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)!