FRIENDS OF THE PILANSBERG
GEOLOGICAL
SITES
Fops
Fops has been involved in the upkeep of the Geological sites for some time. You will have recently noticed quite a lot of work being done at the sites in both cleaning of the sites and the new signage at the sites.
Here is a little history on the geology of the park.
Most people think that the Pilanesberg is the collapsed crater of an extinct volcano, something like the Serengeti Crater, and that we drive around the bottom of this crater and that the hills around us are the walls of the volcano.
The truth is much more astounding. Geologically, this area is world famous and is known as the “Pilanesberg Alkaline Ring Complex”.
There are only two other sites like this in the world, namely in Russia and in Greenland. Both of these are not clearly defined as this site.
The process started about 1300 million years ago when molten rock caused a pool of magma below the earth’s crust called a “hot spot”.
Pressure built up and the volcano erupted spewing out a shower of molten rock, gas and ash. This happened many times over a period of around 1 million years. After the eruption the crust collapsed into the magma chamber and magma was pushed out as lava, covering the landscape.
Eventually these eruptions formed a volcano around 7 km high and 25 km across; one of the largest volcanoes on earth!
The centre of the eruptions is the Mankwe Dam region. As this enormous volcano formed and magma was pushed up, some of the magma filled circular tracks inside the body of the volcano. This intrusive igneous rock never saw the light of day and eventually cooled down and solidified in so-called ring-dikes.
The ring-dikes of Pilanesberg are composed of different chemicals, namely foyaites, also called nepheline syenite and red syenite. The rate of cooling and the composition of the magma determines the type of rock formed in each dike. For instance, white foyaite has particularly course grains and is formed when magnets cools slowly. Red syenite forms when the magma contains plenty of water.
In the early, explosive stage, tuff was formed. This is ash that was catapulted into the air and then settled on the ground with the heavier courser bits on the bottom and the finer ash on top.
Time, the great leveller, eventually eroded the volcanic rocks and exposed the inside of the volcano. The circular dikes inside the body of the volcano are more resistant to erosion and are the circular hills we see in the Park today.
An extract from Peter Derich’s book “Peter’s Guide, Pilanesberg National Park”
G1 - Volcanic Tuff
G2 - Nepheline Syenite
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G3 - Lava
G4 - Kimberlite
During volcanic eruption rocks shatter into countless fragments of variable size that are catapulted high into the air. Upon falling back they solidiy and form a tuff.
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G1 can be found on Dithabeneng Drive just before Phiri Link as you approach from Manyane side.
Syenite is formed from molten magma just below the surface of the Earth. White crystalsof the sanidine feldspar occur as very thin flakes in the rock.
G2 can be found on Dithabeneng Drive just past Malatse Dam as you approach from Manyane side.
Molten rock, known as magma, originates from deep witin the Earth. Magma that erupts from a volcano and flows onto the Earth's surface is known as lava.
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G3 is situated at the top of Korwe Link.
Kimberlite is potentially a diamond-bearing rock. Diamonds were formed in the mantle of the Earth about 15km's below the present surface.
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G4 is situated at the junction of Tshwene Drive and Nkwe Link.
G5 - White Foyiate
G6 - Green Foyiate
G7 - Syenite
G8 - Fluorite
This course, grained rock crystalised from magma that cooled slowly beneath the Earth's surface. The rocks are now exposed in the hills because of weathering and erosion over the past 140 million years.
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G5 is found beneath the hill opposite Tshwene Pan.
Green Foyiate is a rather uncommon rock variety. It contains flaky and needle-like aegirine crystals, giving the rock an unusual layered appearance.
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G6 is located below the hill on Sefare Drive just as you approach Nare Link.
The syenite rock displays a mesh-like arrangement of light-red orthoclase feldspar crystals, which make up most of the rock. Black specks , scattered throughout the rock are magnetite (iron oxide).
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G7 is located below "Red Rocks" on Kubu Drive.
Fluorite extracted from this old Moepo Mining site has a 'Fir Tree' or 'herring-bone' texture, which is rather unique. The fluorite, which is composed of calcium and fluorine, is visible as a purple mineral.
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G8 is situated on Kubu Drive just north of Kubu Dam.
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G9 - Red Foyaite
G10 - Green Foyaite
G11 - 'Ledig' Foyaite
G12 - Red Syenite
This is an even-grained variety of the red foyaite comprising the potassium-rich feldspar, small amounts of nepheline (which weathers rapidly giving the rough appearance) and the black crystals are those of aegirine.
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G9 is located on Moloto Drive by the picnic site.
The ring mountains of Pilanesberg are made up of several varieties of foyaite. They have similar mineral composition, but with differing amounts of individual minerals and overall colour of the rock.
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G10 is situated at Bakubang Gate.
The variety of foyaite was named 'Ledig' foyaite after the farm where the rock was first described, situated in the southern part of the park.
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G11 is located on Tshepe Drive close to Kwa Maritane Lodge.
The red syantie largely contains potassium-rich athocclase, feldspar, chlorite, calcium and fluorine. Syenite is igneous rock formed from magma.
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G12 is located on Tshepe Drive just north of G11.
G13 - Uranium-Mineralised Tuff
This rock, as in G1, is known as volcanic tuff. This particular tuff contains some rare minerals and is mildly radioactive due to its uranium content.
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G13 is located on Tshepe Drive just before the river crossing as you approach the Kwalate Drive junction.
G14 - White Foyaite
G15 - Dykes in Red Foyaite
The three main minerals comprising this rock are white potassium-rich feldspar, smaller amounts of nepheline and the dark coloured mineral, aegirine.
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G14 is located at the Fish Eagle picnic site.
The red rock exposed in the streambed at this locality is red foyaite. The grey bands cutting the red foyaite are dykes of tinguaite, a fine-grained variety of foyaite. Examples can be seen in the riverbed at this site.
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G15 is situated at the viewpoint at Mokorwane Dam.