Hueco Tanks Earthcache EarthCache
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Welcome to Hueco Tanks State Park and Historic Site. This earthcache will teach you about some of the processes that have shaped this area. There is no physical cache container to find; to log this earthcache, you will need to send us the answers to the questions below using the email or send message links in our profile. The questions are repeated in the unencrypted hint for your convenience. No tour is necessary to visit this cache, but there is an entrance fee to the park.
Cell phone reception may be limited at the park. App users may want to save this cache so they can view it offline at the park.
The entrance fee is currently (as of July 2015) $7 for visitors 13 and older; children 12 and under are free. Only 70 visitors are allowed in the self-guided area of the park at a time, so reservations are highly recommended on weekends and from November through March. Reservations are available at 512-389-8911. Call the park headquarters at 915-857-1135 for other information. You can take unguided tours of the area around North Mountain (where this cache is located) after watching a short orientation video. Guided tours are also available for an additional $2; you can only access West Mountain, East Mountain and East Spur with a tour guide. Camping is available with a reservation. Pets are welcome on paved surfaces and the picnic area trail, but must be on a leash.
The park is open 7 days a week year-round. During the winter season (1 October to 30 April) the park is open from 8 a.m. - 6 p.m. daily. During the summer season (1 May to 30 September), the park is open from 8 a.m. - 6 p.m. Monday to Thursday and from 7 a.m. - 7 p.m. Friday through Sunday.
"Many times when scouting in the Sacramento and Guadalupe Mountains I have camped for the night in the Huecos. Sometimes the water in the tanks had all been used up by travelers but there was always plenty of good cool rain water twenty five feet above the main ground tanks. Often I have watered my entire command by scaling the mountain to those hidden tanks and, filling our boots and hats with water, poured it on the flat, roof-like rocks so it would run down into the tanks below where our horses and mules would be watered in good shape. The City of El Paso, I am told, now has a fine graded road to those historic mountains and many of its citizens enjoy an outing there." -- James Gillett, "Six Years with the Texas Rangers"
HISTORY OF HUECO TANKS
The geology that formed Hueco Tanks resulted in hollows in the rock that collect rainwater, a precious resource in the dry Chihuahuan Desert. Animals gathered here for thousands of years before humans roamed the landscape. Beginning around 11,000 years ago, humans gathered here for both the water and the animals it attracted, even though back then the area was not as arid as it is now. The Jornada Mogollon people lived here around 850 years ago and left more than 3,000 pictographs, or paintings, on the rock, including over 200 painted masks, the largest collection in North America. In 1858, the tanks were used as a relay station for the Butterfield Overland Mail. The area became part of the Texas State Park system in 1969 and opened to the public in 1970.
GEOLOGY OF HUECO TANKS
Many of the mountains in the area, including the Franklin Mountains to the west and the Hueco Mountains to the east, are made of sedimentary rock (limestone and other carbonate rock) deposited when this area was covered by the Permian Sea and then uplifted by fault action. The Hueco Tanks, however, are not sedimentary. So where did this rock come from?
Based on current scientific theory, between 38 and 32 million years ago, the area around El Paso and Juarez experienced extensive volcanic activity. In some places, magma rose to the surface and produced lava flows, but not so here. About 34 million years ago, magma pushed into the surrounding limestone here, but it did not push through enough to break the surface. Instead, the magma remained below the surface and slowly cooled, crystallizing into a granite-like rock called syenite. Gradually, the layers of rock above the syenite intrusion eroded away, exposing the Hueco Tanks formation.
WEATHERING
The splitting and rounding of the syenite rock around you was caused by a process called weathering. There are three types of weathering: chemical, mechanical, and biological.
CHEMICAL WEATHERING is the breaking down of the material forming the rock itself, forming new chemical compounds. Common chemical weathering processes are solution, oxidization, hydrolysis, and hydration.
SOLUTION: In this process, also called carbonation, water dissolves the rock itself. As rain falls through the atmosphere, it absorbs carbon dioxide and becomes acidic. Rocks that contain calcite, such as limestone and marble, are soluble in acidic water. The slightly acidic water dissolves the rock, much like dissolving sugar into water. The calcite does not change while it is dissolved; it can later precipitate and reform as a solid; this is how stalactites, stalagmites, and other rock formations are formed in limestone caves.
