Tuesday, October 10, 2017

#Neatrock Entry For Earth Science Week

SciFri Science Club is hosting a #Neatrock challenge as part of Earth Science Week.

Here are my two entries:

Megascopic #neatrock:


This is a migmatitic gneiss from the Greater Himalayan Sequence, Darma Valley, Kumaon Himalaya. Migmatite means a mixed rock made up of a metamorphic host and a newly formed igneous rock. During continental collision, metamorphic rocks buried to great depths and subject to high temperatures may partially melt to form granite magma. The granitic melt segregates into layers. The resultant rock is composed of the original metamorphic host rock such as a gneiss (dark bands)  and granitic igneous layers (lighter bands). This migmatite formed during the Miocene.

Microscopic #neatrock:


This photomicrograph of a Late Ordovician limestone (Fernvale Limestone) from Georgia, U.S.A.  is close to my heart. It formed an important part of my PhD work.  I have stained the thin section with a Potassium Ferricyanide dye. Calcite containing minor amounts of iron (Ferroan calcite Fe+2) is stained blue. Non Ferroan calcite is unstained.  In the center of the photomicrograph is a non ferroan 'dog tooth' spar. It is a calcite crystal with a shape resembling a canine tooth of a dog.

This calcite has a pendant habit. It is hanging from the underside of a particle, in this case a piece of an echinoid shell. Such pendant crystals precipitate in a vadose zone i.e. above the water table.  In this environment, pore spaces are not completely filled with water. Rather, films of water coat grains and form drips. These drips become saturated with calcium carbonate and calcite precipitates from them.  Just like a larger and more familiar stalactite in a cave! Except that this micro-stalactite in tiny..tiny.

Development of a vadose environment indicates that sedimentation was interrupted by a large sea level fall. The sea bed got exposed to rain and a fresh water aquifer developed in the sedimentary deposits.  A tiny 'dog tooth' spar can tell us a fair bit about sedimentary basin evolution and sea level history.

Wednesday, October 4, 2017

Geo Week 2017, Pune

 Geo-Week 2017 Pune

Starting October 9th 2017, a week long geo-activity program for the public is going to be held at Raja Ravi Varma Art Gallery, Ghole Road, Pune.

It is being organized by the Center For Education and Research in Geosciences (CERG) along with Fergusson College, Pune. CERG is a citizen outreach initiative taken by students and professionals from the Pune geology community.

Take a look at the poster.


The inaugural talk by Dr. R. Shankar of Mangalore University will be on October 9th at 11.30 am . The topic is Paleoclimate Studies of Lake Sediments from South India. The venue is Raja Ravi Varma Art Gallery.

There is another lecture scheduled on October 14th  at 7.40 pm by Dr. S. N. Rajguru. The topic is Prehistoric Environment of the Mula Mutha River, Pune. This talk will be held at the Amphitheater on Fergusson College campus.

There is also a geology exhibition, art and essay competitions for school children, earth science themed film shows and a workshop on QGIS. The exhibition is at Raja Ravi Varma Art Gallery, while most of the films will be screened at the Amphitheater, Fergusson College. Check the website for schedule details.

Pune geology and science enthusiasts, share this with your friends and do stop by and support this initiative!

Geo-Week 2017, Pune

Friday, September 29, 2017

The Bay Of Bengal Once Touched Sikkim

See this satellite imagery of the Himalaya.  The Indian State of Sikkim occupies the region just east of Darjeeling.


The Siwaliks (green arrows) appear as a forested linear band forming the southernmost hilly terrain of the Himalaya. The hills abut against broad alluvial plains. Rivers traversing the Himalaya carrying enormous sediment load encounter a gentler gradient upon exiting the hilly terrain. A loss of stream power results in sediment being dumped in the channel, so much so, that rivers get chocked on their own sediment. As a result, channels split and bifurcate forming a braided river system. These rivers  also suddenly change course, abandoning their channel and carving out new ones. Such course changes may occur during floods or by tilting of the land by structural movements.  Over time, the deposits of these ever changing rivers coalesce to form cone shape aprons of sediments known as alluvial fans. These rivers like the Kosi and the Tista, which flow transverse to the mountain range, meet an axial river like the Ganga and the Brahmaputra flowing parallel to the mountain front. The axial river flows into the Bay of Bengal.

The Siwalik hills were once these type of alluvial fans.  Just as today, during Miocene and Pliocene times, sediment was being deposited in front of the rising Himalayan mountains. Beginning about half a million years ago or so, these ancient alluvial fans were crumpled up and uplifted to form the Siwalik ranges. Active alluvial fan formation shifted southwards to its present locus. This process continues. In a few million years, the present day alluvial fans deposited by rivers like the Kosi and the Teesta will be deformed into a newer mountain range south of the Siwaliks. The Himalaya are growing southwards.

How do we know that the Siwaliks were once alluvial fans? Geologists rely on analogy, comparing the Siwalik sediments with what is accumulating in the present day alluvial fans. They find a striking similarity. Siwaliks are made up of alternations of coarse gravel layers and finer sand and silt layers with characteristic bed orientations and structures like cross beds and rippled sand. The gravel layers are inferred to be the river channel deposits while the finer sand and silt layers are the river bank, levee and floodplain deposits. An important finding made throughout the length of the Siwalik ranges has been the paleo-current directions preserved in the rocks.  Geologists have measured the orientation of bedding and ripple marks and found out that rivers were flowing south and south east i.e. perpendicular to the mountain chain. There is no evidence of an axial river like the Ganga in these Siwalik sediments. The thinking is that such an axial river must have flowed much to the south of the region of deposition of Siwalik sediments.

