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. 

Source: Cripps J.A. et .al 2005

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.

Transport Options for Indian Cities

The city of Muenster, Germany has this message for urban transport planners in India:


I got this from Tim Harford's blog who got it here. Despite the quibbling over camera zoom and angle and whether the picture depicts transport patterns realistically, I think the poster makes a great point. The city compares the space used by various modes of transport in moving a comparable number of people. Here are the stats for moving 72 people

Bicycle: 72 people are transported on 72 bikes, which requires 90 square meters.

Car: Based on an average occupancy of 1.2 people per car, 60 cars are needed to transport 72 people, which takes 1,000 square meters.

Bus: 72 people can be transported on 1 bus, which only requires 30 square meters of space.

Besides space occupied there is also the problem of pollution. Bicycles don't emit pollutants and buses pollute less per passenger km traveled than other motorized transport. Yet currently we tax private vehicles less than public transport buses creating distorted incentives for vehicle choice. The various taxes that favor private vehicles more than bus public transport are nicely summarized in this letter from the Centre for Science and Environment to finance minister P. Chidambaram.

Rickshaws and Vehicular Pollution

Part 3 of a six part series on Pune city pollution and environment. Updated every Tuesday.

Didn't you see my right foot?! In the middle of an argument over a minor accident the rickshaw driver claimed he had poked his right foot out to indicate that he was turning right. Pune has around 60-70 thousand three wheeler rickshaws. They are no doubt a very convenient form of taxi service. To many harried customers, rickshaw drivers are rude and indisciplined, but I don't really find their behavior on the road to be any different than that of private vehicle owners. This is a country where everyone owns a piece of the road, so rules be damned.

But rickshaws do pollute a lot. Below table shows pollution amounts in tons/year for different pollutants for a total of 62, 600 rickshaws, compared with other vehicle types. Source: Emission factors from S. Guttikunda, World Bank. Rickshaw numbers from N. Iyer, Consultant Bajaj Auto. Values represent combined sum for two stroke and four stroke rickshaws. Other vehicle numbers, from Pune Municipal Corporation, Env. Status report 2005.

As I showed in my previous post, buses pollute more on an absolute basis but are more fuel efficient on a per passenger km basis. What about rickshaws? Below graph shows per passenger km pollution for a trip of 100 kms. It shows that rickshaws pollute more than buses and two wheelers on a per passenger km basis.

Source: Emission factors for CO2, PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda. Emission factors for rickshaw, S. Guttikunda and N. Iyer.
Assumed Occupancy: 2 wheels-1, Car-2, Bus- 40, rickshaw - 2.

To tackle rickshaw pollution, the Road Transport Authority has made a policy decision to convert existing 2 stroke and 4 stroke petrol rickshaws to natural gas powered vehicles. How much reductions in emissions will be achieved by conversion of petrol rickshaws to CNG? (compressed natural gas). Below table shows reduction in emissions in tons/year for rickshaw and for comparison reduction in emission for PMT buses.

Blank fields indicate lack of data for emission factors. However, SOx emissions are expected to decrease by large amounts for CNG rickshaws.

A conversion of existing fleet of 2 stroke and 4 stroke rickshaws to CNG has the potential to reduce CO and HC emissions by around 95 percent and PM emissions by around 40 percent! There would be significant reduction in CO2 (by about 30,000 ton/yr, or about 20%). Public transport contributes to a substantial portion of total vehicular pollution. My admittedly rough estimates suggest that public transport including rickshaw, PMT buses and private company buses (about 8000 of them according to the municipal corporation) contribute about 40% of total SOx emissions, 30% of PM10 emissions and about 35% of CO2 emissions. Conversion to cleaner fuels will definitely lead to improvements in air quality, especially if the several thousand private company buses are also included. But changing fuel quality of public transport alone may not help in the long run. In Delhi, after significant improvements in air quality due to conversion of public transport to CNG, the latest news is that pollution is on the rise again, due to a heavy influx of private vehicles, many of them running on diesel. Pune faces similar problems with about 6000 new vehicles being registered every month. And with the eminent arrival of the much touted 1 lakh rupee car in the next few years, gains in reducing emissions by cleaner fuels in public transport vehicles may be swamped by the enormous increase in private vehicles. A more comprehensive public transport system needs to be pursued with urgency combined with pricing initiatives to make driving private vehicles more expensive.

