Granite Roadcut Earthcache EarthCache
Granite Roadcut Earthcache
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This wide shoulder pulloff near South Lake Tahoe provides a 3-season Nevada Earthcache that explores the properties of the granite found in the Tahoe basin.
This earthcache is well above snow-line and the wide shoulders are used in the winter for snow removal thus, it should only be attempted after the snow has melted.
As you look at the granite rocks in the roadcut you should notice that the rocks are composed of minerals that you can see (a phaneritic texture is the scientific word). Thus it can be rightfully assumed that granite is an igneous rock. If you examined the rock, you can see it is a mixture of light and dark minerals. The phaneritic texture(the ability to see the individual crystals/minerals) reveals that the rock was cooled slowly, within the crust of the Earth. Together, this texture and the light color of the rock tell you that these rocks are probably granite.
The formation of granite is a complex process. It can be explained by plate tectonics. Almost all granite forms at convergent plate boundaries where subduction is taking place. Subduction is the downward plunging of one plate under another. Granite forms in two steps. In the first step, the subducting plate dives into the Earth's mantle, heats up and begins to melt. This creates magma. Magma rises, and causes melting in the rock above it, making more magma. When this magma cools, it makes igneous rocks like diorite, not granite.
In the second step, subduction, melting, and magma formation continue. However, when the magmas formed in this step rise upward, they cause part of the diorite formed in the first step to begin to melt. This, combined with melting of surrounding sedimentary and igneous rocks, makes magma which cools to become granite.
As a rule, igneous rocks that cool quickly (minutes to hours, usually above ground in the presence of water) turn into rock with little or no crystal structure (extrusive). Those that cool slowly (years+) form crystals, usually below ground (intrusive).
Granite is light in color because it is composed largely of minerals that are rich in silica. These include quartz, feldspar (orthoclase and plagioclase), and muscovite (a mica). Quartz is milky white, feldspars are pink, white, or gray, and muscovite is silvery. The dark color of granite comes from dark colored minerals, including hornblende, augite, and biotite (another mica). These minerals are green, brown, and black in color.
Granites actually come in many different colors. This variety in granites is caused by differences in mineral composition, especially in the feldspars. If feldspars are potassium-rich (orthoclase) the granites may be tan to reddish in color. If feldspars are calcium-rich (plagioclase) light to dark gray colored.
Logging Requirements:
Send the answers to #1-#5 to me through my geocaching profile.
1. List the name "GC1YH3K Granite Roadcut Earthcache" in the first line of your email. Also, list the number of people in your group.
2. View the roadcut in front of you. Based on the description, is this type of granite an orthoclase or plagioclase?
3. There is a large vein at N 38 58.467 W119 52.618, also of granite that is distinctly different than the surrounding granite. How is it different? Why?
4. Anywhere at this earthcache there is granite that has been eroded into sand. Pick up a handful of sand and examine the granuals. Based on the information above, is granite an intrusive or extrusive rock?
5. (Per current gc.com guidelines, photos are no longer allowed to be required. HOWEVER they are encouraged, since they can help clarify that you have visited the location if your other logging requirement answers are vague). Post a picture of yourself and your GPS with your log that shows the historic marker sign that is on the other side of the highway from you. DO NOT show any of the pertinent information in your picture or your log may be deleted.
Congrads to mattbrady775 on the FTF (9/2009)
I will only respond if you have incomplete logging requirements. Go ahead and log your cache
Resources:
Igneous Rocks (for kids) -- (visit link)
HowStuffWorks.com -- (visit link)
Additional Hints
(No hints available.)