La Roche qui pleure - Amazing Mauritean Geology
The southernmost point of the island of Mauritius impresses with its wildness
On the Coast just beyond Le Nefis Gris Gris, a viewpoint where black cliffs drop away sharply. The name Gris Gris is associated with local Witchcrfat. There are deep chasms in the cliffs surrouding the beach which lead to a distinctive headland knwon as La Roche Qui Pleure. (The Crying Rock), so called because one of the rocks here is said to resemble a crying man. You can walk oout on th headland to look for the face but take care, as the path is steep and the rocks uneven. The rockk you can look for is on the furthest point, facing to the sean. When the waves roll in water drenches his face and the he is crying.
There is no coral reef on the coast to protect the beach and bay. The ocean is almost always very agitated here and you can watch and hear the waves rolling towards the shore with full force. The violence of the sea has also made the basaltic cliffs very impressive. Bathing is forbidden here, there are also several signs to indicate this.
Here the erosion clogs the rocks every day.
Erosion
There are two different forces to form rocks: Weathering and Erosion.
Weathering is the breaking down of rocks by the action of the weather, plants or chemical action. In happens in situ - this means 'without movement'. This is the main difference between weathering and erosion. Erosion refers to rocks being broken down and moved away.
Marine erosion is the wearing away and removal of rocks by the action of the sea.
Winderosion: Wind is particularly erosive if it carries a lot of material (dust, sand) with it, which then gnaws at the rock on the ground like a sandblaster
Marine Erosion were defined in four main processes: They are corrasion (abrasion), attrition, solution and hydraulic action.
Corrasion (abrasion) this is where the bits of rock and sand that are carried in the waves are hurled at cliffs and grind them down. It is sometimes described as sandblasting.
Attrition is where the waves cause rocks and pebbles that they are carrying to smash into each other and break down. They become smaller and rounder.
Solution is where acids contained in sea water slowly dissolve certain types of rock. Sea water is very corrosive and slowly dissolves chalk and limestone. This increases the size of the cracks and joints in the rock so that the other forces of erosion can become more effective.
Hydraulic Action is the constant force of waves crashing on the shore. When waves crash against the cliffs they force air into cracks in the rock. The air is trapped, and pressure builds up. As the waves move back, pressure is released and the trapped air expands. Small explosions take place and weaken the rock, blasting fragments away.
In wind erosion, a distinction is made between deflation and corrasion.
Deflation denotes the blowing away of fine particles that have accumulated during weathering. Corrasion denotes the active erosion, i.e. the abrasive effect on rocks and stones due to particles carried along.
Three factors significantly influence the process of wind erosion:
- the soil and its susceptibility to erosion (erodibility),
- the weather (wind speed) and
- agriculture and cultivation by humans.
Tafoni
Additionally you can see Tafoni. The name Tafoni (singular: Tafone) comes from the Corsican taffoni (window) or taffonare (perforate), or pietra taffunata (perforated stone).
So far, there has been no consensus on the creation of Tafoni, and their development processes are controversial. It is believed that these distinctive structures result from both physical and chemical weathering, i.e. salt, wind and solution weathering.
One view is that Tafoni are physical weathering forms that can be attributed to salt weathering, but the origin of which depends on the type of rock, topographic location, exposure, the dryness of a climatic type and, above all, the possibility of supplying salts, especially salts transported by wind from the sea, from salt lakes or playas with superficial saline sediments. Tafoni are concave, cavernous, usually open to one side spherical to kidney-shaped cavities from a few centimeters to several meters in mostly coarse-grained rocks, predominantly granites, gneiss, greywacke and sandstones, which are reminiscent of honeycombs.
Tafoni result from core weathering from the inside out, whereby the chemically released binder (Fe, Mn oxides) is washed away by water or blown out by wind, thereby creating cavities and leaving a hard bark (weathering bark). Sometimes these weathering barks are preserved and form overhangs.
Tafoni form in all climates, but more often on coasts of the sea with clear tides or strong surf
To log this Earthcache:
Look carefully at the coordinates.
1. Which traces of erosion can you see and how would you describe them (shape, structures, different types of rock, possibly layered, ...)?
2. In your opinion, is there a predominant type of erosion that has led to the formation of these formations? What observations do you derive from this?
3. Now look at the tafoni. What is the average hole size you can find here in the rocks.
You can imediately log this earthcache. I will contact you, if the answers will not fit. Pictures are welcome, if the did not spoil the answers to the questions.