Madison County Freedom Rock:
The Madison County Freedom Rock was designed by Ray Sorensen. The rock depicts paintings of several Madison County servicemembers and contains a mural of John Wayne. John Wayne’s birthplace is located just feet from the rock. Sorensen made this monument from a strong, durable, igneous granite boulder. Freedom rocks can be found in each of Iowa’s 99 counties.
What is Granite?
Granite is an igneous rock formed from magma. When molten rock cools underneath the surface of the Earth, it solidifies, and crystals grow. This rock is commonly made up of quartz, feldspar, and mica. Geologists estimate that most granites comprise 10-50% clear, transparent quartz and 45%-90% feldspar (a mineral that can have a pink, beige, or orange hue). Granite that is found near the surface is due to uplifting. This process occurs because tectonic plates push rock from the lithosphere toward the surface.
Veining in Granite:
A vein is when minerals crystallize and precipitate after being carried by an aqueous solution. This geochemical process begins after cracks develop in rock. Cracks can occur due to the effects of tectonic uplift or by weathering. Ice abrasion, water movement, or chemical weathering processes in the environment can create fractures in granite rock. Veins that form in granite rock can take two forms: open-space filling or crack-seal veins.
Open-space filling occurs primarily in brittle rock deposits. The veins that form are cavity-like and can take the form of geodes. Volcanic geodes formed in granite are due to gas bubbles in magma. Crystals like amethyst, a type of quartz with a purple hue, and calcite, a white/clear mineral, can often grow in these spaces. Open-space filling often occurs in extrusive igneous rock (like basalt) or lava that solidifies into rock once it comes in contact with the atmosphere. It can sometimes form in granite, although it is a rare occurrence. Granite and other types of intrusive igneous rock cool underground and often contain large crystal growth. In this process, rarely are any pockets of air suitable for open-space filling forms.
Crack-seal veining occurs when a mineral-saturated solution crystallizes between small fractures of rock. Crack-seal veins are often striped or banded in appearance. Pressure often fractures the rock while the newly crystallized bands of minerals “seal” the fractures in place. In granite, the minerals precipitated in this process are clear quartz or beige-colored feldspar.
Logging Tasks:
- Notice the appearance of the granite on the unpainted portions of the boulder. Search the rock for a vein. Does this boulder contain open-space filling or crack-seal veins?
- When observing from a few feet away (and without touching the boulder), what types of crystals do you notice to have formed in the vein?
- Estimate the width of the vein.
- As of June 2019, earthcaches now have required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://www.hmdb.org/m.asp?m=177142
https://www.geologyin.com/2014/11/veins-and-hydrothermal-deposits.html
https://www.geologyin.com/2015/11/what-is-granite-and-how-is-it-formed.html
https://www.fossilera.com/pages/how-do-geodes-form
https://geology.fandom.com/wiki/Vein
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