Saturday, January 21, 2017

Book: Indica- A Deep Natural History Of The Indian Subcontinent

I am not doing a general book review of Pranay Lal's book Indica: A Deep Natural History Of The Indian Subcontinent. For that, I recommend this fine literate piece by Pratik Kanjilal published in the Indian Express. And Prabha Chandran writes about it in the Huffington Post. Both are aimed at the general reader.

No one has, as far as I know, written critically about the science content of the book. I read through the book and have some comments on the geology.

Before I start, let me say that I enjoyed this book. Pranay Lal has read widely, traveled far, and has had immersive discussions with geologists and paleontologists.  The best sections of the book are when he is writing about the many fossil finds preserved in Indian sedimentary basins and their importance in interpreting paleo-geography, ecology and evolution. He certainly appears more comfortable writing about these themes than he is about geology.

There are many problems with the geology writing. Some are easy-to-fix errors, while others will, in my opinion, require some rethinking on the more effective presentation of ideas and processes.

Let's begin with the easy to fix errors-

1) Page 12: Ref: Structure of the earth-  "The innermost shell of the "core" was composed of iron  and nickel and was surrounded by a larger but less dense mass of molten iron called mantle". - The mantle which is the layer of the earth between the crust and the core is not molten. It is solid and is made up of silicates and not iron. The core itself has two layers, a solid inner core and an outer fluid layer made up of iron and nickel.

2) Page 13: Ref: Age of corals in Rajasthan and Kutch- " This coral colonized the seas about 380 million years ago". There are no 380 million year old sedimentary rocks in Rajasthan and Kutch (Devonian Period). This may be a typo. There are Jurassic age corals in Jaisalmer. They are about 170 million years old.

3) Page 45: Ref: Banded Iron Formations- "Both ferric iron and ferrous iron began to settle as successive bands at the bottom of the iron-rich seas and lakes as oxygen levels fluctuated. Once, deposited, the layers hardened one above the other and gave the appearance of a layered cake- thin strawberry-jam-coloured striations of highly oxidized iron (ferric oxide, Fe2O3) and dark coloured chocolate lines of less oxidized iron (ferrous oxide, FeO)" - In the vast majority of Banded Iron Formations the strawberry coloured striations are forms of silica, either chert or jasper. The dark coloured layers are hematite or magnetite ( ferric oxide Fe2O3). Ferrous iron (divalent) is usually in a dissolved state. Ferric oxides or hydroxide minerals and compounds form following oxidation of this dissolved ferrous iron. Some ferrous iron is trapped in carbonate and sulphide minerals.

4) Page 57: Ref: Coral mineralization- "When hard-bodied marine animals like corals evolved (around 2 to 1.7 billion years ago)" - Multicellular animals originated in the Neoproterozoic likely between 700 and 600 million years ago and acquired hard parts (mineral skeletons) by around 550 million years ago.

5) Page 57: Ref: Limestone formation- " ..the vast accumulation of shell and coral got pressed together into minerals like calcite and aragonite" - organisms combine Ca and CO3 ions to precipitate minerals like aragonite and calcite to build their shells. This accumulation of shells when pressed forms limestone rock.

6) Page 59: Ref: Picture of Cruziana- " This 565 million year old fossil is of Cruziana, one of the earliest multicellular animals and an ancestor of the trilobite which lived in shallow seas". Cruziana is an ichnofossil. It is a name for an impression of a particular shape made by trilobites disturbing the sediment on the sea floor (bioturbation). Cruziana is not an ancestor of the trilobite, it is evidence of the presence of trilobites. These ichnofossils from Rajasthan are in Cambrian age rocks and so have to be younger than 542 million years.

7) Page 60: Ref: Evolution of complex multicellular organisms and animals - " About 570 million years ago, a few enterprising organisms developed a new reproductive strategy - sex! Sex opened up a plethora of possibilities"  -  Sex evolved once in the unicellular eukaryote common ancestor of fungi, plants and animals more than a billion years ago. The oldest fossil evidence of a sexually reproducing multicellular organism is the protist Bangiomorpha pubescens. It is 1.2 billion years old. Preserved filaments show differential spore/gamete formation. So, sex evolved hundreds of millions of years before the evolution of animals.

