Trimble Knob Earthcache EarthCache
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Volcanic eruptions in Virginia are considered extremely unlikely, since the most recent eruptions were in the Middle Eocene period millions of years ago. Nevertheless, volcanic rocks are found in a number of areas in Highland County, Virginia and adjacent Pendleton County, West Virginia. These rocks have puzzled geologists since the 19th century because they are so rare in the Valley and Ridge area of Virginia, which consists primarily of folded and faulted sedimentary rocks, not igneous rocks.
The largest of these mysterious igneous features is a small conical-shaped hill just south of Monterrey known as Trimble Knob. Trimble Knob is considered by geologists 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 surrounding rock (in this case, Devonian Needmore Shale). Diatremes are carrot or funnel-shaped with steep walls that taper at depth. They are different from volcanic plugs, in which magma slowly cools and does not mix with surrounding rock.
Diatremes form underground, but become exposed when surrounding softer rock erodes away. Because the Trimble Knob diatreme has weathered much less than the surrounding shale in the 35 million years since it was formed, it now stands higher than the surrounding land, forming a cone shaped hill that makes it look like it was once a volcano. The question for geologists is how Trimble Knob was formed.
The geologic history of the Valley and Ridge province of Virginia is quite complicated. Over many millions of years, ancient oceans produced sediment that turned into rock, and this rock was folded when continents collided millions of years ago. At other times, the crust was pulled apart, creating a rift.
The vast majority of surface igneous events are related to shifting of tectonic plates, 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, geologists believed they were formed during the Triassic period, related to the fracturing of the ancient supercontinent Pangea, and the opening of the Atlantic Ocean.
This was a logical conclusion because they saw that the rocks cross-cut both the sedimentary rocks and the extensive folds of the Valley and Ridge. If the igneous rocks of Highland County were produced during the Triassic/Jurassic era, they would be between 140 and 230 million years old.
In 1969 two geologists named Fullager and Bottino studied the rocks using isotopic dating methods and found that the rocks were formed much more recently, during the Eocene epoch. As a result, Trimble Knob is now considered to be the youngest known igneous body east of the Mississippi River at only 35 million years old. These dates mystify geologists, because there was neither a rifting event nor a subduction zone of the east coast of North America at that time.
Geologists disagree on how Trimble Knob was formed. There are several theories:
1. A regional basement fracture known as the 38th parallel lineament allowed igneous activity to bubble up during the Eocene.
2. A nearby intrusive supplied magma for igneous activity in Highland County. This same intrusive currently provides heat for hot springs in Bath County, Virginia and surrounding counties.
3. A global shift in tectonic plates during the Eocene era was responsible for igneous activity in Highland County.
4. Diverging continents caused higher density materials to sink and compensated by allowing magma to rise beneath Highland County to compensate for mass redistribution during the Eocene era.
And in 1993, some scientists theorized that the rocks formed due to a combination of the above causes. They hypothesized that during the Eocene era 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 over a period of about 12 million years. Although this is the most widely accepted theory at this time, scientists do not currently know for sure how Trimble Knob formed.
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Logging requirements :
The coordinates take you to the former site of the Highland Maple Museum, just south of beautiful Monterey, Virginia. There should be a parking area here - let me know if this has changed. Across U.S. 220 and a little south, Trimble Knob dominates the landscape. Trimble Knob is located on private property.
1. Post a photo of yourself with your GPS at the posted coordinates showing Trimble Knob in the background.
2. To log this cache you must E-MAIL me the answers to the following three questions. Do not post your answers. Then you may log the find; I will let you know if anything needs correcting.
• What is Trimble Knob and why is it not a volcano? NOTE - As of March 16, 2015, ANY posts that claim this is a volcano WILL NOT get credited with the find. No exceptions! You gotta read the writeup to get the credit. I don't care what the title to the other geocache says!
• How old do geologists believe Trimble Knob to be, and why is that age important?
• How high do you estimate Trimble Knob to be?
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Links and Resources:
Background on Virginia Geology. (visit link)
Fichter, Lynn S. and Steve J. Baedke. (2000). "The Geological Evolution of Virginia and the Mid-Atlantic Region." (visit link)
Rakovan, John. “Diatreme.” Rocks and Minerals, March/April 2006, V81, p. 153. (visit link)
Southworth, C. S., K. J. Gray, and J. F. Sutter. (1993). Middle Eocene intrusive igneous rocks of the Central Appalachian Valley and and Ridge Province — setting, chemistry, and implications for crustal structure. U. S. Geological Survey Bulletin 1839, J1-J24. (visit link)
Tso, J. L., R. R. McDowell, K. L. Avary, D. L. Matchen, and G. P. Wilkes. ( 2004). Middle Eocene igneous rocks in the Valley and Ridge of Virginia and West Virginia. U. S. Geological Survey Circular 1264. (visit link)
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