Monday, February 8, 2010

Tafoni Occurs In Basalts Too

First things first. Go over to Michael Welland's blog - Through the Sandglass and congratulate him. His superb book Sand:The Never-Ending Story has been awarded the John Burroughs medal for excellence in natural history writing.

Sometime back Michael wrote a couple of posts on Tafoni, the beautiful enigmatic natural rock weathering sculptures found commonly in sandstones but also in other types of lithologies. The images that stuck with me from his posts were of desert landscapes and sandstone sculptures....so I got a pleasant surprise when I came across Tafoni in basalt on my recent field trip to the west coast.

Tafoni are ellipsoidal..bowl shaped natural rock cavities...go here to learn more.

They commonly occur in locations where salt weathering is intense. Water percolates through natural cracks in rocks and precipitate salts. These crystallizing salts exert pressure on the surrounding rock and little by little the host rock gives way to form holes and pits. Besides salt weathering the natural variation in hardness in rocks masses may also promote this differential weathering.

The location was just south of the town of Alibag ( Alibag incorrectly shown...it is directly west of the annotation... on the coast), near Korlai fishing village.


There are wave cut basalt benches exposed at low tide which are intruded by dikes. These basalt flows are studded with secondary silica and zeolites mineralization and ...Tafoni.

Here are some examples:

A close up of the beautiful honeycomb structure


Pits and closely spaced joints..


Associated silica and zeolite geodes


Pits...large and small..


Cluster of Tafoni on vertical and inclined surfaces..separated by regions which lack Tafoni..


Besides being located in the intertidal zone where constant wetting and drying and salt weathering is strong..there may be a couple of other reasons why Tafoni is localized to these flows.

First, these flows are not one homogenous rock mass. They are compound flows...i.e. made up of smaller flow units...small and big blobs of lava that overlap, interfinger and coalesce to form a larger compound flow representing one macro eruptive episode. That means there are flow unit scale variations in cooling rates and vesicular content that makes individual flow units unequally suseptible to weathering.

And second,  is the presence of dikes. Not directly..but these dike swarms are concentrated in regions where the crust has undergone extension...leaving the rocks riddled with closely spaces fractures.

That makes them vulnerable to dissolution through circulating water and chemical weathering.

9 comments:

  1. These are spectacular, Suvrat! My impression is that it's not common to find tafoni in basalt - any idead of whether that's correct or not?

    And many thanks for your comments.

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  2. Michael- I have not come across before this field trip examples of Tafoni in basalts although this website does mention basalts ...so it was a good find. they are more common in coarser grained rocks susceptible to granular disintegration and basalts being generally fine grained....

    i think the pervasive vesicularity of these basalts and some porphyry textures may have promoted this type of weathering at this location.

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  3. Did you say that these are in the intertidal zone? Tafoni are wiped out by wave action and typically occur above, but near the water.

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  4. Andrew- these features are definitely in the intertidal zone...I saw the same outcrop submerged at high tide. :) ...they have been previously reported from the intertidal zone too..

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  5. Its good to know the name of the structures. One does come across these often but one is often left wondering about how they form. Thanks for the information.

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  6. Hi Suvrat,
    Thanks for this nice bit of info :)
    I came across this, during my trip to Harihareshwar, few months back.
    Harihareshwar is about 50-80 km south of Korlai out here.

    The formations were at head level hence above the inter-tidal zone.
    It is weird that we find so much basalt despite that most of Konkan is laterrite.

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  7. that's a great example of Tafoni Khalil..thanks.

    yeah laterite is pervasive it has formed by the alteration of basalt.. but basalts do outcrop especially along the rocky coasts and hills.

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  8. I think I have seen similar rocks at Manori too.

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