Tafoni Weathering EarthCache

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Hidden : 8/9/2012
Difficulty:
1.5 out of 5
Terrain:
2.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

This is an EarthCache, so there is no cache container hidden at these coordinates. An EarthCache is meant to provide an earth science lesson by having you make observations and reporting these to the cache owner. For more information about EarthCaches, visit EarthCache.org

Flowing water is one of the most powerful forces shaping our world. From the valleys of our Three Rivers region, to the majesty of the Grand Canyon, the erosional effects of flowing water are everywhere evident. Sometimes, if you look closer at the rocks around us, you can see that erosion isn't caused only by the force of mighty rivers, but also by a slow and patient trickle.

Not all the rainfall and snowmelt runs off a hillside to become streams and rivers. Some of this water is absorbed into the ground, where it slowly percolates its way downhill through layers of porous rock. Some rock layers are more porous than others, so the groundwater prefers to travel in those layers until it emerges as a seep, spring, or artesian well. During its slow journey through the rocks, the water can dissolve some of the minerals in these rocks, leading to "mineral springs" popular for their reported health benefits. The ability of water to dissolve these minerals can also lead to weathering of those rocks.

(Brief technical note #1: Geologists refer to the breakdown of rocks as "weathering" and the transport of the broken bits as "erosion". In a river, the weathering and erosion are occuring simultaneously. Weathering may cause a buildup of broken rocks at the bottom of a cliff, and erosion may not have moved them away yet.)

This photo is an example of what has been called honeycomb weathering, cavernous weathering, stone lattice, or tafoni weathering. Scientists have proposed many ways this weathering could occur: wind erosion, frost shattering, or cold air blowing on sun-heated rocks. More recently, they have focused on mechanisms involving groundwater - either the groundwater dissolving the cement that holds the rocks together, or groundwater evaporating at the surface leaving behind dissolved minerals. Since this type of weathering is seen all over the world, in many climates, it is possible that no one mechanism explains all the occurrances. This is one topic you will investigate for this Earthcache.

No matter how tafoni are formed, there must be an Initiation Phase, followed by Growth Phase. (Tafone is the name for one of the cavities, and tafoni is the plural.) In almost every case, tafoni seem to start at imperfections in the rock - these could be cracks in the rock, a place where grain size is different, or where two different types of rock beds meet. (Brief technical note #2: Where different types of rock layers are present, geologists talk about "bedding planes", which just means how the layers are oriented. There can be horizontal bedding planes if all the layers are flat, or tilted bedding planes, if the layers have been shifted over time.) Once the small tafoni form, they can grow toward each other, eventually joining together to form larger tafoni. At this Earthcache, you should see tafoni ranging in size from a few millimeters to a few meters. If wind, frost, or sun were all it took to create tafoni, you should expect to see tafoni anywhere there were rocks with cracks in them that were exposed to the weather. There must be something more.

When groundwater seeping through porous rock meets a crack, it finds a much easier path to follow. Also, if the two types of rock meeting at a bedding plane have different porosities, the groundwater prefers to travel through the more porous layer. In either case, more water is flowing in these areas. We can now look at the two proposed mechanisms of groundwater weathering.

Mechanism 1. Groundwater flow can dissolve the cement that holds the rock together, making the rock softer. In this case, the rock becomes softer along the imperfections because the higher water flow dissolves more of the cement holding the rocks together. Any agitation, such as wind or ground vibrations, can cause small particles of the weakened rock to be dislodged, forming a small pit. Since the base of the pit is deeper than the wall, water from inside the rock will flow first out of the base of the pit, causing even faster softening of the rock in the pit, causing even faster weathering of the pit, causing even higher flow, etc. The walls of tafoni formed this way would be fairly smooth in appearance.

Mechanism 2. Groundwater with dissolved minerals evaporates at the surface of the rock, leaving behind the dissolved minerals in the rock as mineral deposits. If enough dissolved minerals are deposited, they start to form larger crystals just under the surface of the rock, breaking up the structure of the rock. The rock will start to flake off in thin layers or shells. (Brief technical note #3: When layers of rock weather out of the inside of a cavity, geologists refer to this as "negative exfoliation". Try dropping that phrase into a conversation!) Again, as the pits form, increased flow at the base of the pit will accelerate growth of the tafone.

Now that we know what to look for, it's time to investigate our example of tafoni weathering. At mile marker 36 of the Yough River Trail, just north of Cedar Creek Park, is a sandstone outcrop (on the left, if you're coming from the park). This sandstone is part of the Monongahela Group, formed during the late Pennsylvanian Period, about 300 million years ago. There is plenty of parking at the waypoint coords, and you can follow the bike trail to the Earthcache. The sandstone is up a short but steep slope that can be slippery at times. DO NOT TOUCH THE TAFONI - they can be fragile and you wouldn't want to ruin this unique formation. Bring a small ruler and a tape measure.

Your first task is at the far left end of the outcrop (as you face it from the trail), around the corner and uphill a little. A little above head-high are some small tafoni, still in the Initiation Stage. Task 1. What is the shape of these tafoni? About how big are they? What is their orientation - vertical, horizontal, random,etc.

Your next task is a few feet away, back at the corner of the outcrop. A large tafone is just above your head here. Task 2. What are the approximate dimensions of the large tafone, including depth? Is the floor level or sloping? What is covering the bottom - sand? flakes? Are there smaller tafoni inside?

Continue to your right. Just have some fun checking out all the different tafone, but think about things like their size and orientation, whether they are smooth, what type of debris is there, etc. Task 3. What is the approximate length of this large opening?

After a small unweathered area, there is another, even larger opening. Again, examine the weathering, and this time be sure to check out the roof. Task 4. What is the length of this opening? Why do you think there is an overhang with little sign of weathering?

The outcrop continues on, and after one more large section of tafoni, they seem to disappear. A little further on are two medium tafoni above head-high, but none after that. This same sandstone appears to the right and left of this outcrop, but no tafoni are visible. Task 5. Why do you think the tafoni appear only at this outcrop? How do you think they formed (which mechanism)?

Please send me your thoughts in an email as you log your cache. Post your photos if you'd like, just try not to include spoilers. As always, cache safely and have fun!

References:
Groundwater flow diagram - http://www.emporia.edu/ksn/v09n4-May1963/index.html
General info: http://www.tafoni.com/Welcome.html

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