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.

Is Mount Kailash The Oldest Mountain In The Himalaya?

No. It is not.

Although, according to this tweet it is.

The person tweeting as CBG-san (@OnlyNakedTruth) uses Pranay Lal's book Indica: A Deep Natural History Of The Indian Subcontinent as the source (page 268-269), and also refers to an analysis done on Mount Kailash rocks. I could not make out the source of the table of analysis. They show the age of Mount Kailash Formation as ranging from around 30 million years to around 10 million years old.

Let me get one technical point out of the way. Geologically,  Mount Kailash is not in the Himalaya. It is part of  the Asian continental plate. These mountains are known as the Transhimalaya. Locally, these ranges are also called the Gangdese Shan. The Himalaya are the deformed and uplifted rocks of the Indian plate. This is a quibble though. I appreciate that the larger point is whether the rocks of Mount Kailash were uplifted very early during the India-Asia collision process.

The Mount Kailash range is made up of thousands of feet of sediment of the Kailash Formation, sitting on granitic rocks of the Gangdese batholith. These granitic rocks formed within the southern edge of the Asian continent. As the Indian plate dived underneath Asia, magmas formed deep inside the Asian plate. Blobs of this magma rose and solidified in the subsurface of the Asian continent forming the Gangdese batholith (a large body of granite). This magmatism took place between 100 million and 45 million years ago .

There are new dates available now for the Kailash Formation, which was deposited on top on this granite. Radiogenic dating of lava flows inter-layered with sediment indicates that the Kailash Formation accumulated between 26  million years and 21 million years ago.

This timeline indicates that around 26 million years ago the southern margin of the Asian continent and the India-Asia collision zone subsided. The nature of the sediments indicates that a long chain of lakes formed in narrow depressions. These lakes were receiving sediment eroded from elevated ranges to the north. Organic matter accumulating in these lakes have been transformed into coal layers. There is also an absence of pollen grains of temperate or high-altitude plant species. This sediment composition points to a lower elevation and warmer water setting of these lakes, which geologists speculatively place between 1000 m to 3500 m. Presently,  Kailash Basin sediments are exposed at altitudes greater than 6000 m.

The graphic below shows the depositional environment of the Kailash Formation

Source: DeCelles et.al. 2016- Oligocene-Miocene Great Lakes in the India-Asia Collision Zone. 

The rocks that make up Mount Kailash are younger than 26 million years. They formed nearly 30 million years after the collision of the Indian and Asian continents.

High topography already existed along several belts in the collision zone before 26 million years ago.

First, the southern margin of the Asian continent must have been elevated perhaps as early as 45 million years ago, since this terrain was the source of sediment into the Kailash Basin. More direct methods of estimating elevation also suggest high elevations in this region by  35-40 million years ago. The ratio of the two isotopes of oxygen (O18 to O16), bound in calcium carbonate minerals,  is temperature dependent. Measurements from southern Tibet indicate paleo-elevations of around 5000 m by 35 million years ago.

Second, the zone of India-Asia collision (Indus-Tsangpo Suture), and the Tethyan Himalaya belt (the northernmost Himalayan ranges) had been uplifted by 45-40 million years ago. The evidence for this comes from the composition of foreland basin sediments to the south. As India collided with Asia, a depression formed in front of the rising mountain chain. This foreland basin (which later was uplifted to form the Siwalik ranges) began receiving sediment derived from the erosion of the newly uplifted Himalaya.

Eocene age (45-35 million years) sediments in this foreland basin contain rock fragments and minerals inherited from the Indus-Tsangpo Suture and the Tethyan Himalaya. Younger foreland sediments of Early Miocene age (between 24 and 15 million years) contain fragments of the Tethyan Himalaya as well as the newly emerging Greater Himalaya.

The timing of uplift of the Greater Himalaya is also hinted at by geochronology. The radioactive clock inside muscovite (a type of mica) starts ticking below around 350 C. Clocks in other minerals like zircon (zirconium silicate) and monazite (rare earth phosphate) are set at higher temperatures, at 700 C and 600 C respectively. A sample may contain all three of these minerals, as Greater Himalayan granites and gneisses often do. Their dates of formation track a cooling history, as the rock is uplifted from deeper crustal levels to shallower regions.

Such work by geologists have shown that the Greater Himalaya were exhumed between 21 million and 16 million years ago. Exhumation in such collisional settings is linked to rapid surface erosion and formation of topography. Mike Searle's book Colliding Continents: A geological exploration of the Himalaya, Karakoram, & Tibet describes these different methods for assessing rock ages and cooling histories.

A different geochronology method known as fission-track dating,  that measures radiation damage in crystals of zircon (zirconium silicate) and apatite (calcium phosphate) to estimate when the rock cooled below 200 C to 100 C indicates that the Kailash Formation was uplifted later than 17 million years ago.

Ranges on the Asian continent (Gangdese Shan), as well as the Indus-Tsanpo Suture in the collision zone and the Tethyan Himalaya belt to the south on the Indian plate, are older than Kailash. The Greater Himalaya was uplifted around the same time as the Mount Kailash Formation.

This evolution of topography in the Himalaya and along the southern margin of Asia is shown in the schematic below. Orange arrows indicate transport of sediment from source to basin. Black arrows show fault motion.

I've always been struck by a disconnect in Pranay Lal's book. His end notes are detailed and summarize the state of research fairly well. However, there are many basic mistakes in the main text. I made a list of the many geology errors in his book in an earlier post titled Book: Indica- A Deep Natural History of the Indian Subcontinent.

It is simply not accurate to say that Mount Kailash is the oldest mountain in the Himalaya. The southern margin of the Asian continent was elevated soon after the India-Asia collision, probably by 45 million years ago. But the Kailash Formation did not rise until after 17 million years ago. It is a much younger component of the Gangdese Shan or Transhimalaya.  Mount Kailash's classic pyramidal shape evolved during the ice ages of the Quaternary Period beginning 2.58 million years ago, when glaciers dug out valleys and cut back slopes, forming smooth sided and sharp edged peaks.