This is a rich conversation between paleontologist Prof. Ashok Sahni and Dr. Devapriya Chattopadhaya of the Indian Institute of Science Education and Research, Pune.
It was so refreshing to hear Prof. Sahni talk candidly about the state of paleontology research in India, preserving fossil sites, motivating students, the need for Indian scientists to proactively engage with the public about their work, and the importance of building bridges between research and societal needs.
Prof. Sahni comes from an illustrious line of scientists. His uncle was the paleobotanist Dr. Birbal Sahni after whom the Birbal Sahni Institute of Palaeosciences, Lucknow is named. And his father was also a palaeontologist. His mother was not keen on him taking up geology, admonishing him that there were already too many rocks in the house. However, he persisted.
I've expanded on the link I posted last week on Darwin's Atolls.
A beautiful theory has been undone by ugly facts...well, let's just say facts.
How do coral atolls, those shimmering ring shaped islands set against the blue tropical ocean, form? Charles Darwin had pondered this question on the H.M.S. Beagle as she sailed across the Pacific and Indian Oceans in the early 1830s. He famously reasoned that coral colonies begin growing in the shallow waters surrounding oceanic volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs.
But he just assumed that the present day corals atolls are growing on a volcanic foundation.
Actually, most are not. Tropical region atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years.
Darwin at that time didn't know that the climate over the past 2.6 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes. The story begins in the Pliocene around 5 million years ago. A long period of sea level stability resulted in reefs and other calcium carbonate sediments accumulating on the shallow waters above sea floor ridges. Thick deposits formed flat topped carbonate banks. The earth's climate began to change around 2.6 million years ago as polar and high latitude ice sheets expanded and then withdrew. As a result, sea level started falling and rising cyclically.
These repeated sea-level fluctuations amplified around half a million years ago. When sea level fell, rain water dissolved the exposed carbonate flats to form an uneven karst topography comprising a depressed bowl with an elevated rim. Subsequently, when sea level rose, new coral growth began in the optimal water depths above this jagged limestone rim. With every sea level fall and rise, erosion and new coral growth accentuated the relief between the central depression which became the lagoon and the rim which was now made up of towering stacked coral reefs. The modern looking atoll evolved in this manner.
A recent detailed study
by Andre Droxler and Stephan Jorry compiling decades of research on the
Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.
The image above shows a portion of the Chagos atolls in the Indian Ocean. Notice how the rings of coral reefs have formed on a submerged flat 'mesa' or table land. These are the vestiges of the carbonate banks that formed during the Pliocene and later became the foundation for the modern atolls.
Great scientists get it wrong frequently too. Darwin erred in his too loyal adherence to the principle, 'Present Is The Key To The Past'.
He observed that some volcanic islands have coral colonies growing around them. He extrapolated this condition to the past and theorized that atolls began as reefs fringing volcanic islands. He couldn't imagine how otherwise corals could grow in the middle of the ocean without them having a shallow water foundation to colonize.
Uniformitarianism has been a very helpful paradigm in understanding many aspects of the past, but geologists have learned to apply it with caution. During the long 4.5 billion year history of our planet, critical combinations of atmosphere and ocean compositions and continental configurations coupled with the evolving biosphere have resulted in unique geologic products and processes which have no modern analogue.
Darwin's atolls offer us some lessons about our future too. Ten thousand years of relative climate stability has allowed civilization to flower, but has also lulled us into a dangerous complacency about nature and its permanence. The idyll of these atolls is ephemeral. Their very foundations are testimony to rapid environmental change and great dyings of marine ecosystems. Such changes await us in the future too, now hastened by our own agencies.
Several hundreds of years from now our descendants may well be looking on at a planet where many of Darwin's atolls have disappeared under the sea and our forests and croplands diminished by fire and dust. Will it be possible to modulate this coming change? For that, we must heed the signals from our geological past. Our present behavior will be the key to our future.
