Sunday, December 22, 2019

Glacial Saraswati?: New Data On An Old Question

Did a perennial glacial fed river flow through the Indus Civilization region of what is now Haryana and Rajasthan? Previous work on the fluvial history of this region had indicated that a distributary of the glacially sourced Sutlej was flowing through a network of paleo-channels buried under the river now known as the Ghaggar until around 8,000 years ago. The Sutlej distributary system then died out, turning that river course into a smaller monsoon fed channel system.

For a more detailed history of research on this topic you can follow this link - Ghaggar /Saraswati Posts.

Recently, in November 2019, Anirban Chatterjee and colleagues published new data on deposits of grey sand in the subsurface of the Ghaggar channel and adjacent floodplains. The youngest of these deposits are 4, 500 years old. Geochemical fingerprinting points to High Himalayan granites and gneisses as their source. This likely extends the glacial phase of the Ghaggar to more recent times, until about the beginning of the urbanization of the Indus Valley Civilization (IVC).

Here is the abstract:

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river’s uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar’s perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using 40Ar/39Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.

The geological work looks to be sound. The data on sediment fingerprinting overlaps with what we know about High Himalayan geochemical signatures and present day Sutlej sand composition.

I do want to comment on another sentence from the abstract (emphasis mine)-

"This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks"

Saraswati is the name given to this river by the Vedic people. Correlation of the river's perennial phase between 9,000-4,500 years ago with Saraswati is valid only if  you can demonstrate that the Vedic people were inhabitants of this region from before 4,500 yrs ago. Geological studies cannot establish this. A combination of archeology, linguistics (cracking the Indus script would be nice!) and genetics will eventually answer that. The other scenario is that the Vedic people could have migrated into this region much later and began venerating a smaller monsoonal Ghaggar as Saraswati. Work by Liviu Giosan and colleagues suggests that stronger monsoons over the Siwaliks sustained sufficient flow in the old channels of the Ghaggar until the late IVC period (~1800-1600 B.C).

When did this river come to be called the Saraswati is still an open question.

Two recent genetics papers using ancient DNA recovered from the IVC site of Rakhigarhi and from Central Asia argue that people from the Pontic-Caspian steppes migrated into South Asia between 2000 -1500 B.C. bringing with them the Indo-Iranian branch of the Indo-European language family. These would presumably be the Vedic people.

Read these papers. They are interesting.

1) On the existence of a perennial river in the Harappan heartland.
2) The formation of human populations in South and Central Asia.
3) An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers.

Friday, December 20, 2019

Readings: Erectus SE Asia, Devonian Fossil Forest, Archean Iron Formations

Some selected readings:

1) New dates of Homo erectus from Ngandong Java shows late surviving populations until 117,000 to 108,000 years ago. A short clean summary by Razib Khan on SE Asian hominin diversity.

Southeast Asia during the Eemian was a hominin paradise.

Paper: Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago.

2) Exquisite preservation of one of the earliest forests from the Mid Devonian ( ~385 million years ago) of New York containing a modern looking root system.

Paper - Mid-Devonian Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests.

Write up : The World’s Oldest Forest Has 385-Million-Year-Old Tree Roots.

3) Before around 2.3 billion years ago there was very little oxygen in the atmosphere. This was a time before the evolutionary invention of oxygenic photosynthesis wherein bacteria harvest electrons from H2O and release oxygen as a byproduct. Instead, during this time another photosynthesis pathway known as photoferrotrophy was prevalent. Here, bacteria use light and ferrous iron (Fe+2) to fix CO2 as biomass, releasing ferric iron (Fe+3) as byproduct. This ferric iron then accumulated to form large iron deposits. But these deposits lack organic matter. How to explain this if the iron was being produced from a biomass? Scientists point to a role of silica. At that time the oceans were saturated in free silica. Experimental work shows that in the presence of free silica cell surfaces repel iron hydroxides, thus creating a source of organic matter free iron deposits. This organic matter then was acted upon by methane producing microbes. The methane released kept the temperature of the earth warmer than it would have been under a dim early sun.

Fascinating story of the feedback between geology and evolution.

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.

Saturday, November 30, 2019

Field Photos: Natural Arch, Lava Channel NE Of Pune

Last Saturday I went for a field trip organized by the Centre for Education and Research in Geosciences. This is an outreach effort initiated by geologists Dr. Sudha Vaddadi and Natraj Vaddadi along with the student community from various Pune colleges. They undertake these programs regularly through the  year. We explored the Deccan Plateau region northeast of Pune.

On Pune Nasik Highway we turned east at Ale Phata. Our first stop was a little past Gulunchwadi . Across the road an inclined dike intruding into older basalt flows is visible. And a groundwater seep is seen along the contact between two basalt flow units.

Such water seeps can slowly weather and remove rock eventually creating larger passages and undercuts. We saw that just a few minutes ahead. Besides a small roadside temple is a steep stairway leading you into a streambed below. There you come across a wondrous natural arch.

