Thursday, November 3, 2011

Field Photos: Driving Through A Basalt Countryside

The most prominent topographic feature of the Deccan volcanic province is the Western Ghat Escarpment that runs north south, roughly parallel to the west coast of India. The escarpment is the edge of a deeply dissected easterly tilted plateau, a result of Cenozoic crustal movement. Near Pune, several streams flowing eastwards through narrow valleys have been damned and the backwaters of these dams have some great views of a basalt landscape comprising deep valleys and spectacular exposures of basalt cliffs.

Last Sunday I drove with some friends along the backwaters of Mulshi dam. In the interactive embedded map below (Pune in south center) the western ghat escarpment is the arcuate north south ridge line on the left side of the image. You can count more than ten dams situated to the east which have flooded narrow valleys. If you pan north south you will see more reservoirs. The water is used to generate electricity and for urban and agricultural use.

View Larger Map

All pictures courtesy Bharat Parikh and Rajesh Sarde.

The countryside was lush. Wild grasses in full bloom.

A wide grassland meets distant peaks and ridges.

Steep basalt cliffs. Notice the layering of the lava flows.

The edge of the plateau is penetrated by fracture sets oriented in the NNW-SSE direction parallel to the coast and a NE-SW set. Some of these represent extensional stresses developed during rifting of India from Seychelles around 66 my to 65 my years ago and may have been conduits for the magmas. There are plenty of dykes intruding the lava pile oriented in a north south and north-east south-west direction. There are fracture sets too which may represent a later Cenozoic stress regime related to the uplift of the province. These provide a structural control on drainage.

The image below shows a north-east south-west oriented fracture set (red arrow) which has controlled the dissection of a deep valley. I call it Anil's valley in honor of my friend who told me about it. White arrows point to the NNW-SSE oriented lineaments. They may represent dykes or fractures.

 A view of Anil's valley. It is a box canyon with near vertical walls of basalt.

The main basalt flow type in this region are compound flows. These are made up of discrete blobs of lava that overlapped and coalesced with other blobs of lava to form a compound unit. During one eruptive episode several compound units may be stacked to form a thick lava pile.  These compound flows have weathered over time into a steep hummocky topography with domal summits accentuated by erosion along fractures. (Picture below taken on a previous field trip in February 2010).

A lonely road in the woods.

Bed load! A dry bouldery stream bed. Its just three weeks after the rains stopped and the streams in the upland areas are dry already. Small farming communities make by mostly through water in the monsoons and then rely on small springs to water their fields. Life gets quite hard here after the rains.

Food of a civilization. Yellow ripe paddy grown in the lowlands near the water source. Lava flows in the backdrop. You can smell the fresh rice as you pass by.

Until next time..


  1. Neat post ... nice that you included aerials and explanations with the photos, thanks.

  2. Now I'm homesick :( But thanks for the photos and the nice description.

  3. thanks for visiting Hollis and Amit..

  4. Love this post. Amazing that you can see all this just outside booming, crowded Pune. Beautiful photos too!

  5. Reminds me of some parts of the Columbia River basalts. Situation is the same: head east out of Portland for an hour and you're into BBB.

  6. Hi Survat, I have read in many places that the Western Ghts initally formed an escarpment touching the indian ocean to the west, probably when it abruptly broke off from the africa continent.
    But could you share me some mpossible ethods on how the Konkan Coast or i must say the Konkan Plains came into existence to the west of these Ghats ??? Their max width may be 30-40 Km..

  7. Malcolm - hope to visit the Colombia basalts some day !

    Adrian- when India broke away from Madagascar and then Seychelles that rifting would have produced a fault scarp and an elevated western margin. the present western ghat escarpment though is a retreated scarp. that means over time erosion has kept removing material from the cliffs making them retreat eastwards leaving a low level broad surface at the base which is the konkan coastal plain.

  8. Thanks Suvrat..

    Always loved your spontaneous answers.. very precise :-)

  9. Hi Suvrat, Am a Landscape Architect and working on a garden for a plot in Girivan which is near abouts the area you are talking about in this post. Soil excavations in the plot during house building have thrown up rounded boulders in a moorumish soil. Would these be deposited material? Also noticed some ant/termite hills on the plot which generally indicate the presence of water....the client has been looking at ground water...a borewell to be precise but the local borewell people could not penetrate beyond the rounded boulders with whatever technology they had. Could you tell me about the geology of this area? Urmila

  10. Urmila- those rounded boulders are a result of spheroidal weathering of basalt. imaging a rock layers cut by two sets of cracks at right angles to each other. so the rock layers is cut up into cubes. water percolates through these cracks and the weathering process smoothness the edges giving a spherical shape to pieces of rock. so not deposited but weathered rock. the geology over there is soil/ murum underlain by solid basalt.. not sure why the borewell people could not penetrate and not sure how predictable is the presence of ant/termites :)

  11. Thank you Suvrat! Perhaps the borewell people were just not equipped...or else wanted to hike their charges(!).Was not there then...