Basalt Karst Boulders (Norfolk Island) EarthCache
Hidden : 3/20/2016
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:



Norfolk Island’s amazing basalt karst boulders are very rare in the world and only known in Northland and Auckland in New Zealand, in Hawaii and are well developed here on Norfolk Island. Many examples form centre pieces in people’s gardens and some have been used to beautify the main township of Burnt Pine. There is an excellent example placed on its end at Kingston with a plaque mounted on it.
These basalt boulders are fresh core stones from inside weathered lava flows. Lava flows tend to weather along the cooling joints within them in a process known as spheroidal weathering. The rock along the joints becomes rotten first and the fresh, rounded basalt core stones are those parts that are furthest from any joints. Erosion removes the soft, weathered soils and rotten rock and the fresh rounded boulders are left on or near the surface.
Boulders that have been exposed on the ground, under forest cover, for many thousands of years (possibly tens to hundreds of thousands of years) have gradually dissolved on their surface to form karst patterns. These are mostly solution potholes and solution basins on the top of the boulders and fluting running vertically down their sides. These patterns are typically found around the world on limestone rocks made of calcium carbonate that dissolves quite readily in slightly acidic percolating water.&
Basalt on the other hand is considered to be inert and is not usually considered to be able to dissolve. Clearly however it can dissolve in slightly acidic water given long enough time in relatively warm conditions. It is believed that the slightly acidic water is derived from rotting leaf litter that forms humus and humic acid. This occurs on top of the boulders, particularly where there is vegetation growing on it under a forest canopy. The humus collects around the plant roots on the top of the boulder and gradually over time these dissolve a pothole or basin in the surface where the slightly acidic water is trapped and overflows as seeps down the side of the boulder gradually etching out the runnels and flutes.

Tasks
To claim this Earthcache please email me the answers to the following questions.
1. What type of karst is the feature of the boulder at the published coordinates:
deeply dissolved basins; fluting on the sides; or small circular basins?
2. What do you think has caused this feature?
3. Photographs with your log are welcome

References
Hayward, Bruce Basalt Karst in Norfolk Island
https://www.researchgate.net/publication/280311840_Basalt_karst_in_Norfolk_Island

Additional Hints (No hints available.)



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