And what about evidence of a delta? Where did these Miocene and Pliocene rivers meet the sea? The logical geographic place to look for a coast would be towards the east. And in fact, that evidence has come from the Siwalik sediments of West Bengal and Sikkim. In a really interesting paper published recently in Current Science, Suchana Taral, Nandini Kar and Tapan Chakraborty describe sedimentary structures and marine trace fossils from Middle Siwalik sediments exposed along the Gish River and its tributaries in the Tista Valley. Siwalik rocks in the central and western part of the Himalaya show current structures that indicate south flowing rivers. In this easterly location however, the sediments show evidence of being deposited in a wave influenced environment. Sedimentary structures like wave ripple laminations and hummocky-swaley stratification indicate deposition in wave dominated marine bay.  Paleo-current indicators like ripple marks preserved on sandstone surfaces show a south as well as north directed current. This suggests an environment influenced by tides and north directed waves. Associated sediments show indicators of different delta environments like distributary channels, delta mouth bar and delta flood plain deposits.

Apart from current direction indicators, the sediments contain plant fossils indicative of mangrove vegetation and brackish water environments. They also contain trace fossils i.e. impressions and burrows made by creatures moving and disturbing the sediment surface. Cylindrichnus, Chondrites, Rosselia, Taenidium, Skolithos, Planolites are some of trace fossils reported in this study. The assemblage of trace fossils is similar to those reported from marine settings.

All this suggests that during the time of deposition of these Middle Siwalik sediments in Late Miocene-Pliocene times, about 5-10 million years ago, a branch of the Bay of Bengal had invaded as far north as present day Sikkim. Rivers carrying sediment from the Himalaya were debouching them in a delta and a shallow marine bay. The Sikkim Middle Siwalik strata are ancient deformed delta and marine deposits.  

A paleo-geographic reconstruction of this eastern part of these Siwalik depositional environments in shown below.


 Source: Suchana Taral, Nandini Kar and Tapan Chakraborty 2017

The  upper graphic shows the reconstructed delta and marine depositional environment. The lower graphic shows the regional paleo-geography. The pin shows the environmental location of the study area. The yellow rose diagram shows the paleocurrent directions measured in the Siwalik sediments.

Interestingly, some earlier work by geologists has shown that in Late Miocene times the Brahmaputra was flowing along a much more easterly route towards the Bay of Bengal. They used sand thickness and sand/shale ratios from wells drilled in the delta and found lobate sand bodies, which they inferred were brought in by a large river flowing from a ENE source. Their interpretation is shown in the graphic to the left (Uddin A. and Lundberg N. 1998). At the time the Shillong Plateau did not exist. The river flowed into the Bay of Bengal from the Upper Assam valley and through the Sylhet depression in to the Bengal Basin. The uplift of the Shillong Plateau in Pleistocene times forced the Brahmaputra to turn west and wrap itself around the newly emerging uplands.

Since Pliocene times, the tremendous amount of sediment being delivered by Himalayan rivers, coupled with Pleistocene sea level fall, has caused a retreat of this arm of the Bay of Bengal southwards.

In the satellite image below, based on the location of the Sikkim Siwalik deposits and other work on the Bengal Basin paleogeography, I have drawn in brown the coastline as it would have existed 5-10 million years ago. The ancient drainage systems are shown in blue. South directed arrows shows the extent of the growth of the Bengal/Bangladesh alluvial plains and delta and the retreat of the sea since then to its present location.


Pretty amazing finding.

Monday, September 25, 2017

Evo Devo Musical Video

This is cool!

How do we develop from one cell to a complex multicellular creature? Tim Blais who runs A Capella Science has a musical video out explaining the genetic basis for this wondrous transformation. This is a field of study known as evolutionary developmental biology.



Book recommendation: Sean B. Carroll's Endless Forms Most Beautiful: The New Science of Evo Devo is a good introduction.

Tuesday, September 19, 2017

Environment Links: River Issues In India

Sharing a few interesting and informative articles I came across in the past few weeks on rivers.

Endangered Himalayan Rivers: This one is from 2012. A large number of dams are planned on the Alaknanda and Bhagirathi rivers in the state of Uttarakhand.  Parineeta Dandekar writes about the weaknesses and bias in the Environment Impact Assessment process.

Rally For Rivers Plan. Will It Help?: The Rally For Rivers campaign by the Isha Foundation is calling on creating a 1 km wide tree plantation along the river banks. This, they claim, will help rejuvenate India's dying rivers. Veena Srinivasan, Sharad Lele, Jagdish Krishnaswamy and Priyanka Jamwal with the Ashoka Trust for Research in Ecology and the Environment, Bengaluru examine their claims in detail and find them wanting.

Caution Warranted For River Linking Project: The gargantuan river linking project envisages a series of dams and canal systems to transfer water from Himalayan rain and snow fed river basins to the drier Peninsular rivers in the south. Is it worth it?

Reuter's Erroneous Reporting On The Ken-Betwa River Linking Project: Two rivers in Madhya Pradesh and Uttar Pradesh are to be linked. SANDRP clarifies that the permissions process has yet to be completed. The two states don't even have a water sharing agreement! Reuter's screwed up.

Environment Ministry Panel Reject's Uttar Pradesh's Religious Smart City Plan: I'm including this to give an example of the utter indifference to ecology and environment shown by "planners and developers". The plan is for a smart city to be built inside the Hastinapur wildlife sanctuary, along the banks of the Ganga, which would have destroyed dolphin habitat and river ecology along a 7 km stretch. How does one even come up with such ideas? Fortunately, the usually pliant Environment Ministry has balked at approving this outrageous plan.


Wednesday, August 30, 2017

Mapping: In Praise Of The Triangle

Jerry Brotton in his book A History Of The World In 12 Maps writes about France's National Map Project. Begun around the 1670's upon the establishment of the Academie de Sciences and the Paris Observatory and headed by the astronomer Cassini I, it first attempted to create an accurate geodetic survey of France using the latest surveying instruments. At the heart of the survey was the calculation of distances and directions using the method of triangulation. Latitude was calculated using a quadrant that measured the altitude of celestial bodies. Then, using a measuring stick, a baseline of a known length was established. A third point on the landscape was sighted. The angles between the three control points were measured. Using trigonometric tables the lengths of the remaining two sides of the triangle could be calculated.