1) A beautiful theory has been undone by ugly facts. How do coral atolls, those shimmering ring shaped islands set against the blue ocean, form? Charles Darwin had famously reasoned that coral colonies begin growing on the slopes of volcanoes. Eventually the volcanoes sink into the ocean, while the coral keep growing upwards. The central area where the volcano existed becomes a deep lagoon, surrounding by a ring of coral reefs. But he just assumed that the present day corals atolls are growing on a volcanic foundation. Actually, most are not. Tropical region reefs and atolls rest on an earlier generation of coral and limestone. These in turn have grown on an even earlier layer of coral growth and so on through the past few million years.
Darwin at that time didn't know that the climate over the past 2-3 million years had shifted periodically between glacial and inter-glacial phases resulting in sea level changes, and how these repeated sea-level fluctuations can create environments where corals grow during a sea level rise or later dissolve during a sea level fall to form a karst landscape. This jagged uneven surface in turn becomes the foundation for a new generation of corals. A new detailed study of the Pacific Ocean and Indian Ocean reefs demonstrates this elegantly.
2) Did Homo sapiens enter India prior to the devastating Toba eruption that took place about seventy four thousand years ago or after? This question is of interest in elucidating the timelines and dispersal routes of our species from Africa. Homo sapiens had reached Australia by around 60,000 years ago with India being one obvious migration path. There are no skeletal human fossils from this time period in India and stone tools have been variously interpreted as belonging either to Homo sapiens or an earlier archaic human. There were few accurately dated sites from the time period of 80,000 years ago to 50,000 years ago. Now, some new work from the Son Valley, Madhya Pradesh, shows long term human occupation in north India from pre Toba eruption times. The layers containing stone tools span from about 79,000 years ago to 65,000 thousand years ago. The tools resemble those from the Middle Stone Age of Africa, Arabia and Australia and are interpreted to have been the handiwork of Homo sapiens.
3) The ecologic context of the evolution of our genus Homo is of great interest. A recent study focuses on using carbon isotopes to tease out dietary shifts in herbivore fauna living in East Africa in the late Pliocene to Early Pleistocene. Analysis of herbivore teeth from 3.6 million years ago to 1.05 million years ago reveals a shift from C3 derived food (woody vegetation) to C4 derived food (grasses), first around 2.7 million years ago and again later around 2.1 million years ago. Woodlands were giving way to more open savanna, a change that coincides with the evolution of Paranthropus and Homo.
A while ago a friend commented that she had learned in school geography class that Himalayan faults are the youngest. Now, earthquakes in Bhuj, Gujarat, and Latur,Maharashtra, in recent times tell us that there is active faulting going on elsewhere in India too. But I could understand what she was trying to say, that the Himalaya is a growing mountain chain with active faulting. I think a lot of non-geology folks appreciate this point. What is not that well known is that within the Himalaya the locus of active faulting has shifted southwards over geologic time. The Himalaya is the northern margin of the Indian continental crust which has been broken up into blocks or litho-tectonic units and stacked by major faults. From north to south these faults are, South Tibetan Detachment, Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust. There are plenty of minor faults between these major breaks.
As the Indian continent collided and underthrust Asia, slices of its crust were pushed up in the following order; The Tethyan Himalaya along the South Tibetan Detachment (45-35 million years ago), the Greater Himalaya along the Main Central Thrust (24-15 million years ago), the Lesser Himalaya by the Main Boundary Thrust (and many subsidiary faults, 11-5 million years ago), and the Siwaliks by the Main Frontal Thrust (and some subsidiary faults, 1 million years ago to recent). All these faults merge at depth with a north dipping (sloping) master fault known as the Main Himalaya Thrust along which the India plate is underthrusting or sliding underneath Tibet. The Himalaya is deformed Indian crust riding atop the MHT.
Why did faulting activity migrate southwards in this growing orogenic (fold and thrust) mountain belt? Geologists give a mechanical explanation of this style of mountain growth using the Critical Wedge Model.
It will be worth pausing this post to watch a video made by Middlebury Plate Tectonics on Critical Wedge Theory. Email subscribers who cannot see the embedded video may watch it at this link- Critical Wedge Theory- Himalaya.
