Saturday, August 17, 2019

Darwin: Victorian Menage A Trois

In 1864, a few years after the publication of the Origin of Species, Darwin, fighting a bout of illness took up some botanical work on Lythrum, a genus of flowering plants known as loosestrifes. He had been breeding them on and off for many years. His interest was in their sexuality as they gave him a deeper understanding on the evolution of sterility and reproductive isolation. His theory of population divergence and the origin of new species depended upon the evolution of traits that prevented individuals and populations from mating with each other.

Lythrum was known for its triple sexuality. There were three kinds of flowers. The female stigma could be tall, medium or short-styled. Each was accompanied by two sets of male stamens.  If the female was tall, the males were short or medium sized.

Darwin realized through his breeding experiments that greater the difference in height between these sexual organs, the greater the frequency of sterility. Mating between the unequal sized sexes in the same plant produced sterile seeds.  This was evolution preventing inbreeding and favoring cross pollination.

Adrian Desmond and James Moore in their biography, Darwin: The Life of a Tormented Evolutionist, write about Darwin's mischievous side:

" Talk about illegitimacy might have shocked the ladies guilds, at least coming from Erasmus Darwin's grandson. (Grandfather's own bastardizing experiments were still supplying tittle-tattle. At this moment Darwin suspected the widow of a botanical friend, Francis Boott, to be an illegitimate granddaughter of old Erasmus). But Darwin was stolid, methodical, reducing the love of the plants to cold, clinical calculation. Sterile seeds counts somehow fitted an unromantic, data-crunching age. Not for him Erasmus's flowery personifications, as styles and stamens bent to embrace in a kiss:

Two knights before thy fragrant altar bend,
Adored Mellissa! and two squires attend.

Still, he could tease. Solicited by a Mrs Becker for something edifying for her ladies' literary society, he posted 'On the Sexual Relations of the Three Forms of Lythrum salicaria'. Goodness knows how many red faces left after hearing that 'nature has ordained a most complex marriage-arrangement, namely a triple union between three hermaphrodites,- each hermaphrodite being in its female organ quite distinct from the other two hermaphrodites and partially distinct in its male organs, and each furnished with two sets of males' ".

There is no doubt that Darwin was an outstanding thinker. But he also was a hands on guy. His thinking was not vacuous or overly speculative.  He was a tinkerer and putterer.  He produced data. By cross breeding plants. By dissecting barnacles. By co-opting pigeon breeders and noting down the variation in the size and shapes of different breeds. Documenting tiny differences between individuals within groups was crucial to his case that complex structures could evolve by incremental changes over generations.

From life's little details he built the grandest theory of all.

Friday, August 2, 2019

Secondary Mineralization In Deccan Basalts: Timing And Precipitation Environments

When and how did these minerals form?

These are the world famous zeolites and other secondary minerals (green apophyllite) that fill cavities and cracks in Deccan Basalt lava flows.  Zeolites and other secondary minerals like apophyllite are calcium, sodium, potassium bearing alumino-silicates with varying amounts of water and other volatile elements like fluorine (apophyllite). They are prized by mineral collectors and by petrologists who study them to understand the geologic conditions that affected the lava after their eruption. This has broader implications for understanding the initiation and evolution of fluid circulation systems during the burial and exhumation of the lava pile.

A recent study has taken a step towards understanding the timing and precipitation conditions of these secondary minerals in basalts.

Exceptional Multi Stage Mineralization of Secondary Minerals in Cavities of Flood Basalts from the Deccan Volcanic Province, India - Berthold Ottens, Jens Götze, Ralf Schuster, Kurt Krenn, Christoph Hauzenberger, Benkó Zsolt and Torsten Vennemann. 

The most exciting part of this study is the publication of  absolute ages of mineralization of apophyllite using Rubidium-Strontium (Rb-Sr) and Potassium-Argon (K-Ar) radiometric methods. As far as I know , these are the first ever published absolute dates of secondary mineralization in Deccan lavas.

The scientists studied lava flows from the famous Savda quarries near the town of Jalgaon and also from quarries near the town of Nasik, both in the state of Maharashtra.The photograph below shows a portion of the lava sequence that was studied in one Savda quarry. The occurrence of secondary minerals in lava cavities is shown in the lava profile to the right. All the following images in this post are from the paper linked to above.

This study concentrated on examining the mineral sequences found in the large cavities in the central portions of a lava flow. Smaller amygdules (fully filled cavities) and vesicles (partially filled small cavities) occurring at the bottom and top of lava flows won't contain the entire sequence of minerals.

The investigation revealed three broad stages of mineralization. Stage 1 sequence precipitated first. It consists of an iron-magnesium and potassium bearing clay layer (containing celadonite and smectites) coating the walls of cavities and microbial films and filaments.  Stage 2 sequence consists of calcite (calcite 1), fine grained zeolites (zeolite 1), and plagioclase, followed by a layer of chalcedony and quartz.  This is followed by a second generation of larger calcite (calcite 2) and zeolite (zeolite 2) crystals. The common zeolites, both in zeolite 1 and zeolite 2 stages are heulandite, and stilbite with additional mordenite observed in zeolite 1 assemblage. Stage 3 sequence is made up of a third generation of calcite (calcite 3) , along with apophyllite and rare powellite (calcium molybdate). 

This multi-generation mineralization sequence could only be ascertained by carefully noting down the mineral sequences appearing in hundreds of different large cavities. Such a broad examination is necessary since the entire sequence may not have crystallized in any one cavity. The two pictures below demonstrate this problem.

On the left is a cavity which shows two generations of calcite (Stage 2) grown on a clay-chalcedony substrate (Stage 1). On the right is apophyllite (Stage 3) directly overlying Stage 1 mineral encrusted bio-filaments. In both these cavities, the Stage 2 zeolites have not precipitated. In some other cavity one might encounter a sequence of Stage 2 large zeolites overlain by Stage 3 calcite, but not the earlier calcite and zeolites, nor the Stage 3 apophyllite. Reconstruction of the entire sequence thus requires an examination of a large number of cavities in order to work out the true order of mineral succession.

This reconstructed mineral paragenetic sequence is presented in the graphic below.

The researchers used fluid inclusion studies and carbon and oxygen isotope analysis to narrow down the temperature conditions during precipitation (fluid inclusion) and the source of mineralizing fluids (isotope analysis).

Fluid inclusions are tiny volumes of fluid that get trapped during crystal growth. The fluid may be a liquid or a vapor or both. For ascertaining temperature of entrapment, a bi-phase inclusion is selected. The sample is heated until it reaches a temperature where the inclusions change from a heterogeneous (bi-phase) state to a homogeneous (one phase) state. This homogenization temperature (range) is taken to be the temperature of fluid entrapment i.e. initiation of crystal growth.

