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Joburg Volcano

A cache by Carbon Hunter Send Message to Owner Message this owner
Hidden : 07/02/2008
2.5 out of 5
1.5 out of 5

Size: Size:   not chosen (not chosen)

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Geocache Description:

Located in a small park at the top of the Bruma Lake area. Park just after the boom gate next to the security guard (who can keep an eye on your car).

There are actually three distinct features worthy of an Earthcache here – although one is submerged most of the year, and the other is a little dangerous to get to being in the river bed. So I will concentrate on the one aspect (the laminated volcanic ash deposits), but mention the other two feature for those that may be inclined (or fortunate due to river flow conditions) to seek them out.
NB: Please note that open streams can be DANGEROUS, especially in periods of high flow and the summer when flash floods can come downstream very quickly due to the concentrated catchments system from storm water collection. Please also note that urban water sources in Johannesburg can contain a lot of undesirable matter – so minimize (or completely avoid) contact with the water for health reasons! There is a large grassed area for kids next to the stream – but ensure they are supervised.

VOLCANIC ASH – This is visible below the sign and a little further upstream just after the sharp turn to the right. Lava outpourings from any volcano are often interspersed with periods when great quantities of ash are released from the vent. An accumulation of such ash from a very ancient volcano (the Ventersdorp lava in this area is about 2200 million years old), is exposed as a flat outcrop in the stream here at this locality. The deposit is very well layered, due to the natural separation of coarser from finer particles during their descent (after being blown into the air on eruption) and deposition. Although originally deposited in horizontal layers, these rocks have since been steeply tilted, so that the layering appears as different coloured and “hardness” banding across the outcrops. If you look at the outcrops closely (?!) small volcanic “bombs” or lapilli can be seen in some of the layers. These were formed by small fragments of molten volcanic material that solidified on the air and fell back to be included in the ash.

LAVA FLOW RESIDUE – This outcrop is below the water level much of the year, but you may be fortunate to spot it (or some parts of it on the banks). On the western side of the bridge, in the stream, a contact between two Ventersdorp lava flows can be seen. It is marked by a zone in which white spherical amygdales are heavily concentrated. So what is an amygdale you may ask? Deep in the earth, molten rock can dissolve large quantities of gas and water vapor because of the high pressures caused by the heat and overlying rocks. On reaching the surface (or close to surface) where pressure is much lower, the gas and water (steam) vapour can no longer remain dissolved in the molten rock, and bubbles of gas form in the melt. This is very similar to what happens in a cool drink/soda when the bottle is opened and bubbles suddenly “form” in the drink. As the lava flows along the surface of the earth, the bubbles rise to the top of the lava flow, some of them bursting on the surface, but many of them remain trapped in the thick molten lava as it cools and solidifies from a flow, to a viscous “treacly” consistency, to rock. In time, the cavities left in the lava can become filled by minerals that are deposited out of water percolating through the rocks. The mineral is usually silica, or the mineral quartz, to form small white spherical amygdales. The silicon filling is occasionally beautifully banded and is then known as agate. Each lava flow will eventually become buried beneath subsequent lava flows, but the zone of amygdales remains, and will define the upper layers of that particular lava flow (and subsequent flows) as a tracer.

BOULDER CONGLOMERATE – This outcrop lies a little upstream of the board and consists of lava boulders occurring in a matrix of sand sized particles, also of volcanic origin, to form a conglomerate. This conglomerate was formed when an ancient river flowed across the surface of that was covered with volcanic ash (in a similar position to the current river. Some of the material from these ash beds was then washed out by the water and deposited further downstream along the slope. Again – this exposure is in the river bed and is not always accessible.
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).

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.

1) Take a photo of you and your GPSr at this spot with the river and “board” in the background. [Optional]

2) Look around the cache area, and surrounding areas. Explain what you observe regarding the rocks in the river. Look especially at the way the volcanic ash layer has weathered and why do you think this occurred?

3) Look at the river banks just upstream (where it makes a sharp turn to the right). Why do you think the one bank is so steep and the other so shallow? Try and figure out how it formed. Look around and see how the river has “changed” course over time and see if you can spot historical stream channels upstream too.

Additional Hints (Decrypt)

Erzrzore gb fraq lbhe rznvy! Qb or pnershy bs gur jngre! Jngpu puvyqera arne jngre rqtr.

Decryption Key


(letter above equals below, and vice versa)

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