FIGURING
out
RHYOLITE
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Out of the 'oven' rhyolite is hard, brittle and shatters unpredictably when you take a chisel to it. We were going to learn more about its awkward properties the hard way.
In marked contrast to sandstone, RHYOLITE is exceptionally difficult to tame. It is made up of hot, viscous lava that sludges down the sides of a volcano like porridge and then rapidly cools.
In the process of being ‘cooked’, rhyolite acquires a crystalline lattice that behaves quite differently from the pressed-together homogeneity of sandstone.
The spinoff rock from rhyolite that I’ve mentioned earlier – welded tuff – is made of ultra-fine ash that’s been blown explosively out of the caldera of a volcano.
Think of dense, hot clouds of ash drifting and settling slowly and hardening into rock. Tiny particles of ash are literally ‘welded’ together by immense heat.
There was once an old rusted board near the Piao announcing that the welded tuff on Chidiya Toonk hill has been notified as a National Geological Monument, but the board has rusted away and disappeared now.
With a hand lens you can make out glassy specks that are diagnostic for WELDED TUFF
These igneous rocks may be geologically significant but no one in Jodhpur seems to have found much use for rhyolite or welded tuff except as packing and fill for foundations. We wondered if there was some interesting way of ‘celebrating’ the volcanic substrate that the Park stood on but was otherwise almost completely neglected.
One possibility we wanted to explore was to try and use rhyolite 'interestingly' to build a wall that would separate Singhoria Pol and our Visitors Centre compound from the road that ran alongside.
As you drive steeply up from the city to the Fort, you pass imposing flat-faced cliffs of rhyolite on your right.
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You can see what an important visual element the rhyolite was
even for the creators of this tourism poster from the 1940s
All of Rao Jodha Park too is made up of eroded hills and valleys of rhyolite, with traces of welded tuff.
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In some places the rocks look shattered and reassembled, like they do in this picture.
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In other places, you can still clearly see the characteristic columnar structure that persists even after a lot of erosion has happened.
I found the massive rhyolite formations along the road fascinating. Their flat planes meet at obtuse angles and catch the light beautifully.
Coming up from the city, we spotted some large-ish boulders lying by the side of the road and I thought we could try and build our low wall like a miniature version of these tall cliffs.
Done well, it might even look like our little wall had always been there...
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You can see rhyolite columns like these ones all along one side of the road leading up to Mehrangarh
GOLAK KHANDUAL
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The wall didn’t need a foundation – there was already a low stone parapet rising above the level of the road and because the rhyolite boulders were so big, we wouldn’t need to build up more than 2 or 3 courses at most.
I hoped to be able to make the boulders ‘sit’ steadily on their own without any cementing material. To do this we’d have to choose our boulders carefully and align their ‘noses’ so that they presented flat faces all the way down the rank. Just like they do naturally, in the huge cliffs.
Some of our rhyolite boulders weighed a few tons. Pradip had been working with the Khandwalia miners in the Park for some years and told me how phenomenally good they are at handling seriously heavy rocks.
Six Khandwalias helped to load the boulders onto trucks and unload them again. They used cranes, of course, and a block and tackle for the finer adjustments, and did a marvellous job of making these huge boulders fit together so that they were reliably stable.
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It turned out to be a piece of cake in the end but only because of the tremendous skill of these wonderful Khandwalias! And this was only one of the many jobs at which they proved to be indispensable.
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We began planting up the Visitors Centre by planting in our rhyolite wall. This is what it looked like from inside the Visitors Centre in June of 2011
Pradip needed the boundary wall to double as deep planters because there was barely any soil inside for him to plant in.
I decided not to use any cementing materials to close the gaps between the boulders so that the receptacles stayed interconnected. On the outside too, we used mud gaara instead of hard-setting lime mortar in the hope that plants would grow out of the joints.
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It didn’t seem at all right to have the ripples running all the way through under the Pol. They were looking great outdoors, but... inside?
We wondered if a smooth, polished surface would work, as contrast. Might the rhyolite that we had all around us in the Park behave like a good little boy and allow itself to be neatly sliced up, buffed and polished?
There were no precedents, no one we knew or had heard of who had polished rhyolite. Perhaps granite could give us a hint of what to expect. After all, rhyolite is formed of much the same minerals, the major difference being rhyolite’s much smaller, almost invisible, crystals.
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Our newly laid sandstone walkway rippled right up to the Pol, then stopped abruptly for the 20 feet or so that lay under the roof, and then continued again on the other side.
We then sent off a hunk of maroon rhyolite with Devinder, our Park supervisor, to be cut into 6 x 6 inch tiles. He returned an hour later clutching a half-cut rock, looking defeated and upset.
He had taken the rock to a sandstone cutting machine that uses circular diamond saws drenched with water (to counter the heat generated by friction). Devinder told us they broke 3 diamond saws trying to slice through the rhyolite and then sent him away rudely, refusing to try any more.
All he had to show for his pains was 4 little tiles!
We urged him to try again. Someplace else.
Same story, only this time he managed to come back with a few more tiles. He tried again in yet another place. And again. Each time, he brought back a few more sliced up tiles.
(Later we learnt that to incise a crystalline rock as tough as rhyolite you need to grind rather than cut it, but we didn’t know that at the time.)
All we could do then was hope that the sum of all Devinder’s sorties to different cutting facilities would yield all the tiles we needed.
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We got there in the end!
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We polished the rhyolite with conventional floor-polishing abrasives of different grit-sizes.
When we washed the creamy liquid off the floor after the machines had done their job, we were in for a surprise: by happy accident, we had made ourselves a gleaming red rhyolite floor with the most beautiful swirls and patterns you can imagine!
Not red exactly – china rose, cerise, rubicund, cyclamen, hot magenta... We quickly ran out of names for hues in the red part of the spectrum!
So what next?
The Park needed Gates