Fluid inclusion analysis of calcite 1, calcite 2 and quartz indicated temperatures between 94 deg C to 173 deg C. Inclusions in stage 3 calcite and apophyllites indicated higher temperatures between about 140 deg C to 244 deg C.

The carbon isotopes of calcites (presented as the ratio of C13 to C12) showed depleted values due to enrichment of the lighter C12 isotope. This was taken to indicate a substantial biogenic contribution for the carbon (the lighter isotope is preferentially taken up by organisms during photosynthesis). The oxygen isotope values for calcite 1 and calcite 2 were enriched in the heavier isotope O18. Values of delO18 were + 14 - +15 for calcite 1 and +19 - +27 for calcite 2. DelO18 is a measure of the ratio of the two isotopes of oxygen O18 and O16. Such enriched values indicate magmatic source fluids, although the slightly lighter values of calcite 1 suggests mixing with meteoric water, which is enriched in the lighter isotope O16 as compared to magma and sea water (meteoric water is derived from rainfall and ends up percolating through rock as groundwater). Isotope analysis of calcite 3 is not presented in this study.

Rb-Sr and K-Ar geochronology of apophyllites shows that precipitation of apophyllite took place repeatedly as discrete events spread over a large time span. The Nasik sample yielded ages of 58 mya (million years ago) and 21 mya. Apophyllites from Savda quarry near Jalgaon yielded ages of 45 mya and 27 mya.

The observed mineral sequence along with data from fluid inclusions, isotopes and geochronology have enabled the researchers to propose the following sequence of events depicted in the schematic below.

After the eruption of a lava flow, interaction of the hot lava surface with meteoric water resulted in filling up of cracks and cavities with water, accompanied by the alteration of mineral olivine, plagioclase and volcanic glass.  Fe, Mg, Si, Al released from the rock was recombined to form clay mineral and iron hydroxide coatings on the walls of cavities and on microbial films and filaments.

As the lava layer got buried under younger lavas, the composition and temperature of the fluids evolved resulting in the precipitation of calcite 1, fine grained zeolites 1 and chalcedonay and quartz. The oxygen isotope values of calcite 1 indicates a mix of magmatic residual fluids and meteoric water. Continued burial resulted in a diminishing contribution from meteoric water. Zeolite 2 and calcite 2 phases precipitated from magmatic residual fluids as indicated by the oxygen isotope values which are enriched in the heavier isotope. Maximum burial temperatures at this stage have been estimated to be about 150 deg C.

This interpretation of Stage 1 and Stage 2 mineral assemblages having formed immediately following volcanism and during burial is in line with previous thinking regarding secondary mineralization in Deccan basalts. Data about the timing of Stage 3 however has thrown up a surprise.

Geochronology indicates that precipitation of stage 3 minerals like calcite 3 and apophyllite took place much later. Deccan volcanism ended by 64 mya across most of the province. Mineralization ages of 47 mya and 27 mya indicate that by this time considerable erosion of the lava pile would have resulted in exhumation of deeper layers and much lower burial temperatures. The fluid inclusions in calcite 3 and apophyllite indicate crystallization at temperatures between 144 deg C and 244 deg C. Some earlier work by Shrikantappa and Mookherjee on fluid inclusions in apophyllite from Savda indicate even higher temperatures reaching 280 deg C. Such boiling conditions at a shallow burial level implies the formation of a hydrothermal system. The presence of powellite, a calcium molybdate, indicates oxidizing fluids. Such a system must have formed repeatedly at widely separated time intervals. The study does not put forth an explanation of the geological events that could have triggered the formation of these high temperature fluid circulation systems.

In summary, Stage 1 and Stage 2 involves a locally formed circulation system. Elements were scavenged from adjacent regions of the lava flow and incorporated into growing secondary minerals. Stage 3 involves a larger circulation system. The concentration of vanadium in basalts is low, in the range of few tens of ppm (parts per million). In Apophyllite, the concentration of vanadium is on the order of 3000 ppm.  This suggests that fluids attained this element concentration by circulating and reacting with a large volume of basalts over a widespread area.

The researchers have stressed that their proposed explanation applies to the specific mineral sequence observed at Savda quarries near Jalgaon and the lava flows near Nasik and is not to be taken as a general explanation for secondary mineralization covering the entire Deccan basalts. That would require much more extensive sampling from different regions and stratigraphic levels. The observations from other studies and the mineral sequence I have personally observed in the Pune area though does suggest that Stage 1 and Stage 2 sequences at least are common everywhere, although the specific combinations of zeolites may vary. Early near surface reaction of hot lava with groundwater and then progressive burial with mineralizing fluids getting contributions from both meteoric water and magmatic residual fluids would have been a common trajectory of fluid rock interaction across the volcanic province.

Such a timing of mineralization, contemporaneous with volcanism and continuing after burial, has been noted from lava provinces in Iceland and Iran too. The Stage 3 event though is much younger and would depend on later geologic triggers that may differ from place to place. In eastern Iceland for example, a late stage of mineralization has been linked to heat provided by the intrusions of dikes ( sheet like bodies of magma injected along fractures). The geochronology of apophyllites in the Deccan Volcanic Province needs to be validated by more such work.

Some questions do remain.

First, nearly 20 to 40 million years after Deccan volcanism ended, what could be the source of heat for the initiation of the late stage fluid circulation systems that precipitated calcite 3 and apophyllite?

And secondly, the lack of carbon and oxygen isotope analysis of calcite 3 prevents us from identifying the source of precipitating fluids. That is a lacunae that future studies must aim at filling.


Srikantappa, C.; Mookherjee, A. Water, Aqueous, H2O-CO2 and Gaseous Inclusions in Cavity Minerals in the Basaltic Lava flows around Pune, India: Evidence for Boiling. In Proceedings of the Second Meeting of the Asian Current Research on Fluid Inclusions (ACROFI–2), Kharagpur, West Bengal, India, 12–14 November 2008; p. 176.


Saturday, July 27, 2019

Konkan Road Trip Photos: Murud Dabhol Tural

Last week beginning Monday July 15th, I took a four day road trip to Konkan, India west coastal plains. We went first to the small village of Murud and then drove south via Dabhol to Tural highlands.

The phrase 'coastal plains' is something of a misnomer since between the high Western Ghats and the Arabian Sea there are hill ranges with altitudes reaching 50 m to 200 m ASL. Tural is a community living on one of these ranges. We stayed there in the family home of a friend.

The map below shows a portion of the Konkan region through which we traveled.

The region had come alive due to the monsoons, although that week we caught a small break in the rains. It did rain heavily in short bursts, but there were enough interludes to go for long walks and enjoy the sun too.

Some pictures of landscapes that we came across.

1) The coast near Murud. After a brutal summer, the feel of cool winds and sounds of monsoon waves crashing on the shore was very refreshing.

2) Lonely stretch of a shimmering beach near Murud.

3) Loading our car on to the ferry at Dabhol.

4) Colourful fishing boats at Dabhol jetty.

5) Continental erosion writ in mud! River Vashishti meets the Arabian Sea.

6) Rice fields in a quiet community in Tural highlands.

7) Tural highlands is capped by a flat surface.

8) This plateau cap is made up of iron rich laterite. It formed during late Miocene times (~10 million  years ago) by prolonged chemical weathering of the underlying basalt rock and pediment (layer of weathered rock debris) . The picture shows the hard laterite surface, which would have been a low lying peneplain in late Miocene times.

9) Subsequent to lateritization, the western margin (Konkan coastal region) underwent some uplift, resulting in the formation of a plateau or 'table land' as it is commonly called. As the land rose, invigorated streams cut into the laterite surface forming deeply entrenched channels.  The picture below shows a close up of the laterite plateau dissected by a dendritic stream network (blue arrows).

10) The evolution of the Konkan coastal region from a low lying undulating surface undergoing lateritization, to an uplifted and dissected plateau is depicted in the schematic below.

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

11) The laterite is a commonly used building material in this region. Small quarries pockmark these highlands. The picture shows large bricks of laterite. The plateau cap is hard laterite that can't be cut into regular brick shaped pieces. Below this crust though is a softer iron rich soil. This semi indurated material is cut into brick shapes and left to dry. It hardens upon dehydration into a usable stone.

12) We took long walks in cool lush forest patches.

13) Deep in the forest we visited my friend's family temple, a hidden jewel with a spring fed bath. These temples act like a social glue, bringing families and communities together on religious and other occasions.

14) On the way back via Kumbharli Ghat we caught sight of the majestic Western Ghat Escarpment.

until next time! 

Thursday, July 11, 2019

Groundwater Must Be The Focus Of India National Water Policy

India's Water Management Crisis

A piercingly clear essay by Himanshu Thakkar on why India must realign its water resources priorities from big dams and river linking projects to protecting, managing, and regulating ground water.

Just take a look at the numbers:

"Most of the water that India uses today comes from over 30 million wells and tubewells. Irrigation is India’s biggest user of water and over two thirds of irrigated area gets water from groundwater. 85% of rural domestic supply, over 55% of Urban and Industrial water supply comes from groundwater. The graph of % of water in each sub sector coming from groundwater has been going up for at least four decades. In fact, some estimates show that over 90% of additional water India used in last four decades have come from groundwater. It sounds like an immitigable blessing. That’s not how blessings work, unfortunately.

Central Ground Water Board’s data shows that in about 70% of areas, groundwater is depleting and at many places it has exhausted or is on verge of exhaustion. The quality is deteriorating. Warnings have been available for decades now, but the government has done little to address the emerging crisis.

In fact, India’s water resources establishment, lead by the Big dam ideologues at Central Water Commission have ensured that the government do not even acknowledge that groundwater is India’s water lifeline"....


Some States have taken initiatives to manage ground water. Maharashtra recently passed the Maharashtra Ground Water Act which provides a framework for management and regulation of ground water. How much diligent enforcement of the rules actually takes place remains to be seen.

Additional Reading:

The Maharashtra Groundwater (Development and Management) Act 2009 - Shashank Deshpande, Deputy Director GSDA.

A Decade Of The Maharashtra Ground Water Legislation: Analysis Of The Implementation Process - Sanjiv Phansalkar and Vivek Kher.

Monday, July 8, 2019

Papers: Indus Civilization- Resilience, Fragility And Rural Complexity

Diversity, variability, adaptation and ‘fragility’in the Indus Civilization- Cameron A. Petrie

We are inheritors of a rural civilisation’: rural complexity and the ceramic economy in the Indus Civilisation in northwest India - Danika Parikh and Cameron A. Petrie

These two recently published papers are worth reading.

The first one reviews settlement patterns, water availability, agricultural strategies and craft production in urban and rural Indus settlements. It draws inferences on the type of power structures and hierarchies that may have prevailed within cities and villages and between different regions. And there is the perennial question on the link between climate change, water stress and the decline of urban sphere of the Indus civilization. There were different response from the urban and rural spheres to environmental stress, with the more flexible and adaptable rural lifeways showing more resilience and sustainability.

"Petrie et al. (2017; Petrie 2017) have suggested that the weakening of the ISM around c. 2200–2100 bc meant that the climate in the subsequent period became ‘unpredictably unpredictable’. By this we meant that before and during the Indus urban phase, populations were familiar with ‘predictable unpredictable’ conditions and their farming strategies were tailored to make use of water supplied by combinations of rainfall, inundation, small-scale irrigation and/or lifted water (cf. Miller 2006). Populations in specific areas across the Indus zone might have been able to survive one, two, or even more years of drought, either through reliance on their own resources, or through support from other regions. However, when this range was exceeded, such as when populations were faced with protracted periods of drought, the local and medium-to-long range provisioning and support networks may not have been able to sustain the status quo. I have suggested that in such a situation, farmers may have had to engage in constant risk mitigation, thereby reducing opportunities to produce surpluses, and in such situations it is possible that living in large groups (i.e. urban centres) was not an option".

The Indus cultural sphere lasted a long time after its cities declined. In the graphic below the upper left and right panels show distribution of settlements during the urban phase with modern winter (left) and summer (right) rainfall contours overlain. The bottom panel shows the post urban settlement patterns. There are denser habitations nearer the Himalaya front in the post urban phase. This shift from Rajasthan, Cholistan and Haryana eastwards and closer to the Himalaya foothills followed more reliable monsoons in that region. Gujarat on the other hand wasn't depopulated as much suggesting regional differences in monsoon strength and varied water harvesting strategies. However, the urban center of Dholavira and nearby settlements were abandoned.

Source: Cameron Petrie

Even the decline of the cities was not a sudden event. Indus societies did not collapse due to any one catastrophic environmental change such as one big river changing course or a very rapid decline in monsoon. Urbanization was at a peak between 2600 B.C and 1900 B.C. But at Mohenjodaro for example, signs of abandonment and depopulation begin by 2200 B.C. On the other, Harappa continued to be occupied throughout the urban phase and well into the late Harappan Phase, although analysis of skeletons do suggest increasing physical stress.

The second paper by Danika Parikh and Cameron Petrie concentrates on bringing out the complexities and variation in rural lifeways and economies. Ceramic products from four Indus age villages in Haryana are analyzed and described and some interesting inferences drawn on urban rural (in)dependence and the socio-economic role of villages in the larger Indus sphere.

"The regional rural ceramic economy innorthwest India was clearly complex and shows a considerable degree of variation. Rural communities produced some ceramic forms similar to Classic Harappan forms, and others that were quite different, and they used some decorativemotifs that were common and others that we had previously not seen. This pattern of similar ceramic forms but different techniques and decoration is particularly interesting, given what we understand of how pottery production is learned. Pottery forming is often learned through ‘vertical transmission’, inter-generationally; shape and decorative motifs are more easily imitated and are often transmitted horizontally, or peer-to-peer (Knappett 2011, 106–107; see also Gosselain 2000). The use of different techniques to produce the same forms suggests that Classic Harappan and Haryana Harappan ceramicmaterial was not produced in the same workshops, and that these potters are unlikely to have been members of the same communities of practice".

