EDUCATIONAL LOGGING REQUIREMENTS
In order to substantiate your visit and comply with the educational requirement for Earth Caches you have to submit your answers to the following questions to the cache developers via their profile:
For purposes of logging this cache you must answer the following five questions.
1) What is the predominant colour of the granite?
2) Based on the colour would the granite consist mainly of Orthoclase or Plagioclase feldspar?
3) What is the second most noticeable colour? What constituent of granite would be this colour?
4) Granite includes quartz. Is the quartz noticeable? If not, why?
5) Look at the crystals in the stone. Measure their length, average their size and then state whether the rock is coarse, medium, or fine grained using the following scale coarse crystals over 5mm in length, medium crystals between 1-5 mm in length , or fine crystal less than 1mm in length .
As an optional question, but only if you have logged the Prince of Wales Obelisk #1 cache. What is the major difference between the rock making up the obelisk and the rock making up the trough? Give reasons.
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The Prince of Wales Obelisk was brought to South Africa by the founder of the Eastern Province Herald, John Paterson to be placed on the grave of his partner, George Kemp. The Kemp family donated it to the city. The City decided to erect it in front of the city hall Market Square to commemorate the Prince of Wale’s marriage to Princess Alexandra. This was done on 22 May 1863. Four water troughs were added in 1877/8 and were filled with water during a “tap ceremony” when water was first piped to Port Elizabeth. The obelisk was removed in 1921 and stored until 1975 and now stands at Bay World. One of the water troughs was placed on the lawn at the Walmer Town Hall. The location of the other three troughs is unknown. You are now standing at the Obelisk
While the history and the origin of the Obelisk is fascinating, this time we are interested in the rock that it is made out of.
Granite is a common type of felsic intrusive igneous rock which is granular and phaneritic in texture. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Felsic is derived “Fel” from feldspar, and “sic” from silicon, generally light coloured.
Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist.
Many rocks identified as “granite” using the introductory course definition will not be called “granite” by the petrologist - they might instead be alkali granites, granodiorites, pegmatites or aplites. A petrologist might call these “granitoid rocks” rather than granites. There are other definitions of granite based upon mineral composition.
The chart below illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles can each have a range of abundances.
The word “granite” is used by people who sell and purchase cut stone for structural and decorative use. These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments and many other products.
In the commercial stone industry a “granite” is a rock with visible grains that is harder than a marble. Under this definition gabbro, basalt, pegmatite, schist, gneiss, syenite, monzonite, anorthosite, grannodiorite, diabase, diorite and many other rocks will be called “granite”.
Granites can be predominantly white, pink, or grey in colour, depending on their mineralogy. By definition, granite is an igneous rock with at least 20% quartz and up to 65% alkali feldspar by volume. Granite differs from granodiorite in that at least 35% of the feldspar in granite is alkali feldspar as opposed to plagioclase. Potassium feldspar will many granites a distinctive pink color, while plagioclase feldspar which contains sodium or calcium would be white. Mica would appear in granite as black speckles.
The Obelisk is made of a granite of a very distinctive colour.
To claim this earthcache as a find please send me an email with the answers to the five questions, your caching name and your email address through my profile.
You may log your find once you have visited the site and determined the answers to the questions. Logs not supported by the necessary email will be deleted. Please do not reveal the answers in your log!
The information to answer the questions can be found by looking carefully at the stone and reading the description of the cache
References
Wikipedia
Geology.Com for the diagram
Afrikaans Wikipedia