Ajanta caves - a historical wonder with a touch of geology

Recently I was very lucky to visit the Ajanta Caves, located on the Deccan Plateau. They're called caves, but really they're beautifully carved and decorated sacred spaces chipped out of the bedrock. I've been studying images of the caves for years so it was with great excitement that I was able to actually go see them in person. When I realised there was a geological reason for the caves you can imagine my excitement.

A selfie on the path up to the caves. My excitement is overprinted by the squinty eyed 'the sun is touching me' look. Notes for next time: Carry more water and bring more snacks.

Panorama of the valley hosting the caves. During the wet season, multiple waterfalls flow into this valley.

 

The rock here is vesicular basalt. Vesicles are small holes formed by pockets of gas that exolve from the melt but don't quite make it out of the lava while it solidifies.  This basalt is part of the Deccan Traps, a sequence of flood basalts that erupted at the end of the Cretaceous (66.25 mya). Some researchers have proposed a mantle plume (think Hawaii volcanos) is responsible for these volcanos however this doesn't explain the epic amounts of lava. To support the plume idea, I'd like to see more identification and dating of eruption points. There is some evidence to suggest that the Deccan Trap volcanos were altered by significant meteroite impact which disrupted the internal fluid flow of the mantle and provided additional hot magma.  Personally, I subscribe to the wake theory where fast plate movement creates a wake, which allows the delivery of hot mantle material very close to the surface. Similar to what is seen in Australia's Eastern Volcanic Province, volcanic vents would then form where ever pre-existing faults permit.

What you can see here appears to be calcite (left my acid at home - curses) which has crystallized from groundwater filling in weaknesses in the rock. Interestingly, none of the basalts here exhibit columnar formation. This may be the result of human activity and fast weathering during the wet season though.

Some of the caves have unfinished roofs or floors showing pahoehoe texture. This is the reason the caves exist. Pahoehoe is the ropy texture that forms on the top of lava as the surface starts to solidify but the liquid is still moving. The surface bunches up, a bit like the crust on top of custard, as the lava continues to flow beneath. In this case, the flow was top to the bottom right. The semi-planar form of the pahoehoe indicates the individual lava flows were able to solidify before the next eruption. The slight difference in cooling temperature and grain size means there's a plane of weakness between the flows. This is where the groundwater eventually seeps through, depositing bands of calcite as seen above.

The white calcite in the black basalt showed the cave-makers where weaknesses in the rock were. Looking at the calcite on the sides of the valley wall, it's easy to see this marker has controlled the height of many of the caves. It's a lot easier to create a room when you can undermine the roof and know it'll fail at a specific point.

  

Outside one of the amazingly carved rooms. Though, I've seen beautifully carved basalt in Bali, i wasn't prepared for the rich wonder that I found in every cave.

 

This is about half a meter across - fantastic detail! Basalt, made up of interlocking, fine crystals is a great carving rock. Though it's a lot harder to work than rocks like marble (metamorphic) or sandstone (sedimentary), it's crystal format resists weathering much better.

 

I would never ever take something from a monument like this. However, the authorities are building a new visitors center out of rock that's been quarried nearby. I raided their waste pile for this sample - look at those lovely pyroxenes (largish brownish crystals)!

Cleavage in turbidites, Chewton, Victoria.

During compression, platy minerals align at right angles to the maximum direction of compression. These minerals create a plane of weakness called cleavage which is often highlighted by preferential weathering.

In these turbidites, the mudstones exhibit vertical cleavage. The massive sandstones, with their rounded grains,  won't have cleavage as the grains can't rotate to  reduce exposure to the stress field. Sandy-mudstones show weak cleavage. This is often deflected from the orientation of least-strain as the presence of the sand grains create a more massive and resistant rock.

The image below shows the deflection of cleavage through turbidites in Chewton, Victoria.

Lignite and pyrite

While out drilling not so long ago, I happened upon a unexpected layer of lignite and pyrite in the overburden.

The lignite chips were from ~160m below the current surface and came up as small, black spongy rodlike shapes with a clear woody texture. A number of the chips had positive buoyancy and would float in my rinse water. Samples above and below the lignite, appear to be a form of brown or red dolomitic mudstone as it fizzed on contact with acid. I'm leaning away from a siderite classification, as i also recovered the following:

Imbedded in the mudstone, and proximial to minor lignite, were small grains of pyrite. They sparkled like gold in the sun and had a dark streak. Siderite is prized as it is low in sulfur however the presence of pyrite indicates sulfer enrichment.