8) Page 62: Ref: Animal family relationships- Comb jellies and jelly fish "evolved to become thin, pin-shaped worm like creatures with no arms or legs that wriggled on the bottom of the sea floor". The author is saying the creatures with bilateral symmetry arose from Cnetophores (comb jellies) and jelly fish (Cnidarians). Animal phylogeny reconstructed by genetic analysis shows that Cnetophores are a group which diverged from the animal family tree very early in its history. And Cnidarians and Bilaterans are sibling groups. They share a common ancestor. See this easy to understand essay by Jerry Coyne.

9) Page 154: Ref:  Bedaghat and  Makrana marble- The author says that the famous marble cliffs of Jabalpur (Bedaghat) and the Makrana marble used to build the Taj Mahal are Cretaceous in age.  He writes that Cretaceous sediment made up of calcium carbonate shells were deposited between 145 to 65 million years ago and were cooked by volcanic heat, which transformed these sediments into marble.  However, both these marble deposits are Proterozoic in age.  Calcium carbonate sediments accumulated in seas that covered Rajasthan and Central India in Proterozoic times. These deposits were then metamorphosed into marble during orogenic activity that took place during evolution of the Aravalli mountains (Makrana marble) and in Central India (Bedaghat marbles). Estimates are that deposition and metamorphism into marble took place between 2 billion to 1.5 billion years ago .

10) Page 131- Ref: Mid ocean ridges- "Deep sea trenches on the sea floor are the weakest points on the crust, made up as they are of a thin layer of rock and water above it.. " He goes on to explain that these are the spots where magma melts the crust and flows on to the surface creating new oceanic crust. Technically though, the term "deep sea trench" refers to places where tectonic plates are converging and oceanic crust is subducting underneath another tectonic plate. Lal on the other hand is describing regions where tectonic plates are spreading apart and new ocean crust is being generated. Such places are called "mid oceanic ridges".

11) Page 183: Ref:  Vivekanand rock as meeting place on Gondwana continents - "Geologists call the Vivekanand Rock memorial 'the Gondwana junction' because it marks a place where India, Madagascar, Sri Lanka, East Antarctica were once joined together". The Indian continental crust extends underneath the sea beyond the Vivekanand islet. The continental shelf edge, tens of kilometers away from the present day shoreline, is really the place where India would have been joined to Australia and Antarctica on the eastern margin and Madagascar on the western margin.

12) Page 210, 216, 222: Ref: Magma chambers in Deccan Traps- The authors points out examples of columnar jointing in basalts and calls them remnants of magma chambers. This is incorrect. Magma chambers are present several kilometers below the surface of the earth. If magma solidifies at this depth is won't be called a basalt (it will be called gabbro) and won't develop columnar jointing. These instances the author point out are either thick lava flows or volcanic plugs which have developed columnar jointing on account of cooling and shrinkage. Volcanic plugs are remnants of lava which solidifies in a volcanic vent at the surface.

13) Page 280: Ref: CO2 released by volcanism- " Most of the volume of CO2 in the atmosphere actually comes from volcanism and sea floor spreading. When sea floor spreading occurs, sediments on the ocean floor (including these shells) are dragged deep under the ocean floor where they heated and the trapped CO2 is released". At sea floor spreading centers volcanism releases CO2. Sediments and shells are dragged under the ocean floor at the other end of the plate at subduction zones. As they dive under, they get heated and release CO2.

Some longer discussions:

a) Page 18- Ore deposit formation- The author writes that collisions of meteorites during the early history of the earth up to 2.5 billion years ago kept puncturing the earth's crust releasing metals like iron from as deep as the core. These metals clumped together to form ore bodies. The early earth did go through a period of very heavy meteorite bombardment from 4.1 billion years ago up to 3. 8 known as the "Late Heavy Bombardment". Bombardment continued sporadically after that. No crust from this very early period is preserved as it kept getting smashed and recycled into the interiors. Any ore deposits that formed have also been destroyed.

The earth was much hotter then and after the easing of bombardment, intense magmatism from 3.8 billion to 2.5 billion years ago started forming the first continents. Geologists estimate that nearly 65% -70% of the present volume of the continental crust formed during this phase. The magmatism transferred metals from the mantle to the newly formed crust. 