I liked this passage:

In 1744 the survey was finally completed. Its geometers had completed an extraordinary 800 principal triangles and nineteen base lines. Cassini III had always envisaged printing regional maps as they were produced, and by 1744 the map was published in eighteen sheets. Its new map of France, on an approximately small scale of 1: 1,800,000, shows the country represented as a network of triangles, with virtually no expression of the land's physical contours,and with large areas such as the Pyrenees, the Jura and the Alps left blank. It was a geometrical skeleton,a series of points,lines and triangles following coasts, valleys and plains in connecting key locations from which observations were carried out. Over it all lay the triangle, the new immutable symbol of rational, verifiable scientific method. On Cassini III's map the triangle almost takes on its own physical reality, a sign of the triumph of the immutable laws of geometry and mathematics over the vast, messy chaos of the terrestrial world. The Babylonians and the Greeks had revered the circle, the Chinese celebrated the square, the French now showed that it was the application of the triangle that would ultimately conquer the earth.

Cassini III was the grandson of Giovanni Domenico Cassini (Cassini I). The directorship of the Paris Observatory remained in the Cassini family over four generations.

This surveying method was quickly adopted and adapted by others. The Ordnance Survey began mapping the British Isles using this method in the late 1700's.  William Lambton took the Ordnance Survey's acquired expertise and began the Great Trigonometrical Survey of India in the year 1800, a feat that took nearly 50 years to complete. John Keay's book The Great Arc details that mammoth effort.

Thursday, August 24, 2017

Field Photos: Glacial Deposits Of The Darma Valley, Kumaon Himalaya

During my recent trek to the Panchachuli Glacier in the Kumaon Himalaya, I obsessed about observing changes in metamorphic grade of the Greater Himalayan Sequence on the trek route and also about finding the South Tibetan Detachment fault system. I wrote about this in an earlier post.

But there were other interesting geological observations too. The Panchachuli Glacier has left a thick record of glacial deposits. The river Dhauliganga originates from this glacier. Along this river valley, glacial deposits can be observed to a distance of at least 5 kilometers downstream of the present location of the snout of the glacier, indicating that the glacier was much more extensive in the past. Tributary glaciers flowing out of the ranges east of the Dhauliganga have also left an extensive record in the form of thick fluvio-glacial deposits. These can be observed as far south as the village of Baaling.

We heard anecdotes in village Dugtu about how this glacier was much bigger in living memory and how it has been receding rapidly in the past few decades. On one level such stories are believable because studies of Himalayan glaciers have shown that many of them have been shrinking over the past few decades (ref). This is partly due to anthropogenic global warming, but glacial response to warming may be varied due to local variations in topography, precipitation and wind conditions. Some glaciers don't show retreat while some are actually seen to be expanding. Overall though, there a substantial ice loss observed across the Himalaya. Exactly how much of that is due to recent global warming and how much, as some scientists caution, due to natural factors is still being studied. Sustained warming though will cause these glacier to shrink further over the next century.

There is also a longer geological story of glacial advance and retreat written in these deposits.

I've embedded below an annotated interactive map of the glacial deposits of the Dhauliganga river valley in the Panchachuli Glacier area. This will enable readers to zoom in and recognize the various glacial landforms present in the valley. You can also access it via this Permanent Link.



The annotations depict:

a) The dark blue lines are the snout of the glacier.
b) The light blue lines are the recent terminal moriane fields.
c) The pink lines are older lateral moraines.
d) The yellow lines are outlines of older fluvio-glacial deposits
e) Numbers 1 -12 mark the locations of glacial deposits.

I have mapped only a few representative examples of each of the feature types. Readers can use these to explore similar features scattered throughout the valley. 

Location 1: This is the snout of the glacier. It is a mass of ice and frozen mud. The river Dhauliganga emerges out of an ice cave.


Location 2: Taken from near the snout of the glacier looking downstream. Ridges of the terminal moraine can be seen in the foreground. The arrows in the background outline a ridge of an older lateral moraine. Notice how the ridge decreases in elevation downstream suggesting that the terminus of this older glacial phase in somewhere nearby downstream.


Location 3: The older lateral moraine can be clearly seen as a sharp ridge line (arrow) separated from the valley wall by a depression. 


This moraine top is a few hundred meters above the valley floor implying that the glacier was thicker in the past. When was this lateral moraine deposited? It may be at least a few hundred years old. In the Garhwal Himalaya, similar older lateral moraines close to the glacier has been dated to be several hundred years old. They have been interpreted to be a result of glacial growth and deposition during the Little Ice Age, a period of earth cooling and climate instability that lasted from around the 1300's to the mid 1800's (for more on this climatic episode, I recommend Brian Fagan's book The Little Ice Age: How Climate Made History 1300-1850).

Location 4: A view of the glacier and an older lateral moraine (arrow) on the other side of the valley.


Location 5: Further downstream are thick glacial deposits. The river has incised or cut through these sediments. As a result the deposits form flattish plateaus or terraces that hug the mountain slopes. Village Dugtu, where we stayed, has been built on top of one such glacial terrace. The arrow in the top picture points to an exposure of these glacial deposits. A close up of this deposit is seen in the bottom picture. Notice the extremely ill sorted texture. Such ill sorted sediment deposited by glaciers is called Till. Large boulders are mixed in with  gravel, pebbles and much finer sized rock flour (the light to brown colored matrix).


Location 6: Another exposure of a glacial deposit near Dugtu. Again, notice the ill sorted deposit. However, at the top is a well sorted pebbly layer. This suggests deposition in more vigorous flowing water. Glacial retreat from time to time would have resulted in the establishment of a fluvial regime and deposition in these streams. These deposits may be a few hundred to several thousand years old.


