Folded Mountains and Joint Systems EarthCache

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egroeg: It's time.

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Hidden : 4/30/2013
Difficulty:
3 out of 5
Terrain:
2.5 out of 5

Size: Size: other (other)

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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

If you look at a cliff, or the rocks in the side of a hill or road cut, you can't help noticing all kinds of cracks in those rocks. Big cracks, small cracks, straight, crooked, horizontal, vertical, diagonal.... any kind you can think of. Big deal, right? Well, with a few simple measurements, you're going to discover that there is a lot of geological information to be found in those cracks.

For this Earthcache, you will be investigating a network of cracks found in the top surface of a layer of sandstone. (This is the Burgoon Sandstone of the Pocono Formation from the Mississippian Period.) By measuring the orientation and spacing of the cracks, you will learn something about how and when they formed. This Earthcache will take you on a 2-mile roundtrip hike along a dirt road. The land is administered by the PA Game Commission - it is not a numbered Gameland, but hunting is permitted, so take appropriate caution during hunting season. Take a compass, a tape measure, and you might want a calculator. Access to this cache page (either electronic or a print-out) is recommended.

When geologists talk about what we call "cracks" they might use the terms "fractures" or "joints" instead. (While there are some technical differences in the terms, I am going to use "joints" in this discussion.) If several joints are present and they appear to be parallel to each other, you have a "joint set". You can sometimes see another set of joints at an angle to the first set. When you have more than one joint set, you have a "joint system". Joint systems can form in two or more separate events, or all at once. You will be investigating a joint system for this Earthcache.


Earthcache Logging Requirement 1:At the first location, take a look at the cracks in the rocks you are standing on. Compare the joint system found here to the two diagrams above. Which type of joint system is found here? (Look where the joint sets cross - do they directly cross each other like a checkerboard, or are the joints offset like a brick wall?)

Cracks, fractures, joints - whatever you want to call them - are formed when the rock layer is stressed beyond a certain level. The main stresses are: compressive, tensile (or tensional), and shear. "Confining stress" can be composed of all the other three stresses. (Another stress can come from hydraulic pressure of a fluid in the rocks, such as water. Hydraulic fracturing is also known as "fracking" and has been much in the news.) But what can cause these stresses in the rocks? In this case, it was the stresses due to building the Allegheny Mountains.

The Allegheny Mountains were formed almost 300 million years ago when the continents of North America and Africa collided. This collision resulted in enormous compressive forces pushing the existing North American rocks to the west. In many places, the force was great enough to cause the rock layers to buckle upwards, forming the Alleghenies. (Mountains formed like this are known as folded mountains.) An upward folding of the rock layers leads to an "anticline" formation. The area of greatest folding is known as the "hinge" or "axis" of folding. At our location, the axis of folding follows the direction of Chestnut Ridge to the northeast.
So, compressive forces caused these joint sets, right? Maybe, maybe.... But let's think about what else is happening. When the layers are buckled upwards, they are being bent, and the top part of the layer is elongating more than the bottom of the layer. This puts a tensile stress on the layer. (See the stress diagram.)
OK, so it is tensile stress that is causing the joints, right? Maybe, maybe.... But remember, there are layers above and below each layer, and these layers are also bending. The layer above our jointed layer is bending more than the jointed layer, and the one below is bending less. So our jointed layer, which is bonded to the layers above and below, is feeling a shear stress. (Look at the shear stress diagram above to get a clearer picture of this.)
Have we run out of stresses yet? Not exactly. In going from the newly folded mountain to the current mountain, you lose a lot of material to erosion. (One estimate says about 1500 feet of rock have been eroded from Chestnut Ridge.) The loss of all this material from above the layer (also known as "unroofing") actually reduces the vertical compressive stress and unbalances the confining stess. Some rock layers will try to expand when unroofed.

OK, you've stuck with me so far, and now it's time to do some measuring, and we'll also try to determine what type of stress caused our joint system.

Earthcache Logging Requirement 2a: At the first location, measure the compass bearings of the two joint sets. One joint set runs uphill toward the cliff - call this Set A. The other set runs uphill away from the cliff - call this Set B. You need to subtract 10° from each number to account for magnetic declination. Report these values as a number between 0° and 180°.
Earthcache Logging Requirement 2b: Measure the distance between the joints of Set A in several places and report the average. Repeat this process for Set B. Do not measure the distance between joints of Set A by measuring along the joints of Set B. Measure the distance straight across between joints.
(Bonus Question: Determine the slope of this sandstone layer. By my calculations, this layer would have been about 1800 feet higher at the top of the ridge. Add in the 1500 feet of eroded rocks above this layer, and you find that 60% of Chestnut Ridge is just plain gone!!)


Earthcache Logging Requirement 3a: As discussed above, tensile stresses increase at the top of a layer when that layer is folded. (See the diagram to the right.) If the layer is bent enough, the tensile stresses tend to create joints that are parallel to the axis of the anticline. The Chestnut Ridge anticline runs at about 30° in this area. Does either joint set have a similar heading, within a few degrees?

Earthcache Logging Requirement 3b: A joint system formed from shear stresses has an unusual property: the bisector of the angle between the sets is perpendicular (at right angles or 90°) to the anticline axis. Find this bisector by averaging the two joint set headings. If the perpendicular to the axis is about 120°, are these numbers similar?
Earthcache Logging Requirement 3c: Which type of stress do you think is responsible for this joint system?

Earthcache Logging Requirement 4: Go to location 2 and take a few quick measurements. Does this area seem to have the same joint system?


Congratulations! You have just completed a field exercise that only advanced geology students would normally do. I hope you had fun, and maybe even learned a bit along the way. I'll bet you never look at cracks in rocks the same way again. Cache safely!

Ref:http://www.geo.wvu.edu/~jtoro/structure/joints.pdf http://www.tulane.edu/~sanelson/geol111/deform.htm

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