A recent survey of  five years of research from 2011 to 2016 done by Indian geologists on ore deposits shows that not a single study invokes meteorite bombardment as the cause for ore concentration. Instead, internal forces like subduction zone magmatism, rift magmatism, hydrothermal circulation systems and near surface sedimentary processes are inferred. Now, there may be specific instances where meteorite impacts may have fractured the crust and initiated fluid circulation, but meteorite bombardment is a not a general explanation of metallogeny.

b) Chapter 7- Page 182 and subsequent pages- Pranay Lal discusses the breakup of Gondwanaland. How do continent breakup and what is the force that causes tectonic plates to move and drift for thousands of kilometer? He invokes volcanic eruptions as the cause of supercontinent breakup and the push exerted by magma upwelling through cracks as the force driving plate movement. He refers to a paper by Shanker Chatterjee and colleagues on the subject of India's epic northward journey after it broke up from Gondwana until it collided with Asia.

But an alternate view among geologists is that volcanic eruptions are the consequence of the break up of continents. And plate motion is driven not by the push of upwelling magma/lava but by the pull of cold dense lithosphere which sinks deep into the mantle at subduction zones. A perusal of the paper by Shanker Chatterjee shows that these scientists agree with this "slab pull" notion as the main force of plate motions. Mid oceanic ridge push is a secondary force. Unfortunately, the author does not even mention the slab pull force mechanism.

So, continents break up due to a variety of factors. Indeed, there could be an anomalous build up of heat underneath the continent, which thins and weakens the lithosphere (the rigid plate consisting of the crust and the upper part of the mantle). Hot buoyant mantle impinges the underside of the plate. At this point the mantle is still solid but can flow like silly putty. Continued stretching and thinning of the crust (caused by the slab pull force from a subduction zone at the other end of the plate) results in the underlying mantle decompressing. This results in the lowering of its melting point and magma generation. Magma rises through the fractures of the thinned crust and erupts on the surface.

In this view, volcanism did not prise apart fragments of Gondwanaland one by one. Rather, enormous episodes of volcanism like the Deccan Volcanic Event were triggered by rifting and Gondwana continents moving above anomalously heated portions of the mantle known as hot spots.

c) Page 261: Ref: Himalaya. Here is how Pranay Lal describes the rise of the Himalaya. "The Himalaya rose from below. The rubbing together of the immense plates and the monumental crushing and buckling of land produced a tremendous amount of heat and cause magma from below to ooze out of deep fissures which opened up on the surface. This melted and remelted granite, and pushed it upwards to the surface. As the granite slowly cooled, successive batches of molten granite thrust their way up,forcing the older granite slabs higher. Over time, this process created a pedestal for mountain building. Because the "cooking" process varied (different types of granite are cooked at various depths), the densities of rock slabs differed. This created large cracks or "faults" along places where the continental crust rasped, grinded and pushed slowly onward".

I didn't understand this at all.

Later he says that the Everest is made up of an initial four thousand odd meter foundation of granite overlain by another 3100 meter of sedimentary rock. The granite is 50-30 million years old while the sedimentary rocks are from the Paleozoic era (359 - 252 million years old).

During continental collision, there has been melting of the lower parts of the crust and this terrain has been intruded by pods and lenses of younger granite. Metamorphosed and partially melted Precambrian rock is the main component of the Greater Himalaya. In the Everest -Lhotse-Nuptsu region the granite intrusions are on a massive scale as described by Pranay Lal. These thick intrusive granite and high grade metamorphic rocks make up the base of Everest region. But, these younger granite intrusions are not this thick everywhere. They are present on a smaller scale along certain bands of the Greater Himalaya and are almost absent from the Lesser Himalaya.

Though the author may not mean it, phrases like "successive batches of molten granite thrust their way up, forcing the older granite slabs higher" may be misinterpreted by lay readers to mean that Himalayas formed as a result of magma pushing the crust up to form mountains. This is not how orogenic mountains like the Himalaya form.