Location 7: The glacial terrace on which village Dugtu is built is seen in the lower right corner. Farther away is village Philam built on the thick fluvio-glacial deposits of a tributary glacier originating in the range east of Dugtu. At village Dugtu, the east flowing river Dhauliganga makes a sharp southerly turn. The river has cut through these deposits and the slopes of the valley are thickly forested suggesting the great antiquity of these deposits.


Location 8: A nice view of glacial deposits south of village Baun along a smaller tributary of the  Dhauliganga. Notice the waterfall!


Location 9: A walk right through these thick fluvio-glacial deposits along a forested section of the valley slope. Again, notice the ill sorted nature of the deposits. Glacier are viscous and cannot sort sedimentary particles like water or air can. The result is a jumble of boulder, gravel and rock flour.


Location 10: Another cliff made up of fluvio-glacial deposits. I'm calling the deposits east of Dugtu as fluvio-glacial, since I observed intervals which show layering. This suggest deposition in water, either in streams or in melt water lakes and ponds that form in front of glaciers.


Location 11: A thick sequence of fluvio-glacial deposits along the Dhauliganga river. If you zoom and pan the satellite image you can recognize these terraces  southwards almost up to the village of Baaling.


I did not observe such deposits south of Baaling. However, there are smaller glaciers, such as the Naagling glacier, originating in the ranges on either side of the Dhauliganga. There would be smaller deposits scattered in these tributary valleys.

I have been vague about how old these deposits could be. If we assume that the Panchachuli glacier would have attained its maximum extent in the Pleistocene during the Last Glacial Maximum about 20,000 years ago, then the deposits furthest away from the present location of the glacier would be the oldest. As the glacier recedes one should find younger and younger deposits closer to the active glacier.

A study by Dirk Scherler and colleagues in the Garhwal Himalaya found such a pattern. They studied deposits of the prominent Jaundhar Glacier and the Bandarpunch Glacier in the Tons Valley. I've posted below a map showing the interpreted ages of deposition of glacial sediments.


 Source: Scherler et. al. 2010

Notice how the oldest deposits are further away from the present location of the glaciers (eastern most extremity of the map). These oldest deposits point to the maximum extent of the glacier that was reached in the Pleistocene during the Last Glacial Maximum.  However, the decreasing ages of the deposits upstream aren't the result of a uniform recession of the glacier. Instead, they point to several glacial episodes during which the glacier advanced, then receded, and then advanced again during the Holocene. Their data shows five such episodes of glacial growth dated to approximately 16 ka (ka = thousand years ago), 11-12 ka, 8-9 ka, 5 ka and less than 1 ka.


It turns out that the climate history of the Holocene is not one of uniform warming since the end of the last glacial period. The earth has gone through several minor cooling phases during the Holocene. The well known Younger Dryas Event around 12.9 -11.7 ka is one example.  Some studies suggest cooling episodes around 8.2 ka  and around 4.2 ka . And there is the Little Ice Age during the last millennium.

Another climate dynamic is fluctuating monsoon strength through the Holocene. The authors don't favor the explanation that these periods of glacial growth were triggered by global cooling events.  They argue that glacial growth corresponds to small phases of increased monsoon strength interrupting a longer trend of decreasing monsoon strength. More moisture means more snow and glacial growth. Since the long term trend in this part of the world is one of decreasing monsoon strength, every successive phase of glacial growth was smaller than the previous, resulting in younger and younger deposits upstream. The Little Ice Age deposits (which were likely driven by global cooling and not necessarily increased precipitation) mark the last major phase of glacial growth.

How are these deposits dated? Scherler and colleagues use a technique known as cosmogenic nuclide dating. This technique is one way to date the timing of surface exposure. Glaciers carry rock debris. These form a layer below the moving ice. When the glacier recedes the rock debris is deposited as a moraine or as an erratic boulder. It is exposed to the atmosphere and starts getting bombarded by cosmic rays. Energetic cosmic ray neutrons falling on atoms of minerals like quartz results in spallation reactions. This means that the collision of neutrons is energetic enough to fragment the nucleus. Oxygen bound up with silicon in the mineral quartz gets converted to an isotope of Beryllium (10Be). The amount of nuclides generated this way is proportional to the length of exposure. By measuring the amount of 10Be and comparing it with other isotopes, an 'exposure age' is estimated. This is essentially the age of glacial recession and the deposition of glacial sediment.

Samples have to been selected carefully for this method to give a true estimate of surface exposure and deposition. Care must be taken to avoid sampling rocks that have been repeatedly buried and exposed. Rocks which show signs of being subjected to prolonged glacial erosion are selected since  erosion will remove outer shells of material that may have accumulated nuclides during an earlier period of exposure.  Debris with a polished surface or with striations and grooves generally suggest subglacial transport and prolonged glacial erosion and are preferred samples.

 The figure below taken from the same study shows the reconstructed glacial extents using exposure dates of the moraine sequences in the upper Tons Valley.


Source: Scherler et. al. 2010

Such dating of glacial deposits at other locations in the Garhwal Himalaya (ref) tell a similar story of glacial growth and decay over the Holocene. And what about the Pleistocene? Is there evidence of older glacial cycles in the Himalaya? There are many studies that have identified glacial phases during the Pleistocene as well. For example, in northwest Garhwal, the Bhagirathi Glacial Stage has been dated to 63 ka (ref). And in the Ladakh Himalaya the oldest glacial stage has been dated to 430 ka (ref). Pleistocene ice ages have impacted glacial dynamics in the Himalaya too although more work needs to be done to understand the specific mechanisms of glaciation.

Location 12: Its back to the Dhauliganga valley floor. This moraine ridge (arrows) may be the remnant of an older terminal moraine. It is located about 2 kilometers downstream of the glacier.


The Panchachuli and other glaciers in the Kumaon region to the east of the Garhwal will also have their own history of past glory and recession. How much of the retreat of the Panchachuli and other Kumaon glaciers due to recent global warming?  And what is its fate? Hopefully, someone will study them with more precision in the future.

