Saturday, February 6, 2016

Field Photos- Deccan Volcanics- Buttes And Rubbly Flow Tops

Last Sunday I went for a short trek to Tail Baila, a basalt rock feature about 80 km west of Pune on the Mulshi dam backwaters road from Nive village to Lonavala. Take a look.

This is a "butte". It was once an extensive lava flow (s), but weathering and erosion has removed much of the extensive volcanic plateau to leave behind remnants like this , poking up like skyscrapers.

This is an edge view of the butte. Believe me it is scary when you finally come near it. It is a popular spot for hardened rock face climbers and in fact that Sunday there was a team of climbers which had made their way up to the very top.

These features are not common all over the Deccan plateau. I had put up a satellite image some time back. Here it is below again.

This region close to the Western Ghat escarpment is fractured by N-S and E-W oriented systems. Over eons, weathering and erosion along these weak planes has removed rock material and slowly fragmented the plateau in to a series of mesas, buttes and pinnacles.

On the outcrop scale too are interesting features. Here is a lava flow with a brecciated top.

This happens when a crust forms on newly erupted lava. As the lava moves, the crust breaks into a rubbly (breccia) upper layer. Taking a broad picture, geologists have found that these kind of lava flows with rubbly tops are more common in the upper stratigraphic formations of the Deccan volcanic pile and might be indicating more voluminous eruptive episodes and rapid expulsion of volatiles from the lava. The buttes and mesas and pinnacles of the region I having been trekking in fall in this stratigraphically upper part of the Deccan Province (see fig. on left, credit K. Canter AGI), somewhere I am guessing in the Khandala to Bushe Formations.

Better geo-chronology of  the Deccan eruptive history is now strongly suggesting several phase of eruptions with a low voluminous early phase from about 69 mya to 66 mya, followed by a high voluminous phase beginning around 66.3 mya and lasting for 750,000 years in which about 80% of the Deccan Province lava was erupted. This continental volcanism  lasted for 4 million years, but the lava pile I live on around Pune and seen in the spectacular cliff sections of the Western Ghats all erupted in a relatively smaller energetic time period of about half a million years or so. Its still a long long time from a human viewpoint. Since then, for the last 60 million year or so, the province has been eroding slowly to form the landscape we see today, the deep valleys, the mesas, plateaus and pinnacles. Imagine the enormous amount of rock broken up and material removed to form this landscape, streams and rivers carrying the derived sediment in solution and as clay particles to the Arabian Sea and the Bay of Bengal.

It has been a really glorious winter in the Deccan Volcanic Province. Monsoons have their own beauty, but winter with its brown and earthy grass meadows, bright pink blooms and black rock make for some stunning views as well.


  1. Awesome! Punekars should be made aware of their geological heritage.