As the Indian Plate collided with Asia it delaminated. You can think of this as the plate splitting into two tiers. The lower tier comprising the lower crust and upper mantle slid under Tibet. The upper tier impinged into Tibet and got squeezed, deformed and thickened. The Himalaya is this folded and faulted upper tier. The different Himalayan ranges are slices of the upper tier Indian crust stacked one on top of the other by a series of south moving thrust faults.

 The tectonic structure of the Himalaya with its geological divisions is summarized in the graphic below.

Source: Shankar Chatterjee et. al. 2013

What was the sequence of these thrust faulting events and how do they fit into the three pulses of mountain building that Pranay Lal mentions?  

Leaving the Tibet part aside, the Himalaya most people are familiar with are made up of four distinct geological terrains. I am listing them starting from the north and going south.  Tethyan sedimentary rocks (the ones making up the Everest and many other summits; These sediments  range in age from the Neoproterozoic to the Eocene- ~ 1000 million years to 40 million years, although the entire sequence is not exposed at one place), the Greater Himalaya Crystalline Complex (Proterozoic to Early Paleozoic, 1800 million years to 480 million years, with a younger imprint of metamorphism and granite intrusions), the Lesser Himalaya Sequence (Proterozoic to Cambrian; gneiss and low grade metamorphosed sediments, 1850 million years to 520 million years) and the Siwaliks which are Cenozoic sedimentary rocks deposited from around 15 million years to about 0.5 million year ago. The geological divisions roughly match up with the topographic divisions of the Greater Himalaya, the Lesser or Middle Himalaya and the Outer or Sub Himalaya.

The northern edge of the Indian plate was made up of  Proterozoic rocks, much as it is all across Peninsular India. This Proterozoic sequence was overlain by Paleozoic and Mesozoic sedimentary rocks. There is a more complete sequence of Paleozoic sediments in the Himalaya, since even though most of India was landlocked as part of Gondwanaland, the north edge of what was to become India was open to the Tethys sea all through the Paleozoic and Mesozoic.

As India collided with Asia:

1) Its continental crust impinged on the continental crust of Asia. The Neoproterozoic-Phanerozoic sedimentary cover was folded, faulted and scraped off and uplifted to form an early mountain range made up of the Tethyan sediments.

2) Horizontal shortening of the Indian crust during collision led to crustal thickening and rocks were subjected to high temperatures and pressures. They were metamorposed and partially melted into rocks known as migmatites and intruded by granites. Finally, compressive stresses broke the crust along a major fault known as the Main Central Thrust and uplifted this deeply buried terrain. The thrust moved crustal blocks upwards and southwards. Some geologists believe that the Great Himalaya Crystalline Complex is made up of hot soft rocks from the middle regions of the Indian crust which flowed towards the surface in response to the removal of  crustal cover by erosion. This ductile flow of rock from deep in the crust towards the surface is termed "channel flow" as hot soft rock is confined to a layer or channel between colder upper crust and a more rigid upper mantle.

Either way, with this thrusting and extrusion of high grade rock began the formation of the Greater Himalaya. The main activity of the Main Central Thrust is dated to between 16 million years to 25 million years or so. At about the same time the earlier uplifted Tethyan sediment detached themselves from the underlying crystalline basement and started sliding northwards along a major fault system known as the Southern Tibetan Detachment System.

3) As India continued to press into Asia, compressive stresses propagated southwards. Beginning around 16 million years to 11 million years, the terrain to the south of the Main Central Thrust began to get folded and faulted. Since it was further to the south from the collision zone, it did not experience the high levels of metamorphism and granite intrusions that the rocks of the Great Himalaya did. Eventually, these more distal rock formations were uplifted and moved southwards along the Main Boundary Thrust and associated faults to form the Lesser Himalaya.

4) The rise of the Greater Himalaya and the Lesser Himalaya loaded and depressed the crust in front of them in to a moat. In the alluvial plains, streams and lakes that formed were deposited sediments eroding from the rising Greater and Lesser Himalaya. These were the environments in which a lot of the mammalian evolution and diversification described in an earlier chapter took place. Beginning around a million years ago, maybe a little earlier, these sediments were folded, uplifted and thrust above the Gangetic alluvium along the Main Frontal Thrust to form the Siwalik ranges. The Main Frontal Thrust is still active, and Himalaya earthquakes which originate deep underground rupture along this fault plane. The Himalaya are growing southwards.

