Deep Time at the Convergence of Stone and Sky
Disclaimer: Cell service is slim to none even in the parking area. I would suggest you download the cache for offline viewing before driving up the mountain. Trail maps and wifi are available at the Roan Mountain State Park visitor center at the bottom of the mountain on the Tennessee side. If you want the best experience, go on a clear, sunny day. The view is simply breath taking. The trail is 1.5 miles round trip with 300 feet of elevation gain, heavily trafficked, and considered easy. I would not suggest going up the mountain in winter after any amount of precipitation unless your ok with driving on mountain roads covered in ice.
Time is one of those funny things that most people think they understand; but when you start talking about "Deep time" or "Geological time", things get a bit harder to grasp. This EC will take you to one of the most beautiful places in the country and let you meet some of the current crystaline residents. They've quite a story to tell.
The Backstory:
(The part you read in the parking lot before hiking.)
Over a billion years ago, some unsuspecting rocks were being formed in a land far, far away. How far? Well, that is the question. You see, landmasses tend to get bored and wander around the planet over really long periods of time (plate tectonics). Some of the rocks your going to meet have been witness to multiple (potential) super continent formations and break ups over their long life spans. Since were still piecing together what happened when and where, the following list is a current day (2022) "best guesses" of super continents, starting about the time some of the components of our current rocks formed [Ref 1].
(Ga = billion years ago)
(Ma = million years ago)
(1000 Ma = 1 Ga)
- Columbia: 1.82 Ga - 1.35 Ga
- Rodinia: 1.13 Ga - 750 Ma (Keep your eye on this one for later.)
- Pannotia: 633 Ma - 573 Ma (Hey everyone, wanna meet up at the south pole?)
- Gondwana: 550 Ma - 175 Ma (Arguably not a super-continent itself, but meets up with friends later.)
- Pangaea: 336 Ma - 175 Ma (The one everybody knows.)
Of course, none of this is a very peaceful affair. Often this results in the formation of large mountain chains, intense climatic upheavals, volcanic eruptions, earthquakes, and even changes to the planetary levels of atmospheric gases; and we haven't even mentioned their effects on the oceans. Speaking of mountain building, this area was probably on the eastern margin of the Grenville mountains (the ones that came and went before the current Appalachians). They were possibly one of the largest mountains chains in earth's history and probably formed in stages between 1.3 Ga - 0.98 Ga due to the creation and breakup of the Rodinia super continent [Ref 2].
[Image courtesy of Ref 2]
On top of all that, from time to time the planet seemingly gets tired of all the beach weather and decides its time to cool things off. These events may be relatively mild like an ice age ( Hey, we've got permanent ice caps at the poles! ) or something a bit more... Dramatic. Like turning our planet into a giant ice cube from pole to pole ( Summer? Whats that? ). With this sounding rather fashionable around 718 million years ago, earth decided to dive headlong into one of the longest so called "Snowball Earth" periods and stayed that way until about 660 million years ago [Ref 3].
[Image courtesy Ref 4]
So, how do we know how old these rocks are and their stories? Well, we look at the stuff that they are made up of and we (especially) try to find tiny crystals within the rocks called Zircons. The first part is pretty self explanatory. In the second part, we collect as many of the zircons as we can from certain types of rock, isolate them in a vacuum, then fire a laser into the different parts of the zircon crystals and record the ratio of Uranium atoms to Lead atoms in the parts that get vaporized. Why? Well, Uranium decays to lead at a steady rate over really long periods of time and we can use that decay rate as a clock counting backwards to when the zircons (and by proxy the rock) formed... Sometimes.
For decades, the Carver's Gap Gneiss was thought to be over 1.8 billion years old from the zircon ages recorded within it, making it (at the time) the oldest exposed rock unit on the east coast. It was also believed to be a relic piece of Amazonian crust that had welded onto the North American craton. Recently, these zircon crystals were deemed "detrial" (accumulated from a previously eroded source rock) and the "growth rims" on the crystals show different periods that the current rock was buried deep enough (and hot enough) to be metamorphically altered (allowing the zircons to grow and add layers, sort of like tree rings). Its a fascinating story stretching over 40 years of scientific debate, but too long to provide here [Ref 5] [Ref 6] [Ref 7]. Personally, I'd say this debate isn't finished, but that's just my 2 cents.
The Current Story:
(Picture time)
Ok, so according to the geological display at the Roan Mountain State Park visitor center (undergoing renovation), there are 8 major rock types in the general area. Don't worry, where your going today, there should only be 3 (possibly 4) out of the 8. However, hike further along the Appalachian Trail and all bets are off. To save space I'll only share pictures of the 4 you need to look for.
Augilite Epidote: Not dominant on this hike.
Beech Granite: Not dominant on this hike.
Cranberry Gneiss: Not dominant on this hike.
Magnetite: Not dominant on this hike.
Bakersville Diabase (Gabbro)
Primary color: Black
Majority age: 734 Ma
Formed while continents were rifting apart. Rich in Iron.
Occurs as thin to massive dikes (Cutting across other rocks) and lenticular (lense shaped) masses.
Carver's Gap Gneiss
Primary color: Medium to Dark Grey
Majority age: Previously considered 1.8 Ga. Now considered more like 1.012 Ga - 986 Ma.
Contains plagioclase (think blocky, cloudy quartz looking stuff), garnet, quartz, and one or two pyroxenes (dark silicate material).
