Thursday, January 10, 2019

Cracks In A Rock And The Western Ghat Escarpment

A friend sent me this picture of a section of the Western Ghat escarpment. It is taken from Jivdhan fort, looking north towards the hook nose of Naneghat. This location is about a hundred odd kilometers west-north-west from Ahmednagar town. Naneghat was a mountain pass for travel between the coastal plain and the plateau.

Photo credit: Rajesh Sarde

The yellow bloom makes a pretty contrast with the grey basalt. My geology eye was drawn towards something else; a suspiciously straight flowing stream, which I have highlighted with an arrow.

I looked at a satellite imagery of this location and the stream is seen following a fracture zone (black arrows)  that cuts across Jivdhan fort as well. The escarpment area is riddled with such fractures. They occur as north-south, northwest-southeast, and northeast-southwest (brown arrows) trending sets.


These fractures are regions of shattered rock. That zone erodes away quicker. Water flowing in the linear depressions that form enhance this topographic difference and eventually cut deep straight valleys.

Large fractures or cracks along slopes causes slabs of rocks to cleave away from mountain sides. Slopes retreat due to such rock falls. A large crack is seen in the picture just a few feet away from where my friend took his photograph. At some point a portion of rock will detach itself and Jivdhan fort will become that much narrower.

Look at the zoomed out satellite imagery of this area. The plateau edge has been fragmented into isolated hillocks, mesas and pinnacles by enhanced erosion along fractures oriented in various directions. You can follow some of these fractures (white arrows) to the straight edges of the escarpment suggesting that slab breakoff has played a role in shaping the morphology of the cliff line.


Such fracture systems not only have formed a landscape of mesas and pinnacles but have caused the Western Ghat escarpment to retreat eastwards for at least tens of kilometers from its original location. The escarpment is a legacy of the breakup of the western margin of India with Seychelles at the end of the eruptions of the Deccan Basalts. At that time in the Paleocene (~60 million years ago), continental stretching caused the formation of a series of north-south oriented faults which sloped (dipped) to the west. The westerly block of each of these fault sets sank, created a staircase like crustal structure descending towards the west, with west facing cliffs. The Western Ghat escarpment would have been the easterly most of these cliffs.

See the schematic below which shows this staircase crustal structure of the western margin of India.



The red portion would have been the original extent of the Deccan plateau. It has retreated eastwards over several millions of years. As a result, the coastal plain became progressively broader. Give a thought to the humongous amount of rock that has been removed by erosion.

Along the west coast the erosional  retreat has not wiped clean all evidence of the original plateau. From the coastal plain rise isolated ranges and mesas. The hill station Matheran, where people go to catch the cool wind and a spectacular view, is a fine example.

See the satellite imagery below.


Matheran was where the plateau edge and escarpment once was. It has now moved eastwards (arrows) leaving behind an erosional remnant,  a splendid outlier of the Deccan plateau rising abruptly from the plains.

Let's end with a 3D view of the escarpment along the Jivdhan-Naneghat area.


If you take a flight out of Pune to Delhi, the plane will fly a northerly route parallel to the plateau edge for the first 20-25 minutes of your journey. The Western Ghat escarpment appears as it does in the tilted perspective above, a sinuous line of majestic black cliffs, testimony to the forces of volcanism, continental breakup, and erosion.

A section of this stunning landform deserves to be included in our National Geological Monuments list.

6 comments:

  1. Hi. I am Makarand Ketkar. I am a naturalist as a professional and I am deeply interested in geology and wildlife. I take thousands of students on field every year and teach them about nature and it's various living and non living forms.
    Long back I was told by one my seniors that an oceanic plate subducted under Indian peninsula and tilted it towards east(causing all major rivers to flow eastwards). This subduction accelerated the sinking process of western edge of the plateau. Do you have more info about this event? Thanking you in advance.

    ReplyDelete
  2. Hi Makarand- I think what he may have meant is that the India plate subducted under Asia. There is no oceanic plate subducting under the Indian peninsula. The western and eastern margins of India are divergent margins i.e. they have formed during continental breakup as India separated from Africa, Antarctica and Australia and then moved away. Subsidence of the western margin is because of movement along faults that formed during continental separation and not during subduction. Only the northern margin is convergent i.e. it formed by initial subduction and later collision with Asia.

    There are several plausible reasons for the easterly tilt of the Indian Peninsula.. I have listed them in this earlier post -

    https://suvratk.blogspot.com/2011/09/easterly-tilt-of-deccan-plateau.html

    hope this helps.

    thanks for reading!

    ReplyDelete
  3. Wonderful! Thanks for putting this out there! :)

    ReplyDelete
  4. I enjoyed this post on the geology of the northern Western Ghats as the Nane Ghat area was a favorite haunt of mine when I worked in Pune. In 2014 I worked with some Landsat data to create the attached map emphasizing vegetation that might be of interest to readers. https://ianlockwood.files.wordpress.com/2014/06/landsat_pune_part1a6_14.jpg

    Post is at: https://ianlockwood.wordpress.com/2014/06/01/sahyadri-revealed/

    ReplyDelete
  5. Ian..thanks for the link. very informative maps and blog post!

    ReplyDelete