Finally some thoughts on six seater rickshaws, those horrendous contraptions, which were banished to the outskirts of the city, because they were thought to be "polluting more". Image shows a six seater in Pune suburbs.

I have to admit I have a liking for these beasts. There is no doubt that they offered frequent and cheap transport services, something that the bus transport has failed to offer. In principle they should not pollute more than rickshaws, maybe even less on a per passenger basis, since on average they carry more passengers. But a widespread use of adulterated fuel and very poor engine maintenance has lead to additional pollution. Here's a thought. Why doesn't the PMT offer their own six seater service? Is it necessary that they offer only a bus service? This way at least the fuel quality can be regulated, and drivers can be trained to obey at least a few rules.

But that would mean thinking out of the box!

Part 1. Idling and Pollution
Part 2. PMT buses and Pollution

PMT Buses and Pollution

Part 2 of a six part series on Pune City pollution and environment. Updated every Tuesday.

New Delhi, Asian Games 1982. I have memories of broad, clean avenues and clean air. When I returned after a long gap in 1999, New Delhi was urban hell. My friend pointed out a particularly busy intersection and said, "that's the most polluted intersection in Asia". I went back in 2004, and something strange had happened. Something we Indians are not used to. Pollution had reduced dramatically. Air quality was better. The reason experts tell us, was the conversion of almost the entire public transport fleet (15-20 thousand vehicles by various estimates) of rickshaws, taxis and buses from diesel to CNG (Edit: Compressed Natural Gas).

How much will Pune Municipal Transport (PMT) benefit from a conversion from Diesel to CNG? Graph below shows the potential in reducing emissions (tons/year) by converting the current Pune Bus Fleet of 990 buses (PMT plus rental) from Diesel to CNG. (for the purpose of the calculation I have assumed all 990 buses are on the road everyday, which given the state of disrepair of PMT buses may not be the case).

Source: Emission factors for CO2,PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda. Bus operating kms per day, PMC Env. Status Report, 2005.

A CNG fleet will lower emissions of PM10 (particulate matter) from 135 tons/year to about 40 tons/year (70% reduction), of SOx from 80 tons/yr to about 25 tons/yr (68% reduction) and CO2 from about 65 thousand tons/yr to about 35 thousand tons/yr (46% reduction). There are considerable (about 50%) reductions in nitrogen compounds as well, but I did not have the emission factors available to make any specific calculations. But what about methane? CNG is about 80% methane, which is 20 times more potent a greenhouse gas than CO2. Won't the higher methane emissions offset the lower CO2 emissions? Apparently if one considers the total fuel cycles of diesel and CNG including emissions during fuel production, CNG buses have total greenhouse gas emissions similar to diesel buses. Diesel also emits toxic compounds like polyaromatic hydrocarbons, benzene and 1,3 butadiene that CNG does not.

How does bus pollution compare with other vehicles? Although Diesel or CNG buses may emit more pollutants than a 2 wheeler or car in an absolute sense, are they at least more efficient on a per passenger km basis considering they carry more passengers per trip? Graph below shows the emission of pollutants per passenger km, if a commuter makes a daily trip of 30 kms to and from work. The calculations show that even on a per passenger km basis Diesel buses emit more particulate matter than 2 wheelers. CNG however on a passenger km basis is much cleaner than petrol and diesel vehicles.

Source: Emission factors for CO2, PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda.
Assumed Occupancy: 2 wheels-1, Car-2, Bus- 40

To summarize, conversion of PMT buses to CNG will definitely benefit the city. An even bigger reduction in emissions would be achieved through the conversion of several thousand private buses. There is a move to convert the fleet of rickshaws to LPG/CNG. More on that in another blog.