I am not sure I have a good explanation of how exactly this feature formed. Was there a larger waterfall cascading from the top before? At the same time, groundwater seeping along the contact between the two flow units would have eroded rock material creating large passageways which eventually coalesced. Stream flow got directed along the bed of this large tunnel. Further down cutting by the stream has lowered the level of the stream bed, leaving a stranded 'bridge'. 

Also notice in the satellite picture below that the stream makes an abrupt turn at a couple of different points along its course. Its pathways appear to be controlled by fractures.They would have provided weak zones that focused and enhanced erosion.

From this site we proceeded to Mandahol Dam. A little north of this dam is a ridge line named Mhasoba Zap. Can you spot something unusual in the topography of the ridge?

The sinuous feature is an exhumed river of lava!  It erupted between 67 and 66 million years ago.

Basalt lava is less viscous and can flow for long distances. It can follow a preexisting valley or lows in the landscape, forming a lava channel. The image below is from a USGS monitoring station that has captured a lava channel formed during a recent eruption in Hawaii. 

The view in this photo at Mhasoba Zap is looking upslope. The winding ridge which you can follow up to the isolated hill in the background is the exhumed lava channel. It stands out about 50-100 meters above the adjacent plains. 

And here is the lava channel looking downslope. It continues for a distance of about 3 kilometers 'downstream' before dying out.

The margin of the channel (white arrows) are made up of a basalt which looks a little different from the basalt in the central parts of the channel. The margin rock is reddish in color. A closer look (in the field) will tell you that it is glassy to fine grained.  Lava at the margins cools quickly. This cooled lava gets broken up because of the stresses imparted by flowing lava in the center of the channel. This gives a fragmented character to the margins. The iron in the quenched glassy matrix rusts to impart a orange red hue to the rock.

In a close up I have outlined the base of the channel in orange lines.  Dr. Sudha Vaddadi who has mapped this region when she was working with the Geological Survey of India tells me that this entire ridge is actually a lava tube. The top has been eroded away! She was able to identify the 'roof' a km away downslope.

Lava at the surface cools and solidifies quickly. That leaves a tube or a pipe through which lava is supplied from the vent across long distances. The solid crust insulates and keeps the interior hot, allowing the lava to reach long distances from its source. The photo is of a lava tube from the Reunion Islands, a site of ongoing volcanism.

Photo Credit: Nandita Wagle

Let's take a closer look at the margin rock.   It is distinctive due to the reddish color and the fractured fragmented nature of the rock.  It has also been extensively affected by secondary mineralization. Cracks are filled with (white veins) of fibrous scolecite (zeolite family) and calcite.

Blobs and lava spatter accumulates at the margins, cooling and welding together to form an 'agglomerate'. The close up shows globular masses of lava stuck together. 

In this synoptic view, almost the entire lava channel is visible.  Downslope it breaks up into distributary 'fingers'. 

This really was a fun trip. From this lava ridge we traveled south and saw stalactites at the Duryabai Temple near Wadgaon Durya and then went further south to see the famous potholes in the Kukdi river bed near Nighoj village.  I will write about these features in a later post.

In the embedded map look for Malaganga Temple, Mhasoba Zap, Wadgoan Durya and Nighoj. 

Email subscribers may not be able to see the map. Follow this Permanent Map Link.

Although the Western Ghat escarpment with its spectacular views captures a lot of attention, the Deccan plateau region to the east and northeast of Pune has a lot of interesting geology and landscapes.

Get out there and explore!

Tuesday, November 19, 2019

Kumaon Lesser Himalaya- Lessons In Mountain Building

Mountain belts like the Himalaya and the Alps formed when continental crust was squeezed, deformed and uplifted during the collision of two continental plates. The Himalaya, which is the deformed edge of the Indian continental plate is made up of different terrains. The Tethyan Himalaya is the northernmost terrain whose northern edge meets the Asian continental plate. The Greater Himalaya and the Lesser Himalaya are the two terrains successively to the south of the Tethyan Himalaya.

The Tethyan Himalaya are made up of rocks of Cambrian to Eocene in age  (542 -50 million years old) and have suffered the least burial and metamorphism. The Greater Himalaya are rocks which were buried as deep as 20-25 kilometers, suffering the highest degree of metamorphism and even partial melting. They range in age from the Neoproterozoic to the Ordovician (1000 -450 million years old). The Lesser Himalaya rocks were subjected to an intermediate level of burial and metamorphism. They span the Paleoproterozoic to Neoproterozoic in age (1840 -800 million years old).

The Siwalik ranges that occur to the south of the Lesser Himalaya are made up of sediments that were derived from the erosion of the rising Greater and Lesser Himalaya. They range in age from about 12 million to 0.5 million years.

So, exactly what happens when the crust gets caught up in such a continent-continent collision?

A recent paper by Subhadip Mandal and colleagues in Lithosphere explains the structural architecture and mechanisms of crustal deformation of the Kumaon fold and thrust belt. They propose a resolution of some long standing problems in Kumaon geology, namely the interrelationships between the different fault systems and exposed terrains.

See the map which shows part of the Kumaon and Gharwal Himalaya. Before India Asia collision, the crust between the two orange lines would have been ~575 kilometers wider! ....Squeeeze!

