Welcome to Xiaoliuqiu — An Island of Geological Wonder! (LIMESTONE CAVES)
In this Earth Cache, you will learn about chemical weathering — more specifically something known as dissolution and the "magic" behind this phenomenon when it comes to making limestone caves here at Black Devil Cave. Yep, you read that right! The place where an abundance of limestone caves are is in an area known as Black Devil Cave! So upon learning all there is to know about limestone caves and the forces of "dissolution that sculpted "Black Devil 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 is a Limestone Cave Exactly and Why does the "Perfect" Cave Exist here at Black Devil Cave?
First, we have to understand the unique nature of Xiaoliuqiu Island as a whole. Xiaoliuqiu is a coral island, the only one located here in Taiwan. Basically many, many, many years ago, through the powers of Mother Nature, the coral limestone under the ocean raised and exposed itself and created this island. And with this exposed limestone came a plethora of landscapes — one notably this entire area where Black Devil Cave is as it is the only one with a "full" limestone cave where virtually zero sunlight can enter! Scary, eh? But at the same time so cool, right? An actual real cave in the sense of being in pitch-black conditions! So, how did this "perfect" cave come to be? In simple terms, rainwater seeped into the cracks of the rock and did so over and over and over again, cutting out a hole under those crack and making a cave! Think of it like an ant colony — the ants (the rainwater) dig away (erode) at the soil (the rock) and over time "tunnels" or rather "caves" are carved out underneath the ground, thus giving rise to these very unique landforms.
So Black Devil Cave is a limestone cave, meaning it is composed of limestone — a kind of rock that is susceptible to erosion — particularly chemical erosion when rainwater becomes acidic by reacting with the carbon dioxide in the air and thus carving away or dissolving limestone as it hits it. So, here at Black Devil Cave, you will see the only "perfect" or rather "true" limestone cave as it is completely dark inside!
So now that you know how Black Devil Cave was formed and its composition, let's take it one step deeper here and talk about the cool things of this cave and its speleothems — or morphological features. You have definitely heard of two kinds of these morphological features — stalactites and stalagmites. Stalactites grow downward from the ceiling whereas stalagmites grow up from the ground. Basically, as that rainwater seeps down into the rock and dissolves the limestone, that rainwater "collects" those minerals from the dissolved limestone and as it drips down with those minerals inside it, some of those minerals get "stuck" to the ceiling while others continue to drip down. The ones that get stuck eventually create stalactites while the ones that drip down eventually create stalagmites.
So, to sum up here in a little more detail and geologically kind of way — speleothems form because of water. Rainwater seeps through cracks in the rock. As it passes through the organic material here at Black Devil Cave, it picks up carbon dioxide gas, creating carbonic acid, which causes chemical weathering or the breaking away of the rock material. As this acid passes through joints and cracks in the limestone, the mineral calcite is dissolved from the limestone rock where Black Devil Cave is formed. When this water that now holds the dissolved rock is exposed to the air in the cave, it releases carbon dioxide gas, much like when a bottle of soda is opened. As the carbon dioxide is released, calcite is precipitated or redeposited on cave walls, ceilings and floors as is the case here at Black Devil Cave. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, thus forming a stalagmite.
Speleothems form at varying rates as calcite crystals build up. Several factors can determine the rate of growth. Two important factors are: one — the temperature outside, which you will notice here upon Xiaoliuqiu will be quite tropical at any given time — and two — the amount of rainfall, which can vary of course since Xiaoliuqiu is a tropical island. The shapes of speleothems are determined by how the acidic water enters the cave (by dripping, seeping, or splashing) and how the water stands or flows after entering the cave. And here at Black Devil Cave you will notice that "unique" shapes of the stalactites and stalagmites.
One last thing to note is that most scientists believe that the color of speleothems is determined by the mineral content. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange, and black to the color of speleothems, which is more present here at Black Devil Cave.
So there you have it — a "real" cave in the sense of having virtually no sunlight but once you shine your light upon those walls a whole world of geological wonder awaits!
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Logging Requirements
Xiaoliuqiu is an island of Geological Wonder with so many unique landforms and hidden geological treasures, with one in particular — Black Devil Cave being the only "true" cave of the island.
Please note that this area is open from 7:00 a.m. ~ 5:00 p.m. daily AND that GPS will work fine here. Also, you will need a flashlight and a sense of adventure, so come prepared to have fun!
P.S. The entire length of the cave is about 20 paces or so and you'll be able to see super faint sunlight at any given time indicating the entrance/exit, so don't be too scared while inside.
1. First, as you walk down the stairs to enter the cave, touch the rocks directly to your right (which are the outside cave wall.) Describe the color, their texture, and any other unique characteristics you notice. Based on what you have learned about limestone caves and based on your initial feel of the outside of the cave, how do you think the rocks on the inside will feel and look like? Why do you think this? How did you come to your conclusions?
2. After carrying out the "experiment" and completing Task 1, turn on your flashlight and enter the cave. As you do so, feel the inside rock walls. Describe their color, their texture, and any other unique characteristics you notice. Were your initial assumptions about the rocks correct? Why or why not? How did you come to your conclusions?
3. While inside the cave, walk to and fro, noting the entrance and the exit. Where did you find the water source? Describe it — how big is it and how much water is flowing and/or dripping? Based on what you have learned about this limestone cave, what do this water source and its size tell you about how the cave was formed? How did you come to your conclusions?
4. While inside the cave, identify stalactites and stalagmites. Describe their color, texture, shape, and size. Based on what you have learned about these speleothems, what mineral(s) do you feel are found inside these speleothems? Also, which one did you find a larger amount of — the stalactites or stalagmites? What does this tell you about the cave itself? How did you come to your conclusions?
5. While inside the cave, what "strange" rock formation(s) do you see? What might these "strange" rock formations tell you about the actual use of the cave at one time and how this use could have affected the overall way the cave was changed geologically? How did you come to your conclusions?
6. Optional — Post a photo of yourself or a "Geocaching" item identifying yourself here at "Black Devil 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 Black Devil Cave.