Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering