Friday, July 22, 2011

Geology & Geophysics 1st year field trip to Ambaji, Gujrat, December 2010

Chhabi Jain
M.Sc. 1st Yr, Applied Geophysics (2010-2011)

Department of Earth Sciences
Indian Institute of Technology Bombay
Powai, Mumbai- 400 076, Maharashtra, INDIA

The geological fieldwork at Ambaji (Gujrat) is an annual tradition of our Department. It is a part of the curriculum for the 1st year students of Geology and Geophysics that provides them a hands-on experience of geology. It enhances our understanding of the subject as we actually study the rocks, their constituent minerals and structures we read about in books. The training is so useful for research and field-oriented works that a number of Ph.D. students also joined us.

As Dr. T.K.Biswal said, “Field geology is such a subject that our knowledge about any outcrops or structures is augmented with more and more observations”. And “…it is necessary to understand the field because it provides the basic data in almost all geoscientific studies.” “The fieldwork at Ambaji has even sparked brilliant ideas put forward by students that led me to write interesting manuscripts”, said Dr. Soumyajit Mukherjee.

This year, the field trip to Ambaji was undertaken by the 14 first year students of geophysics, 30 first year students of geology and five of the Ph.D. and M.Tech. students in our department. We were guided by Profs. Tapas Kumar Biswal (TKB) and Somyajit Mukherjee (SM). Four cooks from IIT including Mr. Stefen constituted the support team. TKB has been studing this terrain for two decades or so. This was the 12th batch of IITB students he was instructing in the field. SM, on the other hand, is relatively a new entrant in this area as this was only his second visit. Over the years, the residents of Ambaji have become familiar with geologists from IIT Bombay! Being a tourist place, finding accommodation at Ambaji is very easy.

Since the students of geophysics come from a pure science (physics, chemistry & math) background, this excursion represents their very first fieldwork. They were therefore given special instructions by the teachers about what was to be carried. Students were instructed to wear sun-screen lotion, full-sleeve shirts, full pants, white hats, and sports (or ‘hunter’) shoes with strong grips. We had to drink only bottled water and had to remain careful about snakes!

On 1st December at 8 p.m., our Institute bus dropped us at the Bandra Terminus (train station) where we boarded the Aravali Express at 9 p.m. Chatting, singing and playing chess filled the next 13 hours. In no time it seemed, and much to our co-passengers’ pleasure, the train halted at Abu Road station on 2nd December at right time. We had about an hour travel in a fleet of jeeps to reach the ‘Rewari Dharmshala’ at Ambaji.

A beaming TKB, who had earlier driven to Ambaji with Stephen, greeted us and allotted our rooms. The boys shared the hall with the non-teaching staff. A corner of it was also reserved for cooking (a stove and utensils were brought from Mumbai).

On the first day, we were taken out for lunch as what our teachers called an ‘icebreaking session’. After a (heavy) meal and casual photography, we headed to the famous Ambaji temple. It was grand, made of white marble, with beautifully carved pillars and a sprawling courtyard. The previous batch of students recommended us an ice cream shop named ‘Apsara’ near the temple. We located that shop and earmarked it for future visits!

In the evening, SM made an ‘unusual’ start to fieldwork at a nearby outcrop of granite gneiss where he showed us for about an hour weak foliations and conjugate fractures. For dinner we had hot dal, vegetables, salad and rice. The meal was simple, delicious and homely.

Our field work was not restricted to Ambaji alone, but included many other places around it. Jeeps were hired to travel to all those locations. To cover an optimum area, we were scheduled to start from the dharmshala at 9.00 a.m everyday. To meet this requirement, we had to follow a strict itinerary each day. Those who woke up early made tea ready by 6.30 a.m. Breakfast was served by 8.00 a.m and lunch was packed while we were having breakfast before 9 a.m. We would do fieldwork until lunch time around 1 p.m. We would then resume work up to 3.30 p.m. Back at Dharamshala, we used to fill our cups with tea and sit around the courtyard, relax and chat. Some boys got a cricket bat and a ball. Using a bucket as wickets, they played cricket every day until it got too dark to see the ball! The spectators would seat themselves on the roof and cheer them on. Dinner was ready by 8.00 p.m. Each day we had to submit a resume of our field study to SM. So, until dinner was ready, we would sit in several groups and scribble our reports together. At dinner, we were given our own plates, spoons and glasses, which we had to maintain till the end of the stay. House work was indispensable!