How did the crust get 'shortened' during India- Asia collision? In the schematic that I have drawn below, shortening of a particular length of crust takes place by folding it or by breaking it up into blocks and stacking them. The Himalaya have formed by a combination of such folding and thrust stacking.

Off course, the Himalaya is not one big fold, nor is it a stack of blocks forming a tower like the way I've drawn it. Rather, think of inclined books on a shelf. The books are inclined towards the right, or north. There are 4 books. From right to left they are the Tethyan Himalaya, Greater Himalaya, Lesser Himalaya and the Siwaliks. 

A shelf with books inclined to the right will grow by shelving from left to right. The Himalaya have grown in exactly the opposite manner. 

Imagine four books lying flat forming a chain on a bookshelf.  The rightmost book (Tethyan Himalaya) made contact with Asia and was thrust up. Then the book to its left was thrust up (Greater Himalaya), then the Lesser Himalaya and finally the Siwaliks. Deformation moved from right to left, or in the real world, from north to south.

But enough of abstraction! In the real world.. A  has been crumpled up to form B.  A in the figure below is the original disposition of rock units of the Indian plate. B shows those units as they are today,  folded and faulted after the collisional process. The Greater Himalaya is the topmost pink layer. The Lesser Himalaya layers are shown in green, blue and orange. The Siwalik ranges are fawn colored. The Tethyan Himalaya not shown in this figure. 

Source: Subhadip Mandal 2019

The structure looks like a mangled disordered heap of strata. But there is an order to this apparent chaos. As in our book analogy, these units were deformed in a sequence. The cross sections below shows the sequence of deformation and how the structural architecture evolved.

Source: Subhadip Mandal 2019

The collision of the Indian and Asian plates has been timed to around 55-50 million years ago. The first significant topography formed with the uplift of the Tethyan Himalaya between 45-35 million years ago.  The rocks that became the Greater Himalaya were buried the deepest during continental collision. They were uplifted between 23-16 million years ago. This crustal block was moved along a giant fault system known as the Main Central Thrust. In the two figures above you can see the Greater Himalaya as the pink layer overlying the Lesser Himalaya.

The Lesser Himalaya rocks which show imprints of a shallower buried state were lifted up between 16 - 4 million years ago. This rise of the rocks of  the Lesser Himalaya took place in two broad phases. In the first phase, the oldest rocks of the Lesser Himalaya were thrust up along another big fault system known as the Ramgarh-Munsiyari Thrust. In the figure, these oldest Lesser Himalaya rocks are the thick green layers immediately below the pink Greater Himalaya. Subsequently, more and more of the younger Lesser Himalaya strata got caught up in the deforming pile of rocks. Slices of the younger Lesser Himalaya were moved along thrust faults and stacked in a southerly growing fold and thrust belt.   

Initially, the Greater Himalaya and the oldest Lesser Himalaya were placed atop the younger Lesser Himalaya along very low angle faults in a manner similar to the stacked blocks I showed in the beginning of the post.  Later, the growth of the younger Lesser Himalaya lifted, tilted at steeper angles, and folded the overlying Greater Himalaya and the older Lesser Himalaya thrust sheets in a series of broad domes (anticlines) and troughs (synclines).  

The Greater Himalaya and the oldest Lesser Himalaya domes were more susceptible to erosion. As a result, these domes were removed over time, leaving behind synclinal remnants known as klippen. Isolated outcrops of Greater Himalaya and the oldest Lesser Himalaya rocks sit atop younger Lesser Himalaya at many places along the Lesser Himalaya belt. For example, the town of Almora in Uttarakhand is on a klippen of Greater Himalaya rocks. Further to the east the small town of Askot is on a klippen of the oldest Lesser Himalaya.
And finally, sediments which were being deposited in a southerly moat in front of the rising Greater and Lesser Himalaya rose to become the Siwaliks beginning around 1-0.5 million years ago. 

Mandal and colleagues work clarifies to a great extent the structure of Kumaon Himalaya and the mechanism of how fold and thrust mountain belts are constructed. I have simplified the story here. The paper has more nuanced details of the methods and techniques used to reconstruct a long and complicated process.

When you travel next across the Kumaon region, think of inclined books (thrust sheets) and their sequential uplift.



Monday, November 11, 2019

Articles: Anthropocene, Future Of Science, India's Green Tribunal

Some excellent articles I read recently.

1) What Made Me Reconsider The Anthropocene - Peter Brannen. A lovely essay and one that is really a rethinking of his earlier position wherein he had dismissed the idea of Anthropocene as hubris.

I must share an excerpt:

"For me the essence of a lot of Faulkner is, before you can be something new and different, slavery is always there, the legacy of slavery is not erased, ‘The past is never dead. It’s not even past,’” he said. In Faulkner’s work, memories, the dead, and the inescapable circumstance of ancestry are all as present in the room as the characters who fail to overcome them. Geology similarly destroys this priority of the present moment, and as powerfully as any close reading of Absalom, Absalom! To touch an outcrop of limestone in a highway road cut is to touch a memory, the dead, one’s very heritage, frozen in rock hundreds of millions of years ago—yet still somehow here, present. And because it’s here, it couldn’t have been any other way. This is now our world, whether we like it or not.

