The emergence of geology in the late 18th and early 19th centuries was influenced by two problems beyond the general issues of fossils, strata and the structure of mountains. These were vulcanism and its products, and the erosion of the earth surface (denudation). The next two classes will consider these topics.

The modern volcanoes of Italy had been a subject of study since ancient times. In the late 18th – early 19th cenury literature a growning body of literature described the eruptions, lavas and other deposits of these regions. Curiously, the full impact of these studies was delayed until the 1820s or later. I suspect that this was because modern volcanoes were viewed as relatively minor entities (although it is not easy to explain this view given the size of Mount Etna!).

The most influencial region of ancient volcanism in the development of later eighteenth century geological thought was in south-central France occur in the area of low-relief mountains termed the Massif Central (or the Auvergne and Velay after political entities) consisting of extinct Tertiary volcanoes. The Massif Central volcanoes were part of the stratigraphic succession and included basalt, a very controversial rock type. Remember that the crystalline texture of basalt was not recognized prior to petrographic microscopic investigations, so it was an open question as to whether basalt was a igneous precipitate, and aequous precipitate, or a fine-grained sediment. Just how different is it from graywacke when viewed in hand specimen?

Harris, section 8 will provide you with the general suvey of today’s topic.

Italian Volcanic Regions

Please use the course website page to learn a little about one of the following: Vesuvius, Phlegraean Fields, Stromboli or Etna. The best are the “Volcanoes of Italy”, “Explore Italian Volcanoes” and “Stromboli Online” sites. Please learn about your assigned volcanic region – the critical information is locations, scale, eruptive style, general rock types, and recent eruptive history. Other interesting facts are fine!

Auvergne Volcanics

A few Auvergne maps may help you visualize the regional geology. The Massif Central is an uplifted block that is bounded by faults to the south and east. It dips very gently to the north-northwest. North-south faults divided the region into horsts and grabens that form Cenozoic highs (sites of volcanic activity) and basins (now occupied by major river valleys – some were once lakes). Tributaries off the major rivers have eroded into the volcanic flows along the flanks of the highs. The volcanic eruptions have occurred over the last 10 million years.

1. Scarth & Tanguy (2001)

Scarth and Tanguy present a relatively brief overview of the Massif Central volcanoes, their morphology, and their deposits. They focus on the Chain of Puys which is very useful for us because this chain of relatively recent volcanoes was the focus of the early work of Guettard, Desmarest and Poulett-Scrope. The photos are helpful – you may want to check the web sites linked in the course web pages to see some more examples. You should focus on developing a list of volcanic features such as types of volcanic cones, variation in deposits (not a list of rock types), effects on landscape, etc.  Please do not worry about all the local details or the varied terms used for the igneous rock types (unless you are volcanologist).  Most are similar to basalts, and though some rocks (particularly those that formed the domes) are more silica-rich (trachyte).

2. Short extracts from Desmarest (1774, 1806), Dolomieu (1789), von Buch (1802), Scrope (1825, 1858)

These readings are all quite short (1-3 pages) and serve to illustrate some of the changing ideas about the Massif Central rocks. I recommend reading them in the order listed below (arranged by publication date) – note that some of the online reading files may have 2-3 short articles but you can use the titles to identify the one to read.

As your read these, you will see that there are several interrelated theoretical breakthroughs: (1) the identification of volcanic cones and deposits, (2) the recognition of the igneous origin of basalt (at least some basalts), (3) the realization that erosion was the cause of the present-day discontinuous distribution of lava flows and the degraded nature of some of the cones, and (4) the implications of all this for time.

• Desmarest’s (1774 “The Volcanic Origin of Basalt”) studies followed the pioneering work of Guettard by presenting a detailed description of the Auvergne region and arguments for its volcanic origins. This selection concisely states the case for the igneous origin of basalt.
• Dolomieu (1789 “The Relations between…”) addresses an important conceptual problem of his day. Most geologists considered volcanoes to be the result of near-surface activity (commonly heating due to the burning of coal at shallow depths). He clearly argues otherwise and even challenges the primacy of granite and the solidity of the earth’s interior (!). What are his observations? How does he argue to his rather remarkable position?
• Von Buch (1802 “The Igneous Origin of Basalt”) was probably Werner’s best student. Here he considers the origin of the Auvergne basalts that Werner had considered to be oceanic precipitates. Several interesting points in this short passage. What are his observations? How does this lead to his conclusions? What is the relationship between the Auvergne basalts and the underlying granites? How does this compare with Dolomieu’s (1789) position?
• Desmarest (1806 “Age Determinations in a Volcanic Region”) here turns to questions about the age relations between volcanic cones with different degrees of erosion, lavas at different elevations, and the development of the landscape. Scrope would face the same issue in the 1820s – his summary cross-section may help you understand the geometric relations shown here.
• Scrope (1858 “Origin of Valleys”): Scrope studied the Auvergne in some detail and published a book-length report (1827) that was used by Lyell in his visit.This is taken from the second edition of his book but is little changed from the original publication. He argues for the primacy of aqueous erosion and the significance of substantial amounts of time to understand the present state of the Aurvergne. His dual interests in volcanic features and their erosional modification are very similar to those of Desmarest. His call for “Time! – Time! – Time!” was a precursor to Lyell.
• Scope (1825 “Volcanoes”): Scrope published his book “Considerations on Volcanos” after studying Italian volcanoes. The point of this selection is to illustrate his understanding of the general processes behind volcanic activity. Although we are “jumping ahead” a few decades, but this will help us link the study of volcanoes to some of the theoretical arguments of the 1820s-30s. The text is in three parts:
  • The Source of Volcanic Energy: Previous workers largely attributed volcanoes to the subterranean burning of coal beds (see notes on other readings) or the interaction of water (particularly seawater) with various compounds. Scope prefers heat (“caloric”) as had Dolomieu.
  • Relation of Shape of Rock Bodies to Their Composition: Several issues frame this selection. First is the question of the origin of basalt (typically found in layers or sheets) versus undoubted volcanic products such as “trachytes” (which make volcanic cones). You can appreciate some of this from our discussion of Werner; the fuller significance of this will be clearer once we consider the Hutton model. Second, the origin of mountains was an important problem by this time and this was the reason for his comments on Von Buch’s elevation model (“swelled up like a bladder by inflation from below”). You will want to refer back to this when we turn to nineteenth-century tectonics.
  • Three Modes of Rock Formation: Two interesting points about this brief passage. First, the primary modes of production are quite clear but notice that metamorphism is absent (although it would be suggested within a few years). Second, the final paragraph is a strong plea for modern processes but specifically rules out constant/uniform rates. We will return to this question when we examine the geological models of the 1820s-30s.

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