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 kilometre 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’s geology is somewhat different from Pune’s. 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 Deccan volcanism contributed 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.
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.
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 is being pulled apart. This last scenario is shown below.
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 metres 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 the 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 metres higher than present. Beach rock deposited during this time is present a few hundred metres 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 places 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.