The Anthropocene, for Wing, simply states that humans are now a permanent part of this immutable thread of Earth history. What we’ve already done means that there’s no unspoiled Eden to which we could ever return, even if we disappeared from the face of the Earth tomorrow.

2) Science Must Move With The Times: Phillip Ball. How has society shaped the nature of science over the past 150 years and what is the future course. A very thoughtful essay.

3) Woes of the National Green Tribunal: Are the recent appointments unconstitutional?:  The National Green Tribunal was set up to allow people access to environmental justice. Environmental lawyer Ritwick Dutta documents the way in which this institution is being undermined by the appointment of non-experts in the experts tribunal, by leaving zonal benches vacant, and by the subversion of video conferencing.

Read and weep!

"The situation with the zonal benches is even worse. Though touted as a great innovation, the video conference which is followed for hearing cases in Pune, Kolkata, Chennai and Bhopal does not allow the litigants or their lawyers to effectively make submissions. To make matters worse, speakers are frequently put on the ‘mute setting’ when the hearing is going on. Thus, it frequently happens that while advocates in zonal benches are making forceful arguments, they are not aware of the fact that they are not audible to the Judges sitting in Delhi, since the speaker is on mute setting".

Thursday, October 31, 2019

Geology Of India TV Series- Coral Islands Episode

There is a 13 part series on the geology of India being shown on DD National, a Government of India run channel. In 2016, Vigyan Prasar, an autonomous organization under Department of Science & Technology  had commissioned Pulse Media,  a New Delhi based television production company, to shoot and produce the series.

This is really a commendable decision to give earth sciences the attention it deserves and to try to bring this fascinating field to a broader audience.

But the one episode I've seen so far has been disappointing.

Last Saturday I saw the episode on Lakshadweep Islands and was shocked to see that its depiction of the geological evolution of the islands was factually wrong. The episode describes the formation of volcanic islands (Laccadive Ridge) in the Arabian Sea west of the State of Kerala. The map shows the location of the Laccadive Ridge and adjacent basins relative to the present west coast of India.

 Source: Kerala-Konkan Basin: Directorate General of Hydrocarbons, Govt. Of India.

The audience learns through narrative and a graphic that prolonged volcanism around 66 million years ago resulted in lava piling up on the sea floor and eventually sticking out above sea level forming islands. In the Quaternary Period ( beginning 2.6 million years ago), coral colonies then formed in the shallow water around these islands. The islands then subsided, leaving behind rings of coral reefs encircling deeper lagoons. This explanation (first proposed by Darwin) applies to younger oceanic volcanic regions like the S. Pacific where thermal subsidence over the past few hundred thousand to few million years has promoted the formation of the classic reef and atoll system.

Volcanism in the Laccadive region stopped by 60 million to 55 million years ago. Any thermal effects would have long vanished. In any case, there is no evidence that the Laccadive ridge ever was an island chain. It is considered a submarine volcanic ridge. This ridge actually originated when the Indian continent broke away from Africa. The western continental margin of India was faulted and a series of ridges and depressions were formed due to block movements along faults. The Laccadive Ridge is one such 'structural ridge'. These structures formed by late Cretaceous times (90 million to 66 million yrs ago), and may have been rejuvenated from time to time.

Volcanism then poured lava on top of this ridge and over adjoining regions too. As the sea floor rises here forming a topographic high, the seas above it are shallow. Conditions favorable for calcium carbonate shell secreting organisms have persisted for millions of years. As a result, on this undersea volcanic foundation, a thick pile of limestones has accumulated over the past 50 million years.  Sediments ranging in age from the Eocene to the Pleistocene underlie the present day coral reefs.

The cross section shows the stratigraphy (sedimentary sequence) of the Laccadive Ridge and adjacent regions.

Source: Kerala-Konkan Basin: Directorate General of Hydrocarbons, Govt. Of India.

The present system of living corals has nucleated on a foundation of Pleistocene limestone. They did not form surrounding 'volcanic islands'. Coral blocks and sand originating from dead corals and other shell producing organisms has been moved by currents and has piled up above sea level forming the Lakshadweep Islands. The initiation of coral growth is really tied to creation of appropriate water depths as a result of sea level changes occurring repeatedly over the Quaternary Period due to the waning and waxing of ice ages. It has nothing to do with subsiding volcanic islands.

I was really surprised to see that a factually wrong scenario passed the filters of the subject experts credited in the episode. Did they not peruse the final script? The episodes are of 25 minutes duration. I can understand a need for brevity and simplicity of explanation.  But scientific accuracy is more important and cannot be sacrificed in pursuit of brevity. I do hope accuracy is not a victim in the remaining episodes too. The series is being shown on Saturday evening at 530 pm IST on DD National.

Disclosure: Pulse Media had hired me as a consultant to do background research for this television series. Needless to say I am disappointed that the inputs I had sent regarding the geological evolution of Lakshadweep were not included in the episode.

