Terms to understand:
Supergroup: Rocks of similar age and which occur in the same area are normally grouped together by geologists and the largest such grouping is known as a Supergroup.
Amygdales: When lava erupts from a volcano or from fissures, the lava is extremely hot and bubbly. These bubbles can easily pop forming holes called vesicles. While the lava cools down and starts forming igneous rock, other minerals can fill these vesicles and then these holes with minerals in them are known as amygdales.
Phenocryst: Large crystals which are present within a fine-grained igneous rock.
Porphyritic lava: Lava which contains large grained crystals within a fine-grained rock matrix.
The coordinates will bring you to one of the few exposures of Ventersdorp lava in eastern Johannesburg. Here there are outcrops of igneous rock belonging to the Klipriviersberg Group which form part of the Ventersdorp Supergroup. This rock formed approximately 2350 million years ago during volcanic activity which extended over a vast area of the Highveld – from Ventersdorp in the North West, Evander in Mpumalanga and Welkom in the Free State!
Extent of the Klipriviersberg group, of which this balsaltic lava forms a part.
These basalt rocks exposed here are fine-grained and grey in colour and are composed mainly of albite, epidote and chlorite. They contain large white rectangular crystals and plenty of white spots (with some green spots too). These are feldspar phenocrysts and white and green amygdales respectively. These amygdales contain either quartz (the predominant white spots) or chlorite (the green spots) and were formed when lava bubbled and the holes formed by popped bubbles were filled in by quartzite and chlorite.
Geological map of the area. This Earthcache site is indicated by the red circle.
Other rocks are also exposed at this location which belong to the Witwatersrand Supergroup but are not the focus of this EC. There is also an exposure of white quartzite rock right next to the GSSA sign which should be easy to spot if the grass isn't too long.
LOGGING REQUIREMENTS
In order to substantiate your visit, please submit your answers to the following questions to me via the Message centre or an email. In your message/email please mention the Earthcache which you are sending answers for. Any logs not accompanied by answers within 7 days will be deleted.
1) Describe the basaltic lava rocks – grain size, colour and texture.
2) Take a closer look at the amygdales. Describe their shapes and colours.
3) Amygdales are formed from bubbles in lava although often they are not circular. What do you think causes them be non-spherical?
4) Look for some feldspar phenocrysts and describe their colour, shape and length (use a ruler here). How did you distinguish them from the amygdales?
5) Why do you think the grain size of basalt is so different from the feldspar crystal size?
OPTIONAL: Any non-spoiler photos and/or a photo of you with your GPS by the GSSA sign will be appreciated.
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References:
1) Goodwin, A.M., Precambrian Geology: The Dynamic Evolution of the Continental Crust, Academic Press, 1991.
2) McCarthy, T.S., Stanistreet, I.G., Cadle, A.B. and Durrheim, R., The geology of the Ventersdorp Supergroup (Bezuidenhout Valley) outlier to the east of Johannesburg and its relevance to the tectonics of a Wiwatersrand Basin margin, South African Journal of Geology, 93, 1, 289-309, 1990.
3) Mendelsohn, F. and Potgieter, C.T., Guidebook to Sites of Geological and Mining Interest on the Central Witwatersrand, The Geological Society of South Africa, 1986.
4) Van der Westhuizen, W.A., De Bruiyn. H. and Meintjies, P.G., The Ventersdorp Supergroup: an overview, Journal of African Earth Sciences, 13, 1, 83-105, 1991.