As fuel costs keep increasing, public transport will increasingly be the more cost effective means of transport. Below graph shows Per Km Petrol Cost for Two Wheelers vs Bus Fare Cost per passenger km

Source: Central Institute of Road Transport

The trend indicates that due to increase in petrol prices, public transport per passenger km is increasingly a more efficient mode of transport than private vehicles.

Be sure to read about CNG facts and myths in this fact sheet from the Centre for Science and Environment in Delhi and also about CNG buses and how they compare with Diesel in this fact sheet from the U.S. Dept. Of Energy.

Part 1. Idling and Pollution

An Idle Threat

Part 1 of a five part series on Pune City and environment. Updated every Tuesday.

I cross Law college road everyday on my way to soccer practice. The traffic is an endless stream, and sometime back I told the other coaches that I will be coaching from the other side of the road. At the Prabhat road signal and at Nul stop the back-up is so bad, you could easily miss 3 or even 4 signals before crossing the intersection. Urban legends about these backups are growing. Just last week a woman gave birth in her car waiting for the signal. Most vehicles keep their engines idling during their wait. That gave me the idea for this blog. How much pollutants does your vehicle emit when you wait for a few minutes at a traffic signal? How much does the entire city of Pune emit?

I first had an inkling that the amounts were big when I noticed at truck stops in the U.S. that the truckers never shut down their engines. A little web research told me this astonishing fact:

Trucks in the United States consume about 1 billion gallons of diesel annually while idling! This amounts to about 11 million tons of CO2 emitted annually (Source: U.S. Dept. of Transportation). Overall the transport system in the U.S consumes 8.4 billion gallons of fuel annually while idling. This amount to about 83 million tons of CO2 emitted annually while idling.

Edit: Based on EPA chemistry of fuel combustion 1 billion gallons will emit about 10.1 million tonnes of CO2, but the EPA warns that "calculations and the supporting data have associated variation and uncertainty".

Using vehicle numbers from the Pune Municipal Corporation Env. Status Report, emission factors for various pollutants from a World Bank study and some assumptions about mileage and idling times I did some calculations for Pune. The numbers do not give any comfort.

On a daily basis, assuming a vehicle idles for just 2 minutes every day:

The total fuel consumed in Pune by idling cars, two wheelers and rickshaws amounts to an incredible 19 thousand litres per day! Emissions of greenhouse gases amount to 45 tonnes/day.

How much do all of us on an individual basis contribute? Graphs below estimates how much fuel is consumed and how much CO2, PM10 and SOx is emitted by your vehicle every year if you idle your engine for just 2 minutes every day waiting for the light to turn green.

So, if you drive a regular car you probably lose about Rs 500/- or so yearly on fuel burnt while idling. This 2 minutes is a very conservative estimate I used to do the calculations. Idling time may well be much more. For example, idling your car for 5 minutes every day, you will spend about Rs 1250 per year in fuel burnt and so on. Something to think about next time you are waiting for the light to turn green at a busy intersection. How do you make people change their habits? One way is to appeal to their civic sense, and the other in my opinion the more effective, is to make them aware of the costs of their behavior.

See, where I am going with this? If I alone spend Rs 500/- how much for the entire city? Table below shows how much the city of Pune is paying yearly in terms of fuel lost, CO2, PM10 and SOx emitted, and fuel cost for vehicle idling. This is for total number of cars, two wheelers and rickshaws in Pune, assuming an idling time of 2 minutes every day.









For readers outside India, 1 crore = 10 million, so Pune city spends Rs 34 crore or Rs 340 million worth of fuel burnt while idling. If all vehicles in Pune reduce idling time by just one minute per day we would save about 3.4 million litres of fuel worth about Rs. 17 crore annually! This is equivalent - in terms of fuel saved and CO2 emitted - to removing around 18,500 two wheelers or about 9,300 cars from the roads of Pune.

By the way, you can learn more about idling and various myths using this website from the Canadian Office of Energy Efficiency, designed specifically to help individuals, communities, businesses reduce vehicle idling and pollution.