Monday, October 14, 2019

Papers: Carbonate Sedimentology Tribute To Robert Ginsburg

Dr. Robert Ginsburg, who spent much of his career studying the geological evolution of the Florida Keys died recently. The Depositional Record has an open access special issue in his honor packed with papers on the modern and ancient carbonate rock record.

In Precambrian times (before 542 million years ago), the precipitation of calcium carbonate on the sea floor was influenced by the activity of microbes. Beginning around 530 million years ago, complex multicellular organisms evolved the ability to secrete calcium carbonate as a protective shell.  Since then, limestones have been forming by the aggregation of skeletons of marine organisms. They tell us about past biodiversity and the conditions in which these ancient organisms lived. Understanding the controls on the origin and accumulation of these sediments using present day examples provides useful analogues to interpret the past. Limestones (CaCO3) and dolostones (CaMg(CO3)2) are also important petroleum reservoirs.

The satellite image shows a portion of the Florida Keys carbonate platform.  It is made up of a low energy shoreline with plant stabilized mud flats, quiet sea grass covered lagoons, and towards the southern reaches, an arcuate coral reef system. The sunlight waters provide ideal conditions for a complex community of shell secreting organisms. Broken down shell fragments accumulate either in-situ or are distributed across the platform by waves and currents. Dr. Ginsburg wanted to know the details of these processes.

In the published issue, there are quite a few papers on carbonate depositional environments and the sediment production and distribution processes in action in the Bahamas shallow marine region. These are supplemented by examples from other parts of the world, including the Florida Keys.

... and don't forget to read the warm humorous tribute celebrating the life and work of Dr. Ginsburg written by Eugene Shinn.

"But, what would be his dissertation subject? Someone came up with a catalogue advertising Fellowships at the University of Miami in Florida. When he asked Jack Hough about Miami, Jack admitted he had never heard of the place. In fact, no one that far north had heard of the University of Miami. Bob drove south for the interview anyway. He was on a mission, an idea he wanted to test. He wanted to determine the process by which sediment became rock. It seemed straightforward enough. All you had to do was dig a hole or push a core tube a few feet into the sediment until it stopped. That would be the zone of transition from sediment to rock. He got the Fellowship (about $3,000) and began his research. Of course, the problem turned out to be more difficult than expected, so difficult in fact that many scientists are still working on it today".

One of his seminal contributions was a proposal to explain the cyclic nature of carbonate sedimentary deposits.  It is observed that many thick limestone sequences are made up of a repeated pattern of relatively deeper water subtidal sediments overlain by shallower water intertidal sediment. These couplets are stacked to form deposits that can be hundreds to even thousands of feet thick. Dr. Ginsburg suggested that this pattern arises due to the cyclical shifts in the reduction and expansion of source areas of carbonate sediment.

At times when the open sea area is large, healthy organic growth produces a large supply of skeletons. Breakdown of these skeletons produces carbonate mud. This mud is transported by currents and trapped along the shores causing accretion of tidal mud flats, which grow towards the open sea. This spread of tidal flats in turn eventually shrinks the size of the source region, reducing sediment supply and stopping tidal flat growth. Sediment production in this system then falters. Natural subsidence of the basin reestablishes water depths for optimum organic growth, again resulting in healthy sediment production, and the cycle restarts.

Here is the abstract of Dr. Ginsburg's model published in AAPG Bulletin in 1971.

Title: Landward Movement of Carbonate Mud: New Model for Regressive Cycles in Carbonates:

Repeated regressive cycles are characteristic of the Paleozoic shallow-water carbonates of North America; similar cycles are present, although less abundant, in Mesozoic and Cenozoic strata worldwide. Several of these cyclic carbonates contain major hydrocarbon reservoirs: Permian, Central Basin platform; Mississippian, Saskatchewan; Ordovician and Silurian, Montana. Studies of comparable recent deposits in Florida, the Bahamas, and the Persian Gulf suggest an alternative to the accepted tectonic explanation of these cycles.

The Florida Bay lagoon and the tidal flats of the Bahamas and Persian Gulf are traps for fine sediment produced on the large adjacent open platforms or shelves. The extensive source areas produce carbonate mud by precipitation and by the disintegration of organic skeletons. The carbonate mud moves shoreward by wind-driven, tidal or estuarine like circulation, and deposition is accelerated and stabilized by marine plants and animals.

Because the open marine source areas are many times larger than the nearshore traps, seaward progradation of the wedge of sediments is inevitable. This seaward progradation gives a regressive cycle from open marine shelf or platform to supratidal flat. As the shoreline progrades seaward the size of the open marine source area decreases; eventually reduced production of mud no longer exceeds slow continuous subsidence and a new transgression begins. When the source area expands so that production again exceeds subsidence a new regressive cycle starts.

The seaward progradation suggested by this model should be observable in ancient deposits.