The rural populations were not only engaged in agro-pastoralism. Villages had varied occupations such as functioning as workshops for specialized craft production and as factory sites making goods for larger towns and cities.

Open Access.

Sunday, June 30, 2019

Groundwater Worries: Saving Pune's Hill Slopes


I wrote a short note on my Facebook page on Pune groundwater and its growing importance in response to a citizen's petition to save natural recharge areas that occur on several of Pune's hill slopes. There is a proposal for two roads (the petition mentions only one) to cut across these hills, which will result in the paving over of the recharge surface. In the picture below, the roads will be built at two levels across the slopes behind the green and yellow colored building.

 Forested Slopes of Law College Hill

Besides the threat to groundwater, there are other objections to the road, including its outdated route  and its preference for private vehicles over public transport.

Pune nature lovers and others too... do consider signing the petition and help save part of our precious remaining natural heritage.

Cross Posted from Facebook:

A couple of days ago it rained about 75 mm in central parts of Pune. If you consider a 1 area around your neighborhood, about 75 million litres of water came crashing down in a few hours. If just 2 percent of that infiltrated into the ground, about 1.5 million liters were added to our ground water resource through a 1 sq. km surface area.

Surprised that rock can hold this much water? Those who grew up swimming in the Tilak Tank of old won't be. It was fed by a natural spring. At any one time the pool held about 2.5 million liters of water. The Tilak Tank hole in the ground would have held much more, but excess water was being drained into a nullah. More recently, the Suvarnarekha building on Prabhat Road was demolished, and the builder excavated a hole for a basement. It soon filled up with ground water. It held about 5.5 million liters of water. As the builder started pumping out the water, more kept rushing in. Over a one and half year period up to 100 million liters of water was likely pumped out. All this water held under a few hundred square meter area!

Pune receives about 650 mm of rain annually. That means 260 billion liters of water falls yearly over a 400 area. How much of that is infiltrating into the ground.. 5 percent, 10 percent, 20 percent? No one knows for sure and the amount will be highly variable across a surface. But we do have an idea how much is being taken out. A recent estimate by ACWADAM, one of the leading experts on local hydrogeology, puts our annual extraction of groundwater to be 3-4 TMC, which amounts to 80-100 billion liters of water. This suggests that we are extracting more than the natural recharge, since ground water levels are beginning to dip at many places.

The Pune dam cluster collectively store 826 billion liters of water out of which 315 billion liters are allocated annually to Pune. Our increasing use of ground water at 80-100 billion liters annually, underscores the critical role ground water is beginning to play in our lives.

It is imperative that we redress the growing imbalance between extraction and natural recharge. We can do this by individual action of leaving ground uncovered around our homes, and also by protecting larger swaths of recharge areas where infiltration rates are particularly high. The Vetal - Hanuman Tekdi slopes have been identified by ACWADAM as an important natural recharge zone. They must be protected from being covered over by concrete by the proposed HCMTR and Balbharati roads.

Please sign this petition for saving Pune's tekdi slopes from being destroyed! Say no to the HCMTR and Balharati Roads.



A small addition to the above post. An earlier study by Raymond Duraiswamy and colleagues published in 2009 had identified the Hanuman (Law College) Hill slopes as potential recharge zones. The map below shows recharge potential of parts of Pune as identified using hydrogeologic criteria. The red rectangle (my addition) roughly outlines the hill slopes under threat of being paved over by roads. The study highlights an area inside the red rectangle (Table 20: Balbharati Building) as an ideal site for enhancing recharge and also points out old quarries in nearby places which could be repurposed to store runoff water for recharging the underlying aquifers. Unfortunately, some of these quarries are now being encroached by slums and also being used as dumping grounds for construction debris. 

Source: Raymond Duraiswamy, Vrishali Dumale and Usha Shetty 2009 - Geospatial mapping of potential recharge zones in parts of Pune city.

ACWADAM will be soon releasing a detailed study of the ground water situation of Pune along with maps of aquifers. I will post that information when it is made available.

Thursday, June 20, 2019

Structural Geology And India's Societal Needs

This is a thoughtful essay by Manish Mamtani from the Indian Institute of Technology, Kharagpur, on the need for Indian structural geologists to tailor their research towards the concerns of industry.

Usually, a call for more 'applied' research comes from the Industry side, and so it is refreshing to see an academic ask for a reevaluation of research priorities.

The application of structural geology for society are varied, ranging from better understanding the origin of economic deposits, to assessing geological structures of mountain slopes and their associated landslide risk, to evaluating rock properties for foundations of dams and bridges.

The author worries that recommendations for forging links between academic research and industry  in this subject may remain buried in seminar abstracts and reports unless there is a change in the way research is funded and career advancement evaluated.

"I am sure many of the above aspects that outline the importance of Structural Geology studies to industry/societal issues have already been listed several times in reports of seminars held in the past. Unfortunately, we do not see much progress on the implementation side. One of the ways forward could be setting up of a special program by a funding agency that specifically targets “Applied Aspects of Structural Geology”. This can attract Structural Geology projects, the outcome of which would be useful to society/industry. Indian funding agencies could also consider a special program where two way funding is provided to academicians – partly by industry and partly by the agency itself. For e.g., MoES/DST could act as the nodal agency to bring academicians and personnel from industries like ONGC or Hutti Gold Mines Ltd (HGML) on the same table and they jointly fund Structural Geology research directly related to respective industries. 

In such a collaborative environment, there will be a natural drive in the involved academic to provide solutions to the industry. In the long term, such modus operandi can have a domino effect on the way Structural Geology courses are set, designed and taught in Indian Universities/Institutes. This can also lead to producing students who are better prepared to serve industry and society once they obtain a Master’s degree in Geosciences. But, one has to bear in mind that in doing industry-oriented project work, the “poor” geoscientist will have to sacrifice (to some extent) addition of publications to the “CV”. This would imply delay in career progression, a risk many academicians would not consider worth taking. The onus thus lies on, not only the funding agencies, but also on persons who evaluate career progression of (geo)scientists. Due credit must be given to a geoscientist whose research provides solutions to industry/society even if the “CV” is short on number of publications".

Open Access.

Friday, June 14, 2019

Insufficient Assessement: Pancheshwar Dam Uttarakhand

Environmental implications of Pancheshwar dam in Uttarakhand (Central Himalaya), India.

A warning from earth scientists that sufficiently detailed studies of seismic risks and potential environmental consequences have not been undertaken.

Map from the linked paper shows the location of the Pancheshwar Dam and the future backwaters in red.