Tuesday, August 15, 2017

Links: Human Evolution

Sharing links to interesting articles I have read in the past few months. Better understanding of human evolution is being driven by a) New fossil finds giving valuable insights into morphologic variation and geography, b) DNA analysis of both modern and extinct populations giving us an understanding of genealogical relationships and migration histories and c) better absolute dating of fossils that constraint evolutionary scenarios.

1) What Are Our Best Clues To The Evolution Of Fire-Making? Anthropologist Barbara J King examines the physical evidence of fire making by ancient hominins and presents speculations on how natural fires may have played a role in hominin cultural evolution.

2) A world map of Neanderthal and Denisovan ancestry in modern humans- Phys.Org. " There are certain classes of genes that modern humans inherited from the archaic humans with whom they interbred, which may have helped the modern humans to adapt to the new environments in which they arrived," says senior author David Reich, a geneticist at Harvard Medical School and the Broad Institute. "On the flip side, there was negative selection to systematically remove ancestry that may have been problematic from modern humans. We can document this removal over the 40,000 years since these admixtures occurred."

3) Three new discoveries in a month rock our African origins- Prof. John Hawks on new fossil dating of hominin fossils from Morocco and evidence from archaic DNA from S. Africa that complicates the African story of the origins of Homo sapiens. The scenario suggested here is that Homo sapiens did not evolve due to changes in a population which was genetically isolated from other Pleistocene African hominin groups. Rather there was a pan-African gene flow. This is multi-regionalism within Africa.

4) Out of North Africa- Dienekes argues the exact opposite.. that the Morocco fossils imply that Homo sapiens evolved in north Africa from a reproductively isolated population and that multi-regionalism is wrong.

5) Features of the Grecian ape raise questions about early hominins- Did the hominin clade evolve in Europe and not Africa? Prof. John Hawk's critique of a recent paper suggesting that view. He cautions that convergent evolution is common among different hominin lineages. A single feature, such as the mandible used in this paper, cannot indicate relationships.

6) Early modern humans in Sumatra before the Toba eruption- Steve Drury in Earth Pages summarizes new evidence that indicates early ( more than 70,000 years ago) migration of Homo sapiens into SE Asia. .." Together with the dating of the earliest Australians the Sumatran evidence is at odds with the view, widely held by palaeoanthropologists, that the ‘Out of Africa’ exodus began by crossing the Straits of Bab el Mandab between 74 and 58 ka when global sea-level fell markedly during marine oxygen-isotope Stage 4 (MIS4). A problem with that hypothesis has been that climatic and ecological conditions in southern Asia during MIS4 were unfavourable. But is seems that modern humans were already there and capable of adapting to both the climate shift and to the devastation undoubtedly caused by Toba."

Thursday, August 10, 2017

China Hydraulic Engineering - Yellow River And The Grand Canal

I am reading Philip Ball's excellent book The Water Kingdom: A Secret History Of China. It describes the epic problems of river flow management that China has grappled with over millennia. Enormous floods have always ravaged China. At the site of a large dam at Sanmenxia on the Yellow River is an inscription in honor of the Great Yu ( ~ 2200- 2100 B.C), who as the story goes,  conquered a flood. The inscription says " When the Yellow River is at Peace, the Nation is at Peace". Flood control required enormous civic resources and cooperation and the ability to tame river waters gave political and moral legitimacy to the ruling class.The taming of nature using cooperative people power and as a sign of a strong united society has deep roots in Chinese political thinking.

One particular vexing problem was the very high rates of silt load carried by the Yellow River. A vast area of the Yellow River watershed drains the Loess deposits of north central China. This is a plateau made up of loose friable sand and silt blown in from the Gobi desert of Mongolia. Erosion of Loess fills the river with sediment. There is 300 grams of sediment for every kilogram of Yellow River water giving the river a reddish golden color. Erosion has carved the Loess Plateau in to a landscape of ravines and gorges. The American journalist Edgar Snow in the 1930's described it thus:

" an infinite variety of queer, embattled shapes - hills, like great castles, like ranges torn by some giant hand, leaving  behind the imprint of angry fingers. Fantastic, incredible and sometimes frightening shapes, a world configurated by a mad God - and sometimes a world of strange surrealist beauty. "

High rates of sedimentation meant that the Yellow River bed could aggrade or rise, increasing the risk of the river breaking its banks and flooding the countryside. Dyke building to constraint the river channel began as early as the seventh century B.C. by the state of Qi.

Constant dredging of the Yellow River and the associated tributaries and canal systems ( the Grand Canal) was also required to maintain a channel deep enough for navigation to move armies and grains from south to north.  River channel and canal maintenance acquired a new urgency when Zhu Di known as the Yongle Emperor of the Ming dynasty moved the capital north from Nanjing in the eastern province of Jiansu to Beijing in the early 1400's. The rational was probably to keep the political center closer to the armies amassed on the northern frontier where the Ming faced a threat from the Mongol and Manchurian steppe people. Later in the 1600's, the Manchurians overthrew the Ming and established the Qing Dynasty. Although over time, the Qing assimilated into the larger Han cultural milieu, they felt more at home in the north of the country. That meant the Yellow River and the Grand Canal system had to kept in top navigable order.

Desperation to unclog  the river channel spurred technological innovation. 

An extract:

Removing silt from the Yellow River demanded some impressive technology, not to mention serious organization. The Song government set up a Yellow River Dredging Commission in 1073 which began to deploy boats equipped with dredging tools. The vividly named  "iron dragon-claw silt dispersing machine" was a great rake pulled along the riverbed to agitate the silt and return it to the flow. This principle was extended with the 'river-deepening harrow', a 2.5 metre-long rotating beam fitted with iron spikes, like a thresher for riverine mud. The Ming imperial censor Chen Bangke introduced new techniques in the late sixteenth century, such as wooden machines set rolling and vibrating by the current to constantly stir up the sediment. In the dry season Chen proposed simply digging out the silt manually.