I am not writing a popular book for the lay public.  I realize I may have gone overboard with my Himalaya explanation and am not suggesting that Pranay Lal should include all this in his book. But any explanation should include at least the basic arrangement of the different lithologic terrains and their sequential uplift due to south progressing thrust faulting.

d)  Page 218 - Ref: Satpura mountains were uplifted due to the rise and push of magma leading up to the Deccan volcanism.- This is also a longer discussion but I'll stop on the geology aspects since this post is already too long. Let me refer to an article on the lack of pre- Deccan volcanic uplift in the Satpura region and elsewhere.  Many geologists have concluded that the uplift of the Satpura belt is not due to the push of magma. It occurred much later in the Cenozoic due to the various stresses on the Peninusular Indian crust.

e) I couldn't help elaborating on this:  Why do animals grow large? Pranay Lal mentions an intriguing evolutionary pattern seen in the fossil record. There is a trend towards an increase in body size in the early to mid Triassic following the Permian mass extinction. A second trend in increase in body size in seen in the Cretaceous when some lineages of dinosaurs evolved gigantism. The explanation given is that an increase in atmospheric oxygen levels favored an increase in body size.

Animals have a physiologically demanding lifestyle. That a certain threshold of oxygen will be required for them to prosper over the longer term is a given. It is too broad an explanation and it doesn't tell us why in a group one lineage evolved towards a larger size while a related lineage did not. In the two time periods that author points to, the reason why species evolved towards large size in the Triassic immediately after the Perman mass extinction may be different from why certain lineages evolved gigantism in the Cretaceous.

Let's take the case of body size trends during the early Triassic. Mass extinctions disproportionately cull larger bodied species . Since environmental condition are deteriorating rapidly, larger bodied species with  more energy expensive demands and slower reproduction rates cannot cope as well as species with a smaller body size. Survivor species in the aftermath of mass extinctions are small bodied, maybe as small as their biological limits. From this starting point, even if environmental conditions in the post mass extinction period are neutral in terms of favoring species with a particular body size, the only trend that will emerge is one towards a larger body size, since there is no room to get any smaller.

For Cretaceous too the oxygen hypothesis seems too pat. I could argue that Cretaceous was a time of high carbon dioxide levels. That would mean more food for plants. A lush healthy vegetated landscape means more food for herbivores, conditions favorable for evolution of larger size. Again this is a "just so story". Why did gigantism evolve in the Sauropoda? The answers may lie in phylogenetic heritage i.e. inheritance of ancestral characters which fortuitously proved advantageous, and evolutionary innovations that enabled them to acquire and utilize resources more efficiently than other groups. An avian style respiratory system enabled pnematization (air cavities in bones) of the axial skeleton. This evolved early in Sauropod history. A small head evolved because food was ingested without mastication. These two features enable a long neck to evolve (lighter head, lighter skeleton). A long neck enabled access to food not available to other herbivores.. a cascade of benefits due to inherited and newly acquired features.

Off course, these are ramblings about things that interest me. I have no expectation that Pranay Lal put all these details in his book.  

But, we need books like these to fire imaginations and inspire amateurs and students to go out and explore India's rich physical landscape. A good place to start will be the National Geological Monuments listed by the Geological Survey of India. A surge of visitor interest might put pressure on the government to expand protection to more sites of interest. The destruction of irreplaceable fossil sites and geological structures is a constant theme in Lal's book.

A second edition of this book will be welcome, but one that has gone scrutiny by a discerning geologist editor.

8 comments:

  1. Discussions with readers-

    Copied and pasted from my Facebook post- https://www.facebook.com/suvrat.kher/posts/10154342498980897

    Shyamal Laxminarayanan: I am yet to see this book but one of the things I was looking for recently was some information on paleo-coastlines and I found that an old book by Vishnu Mittre meant for younger audiences served my purpose (not sure if it is outdated or incorrect though) where new books failed (including the Making of India by Valdiya - second edition - would be nice to read your review of that one?) - https://archive.org/details/RocksUnfoldThePast - there certainly seems to be space for something more visually appealing with more illustrations and photographs ...

