Posted by: Rob Viens | March 18, 2012

Neptunists, Plutonists and the Significance of Granite

March 18th – the Beagle sets sail for points south.  Darwin is pleased with his adventures so far:

“Against a strong tide we slowly stood out of the bay of All Saints & took a lasting farewell of Bahia: if I have already seen enough of the Tropics to be allowed to judge, my report would be most favourable; nothing can be more delightful than the climate, & in beauty the sky & landscape are unparalleled in a colder zone.” (Mar 18)

He isn’t kidding about the climate – I’ve been watching the daily weather report in Salvador, Bahia for the last couple of weeks and I have yet to see a day where the high and low temperatures were more or less than about 80°F +/- 3°F. In any case, I think he was very pleased with his first landfall in the New World.

One last Bahia “entry” I’d like to touch on before leaving the Bay of All Saints.  It’s a passage from the Voyage of the Beagle:

“Along the whole coast of Brazil, for a length of at least 2000 miles, and certainly for a considerable space inland, wherever solid rock occurs, it belongs to a granitic formation. The circumstance of this enormous area being constituted of materials which most geologists believe to have been crystallized when heated under pressure, gives rise to many curious reflections. Was this effect produced beneath the depths of a profound ocean? or did a covering of strata formerly extend over it, which has since been removed? Can we believe that any power, acting for a time short of infinity, could have denuded the granite over so many thousand square leagues?” (Voyage of the Beagle)

Half-billion-year-old “Carioca Gold Granite” from Brazil (from OSU Proterozoic Rocks web site):

Carioca Gold Granite

This statement brings ups several interesting geologic concepts, including one that was a big debate in the early 1800’s, and another that is particularly worthy of note since it plays a role in Darwin’s theory of the origin of species (which was already stewing in his head when he wrote Voyage).

First, Darwin makes a reference to the origin of granite, noting that “most geologists” believe it is “crystallized when heated under pressure”. Darwin is alluding to a major rift in thinking that began in the late 1700’s between geologists known as “Neptunists” and “Plutonists”.  The Neptunists, led by the German geologist Abraham Gottlob Werner, believed that volcanic rocks (namely basalt) precipitated out of an ocean and that heat played a minor role in the formation of rocks.  The Plutonists, championed by James Hutton (and later by Charles Lyell), believed that heat played a critical role in the workings of the Earth, and therefore hypothesized that volcanic rock was injected as molten magma from below the surface. This debate almost seems silly today when we can watch magma turn to stone on YouTube at the click of a mouse, but in the early 19th century it was a hot topic (pun intended).

Darwin was pretty clearly a Plutonist, which is one of the issues he had with Robert Jameson, his natural history professor at University of Edinburgh.  Jameson was a very vocal Neptunist who had studied with Werner and taught it as the “only hypothesis” in his classes. I think that in writing this paragraph in Voyage, Darwin is clearly making a strong (though subtle) statement of support for his team – the Plutonists. Since most geologists did agree that granite was crystallized from magma, Darwin’s reference to the 1000’s of miles of exposed granites in Brazil, provides additional evidence for the significant role of heat and magma in the Earth.

Abraham Gottlob Werner and James Hutton:

Abraham WernerJames Hutton

Second, this passage suggests that massive amounts of erosion have occurred in South America. If these granites really do form at some depth in the Earth (and they do), and Brazil is full of granite exposed at the surface, then 100,000’s of cubic km’s of rock must have been striped away sometime after the granites crystallized. This sort of work requires the gradual accumulation of slow geologic processes.  So not only it is amazing to think about the scale of erosion implied by these exposure, but it also suggests the final point…

…the passing of an enormous amount of time – which Darwin poetically called “a time short of infinity”.  Though by the 1830’s the Earth was believed to be relatively old, the actual age was much debated (and typically thought to be less than 100 million years). Hutton actually believed it was nearly “infinite”. Erosion of the scale proposed by Darwin in South America supported a very old age of the Earth.  (Charles Lyell was also a strong supporter of Hutton’s ideas. As a student of Lyell’s school of thought, it is likely Darwin was convinced of their importance by Lyell’s Principles of Geology.)

It is this last concept in particular – that immense amounts of time have passed in the history of the Earth – that gives biological evolution the time needed to create the diversity of species we see today.  Without what geologists call “deep time”, significant evolutionary advances are difficult if not impossible to support. So the importance of an old, slowly changing Earth to the ideas forming in Darwin’s mind cannot be underestimated.

Maybe I read too much into some of these writing, but knowing how smart and observant Darwin was, I’m certain that he is embedding a universe of information into this short paragraph about granites in Brazil. (RJV)

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Responses

  1. […] thought brings us back to our friend Abraham Gottlob Werner (see Neptunists, Plutonists and the Significant of Granite). In the late 170′s he actually came up with a standard color scheme for describing rocks and […]

  2. […] around Rio. They are made of the same granitic rocks that Darwin described earlier in Bahia (see Neptunists, Plutonists and the Significance of Granite).  In fact, it is almost certainly the fact that they are made of granite, a rock that is more […]

  3. […] and timing of this erosion, and even alludes to it when discussing granites in Brazil – (see Neptunists, Plutonists and the Significance of Granite). Granite also forms deep in the Earth’s crust, so it is pretty common to find them in the […]

  4. […] up in preconceived notions, is one of the true indications of a great scientist. (Be sure to see Neptunists, Plutonists and the Significance of Granite for more on this […]

  5. […] Considerations on Volcanoes: The probable causes of their phenomena, the laws which determine heir march, the disposition of their products and their connection with the present state and past history of the globe; leading to the establishment of a new theory of the Earth by George Poulett Scrope (1825) – Read the book OK – that is the best (or worst?) subtitle ever… Scrope was a French geologist, and this book was effectively the first “textbook” about volcanoes and igneous geology. The book (not surprisingly ) makes a strong case for a volcanic origin of rocks like basalt, and conversely argues against the Neptunists (see Neptunists, Plutonists and the Significance of Granite). […]


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