The Ming official Pan Jixun in 1565 or so came up with a solution that has made him one of China's water heroes. He pointed out that if one confines the water flow to a narrow channel, it will have enough strength to scour the sediment off the river bed. There would be minimal need for laborious manual dredging. He may have borrowed the  idea from a Confucian text of the Han era (~200 B.C - 220 A.D) called Zhou li (Rites of Zhou) which stated "A good canal is scoured by its own water"

Chinese philosophical tradition impacted river management strategies. Daoists argued that the river be given room to spread and build wide floodplains in concert with the principle of wu wei which could be read to mean "do nothing" or "having a yielding attitude".  Confucians on the other hand wanted the river to be managed by human  engineering and recommended the construction of high dykes to keep the river channel narrow and constrained.

Joseph Needham, the noted historian of China writes that 'during twenty centuries the two schools contented'.. 'and neither proved wholly successful'.

Highly Recommended.

Friday, July 28, 2017

The Lost Rivers Of The Harappan Civilization- Remote Sensing Analysis

A lot of ink has been spent of this topic, both in the scientific literature as well as in popular books about the Harappan Civilization. The focus of many of these efforts has been on locating the "Vedic Saraswati", a river mentioned in the Rig -Ved. It is described as occupying the region between the Yamuna and the Sutlej and has been identified by many workers as the present day Ghaggar-Hakra.

Settlements of the Harappan Civilization were spread out over quite a large area in northwest India. Hence, it is necessary to map in detail the broader once existing hydrologic networks to assess the relationship between settlements and patterns of water availability and water use. This study uses remote sensing data and image processing techniques to unearth some of the buried paleo-river networks of this region of northwest India.

Twenty eight years of Landsat 5 imagery totaling 1711 multi-spectral images was bulk processed. This use of data covering multiple dates in a year allowed the investigators to reduce visibility issues related to shifting land use and cultivation, changing moisture patterns and variable cloud cover. 8000 km of paleo-river channels were mapped, some identified in previous studies, but many recognized anew.

The paper describes the various image processing techniques used to tease out the spectral signatures of the buried rivers. Don't be alarmed by words like Normalized Difference Vegetation Seasonality Index (NDVSI), Principal Component Analysis and the Tasselled Cap Transform.  One can understand the final interpretations without getting into the details of these techniques. Let me give a brief idea though.

These are techniques has increase the contrast between vegetation, soils with different moisture levels and bare terrain. For example, due to stronger water flow the river channel itself and the levees it builds contain coarser sediment as compared with the adjoining floodplains. These coarser sediment contain less organic matter and are less fertile. Less vegetation grows on the buried channels and levees than on the more fertile fine grained floodplain sediment. This contrast shows up well on data processed using NDVSI which essentially maps the vigor of vegetation. This technique was useful in delineating channels in the northern part of the study area.

The southeast part of the study area is more arid and has less changes in vegetation across the landscape. Here, the Tasselled Cap Transform was more useful in identifying river channels. This techniques organizes the spectral information into several (usually three) main axis or bands of information. One axis contains data variability of reflectance of bare terrain (mineral mixtures). The second contains variability in greeness (vegetation), and the third contains variability of wetness (water and soil moisture). Again, to give an example, this southeast region is fed by rivers draining Aravalli limestones. The calcium carbonate leached away from the rock is precipitated along river channels as a chalky mineral deposit. Buried channels show up as ribbons of  bright reflectance in  the Tasseled Cap Transform brightness band. Imagery of the dry months shows these channels more clearly, since in the wet months, increased soil moisture reduces the contrast of calcium carbonate rich sediment.

A judicious use of such techniques thus enabled the researchers to identify buried networks with different vegetation, soil moisture and mineral brightness properties. 

A map of the interpreted buried rivers is posted below.


Source: Hector A. Orengo and Cameron A. Petrie 2017

Of importance is a critique of some previous studies which attempted to highlight the spatial relationship between river channels and archaeological sites (emphasis mine):

The data provided by these analyses are also important in contextualising previous studies in which palaeo-rivers have been dated using the distribution of known archaeological sites. Notwithstanding the positional accuracy of these locations (see [10,63,64]), which would severely hamper their use for validation purposes, the results for the northern sector of the study area (Figure 3) suggest that proximity to the river might not be a good indication of contemporaneity as the fields close to the river channel might not have been the most productive in agricultural terms. Sites with an agricultural orientation might have preferred to occupy elevations above flooding level in the finer sediment accumulation area. Flooding events and changing river courses could also have had an important effect in the preservation of archaeological sites eroding and burying those that were locatedin the path of new courses or in their sediment accumulation areas.

In addition to these factors, the scale at which these correlations between specific palaeo-channels and settlement locations have usually been published (e.g., [30] (pp. 359–387), [31] (Figure 4.2)]) do not allow the accurate correlation between the two elements. The use of large area (small scale in a geographic sense) site distribution maps for these correlations results in a visual association between the shape of the palaeo-river and the line formed by the grouping of sites, but at these scales the sites could be aligned to a number of palaeo-channels given the number of rivers and the parallel morphology of the drainage in the Sutlej-Yamuna interfluve revealed here. This analysis thus suggests that at these scales it is not possible to co-relate lineal distribution of archaeological sites to any particular palaeo-river that we have documented. The results from previous studies reconstructing the chronology of the hydrological system using the position of archaeological sites and vice versa(e.g., [24,25], [30] (pp. 359–384), and [31]) are, therefore, considered unreliable.


..and

Given the complexity of the hydrological system, the variety in the climatic and weather system of this region, and the diversity of ways that ancient populations are likely to have obtained water, it is unwise to use the date of occupation at specific settlements to date when specific channels carried water. It is essential to date the different palaeo-courses independently to properly reconstruct the evolution of hydrological networks over long periods. The chronologically consistent reconstruction of this palaeo-river network would allow the testing of different hypothetical scenarios of water availability through the use of network analysis in combination to hydrological analysis.