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  2. Discussions with readers-

    Copied and pasted from my Facebook post- https://www.facebook.com/suvrat.kher/posts/10154342498980897

    Shyamal Laxminarayanan: Referring to a picture showing glacial deposits on page 52: "The caption for this reads "Some of the best evidence of the last glacial event in the Indian subcontinent is in Dudhinala, which lies along the Ranchi–Hazaribagh highway in Jharkhand. Here up to several horizontal feet of rocks and boulders lie embedded between alternating layers of sedimentary rock. But soon you may not be able to see this at all. The stones and rocks are being indiscriminately quarried and the ancient rivers have been reduced to effluent drains that lead into the Damodar and Barakar rivers." - Suvrat, why would this be glacial rather than just riverine action?"

    Reply: The sediments have been interpreted of glacial origin because of their extreme poor sorting. So clay, silt, sand, pebble, boulders all make up these rocks. Riverine action will sort sediments into size classes. The pic you attached shows dropstones (white), large boulders carried by glaciers and dropped into lakes and ponds developed along the melt front. Some of these glacial carried sediments can be redeposited in streams and show a mix of characters and are called glacio-fluvial. And glacial layers can alternate with fluvial and lake deposits indicating fluctuation retreat and growth of glaciers. All these features are found in the early Permian Talchir sediments.

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  3. Discussions with readers-

    Copied and pasted from my Facebook post- https://www.facebook.com/suvrat.kher/posts/10154342498980897

    Emmanuel Theophilus: Suvrat, in the para just below the Chatterjee graphic on the tectonic structure of the Himalaya, you mention " the Lesser Himalaya Sequence (Proterozoic to Cambrian; gneiss and low grade metamorphosed sediments,"... Gneiss in the Lesser Himalaya sequence? Shouldn't be there, other than in Nappes that have slid/crept down from the Greater Himalaya, should they? And I also wonder why one rarely finds mention of Limestone in the Lesser Himalaya, when they seem to comprise the bulk of the rocks there? Sorry, perhaps I should ask you this in a separate mail...

    Reply: Emmanuel; The Lesser Himalaya is also made up of a slice of Indian crust composed of Paleoproterozoic to early Paleozoic rocks. So there are Proterozoic gneiss as old as 1800 million years old. They are the basement for the Lesser Himalaya limestone you mention. And you are right, there are ALSO gneisses which are Nappes. These are erosional remnants of Greater Himalaya thrust sheets now surrounded by Lesser Himalaya.. phew... I have emailed you a paper on the stratigraphy of the Lesser Himalaya.

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  4. Thanks for the review, Suvrat. I'm eager to get my hands on this book though some of the inaccuracies you point out beg the question of whether the book had solid technical editing.

    How much does Lal delve into the "less glamourous" aspects of Indian geology in this book? For example, the Dharwar craton and TTGs or perhaps the Southern Granulite Terrane?

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  5. Vivan- don't think the draft was read through by a geologist. Strange since he talked to a lot of geologists and paleontologists. Should have asked one of them to read through the draft!

    He does mention the Dharwar craton but not TTG and Southern Granulite Terrane or greenstone belts. Doesn't mention other cratons either. Very light treatment.

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  6. Discussions with readers-

    Emmanuel Theophilus (via email)- Suvrat, question:

    In Lal's book, p 135, the end of the first long para, he says "The Eurasian Plate crops up in Munsiyari in Uttarakhand,..." and mentions other places in the cis Himalaya. I thought that the Eurasian plate would not be 'seen' south of the Indus-Tsangpo suture. Is Lal right, and if so, where in Munsiari? What type of rock might it be, so that I can keep a lookout?

    Reply-
    I think that is a mistake. I have not come across any map or reference regarding presence of Eurasian plate in the Munsiyari region or for that matter anywhere south of the Indus -Tsangpo suture.