This explanation of cyclicity is known as the autocyclic model, since all the feedbacks are internal to the system. The alternative explanation is called the allocyclic model. In this case, lithologic repeats are thought to result from changes in sea level caused by the growth and decay of polar ice caps due to cyclic changes in the solar radiation received by the earth (Milankovitch Cycles).

Dr. Ginsburg's work still generates a lot of debate.

Dive in!

Monday, October 7, 2019

Geology As A Socially Embedded Science

C.P. Rajendran writes on geology as a socially embedded science, and traces its historical development from a tool to exploit natural resources to present day concerns about sustainability.

"The bottom line of the arguments is that geology fortified by its unique narrative power and reasoning prowess which are the hallmarks of all historical sciences, cannot be seen from the perspectives of physics nor should it be treated as a derivative science. Geology is a ‘preeminent example of a synthetic science’, wherein the geologist employs a suite of logical techniques and tools to understand nature and its components. And, such reasoning powers that depend on the classical hermeneutical methods or interpretative logical procedures offer far superior methodology to find answers in a world of complexities and uncertainties that we now inhabit, be it safe disposal of nuclear waste, climate change or receding groundwater levels".

Fine essay. Do read.

Open Access.

Monday, September 30, 2019

Links: Kimberlites, Ecosystem Recovery, Early Atmosphere, Carbonates

Some readings on assorted subjects.

Enigmatic origin of diamond-bearing rocks revealed

These are volcanic rocks which are the primary source of diamonds. Kimberlite magmas originate from deep in the earth's mantle. A recent geochemical survey has provided insights into the nature of that source. In India, the famous Panna diamonds are derived from the Majghawan Kimberlite which erupted about 1073 million years ago in the Proterozoic Vindhyan Basin.

Diversity decoupled from ecosystem function and resilience during mass extinction recovery

The mass extinction that took place 66 million years ago devastated both marine and terrestrial ecosystems. How long does post-extinction recovery take and exactly how do community structure and ecosystem functions reboot? A study using a 13 million year record of nannoplankton (unicellular protists) spanning the mass extinction has yielded some insights.

The study suggests that essential ecosystem functions such as geochemical cycling of nutrients was established by few hardy species very soon in the extinction aftermath. This recovery preceded by million of  years the reestablishment of species richness.

Did Bacterial Enzymes Cap the Oxygen in Early Earth’s Atmosphere?

Photosynthetic cyanobacteria that expelled oxygen evolved by 2.4 billion years ago. But oceanic and atmospheric oxygen levels remained quite low, about 10% of current levels, until about 400 million years ago.

The Pre-Salt Hydrocarbon Reservoirs of the South Atlantic

A superb example of how an understanding of the environments in which sediments are deposited helps petroleum exploration strategy.  Focus is on the unusual alkaline lake carbonate deposits of Brazil, formed during the Cretaceous when South America and Africa started splitting away from each other.

Tuesday, September 24, 2019

Musings: Two Million Page Visits

Recently, someone who spends most of the day staring at excel spreadsheets told me that geology is a 'dry' subject.

Another typical reaction I get about geology is an inquiry as to why I choose this unusual or fringe field. That too mystifies me. How can a science that is so central to understanding how the earth works be 'fringe'?

Geological knowledge makes the world turn. But the search for metals, oil and coal is not its only utility. For the past 4.6 billion years, the earth has been in a state of constant churn, a dynamic driven by the transfer of chemicals and heat between its interior and the surface. Rocks, organisms, and air feed of each other in an intricate web of energy exchanges. As historians of the earth, we build narratives about this evolution by delving into the rocky archives of past oceans, terrains and climate. We try to understand the processes connecting these different realms on timescales both vast and fleeting.

These history lessons from the deep past give geologists a unique perspective on how the surface of the earth, our home, will change in the near and distant future, changes caused by the interaction of human activity and natural process. Geoscientists will have a critical role to play in solving the big challenges of resource management, environmental degradation and climate change.

Refreshingly, I saw a different attitude towards geology in younger minds. A few weeks back I was asked to judge a school earth sciences projects contest. The children had prepared some wonderful demonstrations of how geology and our daily lives intersect. They were curious about the subject and passionate about applying the science to better our future. This early immersion in earth sciences might just make them more responsible and better informed stewards of our planet.

That day gave me some hope for the coming decades.

I write to tell these richly rewarding stories about geology. Maybe I have succeeded somewhat in my endeavor.

I am forever seeking newer audiences for my writings. A request to you to pass on the link to my blog to your friends. You can also subscribe directly by email or follow me on Twitter.

As for my friend, I am happy to say that he responded well to a treatment of 'one week in the Himalaya', so much so that towards the end of the trip he asked for a geology book list.

Monday, September 16, 2019

Darwin: Citizen Science

After his return to England in 1836 from a five year round the world trip aboard the Beagle, Darwin did not travel again for any extended fieldwork. His home became his study and his laboratory, but he was no lonely isolated genius. His ideas stemmed from data that streamed in from all over the world.