Instructor Dr. Soumyajit Mukherjee explains a small scale structure

The first few basic things which we were taught in the field were measuring attitudes of planes and lines using clinometers and locating ourselves on the map. We measured the fracture planes in the granite gneiss, SM’s first outcrop, near our dharamshala and got them verified by the teachers. At places where folds were observed, we measured axes and axial planes of folds. Next, we learnt how to take back-bearings using a Brunton compass as well as a clinometer. The readings were more accurate from the Brunton, but it was harder to use. Thus, most bearings were taken using the clinometer. By the method of back-bearing, we then located ourselves on the toposheets. TKB pointed out a cuesta hill, which became an easy natural landmark to measure bearings from several locations. We were taught how to handle topo-sheets and how to read contours. From then on, at every new outcrop, we were first asked to locate ourselves on the topo-sheet.

The next area of focus was mineral identification and petrology. Ambaji and its surrounding towns were built on the Aravallis, one of the oldest mountain ranges of India. Rich with its prolonged tectonic history, both igneous and sedimentary rocks of the area enjoyed low-grade metamorphism. I was more interested in an outcrop of mica schist which had a high percentage of quartz. Shining muscovite could be easily recognized! Slickensides on sporadic fault planes led us to deduce their sense of slip Serpentine veins often parallels the slickensides. At places, tourmaline was also found in the mica schist.

Another common rock type in Ambaji was calc-schist. Weathered calc-schist is most easily recognized by its ‘ridge and furrow’ appearance. In this, small, light coloured “furrows” mark where the carbonate in rock has weathered away and the brownish ‘ridges’ consist of more resistant silica-rich layers.

Igneous rocks like basalts and granites were frequently encountered. We studied the Sendra Granite and the Erinpura Granite. Leucocratic granite is pales in colour because it contains a high percentage of quartz alongwith feldspar and dark spots of biotite. We identified individual minerals using hand lenses. These granites are characterized by spheroidal weathering. A number of caverns and crevices could be found in the granitic hills, many hidden behind thorny bushes, desert shrubs and cacti.

When we first examined these rocks, an instant competition started to test who could collect the best sample to take back to IITB! While boys could be seen trying to climb higher up to reach undisturbed samples, girls were seen (borrowing hammers) and breaking rocks themselves to extract an appropriate specimen. We had hung our hand lenses round our neck for convenience and we would study every rock we spotted to locate that finest one!

We also had the opportunity to study some rare rock outcrops such as pillow lava basalt, layered gabbros and anorthosites. Lensoid-shaped pillows could be spotted at many places in the spillites. They had distinct chilled margins where yellowish-green coloured epidotes developed. The outcrop contained sodium rich minerals like albite too. Layered (meta-) gabbro outcrops were rare. The anorthosite outcrops were magnificent. The grain size of plagioclase was over 2 cm.

Students fearlessly cross a river to search a litho-contact!

The hills of Ambaji are a storehouse that has been mined for minerals like Copper, zinc, lead and pyrite. We had the pleasure of visiting one such open-cast mine at Deri. The mine had been excavated extensively and is now exhausted. Until more advanced technology is developed to extract the scanty mineral deposits from the remaining rocks, the mine has to remain closed. The competition was on to collect the shiniest sample and the maximum number of samples of different types! There was purple sphalerite, black galena, and golden yellow pyrite (fool’s gold). The most difficult mineral to identify was chalcopyrite (a copper sulphide). While pyrite and chalcopyrite differed by just a shade, they shone deceptively like biotite in sun light.

Apart from mineral mining and other geologically significant structures, Ambaji is well known for its marbles. While Makrana is India’s best quality marble, Ambaji marble is not far behind. On the last day of our trip, TKB took us to visit a marble quarry. An entire mountainside seemed to have been excavated. At its base, gigantic slabs of marble were being cut, washed, polished and segregated from the unusable rock. The marble glistened pearly white and grey-green impurity minerals made natural patterns on the stone. Adjacent to the quarried limestone was an outcrop where green amphibolites had formed. Marble deposits are fast depleting in the world. The quarry we visited was estimated to be productive for another 50 years only.

