Field Photos: Basalt Landscapes Around Fort Ghangad Near Pune

Its that time of the year when I go wandering in the basalt countryside around Pune. Its always a fine day off exploring the myriad valleys and mountain ranges. Last Sunday I had gone with some friends to fort Ghangad about 80 km west of Pune, situated near the backwaters of the Mulshi  Dam.

The interactive map below shows the location of the fort and a synoptic view of the landscape and geology too (more on that later).



Fort Ghangad- A small mesa landform. You can see the ramparts at the very top.

 The walk begins with a lovely forest trail

As we climb some nice views open up.

Photo-op overlooking a steep valley

Nearing the summit! You can see the ramparts and the last hundred feet or so of these forts in the Deccan Traps are always tough..

Its takes some negotiation along a steep face with the help of cables

And some coaxing from friends!

At the top though is a view to die for :)  Spectacular basalt escarpments, pinnacles (at a distance, the famous Tailbaila, another popular trekking destination) and high forested plateaus

Another view of the many pinnacles in this region.

How do these pinnacles form?

The satellite image below shows the area around fort Ghangad. The terrain is crisscrossed with fracture systems i.e. the basalt rock has large and long cracks oriented in different directions. I have outlined in dotted black lines the two prominent fracture systems. You can browse this image or the interactive map above and discover lots of these fractures throughout the region.

These fractures are zones of weaknesses along which pieces of basalt detach from the main body of rock and fall away. Pinnacles are erosional remnants of once continuous lava flows which have cleaved off along these fractures. You can see this process in action in the picture below. A large slab of rock has detached from a cliff face at fort Ghangad. There is active mass wasting going on at fort Ghangad. Over time, the mini mesa will become a narrower pinnacle! This is something to think about. The immensity of the lava province is apparent when you look at these mountain ranges and massive escarpments. We think less though of how much bigger it was once. These pinnacles are remnants, meaning entire enormous thicknesses of lava flows have been removed by erosion. So has all the rock material to form these valleys. Much of that sediment has made its way into the Arabian Sea. So much sediment has been removed that it is thought (1, 2) that the crust has rebounded in response to the removal of all that weight (denudational isostacy), contributing in part to the great elevation of this region.

Another interesting feature I saw was a dyke (in the satellite image its the orange dotted line which I have offset a little so you can see the valley it forms) intruding the basalt. Many of these fractures systems trace their origin to the tensional forces that split India away from Madagascar (88 million years ago) and later the Seychelles (66 million  years ago). The later event likely triggered the massive basalt volcanism. Some of these fractures in the earth's crust act as pipes bringing magma to the surface. Occasionally, the magma solidifies in the pipe forming an intrusive igneous body known as a dyke. In the picture below the tabular body of rock on the right, the one that looks like a stone wall, is the dyke.

Look closely at the geometry of the cracks in the dyke. You will see a honeycomb hexagonal pattern. It is called columnar jointing, as the six sided (sometimes 5 sided, 8 sided etc) joints form hexagonal columns. These joints are cooling cracks that develop perpendicular to the cooling surface. They develop most commonly in lava flows. There,  since the cooling surface is usually horizontal (contact with the cooler air above), the columns are vertical. In the case of this dyke, the cooling surface was nearly vertical  (the contact between the rising magma and the colder surrounding rock) and the result were horizontal columnar joints.

A close up of the dyke shows the hexagonal geometry clearly.

A feature of regions with such prominent fracture systems is their control over streams. Water finds and enhances weaker parts of the landscapes such as fracture zones. The result is a structural control over drainage with stream orientations that reflects the geometry of the fracture zones. In the picture below just south of fort Ghangad, the differently shaded blue arrows point to two streams which are following the two prominent fracture systems I had outlined in the satellite image. Notice the straight course and the sharp right angle turns the streams take.
 

Geology musings over, we head down a little reluctantly! 

Overall, it was really a fantastic day out with a beautiful countryside to savor along with some great geology. I love these back country roads of the Deccan Volcanics.