We have assessed the likely environmental consequences of the proposed Pancheshwar high dam in  Uttarakhand Himalaya (Indian Central Himalaya) in the light of current geologic and geomorphic   understanding. The study suggests that if executed in its current  format, the proposed  dam  raises  concern  about  safety  and  its sustainability due to seismicity, reservoir-induced  seismicity,  slope instability due to reservoir draw down effect, and unpredictable large volume sediment  mobilization from paraglacial zones. The study therefore, highlights the pressing need to re-assess the feasibility and its  geo-environmental implications through multidisciplinary studies.

During my recent travels in Kumaon I met locals who were also expressing fears over loss of livelihoods as large tracts of fertile land will be drowned. 

Open Access.

Friday, May 31, 2019

Geology Outreach: Darma Valley, Uttarakhand

A couple of weeks ago, in partnership with Deep Dive India,  I had taken a group of nature lovers from Bengaluru to the Himalaya for a geology outreach week. We traveled across a section of the Lesser Himalaya up to the town of Dharchula, and then headed north along the Kali valley and then the Dhauliganga valley to the area around the Panchachuli Glacier from where the river Dhauliganga (Darma river) emerges. The picture on the left shows our group at an outcrop of high grade metamorphic rocks. Picture credit: Asha Kini.

The participants were a mix of IT professionals, Chartered Accountants and Business Management executives. And they were an enthusiastic bunch. This was my first Himalaya outreach attempt and I was a bit nervous. But these people made my job much easier with their curiosity and active participation.

The map below shows our route in red.

Source: Geology, Structural and Exhumation History of the Higher Himalayan Crystallines in Kumaon Himalaya, India- R.C. Patel et. al. 2011

During our journey towards Dharchula and ahead, we drove across and learned about 'Klippen'. Beginning about 23 million years ago and continuing until around 15 million year ago, large faults (thrust faults) moved sheets of the high grade metamorphic Greater Himalaya and the oldest rocks of the Lesser Himalaya southwards, and placed them above lower grade metamorphic rocks of the Lesser Himalaya. Subsequently, erosion removed portions of these thrust sheets, leaving behind outliers or islands (Klippen) of these high grade rocks surrounded by the lower grade Lesser Himalayan rocks. We traveled across the Almora, Askot and Chiplakot klippen on our way to the start of our trekking point, which was north of Sobla. The map above shows the Chiplakot klippen surrounded by Lesser Himalayan rocks. 

Dharchula is situated on the low grade metamorphic rocks of the Lesser Himalaya Sequence. A little north of this town, we crossed into the Chiplakot Crystalline Belt (klippen), which is a  high grade metamorphic belt correlated with the  Munsiyari Formation (see map).  The Munsiyari Formation is considered the oldest unit of the Lesser Himalaya Sequence, made up of rocks metamorphosed to a higher grade. It contains the oldest rocks in the Himalaya, a very characteristic augen gneiss (named after the eye shaped clusters of quartz and feldspar), dated to 1.9 billion years.The Chiplakot Crystalline Belt and the Munsiyari Formation rocks both formed by extensive magmatism that was taking place along the Indian northern continental margin in the Paleoproterozoic (~1.9-1.8 billion years ago). These magmatic events were triggered by converging continental blocks, their eventual collision and suturing leading to the formation of a supercontinent known as 'Colombia'.

Just north of Sobla, we encountered the Greater Himalaya. The Main Central Thrust, known locally as the Vaikrita Thrust (VT), places these rocks on top of the Lesser Himalaya Sequence. We remained in this rock group for the rest of the trip. The Greater Himalaya in this area are made up of garnet to sillimanite grade gneiss, mica garnet schists, and migmatites, intruded by leucogranite sills and dikes. These leucogranites formed by the partial melting of buried Indian crust between 24 million and 16 million years ago. The picture, taken near Baaling village,  shows a leucogranite intruding gneiss. Arrows point to fragments of host rock entrapped in the intrusive magma.

Near Dugtu, we caught glimpses of the Tethyan Sedimentary Sequence high up on the ridges to the east and north of the village. And we found boulders of conglomerates and sandstones dislodged from these Tethyan rocks in small streams joining the Dhauliganga river. We also did a memorable walk along the banks of the Dhauliganga river right up to the point it emerges from an ice cave at the snout of the Panchachuli Glacier.

All along our route we stopped for geology observations at selected locations where lithologic breaks, rock folding, and fault zones could be seen. I gave the group small puzzles to solve, wherein they had to use their powers of observation and reasoning to come up with answers on the type of rocks, the sources of pebbles in streams, and differences between river and glacial deposits. In the evenings, informal discussions continued over piping hot delicious meals of roti, subzi, dal, and rajma.

I won't write in detail about the geology of this region, since I have covered it in an earlier post that I wrote when I visited this region two years ago. Please read that post titled 'Chasing the South Tibetan Detachment'.

I will make one addition to the geology covered in that post. Just north of Baaling village there is a sudden change in lithology. High grade gneiss, migmatites and leucogranites, formed at temperatures between 750-800 deg C, are overlain by lower grade metamorphic rocks (400-500 deg C) made up of slates, phyllites and greenschists (minerals like biotite, chlorite and actinolite). These lower grade rocks are locally named Budhi Schist. I could not see the contact between the two lithologic groups since the hillsides along the trail was covered with rubble and forest patches. The change seems to occur a few hundred meters north of Baaling.

I had earlier put this down to a continuous change in pressure temperature conditions within the Greater Himalaya Sequence. But walking across the lithologic transition one can notice the steep change in pressure temperature conditions as evidenced by the different mineral assemblages of the rocks, the absence of significant leucogranite in the lower grade rocks, the presence of dilation and en echelon fractures (evidence of stretching and tensile forces) in this zone, and the strong contrast in folding style between the two rock groups. Folding in the high grade rocks (upper pic) is manifest as ductile flow of dark and light colored mineral domains into wavy,  sigmoidal patterns, rootless isoclinal folds (light or dark colored mineral domains contorted into isolated folds) and ptygmatic folding of quartz-feldspar rich layers (the more competent quartz feldspar layers gets contorted into tight chaotic folds,while the softer surrounding layers flow around it) . In contrast, the strata in lower grade rocks show tight isoclinal and recumbent folding (outlined in  yellow) which can be traced over tens of meters. This indicates that the two rock groups were deformed at different depths under different rheologic conditions.

These abrupt changes in lithology and presence of extensional stress indicators strongly suggest that this transition is bracketed by a northerly dipping ductile shear zone (deeper crustal equivalent of a fault zone along which rocks are deformed and displaced) which separates lower grade hanging wall rocks (block above fault plane) formed in shallower levels of the crust from deeper crustal level and higher grade footwall rocks (block below fault plane). Lower grade hanging wall rocks juxtaposed against higher grade footwall rocks implies normal faulting.