Interesting work. Open Access.

Saturday, July 22, 2017

Tsunami History Preserved In Indonesian Cave Deposits

How would you know if a coastline had been inundated by a tsunami say 5000 years ago? Well, a tsunami carries sediment stripped from the ocean bed and deposits this material over the flooded coastline, beyond the range of what a regular storm would. The problem is that such deposits have poor preservation potential and over time get eroded away. There are however some environments where such tsunami deposits may get preserved inland. These are estuaries, coastal marshes and lakes. Here, interlayered with normal estuarine, marsh or lacustrine sediment, one may find layers of sand of a distinctly different composition and texture and containing remains of organisms which live in an open marine setting. This implies a sudden incursion of marine waters into these inland coastal settings. The other coastal setting with a good preservation potential are caves. These too get flooded by storm surges and tsunamis and may preserve a record of such events in the form of sand deposits. The picture to the left (Source: Rubin et.al. 2017)  shows sand layers deposited by the 2004 tsunami.

In one such cave on the coast of Aceh, Indonesia a record of the 2004 tsunami along with sand layers deposited by 11 older tsunamis going back to 7400 years ago have been preserved.

Highly variable recurrence of tsunamis in the 7,400 years before the 2004 Indian Ocean tsunami-
Charles M. Rubin, Benjamin P. Horton, Kerry Sieh, Jessica E. Pilarczyk, Patrick Daly, Nazli Ismail & Andrew C. Parnell

Extract:

 We identify coastal caves as a new depositional environment for reconstructing tsunami records and present a 5,000 year record of continuous tsunami deposits from a coastal cave in Sumatra, Indonesia (Fig. 1), which shows the irregular recurrence of 11 tsunamis between 7,400 and 2,900 years BP. The sedimentary record in the cave shows that ruptures of the Sunda megathrust vary between large (which generated the 2004 Indian Ocean tsunami) and smaller slip failures. The chronology of events suggests the recurrence of multiple smaller tsunamis within relatively short time periods, interrupted by long periods of strain accumulation followed by giant tsunamis. The data demonstrates that the 2004 tsunami was just the latest in a sequence of devastating tsunamis stretching back to at least the early Holocene and suggests a high likelihood for future tsunamis in the Indian Ocean. The sediments preserved in the costal cave provide a unique opportunity to refine our understanding of the behaviour of the Sunda megathrust, as well as study in detail the sedimentology and hydrological characteristics of tsunami deposits.

There is one point that cannot be over stressed. The average recurrence time for earthquakes and tsunamis has been estimated to be on the order of several hundred years. However, there is a great variation in the actual occurrence, with several smaller tsunamis occurring just decades apart. While our understanding of earthquake mechanisms and tsunami generation will go on improving, ultimately what will save lives is better preparedness. This includes adherence to structurally appropriate building codes, functioning tsunami warning systems and well drilled and practiced disaster management plans. South East Asia has long neglected these issues and there needs to be a renewed focus on them.

Sunday, July 16, 2017

Olivia Judson On Energy Expansions Of Evolution

Nature Ecology and Evolution has published a fine perspective by evolutionary biologist Olivia Judson on energy availability and evolutionary transitions on earth -

" The history of the life–Earth system can be divided into five ‘energetic’ epochs, each featuring the evolution of life forms that can exploit a new source of energy. These sources are: geochemical energy, sunlight, oxygen, flesh and fire. The first two were present at the start, but oxygen, flesh and fire are all consequences of evolutionary events. Since no category of energy source has disappeared, this has, over time, resulted in an expanding realm of the sources of energy available to living organisms and a concomitant increase in the diversity and complexity of ecosystems. These energy expansions have also mediated the transformation of key aspects of the planetary environment, which have in turn mediated the future course of evolutionary change.Using energy as a lens thus illuminates patterns in the entwined histories of life and Earth, and may also provide a framework for considering the potential trajectories of life–planet systems elsewhere."

Coincidentally, I just finished reading Nick Lane's book The Vital Question, which covers the first three sources of energy discussed in this article. Nick Lane writes about energy currencies of the cell and the constraints it places on the early evolution of life on earth. Why don't bacteria become morphologically larger and more complex?... because there are intrinsic constraints on the energy available for ATP synthesis.  You'll have to read Nick Lane's book for a detailed account but Olivia Judson's essay mentions this and more. The other two, animals and fire, encompass the evolution of complex multicellular life and their impact on evolutionary arms races and ecosystem changes.

..and what about life on other planets?..

"As this is the only life–planet system we currently know of, it is impossible to know how representative it is of life–planet systems in general. But if the development of other life–planet systems requires a similar series of energy expansions, the framework presented here suggests a way to anticipate the paths that such systems might take. For instance, if a planet has only geochemical energy— perhaps because it is far from its star, or because it is a nomad and has no star at all—any life present may have “a limited future in terms of the heights it could achieve”. Or suppose a planet is unable to accumulate oxygen. This could happen if living organisms never evolve a way of splitting water to produce the gas in the first place, but even if they do, the planet itself may have characteristics that prevent oxygen from ever building up. Without oxygen, the geological, ecological and evolutionary potential of a life–planet system is likely to be constrained, even if life forms analogous to eukaryotes in their energy-harnessing power (Box 2) were to evolve. Conversely, some planets might be able to accumulate new forms of energy, and life forms able to take advantage of them, much fasterthan Earth has."

Open Access.

Saturday, July 8, 2017

Field Photo: A Bend In The Rocks

I saw this textbook example of a fold in the Lassar Yankti valley, about 2 kilometers south of Tidang village in the Kumaon Himalaya.


Consider how rocks bend and deform in response to stress. Blue arrows denote the direction of maximum compressive stress perpendicular to the fold axis. As rocks fold, the convex portion of the fold will experience tensile forces and fractures parallel to the axial plane develop. Notice also conjugate stress fractures (black arrow). Since this is a loose boulder I cannot assign actual directions to the stress field.