    There is one way by which Eurasian plate material can be found south of the ITS. After collision and the closure of the Tethys, south flowing rivers eroding rocks of the Asian plate deposited these sediments in basins south of the ITS. Eocene fluvial sediments found in Ladakh and elsewhere south of ITS contain sediment derived from the Asian plate. In fact, by pinpointing the timing of their first appearance in basins south of ITS one can get an estimate of the timing of the terminal India Asia collision and the closure of Neotethys (since the sea disappeared, rivers originating in Asia could carry detritus southwards onto the Indian plate). Studies show that zircons of Asian affinity were first deposited on the Indian plate (Western Himalaya) around 54 million years ago. Based on their Mesozoic age (~115 million years old) and geochemical characters, these zircons are thought to have been sourced from the magmatic belts of the Lhasa terrain. Cool huh!

    Don't think Lal meant it this way though. This is different from saying the "Eurasian plate crops up in Munsiyari". In any case there is nothing close to Munsiyari that is Eurasian.

    I found it strange that he has made so many avoidable mistakes. I wish a geologist had read through a draft copy of his book!

    Follow up question-

    Small niggling question (due to my ignorance) : Do they know when the zircons of asian affinity were deposited by dating the surrounding sediments? And if there was no carbon (or would there have surely been), how would they do that?

    Reply -

    There are two dates here. One is the date when the zircon crystallized out of granite magma intruding the Lhasa terrain. This has been calculated to be ~115 million years and more using radiogenic Uranium trapped inside the zircon. The zircon and the host granite became part of the Asian plate during the Mesozoic.

    Later, post India Asia collision, south flowing rivers eroded and carried that zircon to basins south of ITS on the Indian plate. These deposits are Eocene in age based on biostratigraphy. They have used an assemblage of foraminifera to place them into the Eocene. Now, at this locality, the sediments could not be directly dated. So, how do we know they are 54 million years old? Because somewhere else in the world there are sediments with the same assemblage of foraminifera species, but which are also interlayered with volcanic rocks which contains material that can be directly dated. This could be micas or feldspars containing radiogenic Potassium or Strontium etc. That way, by cross reference, we can calibrate the Himalayan sediments to an absolute age.

    Carbon dating cannot be done on rocks this old. This is because the half life of radiogenic isotope C14 is about 5,400 years. In about 8 half lives or about 40,000 years or so there is too little C14 left in the sample to be measured precisely.

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  7. Pity that. My first impression upon reading the title of the book was that he'd delve into the story of deep Indian time too.

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  8. Discussions with readers-

    Emmanuel Theophilus (via email

    Have you read that paper from Nature that some scientists from China have written, about the fossilized shelled creatures found on the Tibetan plateau, which bear signatures of isotopes of oxygen that indicate that the platueau ca 50 mya was already around 4000 m high? That puts to ef every other thing we have been reading about Himalayan orogeny (at least the dates and so on)? Will dig it up if you haven't seen. Yes, but could you point me to some readings on understanding isotopes esp of oxygen?

    Reply: I've read about those results and some others that indicate that Tibet had elevations of 4000 m or so by 50 million years. I haven't read the paper in detail. This date doesn't conflict with dates of Himalayan orogeny. As the Indian plate drove northwards through the late Cretaceous- Early Eocene (80-55 mya) the oceanic crust making up the front end of the plate slide underneath Tibet. This convergence resulted in the formation of magmatic arcs (there are L. Cretaceous- E. Cenozoic granitic batholiths intruded into the Lhasa terrain) and thrust faulted uplift of the southern Tibetan margin. A present day analogue for this is the Andes mountains where the oceanic crust of the Pacific plate is sliding under the continental crust of the S. American plate. So, southern Tibet / Lhasa terrain by 50 mya would have been what is known as an Andean margin (with ref to the situation along the Andes).

    After this leading apron of Indian oceanic crust was consumed by subduction, the continental crust of India collided with Asia by 50-40 mya or so (our previous discussion on Asian derived sediment south of ITS). It still took another 15-20 million years or so for crustal thickening and faulting along the Main Central Thrust to start uplift of the Greater Himalayan ranges. We can time this by dating new mineral growth along the Main Central Thrust and dating the timing of arrival of sediment in the Himalayan foreland (what was to become the Siwaliks) and the Bay of Bengal.

    Regarding oxygen isotopes, I've attached an article.

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