January 10, 2011

Neptunist vs Plutonist

Brian Romans over @ Clastic Detritus was recently selected as a finalist for science blogging due to his post about Rapid Canyon Formation and Uniformitarianism. The Uniformitarianism vs Catastrophism debate is likely familiar to every geo-blogger, or quite frankly anyone who has taken a historical geology course or read about the 19th century debate. The argument seems quaint and anachronistic these days, but the human condition seems to often unnecessarily dig up old black & white positions on topics that have already moved light-years beyond. I guess cherry-picking ignorance doesn't seem to have a retirement age. This is not to say that we should ignore old debates, as understanding the evolution of the school of geologic thought serves as an important reminder about how methodologies and discussions should and should not be approached.

One item that popped into mind when thinking about Uniformitarianism vs Catastrophism was the preceding debate over the origin of the Earth's crust and its variations of bedrock (IG, SED, META). Neptunism vs Plutonism was a classic geologic debate that showed the value of field work and how direct evidence gathered from it strengthens a hypothesis. Geology and its various branches literally just don't work without field study.

The Neptunist view of the origin of the solid Earth was spearheaded by Abraham Gottlob Werner, a German geologist of the 18th century who instructed at the Freiburg Mining Academy in Saxony. He was considered a competent mineralogist by his peers and students, and his scheme for identifying minerals and ores was pervasive throughout the industry in Europe. Central to Werner's own work was his interpretation of the geologic history of the Earth. His treatise outlined how all rocks of the Earth's crust were mostly of marine origin, deposited or precipitated from a worldwide ocean that once enveloped the entire planet. Sounds like that awful Kevin Costner movie. Certainly many sedimentary rocks are of marine origin, but others are definitely not formed in any way by water. Anyways, Werner's envisioned planetary ocean was placed in Deep Time's Archean (which wasn't yet conceived, but for simplicity's sake lets say Werner placed it in the oldest eon) and characterized as a hot, scalding primordial soup that was saturated with all the dissolved minerals needed to form the basement rocks (Urgebirge), essentially the igneous & metamorphic cores of mountain ranges, cratons, and platform bases.

Abraham Gottlob Werner
circa 1800

Werner's interpretation continued into the next phase with the onset of subsidence and cooling of the old ocean. This phase was marked by the deposition of consolidated, stratified, and fossiliferous sedimentary strata that was structurally deformed in many places. The fossils, he said, proved the planet had become suitable for life, and indeed this may have been an early explanation for the Cambrian explosion of the early Paleozoic era. Above these rocks layers was what Werner called Flotzgebirge, a term used to explain formations and groups of sandstones, shales, coal beds, limestones, and even basalt. But therein lies the problem we can all identify, using our 200 years of compounded knowledge and in situ studies.

So although initially received with great interest and enthusiasm, Werner's ideas soon drew criticism for failing not only to properly explain many features of plutonic & volcanic origin (see below), but what had become of the immense volume of water that once covered the Earth to a depth so great that all continents were totally inundated. The amount locked in continental & alpine glaciers in the late 1700's was not sufficient explanation, given knowledge of the cryosphere in the 18th century, and the topography of the ocean floor was barely mapped (it wouldn't explain it anyways). Neptunism had to pooh-pooh anything to do with volcanism, and some of the ideas bandied about were absurd even for the day. I once read that volcanoes were vents for the expulsion of burning plant matter (coal) from within the Earth; essentially they were labeled by Neptunists as cone-shaped barbecues.

Various volcanic features and forms,
most of which were unexplained or
dismissed by Neptunists
As the 18th century drew to a close, criticism of Neptunism strengthened with visible and indisputable field evidence for the counterargument of Plutonism. Geologists, especially ones investigating volcanic complexes in France's Massif Central (Nicolas Desmarest, Alexander von Humboldt, Rudolph Eric Raspe), were able to clearly demonstrate volcanic origins for basaltic layers and various features such as cross-cutting dykes, sills, tephra layers and structures (*wink* like Pumice Castle *wink*). Geologists who conducted these field studies formulated an opposing view, and came to be the Plutonists. According to the Plutonists, fire was the key to the origin of igneous rocks such as basalt, not precipitated minerals from a marine environment. James Hutton, considered one of the fathers of early geology, was an advocate for Plutonism who stated that classical igneous rocks such as basalt and granite “formed in the bowels of the Earth from melted matter poured into rents and openings in the strata”. Hutton's Uniformitarianism concept also espoused how the agents of deposition and erosion at work in the present have been working since the beginning of the Earth. The Neptunist vs Plutonist debate thus became amplified in the later Uniformitarianism vs Catastrophism debate, with many of the same hypotheses being refined and carried over. I wonder how much the spirit of those old arguments carried into the Geosyncline vs Plate Tectonic debate.

Debate is good for the evolution of a scientific discipline, and though today's debates in geological academia are of a different flavor than the one I've just focused this post on, they are just as important in furthering our understanding of the spheres of our natural world, an understanding that I hope never comes to a completion. If anyone wants to check out some quoted material from early Plutonists, look no further than this page which has explanations of fractionation & igneous differentiation by English geologist George Poulett Scroupe.

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