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 et.al. 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 et.al. 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!