Ideally, shear zones need to be recognized on structural criteria, i.e. the appearance of oriented structures in the rock fabric that indicate the sense of movement. Not having the required structural geology skills, I couldn't document accurately the shear sense (direction of displacement), but previous work carried out on this shear zone shows fabrics indicating a phase of top to the north-northeast normal shear, which means that the hanging wall rocks have been displaced downwards in a northerly direction. 

In the Central Himalaya two strands of the South Tibetan Detachment ( a network of extentional or normal faulting) have been recognized. The shear zone at Baaling likely represents the structurally lower strand of this fault system. The upper strand of this fault zone is present north of Dugtu village and brings into contact unmetamorphosed sediments of the Tethyan Sequence in the hanging wall with lower grade metamorphic rocks (Budhi Schist) in the footwall. 

I'll post below a few pictures of the landscapes around Naagling and Dantu villages. People of the Bhotiya tribes live in this region. We were at about 10,000 to 11,000 feet ASL. These villages  are abandoned for the winter as inhabitants move to lower altitude towns like Dharchula to spend the cold season. People start migrating back in the month of May. When we arrived, only a few families had made their way back. As a result, most villages had an empty feel around them.

1) High grade metamorphic massifs of the Greater Himalaya seen from Naagling.

2) Early morning sunshine hits Dantu Village.

3) Beautiful earthy homes and icy ranges in the background seen at Dantu.

4) Panchachuli Peaks seen from Dantu.

5) Village Goe basking in the sunshine.

6) Golden hues in the countryside around Philum village.

7) The Lassar Yankti valley (tributary of Dhauliganga) seen from Baun village looking north.

8) The picture postcard Baun village.

 9) Realm of the shepherds. Lush meadows with the Greater Himalaya looming all around. Near Baun.

10) Explaining the origin of the Himalaya to the Geo group. Picture credit: Samir Kher.

11) And.. that's me standing at the snout of the Panchachuli Glacier. You can see the river Dhauliganga emerging out of an ice cave. Picture credit: Prakash.

Overall, it was a great learning experience for me. And from the feedback I got, all the participants enjoyed it thoroughly too.

I will be doing this again!

Thursday, May 9, 2019

Links: Petroglyphs, Language, Urban Groundwater, Dams

Some interesting articles I came across past few days.

1) Pleistocene Rock Art in India- New York Times covers the discovery of ancient rock art (40k-10K yr old?) carved on laterite plateaus of Ratnagiri District, S. Maharashtra. Good to see credit given to the stellar work of two amateur archaeologists Sudhir Risbud and Dhananjay Marathe.

Link: Ancient Rock Art In The Plains Of India.

2) Language Evolution- Linguistic analysis suggests that the Sino-Tibetan language family originated about 7200 years ago among millet farming communities in northern China.

Links: Paper - Dated language phylogenies shed light on the ancestry of Sino-Tibetan.
Summary - Origin of Sino-Tibetan language family revealed by new research.

3) Urban Groundwater- This is an issue that is gaining importance as cities in India grow and municipal water supply from surface reservoirs becomes inadequate. S. Vishwanath crunches some numbers on the ground water potential of the shallow aquifer underneath Bengaluru. It comes to more than hundred billion liters! Similar situations exist underneath other Indian cities as well, but urban groundwater has been a neglected area of study. More quantitative understanding of aquifers is needed along with a focused effort to recharge ground water.

Link:  Revisiting The Shallow Aquifer

4) Environmental Implications of Pancheshwar Dam, Uttarakhand - A review in Current Science of environmental concerns regarding the proposed Pancheshwar Dam in Uttarakhand implies that critical aspects of seismicity, slope instability, and high sedimentation rates have not been addressed in detail during the planning stages in the environment impact assessments carried out so far.

Link: Environmental implications of Pancheshwar dam in Uttarakhand (Central Himalaya), India.

Tuesday, April 30, 2019

Eastern Ghats- The New Kid On The Block

We who live in the Deccan Volcanic Province in and near about the Western Ghats generally look down upon the Eastern Ghats. Call them the poor man's mountains. Point out that the Eastern ranges have a more gentle topographic profile than the Western ranges. We smirk at the lack of spectacular escarpments, narrow gorges and the mesas and pinnacles.

But, when it comes to geology, the Eastern Ghats more than holds its own. In fact, it has a much more complicated and interesting geologic history than the Western Ghats, at least the Deccan Volcanic part of the Western Ghats.

The Deccan Volcanic part of the Western Ghats is an elevated plateau which formed by the piling up of lava 66 million years ago and which since has been dissected by rivers, forming gorges, narrow valleys, and high relief. The edge of this plateau is the Western Ghat escarpment. The Eastern Ghats on the other hand is an ancient orogenic belt which formed by the collision between crustal blocks, resulting in the formation of fold mountains.

The map below shows the broad geology of the Eastern Ghat with the inset showing its location within the Indian continent.

Source: Relative Chronology in High-Grade Crystalline Terrain of the Eastern Ghats, India: New Insights: Samarendra Bhattacharya, Rajib Kar, Amit Kumar Saw, Prasanta Das 2011.

The Eastern Ghats is a Late Archean to Proterozoic age crustal block that has evolved through long and multiple episodes of magmatism, metamorphism and deformation.  It contains rocks ranging in age from 2. 9 billion years to 900 million years old. The rocks have some of the coolest names in petrology; charnockites and enderbites, khondalites, anorthosites and syenites along with granitic rocks and  sedimentary rocks like quartzites. Charnockites (and enderbites) and khondalites are granulite grade metamorphic rocks, i.e. they formed at very high temperatures of around 900-950 deg C by transformation of older igneous and sedimentary rocks respectively. Anorthosite is an igneous rock made up almost entirely of plagioclase feldspar. Syenite is also an igneous rock containing potassium and sodium rich feldspars with no or little quartz.

The interesting part is that the Eastern Ghat block was not part of India when these rocks formed. It may have been an independent block in the Archean (more than 2. 5 billion years ago), but at some point it became part of a larger block that is now the Antarctic continent. This region then underwent magmatism around 1.7-1.6 billion years ago, an episode of granulite metamorphism around 1.6 billion years ago in its southern regions, followed by sedimentary basin formation around 1.3 to 1.2 billion years ago. These sediments were then buried, intruded by magmas like syenites,  and subjected to another episode of granulite grade metamorphism around 1.2 to 1 billion years ago. This last episode of metamorphism and deformation was a result of continental movements and collisions related to the formation of the Rodinia Supercontinent.

When did the Eastern Ghats become part of India? Geologists have timed that event to around 500 million years ago, part of the assembly of Gondwanaland.

How did they figure that out? When the Eastern Ghat terrain collided with India in the Bastar region, it caused the Baster region crust to be buried to great depths resulting in the partial melting of that crust. Radiogenic dating of minerals titanite and zircon, which formed in these new melts, give an age of around 500 million years to this melting event.

