A Peek at Silica Valley

Originally shared by Johnathan Chung

A Peek at Silica Valley

Geological formation of quartzite pillars and caves

What Do You See?

I really like the #ISeeTheWorldWithScience  series started by the Science on Google+ Community. Take or select a photo and create an interesting or unique caption based on any underlying science concepts directly related to the picture. The possibilities are endless, and it’s a fun way for curious people to learn something new!

Note that your contribution doesn’t have to be detailed or complex (that’s just my own preference) and can be a quick & simple observation. So click through to the original for more challenges and tidbits, and check out past ISTWWS posts as well:  https://plus.google.com/105917944266111687812/posts/H5eBCR9qhTi

Rock Formation and Breakdown

The primary type of rock seen in the original post below is quartzite and was created by tectonic compression of quartz sandstone [1]. The pillars probably formed mostly by erosion. Ice forming within cracks, plant growth, and rain contributed to fracturing and destruction over millions of years.

Karst development [2] is a type of geological formation where layers of rock are dissolved. Karst is more common for soluble limestone bedrock, but apparently can happen with quartzite under very specific conditions. Generally, quartzite tends to be one of the more highly resistant minerals and rocks to weathering.

Quartzite in Water

At the molecular level, quartz can be hydrated to form silicic acid: 

SiO2 (s) + 2 H2O (aq) = H4SiO4 (aq)

The extremely slow dissolving rate is due to bond breaking and hydration of silica (SiO2) at the surface. Silicic acid (H4SiO4) is very weakly acidic and a minor contributor to the more dominant mechanical weathering process because silicic acid barely ionically dissociates under normal karst conditions.

Cave Formation: Speleogenesis

This karst dissolving process actually becomes a main factor in the formation of quartzite caves [3], which could be near the location depicted in the photo (once again, click through to the original post). They are relatively rarer, though, compared to limestone or dolomite caves [4].

Side note: limestone is made of calcite, which is calcium carbonate or CaCO3. Dolomite is calcium magnesium carbonate, or CaMg(CO3)2. Over 80% calcite content is one factor that tends to favor karst conditions [5].

Disillusioned with Dissolution

If you’re anticipating watching this quartzite chemical reaction in action, don’t hold your breath. Saying quartzite karst is slower than molasses in January is quite an understatement. (That’s two colloquialisms in one paragraph for you!)

In the laboratory setting, an estimate of the rate of quartz dissolution by water at 25°C is 10^(-17) moles per second for each cm^2 surface area [3]. In these isolated conditions, that turns out to be 0.02 micrograms dissolved per squared centimeter of surface exposure over one year (I told you it was slow).

In nature, however, organic acids from dense vegetation in quartzite terrain increase this rate substantially. Microbes in the soil also contribute by releasing (smelly) hydrogen sulfide gas, which turns into hydrosulfuric acid. Hydrothermal conditions from deeper layers can increase the temperature to several hundred degrees Celsius and speed up the reaction too.

In summary, quartzite rock morphology above ground is more likely influenced by mechanical erosion through weathering, while underground changes are due to chemical dissolution and corrosion.

Photo Location

The photo linked to in the original post appears to have been taken somewhere within Zhangjiajie National Forest Park in China [6] and possibly within a smaller popular historic area called Wulingyuan [7]. The UNESCO World Heritage site has some more great information on plants and endangered species found in that area [8].

References:

I recommend reading reference #4 as an interesting and well-written introduction to the factors involved with the science of cave formation.

[1] Quartzite – http://en.wikipedia.org/wiki/Quartzite

[2] Karst – http://en.wikipedia.org/wiki/Karst

[3] Encyclopedia of Caves and Karst Science, p. 622 – http://goo.gl/6oMIz8

[4] How Caves Form by Eniscuola Energy and Environment – http://www.eniscuola.net/assets/3860/pdf_caves_2.pdf

[5] Cave Formation by University of Wisconsin Eau Claire – http://www.uwec.edu/jolhm/cave/caveform2.htm

[6] Zhangjiajie National Forest Park – http://en.wikipedia.org/wiki/Zhangjiajie_National_Forest_Park

[7] Wulingyuan – http://en.wikipedia.org/wiki/Wulingyuan

[8] UNESCO Wulingyuan Scenic and Historic Interest Area – http://whc.unesco.org/en/list/640/

#ScienceEveryday   #Quartzite   #Caves   #Karst   #Geology  

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