Welcome to Xiaoliuqiu — An Island of Geological Wonder! (JOINT FISSURES )
In this Earth Cache, you will learn about coastal erosion — specifically joint fiissures, and how Mother Nature helped form one unique piece of "art" right here along the coast of Xiaoliuqiu Island. And this particular sea gorge here is known locally as "Lobster Cave" as it used to be home to many, many lobsters (or maybe, as I like to think, it resembles a lobster! How cool is that?) So, upon learning all there is to know about joint fissures and the forces of "hydraulic action (and more)" that sculpted "The Lobster Cave", you will be ready to put on your Earth Science Caps and get geological on location here at Xiaoliuqiu Island. And after you answer the task questions and complete all your Coastal Erosion Studies and Observations (which can all be found at the bottom of this page), you can log a “Found It!” for this Earth Cache. I hope you enjoy! Are you ready? Let's begin . . .
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What Exactly is a Joint Fissure and How does a Joint Fissure Form?
Basically, a joint fissure is a break in a rock without displacing it. In other words, a rock is "cut" into two smaller pieces but those two pieces stay where they are and don't "move" or become "displaced" to a new area. So, here at Lobster Cave, you are viewing a place along the coast where there was a discontinuity or a break in the rock formation. And how did this joint form here? Well, by a special kind of stress called tensile stress — a special force that pulls rocks away from each other. But how exactly does tensile force work? First, by fluid pressure — ocean water gets trapped inside the rocks and wants to get out and it does so by "breaking away" the rock to do so which is evident here at Lobster Cave. Also, another way that caused joint fissures to form here was a change in weight of the overburden — the rock that is on top, which is of course compressing or pushing down on the rock material below it. And these rocks below the overburden are of course subject to higher temperatures and higher pressure since they are "burdened" by what's on top of them. So over time, there are eroded by:
⇒ Hydraulic action: The ocean water hits against the rock causing air to be trapped in the cracks and thus causing the rock to break apart.
⇒ Abrasion: The ocean water brings small rocks with it that hit against the other rock, acting like "sandpaper" and smoothing the rock out
⇒ Attrition: The ocean water brings small rocks that hit against each other and break away to form smaller and smaller rocks
⇒ Solution: The ocean water dissolves or breaks down the rocks to turn them into a liquid and then carries them away
And because they erode and become "smaller" and/or change shape, the overburden is lifted, and now causes the rocks under it to cool down and feel less pressure. This causes the rock to change shape, often forming breaks, and as the compression is lifted from the rocks, they are able to react to the tension on them by forming these breaks, or joint fissures, or as I like to call it, a "sea gorge", which is the magic that is Lobster Cave.
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Logging Tasks
Xiaoliuqiu is an island of Geological Wonder with so many unique landforms and hidden geological treasures, with one in particular — Lobster Cave — which in fact isn't a cave at all but a magical sight of how Mother Nature creatures a "sea gorge" right here along the coast!
In order to log a Found It! for this Earth Cache, please go to the Posted Coordinates and the referenced Waypoint (WP 02) to complete the following tasks:
1. First, go to Waypoint 2, which is right above Lobster Cave, and estimate the width of the gap at its largest point between the two rocks. Based on this first initial observation and based on what you have learned about fissure joints and the way they are formed, which erosional process do you feel is responsible for this gap? How does this erosional process cause tensile stress? How did you come to your conclusions?
2. At the Posted Coordinates, which are "inside" Lobster Cave, estimate the height of "cave". Based on what you have learned about fissure joints and taking into account your initial observations during Task 1, how much overburden is there? What other additional erosional processes can you observe that would be responsible for a change in weight of the overburden to cause tensile stress? How did you come to your conclusions?
3. While at the Posted Coordinates, observe the two "pieces" of rock that "separated". Imagine them to be two big jigsaw puzzle pieces. Are you able to put them back together to form a "perfect fit? or are there areas that just won't "fit"? Based on everything you have learned about fissure joints and based on your observations during Tasks 1 and 2, why or why not were you able to put the "puzzle" back together?
4. Based on everything you have learned about fissure joints and based on all your observations, what does the limestone of this area tell you about its "tensile strength" or "susceptibility" to erosion? Also, in which direction(s) will the fissure joint enlarge and what kind(s) of shape(s) will the Lobster Cave take on would you say? How did you come to these conclusions?
5. Optional — Post a photo of yourself or a "Geocaching" item identifying yourself here at "The Lobster Cave!"
You may log a Found It! on this cache after you have completed the tasks and emailed me the answers. If there are any issues with your answers, I will contact you. And as a friendly reminder, please do not post any of the answers in your logs. Thank you for visiting this Earth Cache. I hope you enjoy your time here at The Lobster Cave!