I love it when these big ideas are depicted in simple and clean diagrams. Below is a graphic that shows the separation of the Eastern Ghat terrain from its conjugate Antarctica block called the Rayner Complex.

Source: Eastern Ghats Province (India)–Rayner Complex (Antarctica) accretion: Timing the event- Pritam Nasipuri, F. Corfu, and A. Bhattacharya 2018

Two scenarios are shown. The upper panel shows a composite Eastern Ghat Province-Rayner Complex colliding with the Greater Indian landmass around 500 million years ago, followed by a breaking away of the Rayner Complex. The lower panel shows that the Eastern Ghat Province had broken away from the Rayner Complex by 800 million years ago. It then collided with India around 500 million years ago.

The Indian continent was put together by the collision and welding of several smaller continental blocks, namely Dharwar, Aravalli, Bundelkhand, Bastar and Singbhum. This assembly took place between 2 billion and 1 billion years ago.

The Eastern Ghat block was the last to join India. As recent work suggests, as late as 500 million years ago.

Sunday, April 14, 2019

Human Evolution: Stories From SE Asia

Some recent finds from SE Asia are adding detail to the complex story of human migration and population interaction, and putting a much needed spotlight on the varied geographies and ecology in which human evolution took place.

1) Anthropologist John Hawks writes about the significance of the newly reported Homo luzonensis from the northern island of Luzon in the Philippines. This hominin appears to be small bodied like the 'Hobbit' (Homo floresiensis), which lived about 700 km to the south on the island of Flores. The fossils are at least 50,000 years old and their presence suggests that SE Asia was colonized several times by different hominin populations. How they were related to each other is currently an open and actively debated question.

Link: New species of hominin from Luzon.

2) Denisovans were an archaic group of hominins who diverged from the Neanderthals more than half a million years ago and lived over wide swaths of Eurasia and SE Asia. They interbred with more recent humans entering these regions, beginning about 60,000 years ago. Living Eurasians and Papuan people carry small amounts of Denisovan ancestry. A recent genetic analysis suggests that at an early stage in their history the Denisovans split in to two or three distinct groups, which then genetically diverged from each other. Papuans carry evidence of intermixing with these different Denisovan lineages.

Link: Multiple Deeply Divergent Denisovan Ancestries in Papuans (paper)
Summary: Ancient DNA reveals new branches of the Denisovan family tree.

3) Some of the oldest cave art has recently (2014) been found in Indonesia from the southern part of Sulewasi Island. They are estimated to be around 35,000 to 40,000 years old. A nice summary in Smithsonian Magazine details the discovery. Art forms of this antiquity from Indonesia suggests that a simple story of a singular origin of human symbolic thinking is not tenable anymore. 

Link: A Journey to the Oldest Cave Paintings in the World.

Saturday, March 30, 2019

Palaeontology: Some Recent Spectacular Fossil Finds

Sharing some news on exciting fossil discoveries of the recent past:

1) Early animal evolution is a topic that continues to fascinate. A fossil rich sedimentary deposit from China dated to about 518 million years ago reveals exquisitely preserved soft bodied animals of the early Cambrian. This find, termed the Qingjiang biota, compliments the well known Burgess Shale of Canada and the Chenjiang site in China. It contains representatives of early cnidarians (related to corals), comb jellies, sponges, and many other creatures, and is helping paleontologists answer questions about the evolutionary relationships and timing of branching of animal groups.

Link: Spectacular new fossil bonanza captures explosion of early life.

2) Before the early Cambrian diversification of animals, is fossil evidence of the roots of some animal lineages, contained in the Ediacaran biota of late Neoproterozoic age ( 600-542 million years ago). At one site in S. Australia, a farmer is conserving a rich Ediacaran fossil site, turning it in to an outdoor research museum.

Link: This Australian farmer is saving fossils of some of the planet’s weirdest, most ancient creatures.

3) A 4 foot sedimentary layer in South Dakota contains a jumble of fossils of animals and plants. This 'event deposit' formed instantaneously from material gathered and dumped by a tsunami triggered by a large meteorite crashing into the Yucatan Peninsula, Mexico. Readers will recognize this! It happened 66 million years ago and resulted in the end Cretaceous mass extinction.

Link: Fossil Site Reveals Day That Meteor Hit Earth and, Maybe, Wiped Out Dinosaurs.

..and there is a longer article in the New Yorker on this fossil site and the hard work paleontologists have put in to tease out its secrets..  (thanks to Hollis for the reminder! ).

Link: The Day The Dinosaurs Died

Happy reading!

Saturday, March 23, 2019

Two Short Talks - Deccan Basalts And Geology

My friend Milind Sathe has started an arts and science outreach initiative for children named Khula Aasmaan (Open Sky). He asked me if I could give two short talks, one on my career path and experiences in geology, and the other on Deccan Basalts.

We went to a nearby hill to shoot the videos. An abandoned quarry and the basalt rock made for a pretty and relevant backdrop to the video.

Here are the links. Email subscribers who can't see the embedded video can use the permanent link to go to the Khula Aasmaan web pages for access.

1) Link- Deccan Basalts: Eruptions, mass extinctions, western ghat escarpment, ground water properties.

One correction. I mention that India broke away from Africa about 100 million years ago. It was earlier, beginning about 160 million years ago.

2) Link- Geology: My career pathway and broad interests.

Hope you like them!

Monday, February 25, 2019

The Geology Of Mumbai

Last Saturday I was fortunate to be given a tour of the construction site of  Mumbai Metro Line 3 near Siddhivinayak Temple in Dadar, Mumbai. There, we descended about 100 feet to the floor of an enormous pit, and then traveled south along a tunnel for a kilometer towards Worli, right up to where the Tunnel Boring Machines (TBM) were at work.

It was a fantastic experience.

Geology is not a term you would normally associate with the concerns of a bustling metropolis like Mumbai. Yet, at this enormous construction site, it is at the heart of operations. Progress very much depends on understanding the subsurface rock layers. Their thickness, strength, orientation, and water bearing capacity, pose engineering challenges that need to be understood and solved before tunneling can proceed safely. Far from just being an esoteric pursuit that delves into the earth's dusty past, at this site, every thump of the giant TBM rams home the relevance of geology in our day to day lives.

A friend asked me whether the rocks that the TBM's are encountering in Mumbai are any different from those under Pune. Metro construction has started in Pune too, but only one section about 6 km long will be underground. One of the reasons given for avoiding long underground stretches in Pune is that the rock type is very hard basalt.

Mumbai geology is somewhat different from Pune. I did not see any rock during my Metro visit since the pits and the tunnels had already been lined. But I do have a fair idea of the geological history of Mumbai area.

