An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
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Logging Tasks:
Most answers to the questions are obtainable from the plaque mounted on the structure at the site of G3.
1. What is molten rock known as?
2. What are rocks formed from Magma known as?
3. What rock is displayed on the structure? Describe the colour and what stained it, the size of the grains and how these grains were formed.
4. Take a photo of you and your GPSr through the open window of your vehicle with the structure in the background -see picture (optional). PLEASE DO NOT ALIGHT FROM YOUR VEHICLE AT THIS POINT.
PILANESBERG - AN ANCIENT VOLCANO
Although the geology of the Pilanesberg Complex exhibits the roots of an ancient volcano that erupted some 1 500 million years ago, the concentric mountains are actually erosion remnants of intrusive ring dykes that formed by upwelling magma during and subsequent to the collapse of the volcano. The Pilanesberg Complex, as this group of hills is geologically known, is the best-defined alkaline ring complex in the world. The rings of the hills you see today are the deeply eroded remnants and not the crater walls as most of us may conclude.
ROCKS
The rocks that formed during this early explosive volcanic phase are called tuff (volcanic ash), which comprises layers of coarser fragments at the base followed by layers of finer fragments higher up. Subsequent to this violent eruption there was no longer any support for the brittle, stretched crust above the magma chamber, and the crust collapsed into it. Consequently, magma that was still in the magma chamber was forced upwards along the circular fractures that have developed earlier.
The magma that was forced upwards quietly poured out onto the surface as lava where it solidified. This lava is very fine-grained in appearance because it has cooled down rapidly. The larger, well-formed crystals that occur in the lava are phenocrysts of the high temperature potassium feldspar variety called sanidine, which were formed at depth and were carried upwards by the lava. These sanidine phenocrysts are often aligned parallel to the general direction of flow of the lava.
At the top of the lava flow vesicles (gas bubble cavities) have formed as a result of the gas that escaped to the atmosphere. These vesicles are often filled with secondary mineral matter, for example calcite and are then called amygdales. Amygdales occur as small irregularly shaped white specks in the lava. Part of the magma that was forced upwards along the circular fractures, however never erupted onto the surface, but cooled and crystallized before it reached the surface and hence clogged up the fractures. The rocks that have formed in this manner along the circular fractures are intrusive igneous rocks that are called ring-dykes.
The ring-dykes that formed in Pilanesberg are composed of foyaite (also called nepheline syenite) and syenite. It is these ring-dykes that have formed below the surface, and that have now been exposed at the surface as a result of erosion over the past 1 500 Ma that account for the rings of mountains of Pilanesberg. The foyaites of Pilanesberg do not all have the same mineral composition and therefore have different names.
WHERE IS THE VOLCANO NOW?
Because of the great age of the Volcano events in Pilanesberg, the original form and many of the lavas and volcanic debris (+/- 1000m3) were removed by erosion over the years. What remains to be seen are the roots and fissures through which the lava welled. The volcano probably erupted through several vents, but the central and main conduit for lava would have been the Mankwe Lake.
CONCLUSION
A combination of the unique geology of the Pilanesberg National Park and the varied topography give rise to a wide range of habitats that support a rich biodiversity in the Park. In excess of 50 larger mammal species – including the “Big Five”; more than 354 bird species; 65 reptile species; 18 amphibian species and many other currently inhabit the Park. The Pilanesberg National Park is therefore an excellent geological and eco destination for local as well as foreign travelers with a passion for the environment.
ACKNOWLEDGEMENTSThe Geology of Pilanesberg Prof D.S. Coetzee,Prof R.G. Cawthorn
Congrats to GEO936 on the FTF!