Pilanesburg Dyke Swarm EarthCache
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This cache is located in suburban Joburg and forms part of the sidewalk. Do be observant as vagrant muggles sometimes use this area as a resting place.
These outcrops and boulders are part of the Robinson Dyke, one of the many sub-alkaline and femic (i.e. iron and magnesium rich) dykes belonging to the 1000 – 1400 million year old Pilanesberg Dyke swarm. This swarm extends in a northwesterly direction to the Pilanesberg Complex in the North West Province (around the Sun City / Lost City / Pilanesberg Reserve area) – {see Pilanesberg National Park Alkaline Ring Complex Earthcache by cincol (GC18GG9)}. And then onwards towards Botswana. These dykes, which are commonly composite in nature, have dark femic margins of tholeiitic basalt or andesite grading into a more salic (acidic) centre, which contains finer grained, reddish-grey syenite or trachyte. In this exposure, only the femic (basaltic) phase is visible and the dark rocks have a similar texture to that of a diabase dyke. The dykes intrude the approximately 3200 million years old granitic rocks of the Johannesburg Dome and caused the growth of the large feldspar crystals (porphyroblasts) and the reddish-pink iron discolouration seen in the rocks flanking the dykes. This dyke can be traced intermittently from the old Robinson Deep Gold Mine in the Booysens area to well beyond the Northriding area – a distance of over 20km.
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
A dyke is a vertical igneous intrusion into older existing “bed rock” or country rock (i.e. rock that was already there. A horizontal intrusion is called a sill and a single “shaft” intrusion is called a pipe.
A dike swarm or dyke swarm in geology is a major group of parallel, linear, or radially oriented dikes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event and are magmatic and stratigraphic. Such dike swarms may form a large igneous province and are the roots of a volcanic province.
The occurrence of mafic dike swarms in Archean and Paleoproterozoic terranes is often cited as evidence for mantle plume activity associated with abnormally high mantle potential temperatures.
Dike swarms may extend over 400 km (249 mi) in width and length. The World's largest dike swarm is the Mackenzie dike swarm in the Northwest Territories of Canada, which is more than 500 km (311 mi) wide and 3,000 km (1,864 mi) long.[1]
The number of known giant dike swarms on Earth is small, only about 25. However, the primary geometry of most giant dike swarms is poorly known since plate tectonics are thought to destroy them.
In order to qualify to log this cache, you need to answer the following questions and email the cache owner. Any logs not accompanied by an email will be deleted. Also note that we are not looking for a geological thesis – just your own investigation and learning based on your knowledge and observation. And we trust that you enjoy and learned something.
1) Take a photo of you and your GPSr at this spot with the outcrop visible and include in your log. [Optional]
2) Describe the weathering of these rocks – i.e. are they round, flaky, blocks, angular, large, crumbly (friable) etc. Why or how do you think this occurred?
3) This forms part of a dyke swarm. How would you think a dyke swarm differs from a normal dyke?
Additional Hints
(Decrypt)
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