On Friday afternoon and Saturday I continued my trips into the mountains to learn more about Appalachian geology and paleontology. As we drove toward Monterey, Virginia on US 220, we passed by Trimble Knob, a large conical hill that sticks up prominently above the surrounding landscape.
Trimble Knob is apparently privately owned, with a fence around it, so we couldn’t go up. However, there are roads all the way around it, so we were able to get good pictures, including zooming in on some of the exposed rocks:
Trimble Knob is made primarily of basaltic igneous rocks, which are not especially common in the Appalachians. There are actually a series of comparable structures, as well as igneous dikes, in Highland and Rockingham Counties in Virginia and Pendleton County in West Virginia. Trimble Knob (which is the largest of these structures) seems to be a diatreme. Diatremes are formed when magmas rise up to near the surface and interact with groundwater. The groundwater flashes to steam and explodes, opening a gradually widening crater that fills with a mixture of the cooled magma and fractured pre-existing (country) rock (in this case, Devonian Needmore Shale). The resulting structure is more resistant to erosion than the shale, and ends up as a high-relief feature.
The vast majority of surface igneous events are related to motion at tectonic plate boundaries, either rifting during the formation of an ocean basin or subduction when an ocean basin is destroyed. For decades after the discovery of Trimble Knob and related features, they were assumed to be Triassic in age, related to the opening of the Atlantic Ocean.
This all changes in 1969, when Fullagar and Bottino used K-Ar and Rb-Sr dating to determine that these rocks were Eocene in age, more than 200 million years younger than previously supposed. The structures they dated were only about 47 million years old; Southworth et al. (1993) later reported a K-Ar date of 35 million years (± 0.5 Ma) for Trimble Knob, making it the youngest known igneous structure in the eastern United States.
These dates are rather remarkable, because there was neither a rifting event nor a subduction zone of the east coast of North America at that time. There have been a number of hypotheses proposed for the origin of these rocks (see Tso et al. 2004 for a review). It seems the most accepted proposal was that made by Southworth et al. (1993). They suggest that during the Eocene the direction of motion of the North American plate changed, relieving compressional stress on the crust. This opened pre-existing fractures enough to allow some magma to rise rapidly to the surface, causing a small amount of igneous activity that lasted about 12 million years. The same change in plate motion is thought to possibly be responsible for the formation of the Bermuda Rise at the same time.
We also actually found some fossils on our trip; more on those in a few days.