To summarize, the Himalaya may be abstracted as a wedge of crust which is thicker in the north and thinner towards south. The ratio of normal stress to shear stress controls whether a fault can slip. As crust thickens beyond a threshold value of the ratio, increased normal stress can pin down and lock a fault. Subsequently, the locus of active fault slip migrates towards the region with a more favorable stress ratio, which in the case of the evolving Himalaya orogen has been progressively southwards.
This is a very informative video but I know of many geologists who would protest. Their objection will not be that the Critical Wedge model is wrong but that it doesn't explain all of the Himalaya. They argue that the upper structural levels of the Greater Himalaya were extruded by a different mechanism. The growth of compressional mountain belts involves crustal thickening due to folding and thrusting. The Critical Wedge model explains this as taking place by the brittle breakage of slices of underthrusting crust along faults and their continuous accretion to a growing wedge. Rocks of the Greater Himalaya though show signs of ductile deformation. High grade gneisses were partially melted to form migmatites. Pods, lenses and sheets of granite magma was injected along fractures and planar rock fabric (schistosity).
The picture above shows leucogranite sills (white layers) intruding high grade gneisses near the village of Naagling in the Kumaon Himalaya, Uttarakhand. This partial melting and magma injection was contemporaneous with the extrusion of rock from deeper to shallower levels of the crust.
All this took place beginning about 24 million years ago. Geologists have termed the movement of this hot mushy ductile rock mass as 'channel flow', literally to mean a channel of semi-solid rock that is being squeezed upwards like toothpaste from its container. In this case, the container were two bounding fault systems, the South Tibetan Detachment as the roof, and the Main Central Thrust as the floor. Supporters of channel flow say that the pervasive ductile deformation observed in the Greater Himalaya doesn't support the Critical Wedge mechanism of orogen growth. Instead, they propose that 'channel flow' was a unique phase in Himalaya development, restricted to the Miocene when deeply buried hot crust was being extruded. Over time shallower levels of the crust were incorporated into the growing orogen where colder temperatures permitted brittle breakage of the crust and critical wedge growth. The Lesser Himalaya and the Siwalik ranges can be more satisfactorily explained by this mechanism of southwards fold and thrust propogation.
'Critical Wedge' and 'Channel Flow' are statements on how crust with contrasting mechanical properties responds to compressional forces of tectonic origin and/or surface directed pressure gradients generated due to removal of overburden by erosion.
One final point. The video mentions 'out of sequence' thrusting referring to rejuvenation of extinct fault zones in the rear of the wedge. In case of the Himalaya this means renewed faulting in locales much to the north of the Main Frontal Thrust. This out of sequence thrusting manifest by low level earthquakes is taking place near and just southwards of the Main Central Thrust zone and seems to be driven by enhanced erosion stripping away rock, thereby reducing crustal thickness and normal stress.
Interestingly, I came across a paper by Paramjit Singh and colleagues which has used Apatite Fission Track (AFT) to reveal a pattern to this exhumation. Fisson Tracks is a kind of radiation damage in uranium bearing crystals. It is an ongoing process, but the tracks get preserved only below a critical temperature. The density of tracks is correlated to the time since the rock cooled below the healing temperature. In mineral apatite, fission tracks records the time when the rock cooled below 120 deg C. A young AFT date means that the rock was at around 4 km depth at a more recent time and has been exhumed to the surface much rapidly than a rock recording an older AFT date. AFT dates taken along a north south profile in the Kumaon-Gharwal Himalaya from the Vaikrita Thrust to the Berinag Thrust show a southward younging of dates, indicating sequential uplift and exhumation from north to south since Pliocene times (<5 million years ago). The 'out of sequence' faulting regime seems to be a second cycle of an 'in-sequence' pattern developing in the footwall (structurally underneath) of the Main Central Thrust zone. Similiar studies done by this team of scientists across nearby transects in the same climatic zone in the High Himalaya show that rocks are following different exhumation patterns. Contrary to what the video depicted, that does not sound like a climate controlled phenomenon. Rather variations in local tectonics may be dictating this style of exhumation. Himalaya never cease to be a mystery and a wonder.
