Golden Rock EarthCache
Hidden : 3/23/2011
Difficulty:
2.5 out of 5
Terrain:
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The Golden Rock is one of Myanmar's most sacred sites. Balancing its great weight on the cliff edge, the Golden Rock is an extraordinary natural feature. According to the buddhist legend, it is only kept in place by a hair of Buddha. However, the Golden Rock is a large boulder of granite, which was formed by spheroidal weathering.


Weathering and spheroidal weathering

Weathering in general is influenced by the rock type and structure due to the composition of minerals with different stabilities, by the slope, the climate (chemical reactions usually run faster when it’s warm and humid) as well as by the existence of burrowing animals like rodents, earthworms and ants.
Spheroidal weathering is also known as concentric or onion-skin weathering. During spheroidal weathering concentric shells of weathered rock are formed by fracture of altered weakly bedrock. This phenomenon can be observed on almost all rock types including gneiss, schist, andesite, sandstone, and greywacke and in almost every climate. However, it is most common in granites and basalts. Spheroidal weathered granite boulders have been identified in a variety of climatic regions on all continents except Antarctica.


Process of spheroidal weathering

Step 1
At the beginning granite is already fractured in the subsurface. These fractures become more intense with the time and build a network that splits the granite into blocks of cube-like shape.


Step 2
When the fractures come into contact with ground water, that seeps into the cracks, the building of boulders starts. Since the corners of the granite cubes have 3 joints that intersect, the corners decay most. The decay of the corners results in granite boulders of spheroidal shape. The process – including the rounding – takes place meters beneath the surface (subsurface weathering), where the minerals are weathered into grus (granate sand) by chemical decay.


Step 3
The last step is erosion of the decayed granite (grus) that exposes these spheroidally-weathered boulders at the surface.



But how did the granite get here?

To answer this question, we have to look at how the mountains of Myanmar have been built some billion years ago:


Tectonic plates and mountain building in Myanmar

The earth consists of tectonic plates. Since these plates move steadily, there are 3 types of possible interaction between these plates:

1) The plates move away from each other. In this case, between the plates new crust is produced (constructive boundary). Example: mid-Atlantic ridge
2) The plates collide with each other (destructive boundary). The standard case with destructive boundaries is that one plate descends under the other (subduction zone).
3) One plate slides past the other without any creation or destruction of crust taking place. This phenomenon is called conservative boundary. Example: San Andreas fault along the West coast of California

The tectonic plates are shown in the following figure:


Myanmar is situated at the interface between the Indian and the Eurasian plate. Already in the Precambrian period the Indian plate slided northwards underneath the Eurasian Plate. This subduction zone caused the mountain building in the West and in the East of the country (West: Western fold belt in Arakan/Rakhine region, East: Shan-Tenasserim massif in Shan and Mon region). So these mountains - where the Golden Rock belongs to - are old mountains. Much later the Northern mountains (Himalaya) were built, when both plates collided and elevated crust. This happened only during the Cenozoic period. So the Himalaya mountains are young mountains. Actually whole Myanmar was once born from the sea by subduction of the Indian underneath the Eurasian plate.


Stratigraphy of the Eastern mountains of Myanmar

Precambrian rocks build the basis of the Eastern mountains of Myanmar in the Shan and Mon region. During the Paleozoic period, limestone and tuff have been deposited. During Carboniferous, deposition of granite (early Carboniferous), respectively granite and further limestone (late Carboniferous) accured. During the Mesozoic period in late Jurassic and late Cretaceous as well as during the early Cenozoic period still more granite has been deposited.

The following figure shows the granite intrusion in the South-East of Myanmar which is probably the largest of its kind in Myanmar. The Golden Rock is marked in red.





To log this Earthcache...

Send me the answers to the following questions:
1) Calculate the weight in tons of the Golden Rock. (Assume it as a sphere.) Tell me how you calculated it.
2) There is another relict of spheroidal weathering near the Golden Rock, not more than 200 m away. Find it and tell me its coordinates.
3) The Golden Rock is situated on top of a hill. Can you imagine a reason why weathering takes place faster on top of a hill than in a valley?
4) Look out on your way to and from the Golden Rock. Which sign can you find to show that the hills in the area consist mainly of granite?
Optional: You are invited to post a photo of you and your GPS in front of the Golden Rock. (According to the Geocaching guidelines, a photo does not compensate for answering the questions.)



Enjoy Myanmar - a truly unique and wonderful country!




References:
R. C. Fletcher, H. L. Buss, S. L. Brantley: A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation, Earth and Planetary Science Letters 244 (2006) 444-457
F. Bender, 1983: Geology of Burma, Gebrüder Bornträger Berlin
K. L. Zaw, K. Z. Lat, 2010: Report on Regional Geology of Myanmar, Gadjah Mada University
G. Park, 2008: Introducing Geology, Dunedin Academic Press. Edinburgh
http://earthsci.org/education/teacher/basicgeol/weasoil/weasoil.html
http://alliance.la.asu.edu/slides/stop03/3b/3b.html
http://www.vdf.ethz.ch/service/erde/data/Gesteinskreislauf/Verwitterung/Verwitterung_chemisch_wollsack.htm
http://www.sacred-destinations.com/burma/golden-rock-kyaiktiyo.htm
http://www.mp.haw-hamburg.de/pers/Kaspar-Sickermann/mm/emm005.html




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