When asked about why field trips were organized to Ambaji, SM said that the terrain is a “heaven for structural geologists”. It was true! Fractures, superposed folds and faults were visible in most of the outcrops. The type of these faults could be deduced from their attitude and that of their slickensides. We also classified folds based on the plunge of fold axes and the dip of the axial planes. Dolerite dykes were fairly common in some calc-silicate outcrops. These dykes were boudinaged. We calculated the aspect ratio of the boudins. Shear structures were studied in the Kengura Shear Zones. Here, granites were sheared to mylonites. Sitting over the unusual rock outcrop, our two teachers not only taught us the fundamentals of ductile shear structures, but also supplemented the theory with actual samples of proto-, normal- and ultra-mylonites. In these rocks, quartz grains became ribbons and feldspar clasts elongated by ductile shear. Together, they formed the C-fabrics. Faint S-fabrics could also be made out in many rocks. Stretching lineations were also faintly visible over some of the mylonite foliations in some samples. A thorough search also revealed winged porphyroclasts in the mylonites, which is rather uncommon. Observing these shear structures was very exciting. While searching for specimens and measuring the lineation to find the direction of shear, SM demonstrated how to take ‘oriented’ samples.

The study of geological structures at Ambaji is incomplete without a mention of Surpagla (a neighbouring village, named so because believers discovered there God’s footprints). It is a place to study ‘ideal’ structures found in textbooks. To reach these structures, we had to cross a rivulet. Caught in the spirit of adventure, we students folded our jeans, removed our shoes and splashed our way across. The first rock type we encountered was pelitic granulite, a well banded rock showing migmatitic features- stromatic structures, augens, folds and boudins. The paleosome layers (dark body) consist of mafic minerals such as biotites, garnets, sillimanites and green spinel. Pockets of granites were seen within the granulites. As we climbed and slipped over the steep, slippery terrain, we found calc-granulites displaying their characteristic ribbed appearance. We were taught to identify minerals like diopside and garnet. Pegmatite veins containing small percentages of tourmaline intruded parallel to the slaty cleavage in the country rocks. The veins showed boudins, which were also folded along with the parent rock. Biot’s Law of buckling and disharmonic folding were practically observed and verified. The time spent at Surpagla was the best geological moment in this fieldwork.

Speaking of adventure, another day that must be mentioned is when we visited Mount Abu. It was a day set aside to relax and experience non-geologic joys, though some field work was done there too! Our first destination was the Gabbar hill where we enjoyed a cable-car ride from the base of the hill to the peak where a temple dedicated to Goddess Ambaji stands. While floating in mid-air via rope-way, we got a mesmerizing aerial view of the region. Bare granitic hills with scanty shrubs that sprawled across the horizon were interspersed with sparse townships. These are the Erinpura Granites with an age of ~ 750 million years. Spheroidal weathering was the ubiquitous feature. Next we stopped at Guru Shikhar, the highest peak in Rajasthan. On this is built a small temple to commemorate guru Duttatreya-ji who is believed to have started the guru-shishya tradition, which is ingrained in the Indian culture.

We next visited the historically significant Dilwara temple. Made of beautiful white marble, this Jain temple is an architectural wonder. Each pillar and the ceiling are intricately carved with finest details. Each design is individual and unique. The temple houses statues made of marble and gold.

Towards that evening, we finally reached Mt. Abu. Some students went boating in the pretty Nakki Lake, which is surrounded by lush greenery and houses the Toad rock in its centre. Many devotional places have also been constructed around the lake. A few other students made a beeline for the local market. They roamed the market place in mixed groups, licking ice creams of odd flavours and clicking photographs. Many students bought warm caps and scarves or souvenirs to take back home. Tired from the travelling and wanting a place to sit together and chat, we hunted for a dhaba. The food joint where we eventually had tea boasted of having been the set of many Bollywood films! As sun-down approached, we hurried to secure comfortable spots at the sunset point. These are small platforms built high on the mountain, which offer a magnificent view of the sun while it rises and sets. And truely, the vision was breath-taking. The horizon was oddly divided into a layer of light clouds over layers of dark ones. The horizon was magnificently stratified into alternating layers of light and dark clouds. The sun seemed to be swallowed by the divide, glowing a dull yellow, then bright orange followed by a fiery red and finally sinking into blue-purple oblivion. Peace and calm descended on us as we watched the sinking ball of fire. Still wrapped in the golden glow of its memory, we finally made our way back to the vehicles. It was indeed a memorable day!

