Boulders EarthCache
Hidden : 1/10/2014
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The Boulders Shopping Centre is built on top of and around one of South Africa's heritage sites. A rock outcrop which was part of the early settlements in the country dating back to pre historic times. The centre derives its name from these Boulders.

 

Cross-Section of the Johannesburg Dome
Cross-Section of the Johannesburg Dome





This idealised cross-section of the Johannesburg Dome, between Johannesburg and Tshwane, covers a distance of about 70km and is centred on an inlier of Archaean granite-gneiss and greenstone. On this ancient basement to the south, the Witwatersrand Supergroup was deposited and now dips southwards at about 30ᴼ, while to the north the somewhat younger Transvaal Supergroup dips northwards at around 20ᴼ. On the western side of the dome, is Swartkops. A small, isolated outlier of Witwatersrand rocks emplaced by thrusting from the south.
Taken from: "Geological Journeys by Norman and Whitfield"


At this cache location, the Halfway House Granite Dome’s Boulders can be viewed.

To get to the EC location, enter the basement parking of the mall at the given reference point coordinates. You will not be able to use your GPS to get to GZ after entering the basement parking area. Enter the basement and continue straight down to the second level and park right at the boulders visible from the parking area.

It is not recommended to do this Earthcache outside of the mall’s shopping hours, although the entrance and parking area should be open as the nearby gym is open ‘till late at night. Shopping hours are: Mon-Fri 9am-6pm, Sat 9am-5pm, Sun & Public holidays 9am-2pm. Parking for the first 30min is free.



Granite is a common type of intrusive, felsic, igneous rock which is granular and phaneritic in texture. Phaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic (which is too small to see with the naked eye). This texture forms by slow cooling of magma deep underground in the plutonic environment.
Granite rock consists mainly of quartz, mica, and feldspar. Granites can be pink to grey in colour, depending on their chemistry and mineralogy. By definition, granite is an igneous rock with at least 20% quartz by volume.
Outcrops of granite tend to form tors and rounded massifs. Granite is nearly always massive, hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is between 2.65 and 2.75 g/cm3, its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3-6 • 1019 Pa•s. Melting temperature is 1215 - 1260 °C.
The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock. Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks.
Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar and plagioclase feldspar. True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase, the rock is referred to as alkali granite. When a granitoid contains less than 10% orthoclase, it is called tonalite
Granite is currently known only on Earth, where it forms a major part of continental crust. Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.
Granite is an igneous rock and is formed from magma. Presence of water is required for the formation of granite from basalt.
Taken from: "wikipedia"



To claim this Earthcache please complete the following tasks:

1) Observe the surrounding area and determine whether these large granite boulders are course-, medium- or fine grained?

2) How did the granite dome around this area form?

3) This site is very old and some artifacts of age have been found in the surroundings. There is a display here showing some of the human created artifacts found. What is the item in the middle glass container?

If you like, please share your experience of visiting this Earthcache site by posting some photos (please don’t add photos that will give answers to the above questions)

Please send your answers to 'teamginger@gmail.com' (or use the geocaching mailing facility) for approval of your log (don't post them in your log). Find-logs posted without the required answers will be deleted, thanks for your understanding.
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