Rockin' at Inks Lake! EarthCache
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A metamorphic Earthcache at Inks Lake State Park's Lake Trail (300s campsite and South Fishing Pier area) about the Llano Uplift.
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| Road Map Superimposed Over The Llano Uplift Geology Table |
Pre-Cambrian Time
The geologic record begins in Texas a little over a billion years ago, when thick sequences of coarse, then fine sediment were dumped into an ancient sea bordering a continent. Eventually the continent collided with either another continent or an ocean margin in a plate tectonic event that buried, squeezed and heated the borderlands, including the sediment piles. The collision built mountains and created metamorphic schist and gneiss out of the deeply buried sediments and generated molten magmas which cooled to form granite bodies. Erosion then flattened this range to a table-top by Cambrian time. All these rocks are displayed in the Llano country of central Texas.
Metamorphic rocks form deep in the earth where high temperature from the magma (molten rock material under the earth's crust), great pressure from layers of rock piled on top of one another and/or a shift in tectonic plates, and chemical reactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at 7.4-9.9 miles or 12-16 kilometers beneath the earth's surface. They begin changing at temperatures of 212°F/100°C (temperature at which water boils) to 1472°F/800°C.
Some of the major characteristics of metamorphic rock are:
May have alternate bands of light and dark minerals
May have bent or curved foliation (sometimes split into many layers that look like a stack of pancakes with different mineral grains running through the rock)
May be composed of only one mineral
May have layers of visible crystals
Rarely have fossils
Rarely have pores or openings
Some examples of metamorphic rock are: Gneiss, Marble, Quartzite, and Schist.
Granite becomes Gneiss (pronounced 'nice')—foliated, coarse-grained, non-micaceous (not separated easily into thin sheets). Gneiss is usually light in color. It looks like it has ribbons or stripes of minerals running through the rock. The grain size is usually fairly coarse. Gneiss usually breaks into blocky pieces, not along the layers. Unlike granite, in which the crystals are randomly arranged, the crystals in gneiss are lined up and in layers. Gneiss is a tough and hard rock.
Limestone becomes Marble—nonfoliated, composed of interlocking calcite grains. Often pure white. It may be streaked or patchy gray, green, tan, or red. Marble is fine grained to very coarse grained and crystals are usually easy to see. The rock is soft; it will not scratch glass (quartzite may look like a fine grained marble, but easily scratches glass). The powdered marble will often fizz with white vinegar. If it does not fizz, it may be dolomitic marble.
Quartz sandstone becomes Quartzite—nonfoliated, composed of interlocking quartz grains. If the quartzite is pure quartz it is white. It may have a yellowish to reddish color if it contains iron minerals. Rarely, it is black if it contains a lot of magnetite. Sometimes, using a magnifier, the grains of sand from which it formed can be seen. The rock breaks through the grains, not around them (sandstone breaks around the grains). Quartzite often shows lighter colored flakes on a broken surface, where air is behind a very thin chip. Unlike marble, quartzite is very hard and easily scratches glass.
Shale becomes Schist—foliated, coarse-grained, micaceous (separated easily into thin sheets). Top and bottom layers are usually a silvery, to green, to brown, to black mica, or a green to very dark green chlorite. The micas are often in small flaky crystals. Layers are usually thin, often with lens like layers of quartz between the mica layers. Layers may be somewhat wavy. Grain size varies from medium to coarse. Schist usually splits easily along the layers of mica, unlike gneiss.
To log this Earthcache, please Message me your answers to the following questions: (logs, where no answers were e-mailed within 7 days of the on-line log, will be deleted)
#1. Study the graph Cross Section of the Llano country west of Austin. Start at the given coordinates. Survey the area around you. List the characteristics of Metamorphic rock you see present?
#2. What is the majority of Metamorphic rock you see: Gneiss, Marble, Quartzite, or Schist? How do you know?
#3. Move to Waypoint 2. Compare the two formations present by recording what you see, feel, smell, and taste? Why do you think they are different?
#4. Move to Waypoint 3. Here, you see an “X.” What rock comprises the “X?” Compare the “X” with the other rock by recording what you see, feel, smell, and taste.
#5. Move to Waypoint 4. Do you see water present?
• If yes, what metamorphic characteristic lends to the collection of water?
• If no, could water hold here? Why isn’t there water present?
Describe two other things you see besides rock, and offer a logical reason as to why they are here.
For more information about the topics covered in this Earthcache, visit these links:
What is a gneiss?
Geophysics (to the right of the image under "Connections to:").
What is a rock?
Earth Facts for Kids.
Vernal Pools.
For more Earthcaches about the Llano uplift, visit:
GCZP2N, The Enchanted Rock,
GCZWYY, Indians, Bandits, and a Speakeasy Earthcache,
and GC2BD0D, Packsaddle Schist.
| Congratulations to BulldogBlitz on ¡FTF! |
Bibliography
Allison, David T. “Index of / geography/allison/images.” Department of Earth Sciences, Geology. University of South Alabama, 26 Jan. 2013. Web. 11 Mar. 2013. http://www.usouthal.edu/geography/allison/images/LLANO-1.jpg.
Morelock, Jack. “Highway 71-Llano to Marble Falls.” Geological Oceanography Program. Universidad de Puerto Rico, 2005. Web. 11 Mar. 2013. http://geology.uprm.edu/Morelock/71MrbFl.htm.
Peck, Don. “The Rock Identification Key.” Collector's Corner. Mineralogical Society of America, 2001. Web. 18 Apr. 2013. http://www.minsocam.org/msa/collectors_corner/id/rock_key.htm.
Additional Hints
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