Tuesday, May 10, 2022

Field Photos: Dikes And Gneiss

My friends have been traveling across India and sending me pictures of landscapes and rocks. I am doing field work vicariously.

Posting below a few pictures that I have received.

Dikes Intruding Bundelkhand Gneiss. Pictures by Rajesh Sarde.

These two photos were taken at the Ken River gorge in Madhya Pradesh, near a gharial sanctuary. 

The dark rock making up the floor of the gorge is a dike. It has intruded the pink colored gneiss rock. Notice that the gneiss is fractured. Intrusions follow major weak zones in the gneiss.  Being softer than the gneiss, erosion over time has removed much of the dike, forming a narrow valley.

And in this picture, an arm of the dike known as an apophysis can been seen. It is almost at right angles to the gorge.

The Bundelkhand Gneiss ranges in age between 3.2 billion to about 2.5 billion years ago. The mafic dikes, igneous rocks rich in iron and magnesium silicate minerals, intruded later into the granite gneiss. Recent geochronological work on the dikes suggest two distinct events of dike emplacement, an early episode dated to about 2 billion years ago, and a later one at 1.1 billion years ago. Interestingly, the magnetic signatures frozen in these dikes have been used to make inferences about paleogeography. The results indicate that the north and south Indian crustal blocks which had independent origins were in close proximity by about 2.5 billion years ago. 

The magnetic signatures of the 1.1 billion year old dikes throw up a puzzle. They match those preserved in the Upper Vindhyan strata and intrusive rocks, seemingly constraining the age of the Upper Vindhyan sequence to around 1 billion years. However, recent fossil finds which I wrote about in a recent article for Nature India point to the Upper Vindhyans being much younger, about 550 million years old!

Dikes Intruding the Deccan Traps. Pictures by Rajesh Sarde.

These two photos were taken at the base of Tamhini Ghat, west of Pune, near a popular trekking spot known as Plus Valley. The rocks are about 66-65 million years old.

As in the previous example, the dike has eroded away faster than the host rock forming a narrow depression. Notice the closely spaced jointing pattern or cracks in the dike. 

 In this photo, the sharp boundary between the dike and the basalt rock can be clearly seen.

 Tonalite Trondhjemite Gneiss, Palolem Beach, Goa. Picture by Aneeha

These rocks, abbreviated as TTG, are relicts of early continental crust. They are about 3.4-3.2 billion years old. They represent Archaean age magmatism that formed the lighter continental crust. Such TTG's  are found all across India. They are the oldest component of cratons, the nucleus of the first continents. These magmas are generally granodiorites, rich in sodium and calcium feldspars and poor in potassium feldspars. They were deformed and metamorphosed subsequently in to a gneiss, in the process acquiring a characteristic banding. 

Next time hopefully pictures from my own field trips!


  1. These are spectacular photos! And so cool to get a glimpse of the "first continents".

    1. thanks Hollis. The next plan is to go and visit these sites myself :)

  2. Great photos, Suvrat. I have an opportunity to spend few days in Iceland next month. As a geology enthusiast, particularly looking forward to seeing the exposed mid ocean ridges..What other geology treasures can be observed there- thought you are the best person to suggest. Thanks, Mohan

    1. thanks Mohan. there likely are guided volcanic tours of Iceland for tourists which will take you to see a variety of volcanic features, e.g. active vents, different lava types, dikes etc. Have fun!

    2. Thanks Suvrat. Iceland was beautiful. Spectacular geology seen there. I did hike little bit on a glacier (Vatnajokull) and a small volcano. As we drive around, we see several places where fissures can be seen. I got some nice geology pics to share. Will email you. Mohan