century.

The Neptunists were a school of geologists headed by a German geologist called Abraham Gottlob Werner who believed that there was a distinct sequence of rocks that could be recognised everywhere in the world which could be related to a global inundation of the sea across the earth, followed by a slow withdrawal of the oceans to their present positions. They therefore believed that many rocks were originally created in the sea, including basalt which we now know is a fine-grained crystalline volcanic rock expelled from a number of volcano types.

And the originator of the idea that basalt could be formed by crystallisation in an aqueous fluid such as sea water? It was the same Jean Etienne Guettard who had shown that volcanoes once spewed lava across the Auvergne. Eighteen years after his ground-breaking memoir on the volcanoes of the Auverge, he published a paper "On the Basalt of the Ancients and the Moderns". Despite the fact that various writers had claimed a volcanic origin for basalt, and that he had found plentiful basalt in the Auvergne, Guettard insisted that: "basalt is a species of vitrifiable rock, formed by crystallisation in an aqueous fluid, and there is no reason to regard it as due to igneous fusion."

The proof that basalt really was a volcanic product came from the subsequent mapping of the Auverge by French geologist Nicholas Demarest (1725-1815) whose tireless mapping of the Auverge produced incontravertible proof of the volcanic origin of this region, the various historic stages in the development of the volcanos and the proof of the volcanic origin of basalt. Although the debate between Neptunists and Plutonists as to the origin of basalt raged throughout Demarest's life, he took no part in it, making only allusion to the fact that the answer was clear enough "to those who knew how to look at the actual facts" and asking reproachfully "what would become of natural history and mineralogy, if every question were treated as that concerning basalt had been?"

According to Geikie, when any belated straggler from the enemy's camp came to consult Desmarest on the subject in dispute, the old man would content himself with the answer. "Go and see."

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