GeologyRocks

Hi Roshan

Looks like limestone - what area is it in and is that a cave?  A couple of close-up photos would be useful. 

Cheers 

Gus

Hi Gus,

You are right. Its a natural cave. I have posted some more pics. There are many rocky chambers inside this cave but we could reach just 3 as its too dark and dangerous deep inside.

 Infact i want to write an article on this and post it in my website ( www.CoorgCreek.com )

 Incase the earlier link do not work plz try this: http://www.CoorgCreek.com/rock.htm

It is located in the western Ghats of South India. The place is a hill-station. 7 months rain, 2 months winter, 3 months summer here.

Thanks

Roshan

The rock is definitely limestone but I can't tell you exactly what age. It's possibly Jurassic.  There's a number of caves in the area, including Kavala Caves and some which contain temples.  If you're thinking of exploring them further I would recommend you try to get in touch with somebody with the proper equipment (helmet, lamps, etc), preferably a caving club, but don't go on your own unless you are very experienced. 

I got this reply from some geology expert:

Most rocks of southern India are proterozoic-archean in age meaning they are between 543 million to 3.8 billion years old.

Just a guess, but you were probably along the west coast, near Mumbay or east of there, maybe even a little north.  If my guess is correct the rocks are basalts of the Deccan Traps.  (Okay, I was a bit north.  I looked up the area you were in, the Corg or Kodagu district.  This area lies on the southern fringe of the Decaan plateau, and the hills or mountains around the area are actually the edge of the Deccan.  So my guess was not too far off and everything I said applies.  There are also in the area, some metamorphosed granites (charnokites) and granite gneisses also metamorphic, that would be part of the Dharwar Craton area on the map.  It is likely you were in the area of the Koyna Rift-Dharwar Trend.)

The Deccan Traps are a region consisting of massive flood basalts.  Basalts are the type of volcanic rock that flows from volcanos or mid ocean rifts and comprise the sea floors of the oceans.  At times basalts have flowed out from massive rifts to cover vast areas of the continents.  There are vast Traps in Siberia, Canada, Brazil, and Washington State as well. The dark areas on the Moon, the Mares or Seas, are flood basalts too.

The basalts of the decan traps were deposited when the sea floor spreading center between Africa and India released the flood basalts over a .5-8 million year period.  The Deccan lies between several intersecting rift zones bounding it on nearly all sides.

The crevasses shown in the pictures remind me of similar features in my home state of Virginia.
They could be dykes which are fractures due to tension that once open fill with molten rock that differs chemically from the surrounding parent rock in which the fissures formed.  The infilling rock hardens and leaves marked linear bands of varying width and depth.  This rock usually erodes faster than the surrounding rock leaving linear passageways almost like corridors.

The Deccan possesses many of these and the whole is called the Dediapada dyke swarm. (see attached map)

So if my guess is right, the bulk of the rock you show in your pictures is basalt, and the passages if actually dykes, would be made up of dolerite/diabase and basalt.  The difference is compositional and grain size determined.

You can determine exactly what you were seeing this way.  Basalt is a dense, hard and fairly heavy black rock when viewing a fresh surface.  Crack one of the rocks open.  It may even contain small inclusions.  The diabase/dolerite on the other hand might be slightly crystalline, meaning the individual crystals of the rock are visibel with the naked eye.  It will be pedominately black with some speckly of white.

The outside of both rocks take on a powdery dirty appearance due to the fact that the mafic minerals that make up the rock weather to clay.

Now if my location is entirely wrong, LOL, then you might have been exploring some of the cratonic rocks that bound the Deccan traps.  These too will contain dykes, much like my native Virginia.  The surrounding rock there is Granite, with diabase/dolerite dykes cutting through.

I know the area includes the Deccan Traps but I also know it contains some limestones.  If the rocks contain caves then they are most likely to be the latter.  I suppose the real test would be to see if the rocks contain fossils although not all limestones show visible fossils and if the caves have stalactites, although these can be absent in some caves.

To me these rocks do not quite look like limestone. Fractures do not look as I experience them in my work with limestones. Neiter do there seem to be dissolution features typical of limestone. The surface of the rocks does look too dusty. Either is a marly limestone or more likely some other rock. Of course this distance diagnosis has its limits.

There is a very simple test. Try to get a little 10% HCl at a local pharmacy. Mine gives it to me even for free because they know what I do with it. Break a piece of the rock so you get a fresh exposure and then put 1 or more drops of the acid on it. If it starts fizzing and making bubbles it is limestone. 

You may well be right about it possibly not being a limestone and the acid test should prove it one way or another.  Having said that, you often get dusty-looking limestones underground and fissures like the photos which can be affected by boulder falls.  There's many caves in the UK, for example, which the photos could have been taken in, and Ogof Ffynnon Ddu in the Brecon Beacons has several miles of similar-looking rift passage.  We could really do with some close-ups of freshly-broken rock.

Thanks folks, let me drive through and get some sample first. Thanks again for your comments.

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