Towards the end of the field trip, students were divided into four groups. Each group was assigned a particular region on the toposheet to map. We had to first identify the various rocks constituting that region then map the litho-contacts as far as possible. All that we had learned so far in the field was put to a test. For three days, each group travelled to its assigned destination. In the process, two groups even encountered some hostile villagers who refused to allow the students to proceed further and even threatened to behead them on grounds of trespassing! Despite these minor impediments, the group work was fun. We studied rocks on our own, documented and photographed the geological structures that we encountered, measured attitudes and brought back significant samples.

Towards the end of the trip, we were taken once again to all the locations we had previously visited and worked in. Now that we were familiar with the structures and minerals present at each location, TKB summarized the tectonic events of the entire Delli basin of the Aravallis. The field work ended with the study of a unique outcrop of calc-granulite in which granite intrusion altered (or metamorphosed) the adjoining parent rock to ‘skarns’ (burning involves oxidation). High temperature minerals developed in scarns due to contact metamorphism. That particular outcrop contained one such mineral: wollastonite. It is commercially used as a refractory mineral since it can withstand scorching high temperatures. Since the mineral is very rare, we were instructed to take minimum number of samples to preserve the outcrop.

Academic satisfaction was not the only cause of joy on the trip. Some occasions for celebration fell within the fortnight spent in Ambaji. At the start of our trip itself, a student revealed she has been selected for employment in the Geological Survey of India. We didn’t rest till she agreed to take us out for dinner- though we deeply regretted our choice of restaurant when the food turned out to be inedible! Birthdays of three students, (including Chhavi Jain!) happened to fall during of the field trip. Ambaji being a very small town, great effort went into locating a confectionery shop. Cake was bought on all three occasions and mid-night parties were organized. With lit candles, balloons and colourful streamers, any room in the guesthouse was decorated and the birthday was celebrated there! The following day, everyone was treated at the ‘Apsara’ ice-cream parlour. On the last night of the trip, the students also organized a bonfire. We borrowed some firewood. Frequent addition of paper, cardboard and dried leaves kept it aflame for over an hour. We sang songs around it and played antakshari. A game of dumb charades was also enjoyed.

Even in a fortnight, the field trip taught us more than what we could ever learn from books. Besides geology, it taught us how to work as a team, adjust in a group and to find how to make the best out of even the worst situations. Our analytical skills and observational power were greatly enhanced. The difficult terrains we climbed, the knowledge we gained and the friends we made were the non-geological benefits.

Though the field trip was sufficient in every way, the department is planning some changes in future. To remain at par with current technology, the faculty wishes to include lithocontact mapping using GPS next year. Some also feel that the presence of a geomorphologist and a sedimentologist could improve the field education. A debate is still underway as to whether geology and geophysics students should continue to be taken into the field together or should they learn separately. The field experience would be more fruitful if the duration of work were stretched from a fortnight to a month.

Profs. T. K. Biswal & S. Mukherjee instructed geology to students in field, which was funded by IIT Bombay. SM annotated the text, which underwent some polishing by Chris Talbot (retired from Uppsala University). CT is thanked additionally for suggesting a more appropriate title.

We are privileged to receive Chris Talbot’s following two sets of comments on Chhabi Jain’s account:
"I enjoyed the fascinating account by Chhavi Jain of your field trip. Together you write very well. My suggestions aim to smooth it a little. I have taken the liberty of including Ms. Jain in the address. Let’s hope that she will look at real rocks when making her geophysical measurements.

Chhabi Jain is already a skilled writer, and I have been honoured to add a few suggestions that might improve the readability of a fascinating account. I would have liked to know the trip was 14 days long near the beginning. You inspired me to read about the Guru-Shishya tradition."
~Christopher Talbot (Olney, UK, 2011.02.02)

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