Fissure EarthCache
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
Fissure earthcache is a fun experience and educational field trip along the Cibolo Creek streambed. It should take less than an hour to complete and will show you firsthand how nature makes little rocks out of big rocks.
To log this earthcache, first read the information about fissures below. Next, make the field trip to Cibolo Creek, accomplish the earth tasks and e-mail us your observations. Finally, although not required, we would appreciate you uploading a photo of your adventure.
FIELD TRIP STOPS:
1. Observe the long fissure at N 29° 33.719 W 098° 17.885 (GZ). Sense the enormity of the bolder by walking the full length of the fissure. Hypothesize what phenomenon caused this fissure to form?
2. Observe the fissures at N 29 33.727 W 098 17.888. What type of weathering is at work here? Also, what sediment likely evaporated in this fissure?
3. Observe the fissure at N 29 33.751 W 098 17.918. What specific type of mechanical weathering is at work here?
4. Observe the boulders at N 29 33.723 W 098 17.949. Try to visualize the past and future of these boulders given the present evidence of fissures and weathering.
5. Observe the fissure at N 29 33.676 W 098 17.914. What type of mechanical weathering caused the dramatic contours in the rock surfaces?
6. Finish at N 29 33.663 W 098 17.722 to observe the end result of weathering on fissures.
FISSURES:
The main species of rock around Cibolo Creek is limestone. This sedimentary rock formed in ancient bodies of water such as rivers, lakes or oceans which once covered the area we now call Texas. Sand, mud, minerals, rocks, and even dead plants and animals were carried to these bodies of water by wind, flowing water, ice, or gravity where they sank to the bottom. Over scores of millennium these sediments formed layers that built on top of each other. The weight of all of these layers of rocks, minerals, dead animal skeletons, and plants compressed the lower layers until they hardened into rock. These bodies of water dried up after sedimentary rock was formed; and later, Cibolo creek began to flow through the same area.
Around GZ you will observe a streambed composed of what appears to be a single massive solid rock. As you continue along the streambed on your field trip, you will observe more broken rock ultimately way to boulders, cobble and gravel. Just how did this transformation take place? The answer lies in the name of this earthcache.
In geology, a fissure is a long narrow crack or fracture in rock. Specifically, it is any discontinuity plane in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. Fissures are often caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Once a fissure develops, weathering begins to break down the rock.
The main types of geological weathering around Cibolo Creek are chemical and mechanical. A common type of chemical weathering is Crystal Formation. Most water contains dissolved minerals like salt. When water in rock fissures evaporate, the concentration salt increases, which leads to crystallization. Other minerals can dissolve and then evaporate in the rock causing discontinuities.
An example of mechanical weathering is Frost Wedging. Water expands in volume by 9% when it freezes and as it expands and can exert 30,000 psi on the rock around it. Since water seeks low points, water will drip in and find the deepest spots in the rock. As the seasons change, repeated freezing and thawing can lead to deep vertical fissures as cracks are extended downward. Another type of mechanical weathering is Thermal Expansion and Contraction. Heating causes rock to expand and cooling causes it to contract; both cause fissure cracking. The resulting cracking looks similar to Frost Wedging. A special type of mechanical weathering is biological weathering. If water can enter a fissure, so can roots. Roots exert an enormous amount of pressure on the rocks which lead to wedging also similar to Frost Wedging. Finally, erosion contributes to weathering as flash floods and other water flow acts on rocks like sandpaper on wood. All of these forms of weathering ultimately break down the rock. (Information from Encyclopedia Britannica,Yahoo Answers, WiseGeek, Wikipedia and eHow websites.)
Additional Hints
(No hints available.)
Treasures
You'll collect a digital Treasure from one of these collections when you find and log this geocache:
Loading Treasures