Solution is not limited to sedimentary rock. Solution pans are small-scale, surface weathering features found arund the world in different rock types, including granite and basalt. The pits are small pockmarks, from a few millimeters to some centimeters in diameter and depth; the pans develop from pits by lateral extension in all directions but for the most part maintain a perfectly flat floor.
MECHANICAL WEATHERING
Unlike chemical weathering, which changes the makeup of the rock material itself, physical weathering doesn't change the rock, it just breaks it apart. There are many different processes that can cause physical weathering, including frost wedging, differential expansion and contraction, and unloading.
FROST WEDGING happens when water seeps into cracks in the rock and then freezes. As water becomes ice, its volume increases by 8 to 11%. This can create up to 30,000 pounds per square inch of pressure and further split open cracks in the rock.
DIFFERENTIAL EXPANSION is the result of hot/cold cycles. Simply put, rocks expand when they heat up and contract when they cool down. Different minerals in the rock expand and contract at different rates, and this can cause cracks. Fires and lightning can speed up this process from the rapid temperature changes they cause. This is more common in desert environments, where the difference between daytime and nighttime temperatures can be extreme.
UNLOADING happens when rocks are buried deep below the surface, compressed by the weight of the layers above, and then those layers are removed. This lets the rock expand, and parallel cracks form called joints. The layers of rock then start gradually peeling away like an onion. If the cracks develop vertically, it's called exfoliation; if they develop horizontally, it's called sheeting.
BIOLOGICAL WEATHERING
As the name suggests, biological weathering is done by living creatures, not by geologic processes. Trees can cause weathering -- tree roots can grow into cracks in the rock and further split them apart. Lichen can cause chemical weathering. Animals (such as humans) can also cause weathering by walking on rock surfaces and wearing them away. So can lichen. Although it looks like one organism to the naked eye, lichen is fungi and algae, living together in a symbiotic relationship. The fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering.
LOGGING THIS EARTHCACHE
To log this earthcache, send us a message (visit link) or an email (visit link) and copy and paste these questions, along with your answers. Please *DO NOT* post the answers in your log, even if encrypted; that may result in your log being deleted. There's no need to wait for confirmation from us before you log, but we will reply if you send a message or if you include your email address in the email. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Hueco Tanks Earthcache
2. Do you think the tank at ground zero is an example of chemical weathering? Why or why not? Is it currently holding water? What forms of life, if any, do you observe in it? (Please look but don't touch!)
3. Based on your reading and observations, what type or types of physical weathering affected the rocks here?
4. What evidence do you see of biological weathering? (We saw four different kinds in the immediate area.)
Photos of your visit are always appreciated.
SOURCES
UT Austin, Texas Beyond History, "Hueco Tanks: Crossroads through Time." (visit link)
Texas Parks and Wildlife, "Interpretive Guide to Hueco Tanks State Historic Site," (visit link)
Bruce Cornet, Ph.D., El Paso Community College, "Hueco Tanks Pluton and Sill and Permian Carbonate Complexes, West Texas," (visit link)
Weathering - Dr. John Douglass, Paradise Valley Community College, (visit link)
Biological Weathering, Prof. Martha Mamo, University of Nebraska-Lincoln, (visit link)
Fairbridge, R.W. "Solution pits and pans." Geomorphology, January 1968, p. 1033-1036.
This earthcache is placed with the permission of Texas State Parks, and a geocache permit is on file.
Additional Hints
(Decrypt)
[To log this earthcache, send us a message or an email through the link in our profile and copy and paste these questions, along with your answers. Please *DO NOT* post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will reply to messages and to emails if you include your email address in the message. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Hueco Tanks Earthcache
2. Do you think the tank at ground zero is an example of chemical weathering? Why or why not? Is it currently holding water? What forms of life, if any, do you observe in it? (Please look but don't touch!)
3. Based on your reading and observations, what type or types of physical weathering affected the rocks here?
4. What evidence do you see of biological weathering? (We saw four different kinds.)
Photos of your visit are always appreciated.]