From Darwin: The Life of a Tormented Evolutionist

Down House had become the hub of a correspondence network across the Empire, its tentacles touching every little England. The sack of mail brought gems daily to aid his sexual selection. Botanists from Ceylon to Calcutta sent reports on monkey manes and bearded Indians; mining engineers from Malacca to Nicaragua told of indigenous customs; tile manufacturers in Gibraltar attended to merino lambs; wine exporters in Portugal followed the local tailless dogs; Laplanders measured reindeer horns; New Zealanders heroically tackled the Maori's sense of beauty; and missionaries and magistrates from Queensland to Victoria ceased converting and incarcerating to observe aboriginal ways- with an old Beagle shipmate Philip King helping out. This is what Darwin excelled at: collecting and collating, tracking down facts, verifying,extending his old notebook speculations to embrace the globe.

Darwin had many India connections. His botanist friend Joseph Hooker who had traveled to India in the 1840's had been one source of information on indigenous plants, animals, and people. From 1855 or so, Edward Blyth, curator of the museum of the Royal Asiatic Society in Calcutta became his chief contact. He struck up a lively correspondence with Darwin. Details about monkey manes and bearded Indians would have come from him.

"his large and varied stories of knowledge, I should value more than that of almost anyone" he wrote of Blyth.

Vikram Doctor has written an insightful essay on Edward Blyth's life. It sketches the sharp contrast between the financially comfortable life Darwin lived in England against the hardscrabble existence of Blyth, who managed to stay on for 21 years in India on a salary of Rs 250 a month, supplementing it with a trade in exotic birds and animals.

A love for natural history drove Blyth on and Darwin benefited from that immensely.

Tuesday, September 10, 2019

Books: Timefulness And A New History Of Life

More books arrived in my mailbox.

I have been looking for an updated sweeping history of life survey. Peter Ward and Joe Kirschvink have written just that book, covering topics from the origin of life to mammalian evolution. I'm in the middle of the section on the origin of life, along which I am re-reading the relevant section from Nick Lane's superb book The Vital Question. Flipping ahead, I can see that the authors have developed broad narratives of how ecosystems changed, framed within the themes of catastrophic events and changes in levels of oxygen, carbon dioxide and hydrogen sulphide (very early in the history of life).

After enjoying her first book, Reading the Rocks, I had to order Timefulness, a long essay on the need to be aware of the multiple time scales on which geologic processes operate. Marcia Bjornerud stresses that inculcating time-literacy is vital for a sensible societal response to our epoch of rapid planetary change.

I'll be sharing excerpts from time to time.

Saturday, August 31, 2019

Hot Water Springs Of Konkan- Geological Significance

During my recent trip to Konkan, just north of the Tural area, I came across a sign for a hot water spring.

This is located in the small settlement of Aaravali. The area around the spring has been converted into a tourist spot where locals and tourists come to enjoy a warm bath.

One interesting feature of Konkan coastal belt is the presence of hot water springs arranged in a fairly narrow linear belt from north of Mumbai to Ratnagiri area in the south. They occur somewhat midway between the Western Ghat Escarpment and the coast. Few names from north to south- Vajreshwari, Akoli kund, Ganeshpuri, Pali, Dasgaon, Unhere, Tural, Aaravali, Rajapur. Satellite image shows the area between Dabhol and Ratnagiri. Hot springs are located within the oval. The dark brown undulating line is the trace of the Western Ghat Escarpment.

What is so special about this area? See the map. Black lines are fracture zones, trending N-S, NW-SE and NE-SW. Hot water springs are located in the vicinity of these fractures roughly within the oval. The depicted area is again between Dabhol and Ratnagiri, but this relationship between fracture systems and location of hot springs applies elsewhere along the entire Konkan coastal region.

Source: Neotectonism in the Indian Subcontinent: Landscape Evolution- K.S Valdiya and Jaishri Sanwal (modified).

Water temperatures are between 50 deg C to 60 deg C. Interestingly, analysis shows significant levels of radon gas at measured sites near Tural. Presence of radon gas hints at the reason why there are hot springs here. It points to deep circulation of water.

The crust in this region is made up of a foundation of older Precambrian age granitic rocks overlain by several hundred meters of younger basalt of Late Cretaceous to Paleocene age (67-65 million years old). Radon forms by radioactive decay of uranium. The Deccan Basalts contain only tiny amounts of uranium. Granites on the other hand are enriched in uranium. Radon emission here imply that these fractures cut through the basalt pile and penetrate the 2 billion years and older granitic rocks underlying these basalts. 

Such fracture systems have provided a passageway for groundwater to percolate to great depths. Cool water comes in contact with hot rocks deep below and gets heated. This warmer buoyant water then rises to the surface, forming a hydrothermal circulation system. The cross section shows fracture/fault systems of the coastal region cutting across basalts and penetrating the underlying granitic basement. I have added a few additional fractures to the figure.

What makes the rocks hot? What is the source of heat?  Deccan volcanism ended 60 million yrs ago. It is unlikely that there is any magma underneath to provide heat. Rocks get hotter at depths due to the natural geothermal gradient. Some geologists think that many of these fractures are actually faults along which there is intermittent movement of the crust. This faulting may be causing friction between crustal blocks, generating additional heat in these zones. These fractures and faults are a legacy of the breakup of the India with Madagascar and later Seychelles during and post Deccan volcanism  68 -60 million years ago. This rifting of the Indian crust resulting in oriented fracture systems.