I hypothesize these rocks are the result of a significantly more humid climate where water was able to pool at the surface. In this particular location, a swamp has formed. Dead vegetation was covered by water and sediment before significant decay could occur. The anoxic environment of the swamp has preserved some of the vegetation prior to burial. The water chemistry probably has also contributed to the precipitation of sulfur as pyrite.

Sedimentary structures - mud cracks and rain droplets

Very occasionally, it's possible to find ancient raindrops preserved in sedimentary rocks. These imprints tell us a couple of things, a) the host sediment was either under very shallow water or at the Earths surface so it's highly likely to be of terrestrial (as opposed to marine) origin and b) the overlying sediment was deposited in a very low energy environment so as not to disturb the imprints.

 

The image above displays two lovely modern sedimentary features, mudcracks and rain drop craters. Interestingly, we can see that in the middle of the images the raindrops imprints get fuzzy before stopping. We can therefore assume this is the border of a puddle that existed either before the rainfall or soon after as either the water absorbed the impact of the raindrop creating no mini-crater or the water encroached and wore the imprint away.

Think about how fragile these mudcracks and raindrop craters are, they'd be erased by the next large rainfall if not covered over and protected. The likelyhood of raindrop preservation is very low, so if you do happen to find some take a moment to marvel at the numerous process that had to occur to create that rock!

Willy-willy weather

 

Willy-willy weather is here!

For those not familiar with the Australian vernacular, a willy-willy is a dust devil or small whirlwind. The word itself is an adoption from the Yindjibarndi/ Wemba-Wemba language/s. I've used it ever since I read the Silver Brumby books as a child and like most common words, my understanding of its history and original usage is sadly lacking.

A willy-willy can be small, they're often seen in urban environments where two opposing breezes meet on a street corner. They can also be quite large though I've never encountered the term being used for a Category Anything Tornado.  In Australia they can be responsible for a significant amount of sediment transport and do contribute to topsoil erosion. Areas with limited vegetation coverage, such as this recently bulldozed track, can be especially vulnerable. Sand dunes, with their well sorted and graded particles are also quite susceptible to erosion by willy-willy. The seasonable nature of these natural features should be considered when planning any earth moving activities such as rehabilitation.

Rig cookery - the humble hole plug

This blog was called Rocks and Rocksalt because my partner in crime is supposed to provide foodie reviews while I manage the rock reviews when we travel. One half of this partnership isn't pulling their weight, blog-wise. :)

Instead I present a new topic - FIFO cookery.

If you've ever worked FIFO (fly-in-fly-out) you'll understand how big an impact food can have on your life. If you are lucky, you're in a small camp with a chef who cares enough to create a varied diet on a regular basis. It seems that the larger the camp gets, the less flexibility there is. This is the case with my camp. The same provider runs all of the camps in the Pilbara and offers the same meals to everyone. The menu for dinner is highly varied (i.e. they rotate between 6 ways of serving potatoes and do try to offer at least one green vegetable each night).

The menu for lunch however, is dismal. Don't get me wrong, there is a lot of selection in the bain-marie's however it's the same, unchanging options day in day out. The cooks do try to vary the 'salads' however they always boil down to the same four selections - the Chef's special (which can be assembled from the ingredients in the other bain-maries quite easily), the oil salad, the mayonnaise salad and the inappropriate meat salad (i.e. tuna or bacon added where it isn't required). Recently we've been offered a 'special' option every second day which was often a DIY noodlebox sort of thing. Unfortunately, after about a month, this morphed into pre-packaged ham and cheese sandwiches. Not very inspiring, especially on week two of your swing.

To combat this, many people bring up their own food or have come up with a variety of ways to combine the ingredients offered to create an array of taste sensations. I'll be posting recipes and instructions at least once a month. They will assume you have access the basic ingredients and will highlight any additional item you may need to bring in from the city. Additional items will be limited because the Authors don't believe in (read: are poor at) planning ahead. I'll also break down the recipes into two categories - indoor and outdoor. Indoor will assume you have access to hotwater, ice and a sandwich press.

Outdoor cooking may require some inventiveness on your behalf. I believe people used to heat pies on the elements of lighting plants but this is a safety risk and I do not suggest this method at all. Instead, I highly recommend the use of the humble hole plug. This black container is typically wedged in the top of a drill hole upon completion and should be readily available from your drilling laydown. When sealed and left on the dash of your car in full Pilbara sun, this black-plastic wonder can reach temperatures in excess of 50 degrees (I'll science that up for you in a later post). Better yet, the hole-plug used for 140mm RC holes, can hold a standard round plastic lunch container quite easily. I highly recommend it for all your warming, baking and braising needs.

Set and forget cooking with RC hole-plugs.

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