Like most of Maharashtra, Mumbai too is part of the Deccan Volcanic Province. This enormous area covered by mostly basalt lava formed between 68 million and 60 million years ago, from Late Cretaceous to Early Paleocene times. The bulk of the volcanism, about 80% of it, occurred between 67 million and 66 million years ago, within a time span of a few hundred thousand years. This big spurt of volcanism overlaps the mass extinction that took place at 66.03 million years ago. The main cause of this extinction is the environmental degradation resulting from a large meteorite crashing into what is now the Yucatan Peninsula of Mexico. A lively debate has now broken out on how much did Deccan volcanism contribute to the mass extinction.

At this time the Indian continent was located far south of the equator. The Mumbai region was located around 25 degrees south of the equator. The map below shows in grey the distribution of the Deccan Volcanics in context to the other major geological provinces of India. Insets show the progressive separation of India from Madagascar at 88 million years ago, and later from Seychelles at 65 million yeas ago. The black region in the right inset are the Deccan Volcanics with the smaller fragment being Seychelles.

Source: Sheth H.C. 2007

By 65 million years ago the western margin of the continent began to split apart and a chunk which became Seychelles broke and moved away from the Indian continent.  North south oriented fault systems along the western margin of India caused blocks of crust to subside westwards. The region around Mumbai would have been at sea level by around 64 million years ago. Eruptions had ceased over most of the Deccan Volcanic Province.

A map showing the major tectonic elements of the Indian western margin and the Mumbai area is shown below.

Source: Sheth H.C. 1998

In the Mumbai region though, volcanism continued for the next few million years under conditions which imparted to Mumbai its peculiar geological character. This volcanism differed from the rest of the Deccan Province. 

First, the lava composition was more 'evolved'. The Deccan Province is made mostly of basalt, which is an igneous rock rich in iron, magnesium and calcium silicate minerals. However, in the Mumbai region, besides basalt, other lava types known as rhyolites and trachytes erupted. These lavas are more silica rich and contain the mineral quartz (silica dioxide) and other sodium and calcium silicate minerals.

Secondly, since this region was at or near sea level, some of the volcanism took place under water forming characteristic pillow like lava structures. Volcanism over the rest of the Deccan Province took place in subaerial conditions above sea level. 

Thirdly, the meeting of hot lava and cold sea water caused steam explosions. This resulted in the formation of large amounts of lava rubble which when consolidated forms a rock known as volcanic breccia. Explosive volcanism also generated ash which was deposited in layers known as Tuff. 

Volcanism was also sporadic. In these interludes, in coastal embayments and lagoons, mud and silt was being deposited. Fossils of turtle, frogs, crocodiles, molluscs and various types of plant remains have been recovered from these sediments. A resumption of volcanism would bury these sediments under lava. Repeated episodes of volcanism and sediment deposition has resulted in the formation of a rock sequence made up of different lava types alternating with thinner layers of sedimentary rock (intertrappean sediments). These events took place between 64 million and 62 million years ago. 

The volcano-sedimentary environments of Mumbai are shown in the schematic below. 

Volcanism continued until around 60 million years ago. The famous Gilbert Hill in Andheri, made up of basalt columns, formed by polygonal cracking of lava as it cooled, has been dated to around 60 million years old. This makes it probably the youngest volcanic activity of the Deccan Province.

Finally, the Mumbai rock sequence differs from the rest of the province in its structural disposition. Whereas in the rest of the Deccan region the lava flows are nearly horizontal, in the Mumbai region they show a pronounced tilt (dip) to the west. This feature is known as the Panvel Flexure, as it becomes more pronounced beginning just around the town of Panvel, a few tens of kilometers east of Mumbai (see right panel of tectonic map posted earlier).  

Many explanations have been given for this tilt. One theory is that it resulted from a bending of the lava flows as the crust to the west of Mumbai subsided upon cooling and due to the weight of sediment. Another explanation ties the structure to continued movement along west facing faults which initially formed  during continental breakup. A third hypothesis is that the flexure formed by tilting of the crust along an east facing listric (curved plane) fault now located under the Arabian Sea to the west of Mumbai. Such faults commonly occur along continental rift margins, where the crust in being pulled apart. This last scenario is shown below.

Source:  Sheth H.C. 1998

This tilting occurred after volcanism and sedimentation ended, later than 60 million years ago. The result is that the entire package of volcanic flows and sedimentary strata dip westwards. This Mumbai stratigraphy is shown in the cross section. 
After volcanism and crustal tilting, the next recorded geologic history is from much more recent times, in fact just a few thousand years old.

Early travelers and geographers describe Mumbai not as one land mass, but a collection of seven islands separated by shallow tidal inlets and marshland. This particular configuration of land and sea, is in geological time quite a recent phenomenon, forming  just about 10,000 years ago. Before that, during the Pleistocene ice age, sea level was about 100 meters lower than present. The Mumbai area and almost the entire continental shelf to the west would have been land. The earliest humans to have entered India about 70,000 years ago, following a coastal route from the Arabian Peninsula, would have walked on the now submerged land to the west of Mumbai.

During this sea level low, rivers traversing the Mumbai region would have met the sea tens of kilometers to the west. Sea level began to rise about 15,000 to 12,000 years ago at the end of the ice age. In the next few thousand years, rising sea level inundated the continental shelf and various river valleys, forming Panvel Creek, Thane Creek and Vasai Creek to name a few of the creeks in this region. These creeks are all drowned river valleys of the Pleistocene.

Sea level peaked about 3,000 to 4,000  years ago. The position of the shoreline at this time was about 2 meters higher than present. Beach rock deposited during this time is present a few hundred meters inland at Madh Island. This shelly rock is locally known as Karal. By this time Mumbai became an island locale, with topographic highs remaining as land, with low lying areas becoming marshes and shallow tidal channels.

This then is the geological inheritance of the city of Mumbai,a legacy of  volcanism and sedimentation in Paleocene times and a pronounced sea level rise during the Holocene.

The rock outcrops that tell this story have all but disappeared under the onslaught of urbanization over the last few decades. As modern Tunnel Boring Machines enter Mumbai's underworld, a few pages of this history are again being discovered.

The Mumbai Metro website, in their newsletter Metro Cube, has put up a series of ten articles titled 'What Lies Beneath The Earth' (issues February 2018 - December 2018). This series summarizes the geology beneath each of the sections of the metro route. It is an excellent resource. A perusal of this series reveals that the tunnels are mostly encountering Paleocene age hard basalt and softer breccias and tuff layers. Only at some place are sedimentary layers being intersected. This though is in contrast with the geology underneath Pune. There, only hard basalt will be found.

It is imperative that we save some of this treasure for our citizens to appreciate. Wouldn't it be wonderful if at a few of the underground metro stations, exposed rock panels and a museum like display of recovered rock cores along with a short history of Mumbai geology is displayed? It would make Mumbai's unique geology accessible to citizens and help all of us forge a more enduring connection with our natural heritage.