I have a love-hate relationship with geologic maps which are so often both strikingly beautiful and challengingly complex. I am drawn to them because they are begging to be read, the swatches of color spread across them hint at wonderful geologic stories. But, at this stage, I can only enjoy the hints, I just don’t have the necessary grasp of the science or the language to do more.
My latest adventure with geologic maps came earlier this month after Washington Post columnist John Kelly wrote about two volcanoes in Virginia that saw action during the Eocene Epoch (Hidden Depths: The Mystery of Virginia’s Extinct Volcanoes, December 15, 2012). Mole Hill, just to the west of Harrisonburg, Virginia, and Trimble Knob, just outside of Monterey, Virginia, are the very eroded remains of extinct volcanoes. Courtesy of Google Maps, here’s where they are (the blue markers).
And here's what they look like.
(This image has been released into the public domain by its author, Jstuby and is available on Wikipedia.)
(This image is by Raph Levien and reproduced under a Creative Commons Attribution 3.0 License. It is available on Wikipedia.)
These volcanoes are a geologic surprise and a puzzle. The surprise is their age. Before 1969, the accepted wisdom was that these volcanic remains were no different from nearly all of the other igneous rock in the Blue Ridge and Piedmont provinces that go back hundreds of millions of years. But, beginning in 1969, analyses of the rocks at these sites have produced estimates that place these volcanoes in the Eocene Epoch (about 56 to 34 million years ago), making them collectively “the youngest igneous rocks in the Eastern United States.” (C. Scott Southworth, et al., Middle Eocene Intrusive Igneous Rocks of the Central Appalachian Valley and Ridge Province – Setting, Chemistry, and Implications for Crustal Structure, U.S. Geological Survey Bulletin 1839, 1993, p. J17)
The puzzle posed by these volcanic remains emerges from, again, their age. This area during this Eocene time frame had been thought to be quiescent. But not now, and, so, the debate is on. What caused this volcanic activity? Southworth et al. concluded that the action here during the Eocene was geologically very brief (a few million years), limited in scope, and, on occasion, explosive. They argued that the volcanic activity at this place and at this time might be best explained by a reopening of basement fractures (the basement is the level below which sedimentary rock does not appear) and a global change in plate tectonic movement that occurred around this time.
Kelly’s article features Elizabeth Johnson, a geologist at James Madison University which is located in Harrisonburg, Mole Hill’s backyard. These and other extinct volcanoes have become teaching tools. The focus of her research and, by extension, that of her students, is on the material carried or exploded to the surface. The central question is what does that material reveal about the Earth’s crust and the depth of the mantle here.
After reading Kelly’s piece and some of the available research on these geological phenomena, I went in search of Mole Hill and Trimble Knob. Well, actually I stayed at my computer and set out to find geologic maps with a view of these two sites.
A friend of mine approaches our book club books by reading the first few pages and then skipping to the end to read the dénouement. If the conclusion’s of any interest (and not upsetting), she’ll go back and read the entire book. Though that’s not how I deal with books, there’s a certain logic for a neophyte coming to geologic maps to be forearmed with a sense of what the maps are intended to tell. At least, I have a fighting chance of interpreting the language of the maps.
What follows makes it all seem so much more straightforward than it really was. I’ll even skip going off on how much remains to be done to bring order to the world of maps from state geological surveys and the U.S. Geological Survey, partly because there does seem to have been some progress. A case in point is the National Geologic Map Database being assembled by the USGS. Sadly, though, Virginia is the only state on the East Coast included in the very useful MapView product in the Database. And I have already vented on this topic in an earlier blog post.
Here is a segment of a statewide geologic map prepared by the Virginia Division of Mineral Resources in 1993 (digitized in 2003).
The arrows point to the towns of Monterey (to the west) and Harrisonburg (to the east), not to the volcanic remains which really don’t appear in this map given its small scale – 1:500,000. At this scale, 1 inch on the map equals 500,000 inches or approximately 7.89 miles, which makes it useful for a broad picture, but not for exploring the geology of these two volcanic sites. [Note: I corrected my description of this scale in response to the gracious comment from Silver Fox. See below.]
Maps with a scale of 1:24,000 give enough detail to focus on Mole Hill and Trimble Knob. At this scale, 1 map inch equals 2,000 feet or approximately 0.38 miles. Here is Mole Hill in a 1986 map prepared by the Virginia Division of Mineral Resources (T.M. Gathright, II, and P.S. Frischmann, Geology of the Harrisonburg and Bridgewater Quadrangles, Publication 60, 1986).
The “Tv” symbol in Mole Hill’s orange blob indicates that this is considered a “volcanic pipe.” As I understand it, the pipe is the passageway through which the magma travels upward. Upon the death of the volcano, the solidified magma in the pipe proves more resistant to erosion and so is often the last vestige of the volcano. (Reed Wicander, et al., Essentials of Geology, 2006, p. 90.) I suspect it's important that Mole Hill is bracketed by faults. The dotted lines that originate from a point just to the southwest of the hill are used to represent fault lines that are either covered over or inferred by the geologist preparing the map. “U” indicates the “upthrown side” of the fault (rocks here are displaced upwardly), while “D” is the “downthrown side” (downward displacement). The light pink that surrounds Mole Hill is identified as “Obud” or the upper dolomite unit of the Beekmantown Group. This is Ordovician limestone.
Had I expected to find something similar in the maps of Trimble Knob, I would have been disappointed. As it was, I wasn’t burdened with any expectation, because I invested too much energy struggling to find the appropriate map. That turned out to be Geologic Map of the Monterey Quadrangle, Virginia (Gerald P. Wilkes, Virginia Division of Geology and Mineral Resources, Publication 178, 2011, scale 1:24,000).
I actually don’t think it’s significant that the geologic map depicting Mole Hill appears to differ so much from that for Trimble Knob. Perhaps it’s just a function of a quarter century (1986 to 2011) of evolving standards and practices in making geologic maps, or different views of different geologist authors. Geologists Jonathan L. Tso and John D. Surber describe these igneous features in this part of the Shenandoah as “the old plumbing system that once fed a volcanic complex that has since been eroded away. The larger bodies such as Trimble Knob and Ugly Mountain [West Virginia], may represent old volcanic necks or pipes.” (Eocene Igneous Rocks Near Monterey, Virginia: A Field Study, Virginia Minerals, August/November 2006, p. 9).
The images that open this post and the geologic maps that close it, attest to the fact that we are witnessing the final episode in the life of two volcanoes. Of course, we won't be here when the last chapter ends. I think it's fitting to finish with an observation from geologist and paleontologist Ellis W. Shuler (1881 - 1954) about the life spans of volcanoes. Shuler grew up in rural Virginia during the late 19th century, and, as I described in an earlier post, he opens his book Rocks and Rivers by recounting his first encounter with geology and paleontology. As a boy of 14, just after reading The Last Days of Pompeii, several earthquakes hit the area around his town of Pearisburg, Virginia (165 miles southwest of Harrisonburg). Shuler was convinced that Angel's Rest, a mountain that overlooked the town, was about to blow. The geologist sent from the USGS to explore the causes of this seismic activity befriended the boy and sparked a lifelong interest in geology and paleontology. There's a poetic touch to Shuler's observation about the ultimate demise of a volcano:
In the end extinction comes to all volcanoes. Pressures which force up the lavas are relieved and the explosive gases escape. The volcanic rocks decay to rich soils, and these soils in turn are eroded away. Volcanic necks, the hard inner core or pipe, survive longest, but these too finally disappear; and the plains of erosion sweep across the site. The volcanic episode is closed. (Rocks and Rivers, 1945, p. 170)