The schematic shows the evolution of the Western Ghat escarpment and the coastal region. Earlier, perhaps soon after Deccan volcanism ended, the escarpment was a west facing cliff formed when faulting caused the western block to subside . Subsequent erosion has resulted in this cliff retreating eastwards, creating a coastal plain. Orange lines mark the highly fractured Indian crust.

Source:  Western Ghat: The Great Escarpment of India- V.S. Kale 2010. (modified)

Next time you visit Konkan and take a dip in the invigorating warm waters, remember that grand geological forces of continental separation are responsible for the high heat flow and the ground water circulation systems that arise consequently.

Wednesday, August 21, 2019

Does Volcanism Cause Global Warming Or Cooling?

On million year time scales, does volcanism cause global warming or cooling?

The answer is both, depending upon the longevity of the volcanism and it effluents. Prolonged volcanic emissions over tens of thousands to millions of years of greenhouse gas carbon dioxide will warm the earth's surface. But magmatism and volcanism creates continental crust. During volcanic episodes and after the magmatic system dies, this new crust consumes carbon dioxide in chemical weathering reactions. This draw down of atmospheric carbon dioxide can result in global cooling, as is inferred to have resulted in the Cenozoic beginning around 30 million years ago, after collision of the India -Eurasian plates. Volcanism and tectonic activity can both warm and cool the earth's surface as magmatic arc systems grow and die.

Volcanism also ejects sulfur particles into the atmosphere. These particles block and reflect sunlight away and this albedo effect may result in cooling of the earth's surface. Volcanic ash falling on both land and sea may act as a fertilizer, enhancing organic productivity and further drawing down and sequestering carbon dioxide through increased organic carbon burial. A recent paper published in Geology by Gerilyn Soreghan and colleagues (open access) points to a temporal coincidence between explosive eruptions and glacial conditions during the Late Paleozoic. The researchers suggest that the prolonged icehouse conditions from around 360 million years ago to 260 million years ago resulted from explosive volcanism and effects of sulfate aerosols.

This paper has prompted a thoughtful commentary (open access) by Rice University geologists Cin-Ty Lee and Sylvia Dee on the broader controls of volcanism and crustal weathering on global climate. On the particular question of whether the Late Paleozoic ice age was a result of sulfate ejections, they differ somewhat from the authors of the study. Cin-Ty Lee and Sylvia Dee point out that the residence time of sulfur particles in the atmosphere is just a few years. To maintain a global icehouse for a 100 million year period would require large explosive eruptions every few years over tens of millions of years.

They point to an example of another period of enhanced magmatic activity in the Cretaceous Period. Field evidence from the continental interior of the U.S. shows just about 200 eruptions over a 10 million year period. Only a few of these were large enough to have ejected significant amounts of aerosols into the stratosphere. That is not to say that sulfate aerosol albedo cannot cool the planet. But it may happen over shorter 1000-10,000 year time scales. In case of the Late Paleozoic icehouse, they suggest that the pattern of cooling may hint at the causative factor. Numerous short-lived cooling events would be suggestive of explosive volcanism as the cause.

On longer time scales carbon dioxide will play a larger role in modulating climate. Explosive eruptions and resulting sulfate particle emissions are only a small component of magmatic flux. On the other hand, CO2 degassing is taking place even without eruptive activity. Long lived magmatic activity will result in a warming trend due to an increase in atmospheric CO2.  Post magmatism, a drop in atmospheric CO2 levels and cooling due to silicate weathering also takes place on longer million year time scales.

An extract from Cin-Ty Lee and Sylvia Dee commentary:

More broadly, the nature by which volatiles are exchanged between planetary interiors and their surfaces is rich with complexity. The magnitude and style of magmatism not only controls volatile degassing but also erosion, weathering, radiative balance, and biological productivity. How magmatic processes change through time and with geodynamic states is an area ripe for interdisciplinary research and new discoveries. Soreghan et al.’s work is an example of how investigating these processes from deep time to the present, as well as on Earth and other planets, will force us to rethink how planetary systems operate.

The geologic record shows that enhanced phases of volcanic activity sustained over thousands of years can cause the earth's climate to tilt towards a long lasting greenhouse or an icehouse. People who claim that the warming of the earth's surface over the past few decades is due to natural causes like volcanic eruptions and not fossil fuel burning must understand the time scales and amounts involved. Even big volcanic eruptions that occur every few years emit only a few million tons of CO2. Awkwardly, for anthropogenic warming deniers, these eruptive events may result in a short term cooling phase due to the effects of sulfate aerosols. A recent survey puts the total global emissions due to volcanic eruptions and non-eruptive degassing of magma to be about 0.3 billion tons per year. In contrast, human activity is putting 30-40 billion tons of CO2 in the atmosphere every year.

Explosive volcanism as a key driver of the late Paleozoic ice age.
Does volcanism cause warming or Cooling?

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.