Cloudland Gneiss
Primary colors: Light and Dark grey.
Majority age: Roughly 1.00 Ga - 980 Ma.
Contains among other things quartz, plagioclase (Same as the previous rock), biotite (black shiny) mica, and garnet.
and finally,
Mylonite
Primary colors: Vary depending on host rock. In our case black, grey, and white.
Majority age: Youngest rock type in the lineup. 300 Ma.
Apperance: Cohesive foliated (layered) rock. Identification is tougher with dark colored host rock, easier with light colored. In our case, usually looks similar to squished common gravel.
Since the USGS has not done a 7.5 minute quadrangle survey I've had to rely on research papers to locate generalized rock groupings for verification.
Hit the Trail:
(The part you've been waiting for)
Alright, time to get your hike on and claim that smiley! After leaving the parking lot, follow the AT up towards Round Bald. The trail is fairly gentle the whole way (dodge the tree roots). You've got 3 stops to make, questions to answer at each, and... Well, you'll see. I guarantee pictures are going to be taken, but please try and leave out any of the rock groups listed below.
As your coming towards the end of the wooded area you should see some decently large boulders nearby. In summer they usually have some amount of moss on them. No need to disturb anything, just take a moment to examine them reasonably close and perhaps save a few pictures to your phone to compare them later.
Stop #1 - (36° 6'28.58"N 82° 6'27.28"W) Rocks in the woods:
Question 1: What would you say the primary color(s) of these rocks / boulders are?
A short distance further and after exiting the woods you should see a large boulder directly trail side. Get up close and personal with this one. Grab a few more pictures to compare at the final stop.
Stop #2 - (36°06'26.9"N 82°06'25.8"W) Large trail side boulder:
Question 2: Aside from white, grey, and black, what other color(s) are present on this formation? (Hint: Primarily check the side of the rock you approached from).
Onward to the summit.
Alright, if you hiked here on a clear day, take a few minutes and savor this. Perhaps its just me, but this place is special unlike any other I've ever been to. After your done taking it all in, you should see 2 information signs 180° from each other. Your final stop is near the one facing northward.
Final stop - (36° 6'25.45"N 82° 6'12.97"W) Rock pile at the top of Round Bald next to the north facing "Peaks in the Distance" sign.
As before, take a look at the rocks laying around. Specifically, look for a few that have exceptionally smooth sides, like a giant knife took a slice out of them. Examine them fairly closely, recall everything you learned earlier (or cheat and scroll back up) and answer your final questions.
Question 3: Based upon the colors and textures of these rocks, do they have more in common with the rocks you examined in the wooded area, or the large boulder?
Question 4: If I were to remove 2 options from the earlier line up (Say... Mylonite and Bakersville Diabase) what type of rock would you suppose this is?
Question 5: Based off of the backstory, were the rocks in front of you right now, around for any "Snowball Earth" episodes? (Not necessarily at the surface, but formed yet?)
Ok, thats it. As usual, just message the answers to the 5 questions.
Alright, you know you want pictures, because who wouldn't up here? Please try and stay on the marked trails as much as possible at the bald, foot traffic is causing erosion problems and god knows we don't want anymore areas roped off. Oh, and If you want pictures worth hanging on the wall back home, wait for sunset. Trust me. Until next time...
Works Cited
[Ref 1]
Bradley, D. C. (2011). Secular trends in the geologic record and the supercontinent cycle. Earth-Science Reviews, 108(1–2), 16–33. https://doi.org/10.1016/j.earscirev.2011.05.003
[Ref 2]
R.P. Tollo. Editor(s): Richard C. Selley, L. Robin M. Cocks, Ian R. Plimer (2005). GRENVILLIAN OROGENY, Encyclopedia of Geology, Elsevier, ISBN 9780123693969. https://doi.org/10.1016/B0-12-369396-9/00432-9
[Ref 3]
Hoffman, P. F., Abbot, D. S., Ashkenazy, Y., Benn, et al (2017). Snowball Earth climate dynamics and Cryogenian geology-geobiology. Science advances, 3(11), e1600983. https://doi.org/10.1126/sciadv.1600983
[Ref 4]
Boutaud, A. S. (2018, January 22). When Earth was a Snowball. CNRS News. https://news.cnrs.fr/articles/when-earth-was-a-snowball
[Ref 5]
Carrigan, Charles & Miller, Calvin & Fullagar, Paul & Hatcher, & D, Robert & Brendan, Bream & Coath, Christopher. (2001). Age and Geochemistry of Southern Appalachian Basement, NC-SC-GA, with Implications for Proterozoic and Paleozoic Reconstructions. Faculty Scholarship - Geology.
[Ref 6]
John N. Aleinikoff, Scott Southworth, Arthur J. Merschat; Implications for late Grenvillian (Rigolet phase) construction of Rodinia using new U-Pb data from the Mars Hill terrane, Tennessee and North Carolina, United States. Geology 2013;; 41 (10): 1087–1090. doi: https://doi.org/10.1130/G34779.1
[Ref 7]
Scott Southworth; Shrimp U-Pb Ages of Detrital Zircons From The Carvers Gap Granulite Gneiss: New Evidence For a Missing Record of Sedimentation And Metamorphism In The Appalachian Blue Ridge. GSA annual meeting and exposition. 2010. https://gsa.confex.com/gsa/2010AM/webprogram/Paper177213.html