The graphic below summarizes the typical fracture patterns found in folded rocks. How many of these can you identify in the fold above?


Source: Applied Hydrogeology of Fractured Rocks

My Himalayan treks over the past few years have taken me on a walk across almost the entire thickness of the Greater Himalayan Sequence. As I mentioned in an earlier post, the GHS is bounded at its base by the Main Central Thrust and at the top by the South Tibetan Detachment. It shows an "inverted" metamorphic sequence. This means that the grade of metamorphism increases as one climbs to higher structural levels. Finally, sillimanite and kyanite grade metamorphic rocks transition into a zone of partial melting and leucogranite intrusions. Above this level the grade of metamorphism decreases to biotite grade and then to a finer grained phyllite grade. One conspicuous structural feature of the GHS is that large folds are very rare. Instead, from the base right up to the zone of partial melting the GHS exhibits a homoclinal northerly dip as seen in the picture below.


Within these northerly dipping slabs, small scale ductile folding in high grade gneiss and migmatites can be seen (picture below), but the slabs themselves are not contorted into mountain face scale folds.


Large isoclinal and recumbent folding is present only in the uppermost structural levels of the GHS in the phyllite grade rocks above the zone of partial melting. The picture below shows tightly folded phyllite grade metamorphic rocks north of the village of Baaling in the Darma Valley.

 
And this splendid recumbent fold is exposed at village Dantu.


Why is large scale folding rare to absent over much of the thickness of the GHS? Could the movement of the South Tibetan Detachment cause folding in the underlying phyllites?

These are some of the niggling questions I am struggling with. I still have much to learn about Himalayan geology. I need to go there with a structural geologist!

Finally, a view of the outcrops from which was derived the textbook quality folded phyllite.


Tuesday, June 20, 2017

Quiz: Spot The Granite Intrusion

I came across this glacially transported boulder in the Dugtu village valley near the Panchachuli Glacier in the Kumaon Himalaya.

It is a block of high grade gneiss intruded by a granite. Without scrolling beyond the first photograph, try to work out the contact between the gneiss and the granite.



Answer:

The boulder is encrusted by moss. There is some mineral staining too. And sunlight falling on the rock gives it a speckled appearance.. All this reduces the contrast in color between the gneiss and the granite.

But there is a vital clue in the orientation of structures. Both the gneiss and the granite have a planar fabric imprinted on them.

The fabric of the gneiss is due to the orientation of platy minerals like micas stacked in layers, alternating with layers richer in quartz and feldspars. Assume this is the original disposition of the rock as well. The gneiss layering you see is due to the trace of horizontal planes of separation of different mineral layers. I have outlined some of this planar fabric in brown lines.

The granite has a planar fabric too, but this is due to near vertical fractures. The rock has been broken in to thin slabs  by fractures (red lines) which may have formed during the cooling of the magma. These fractures don't pass into the surrounding host gneiss. Two arms of the granite have penetrated between the gneiss layers forming mini sills.

You can see the contact (black line) between the gneiss and the granite roughly where my wallet is. Here, the horizontal planar fabric of the gneiss abruptly juxtaposes against the vertical planar fabric of the granite.



Thursday, June 15, 2017

Field Photo: Glacial Erratic

Inspired by this xkcd comic:


I saw quite a few of these glacial erratics in the Dhauliganga river valley around the villages of Dugtu and Dantu. Here is my friend sitting on one of them.


This boulder is a high grade gneiss. It is an erratic because the surrounding bedrock is all low grade phyllite and slate. The source of the high grade gneiss boulder is the snow capped range you see in the background. These are the Panchachuli peaks and the Panchachuli glacier has eroded, transported and deposited gneiss rocks all the way down the valley onto a different bedrock.

The photo below shows another erratic from this valley. If you look closely it is a mixed rock made up of high grade gneiss intruded by light colored granite. A big patch of dark grey banded gneiss is visible in the lower right corner of the boulder. The cliffs in the background and the substrate on which the boulder rests is low grade phyllite.


And a long view of village Dugtu with glacial erratics strewn all over the hill slope (blue arrows).


I have been promising a post on the glacial deposits of the Dhauliganga river valley. That post will come soon. Meanwhile, here is a view of some of the moraines I saw near village Dugtu.  Photo taken from near the snout of the glacier facing downstream.


The linear ridge in the center of the photo made up of rust, brown and light colored boulders is a medial moraine. It was formed when two glacial streams carrying debris along their edges joined. As these glaciers receded the debris along their edges (lateral moraines) coalesced and formed a ridge in the center of the valley. You can see the milky white colored Dhauliganga river flowing to the right of the ridge. The blue arrows to the right of the picture high up along the mountain slopes point to older lateral moraines deposited when the Panchachuli glacier was thicker and extended further down in the valley...

more on these deposits later..

Tuesday, June 13, 2017

Books: Origins Of Complexity ; China Water History

These just arrived.

Extract:

I hope to persuade you that energy is central to evolution, that we can only understand the properties of life if we bring energy into the equation..... I want to show you that the origin of life was driven by energy flux, that proton gradients were central to the emergence of cells, and that their use constrained the structure of both bacteria and archaea. I want to demonstrate that these constraints dominated the later evolution of cells, keeping the bacteria and archaea forever simple in morphology, despite their biochemical virtuosity. I want to prove that a rare event, an endosymbiosis in which one bacterium got inside an archaeon, broke those constraints, enabling the evolution of vastly more complex cells. .....Finally, I want to convince you that thinking in these energetic terms allows us to predict aspects of our own biology, notably a deep evolutionary trade-off between fertility and fitness in youth, on the one hand, and ageing and disease on the other.

The last book I read on the evolution of complexity was Mark Ridley's The Cooperative Gene which described the many evolutionary inventions that suppress genomic conflict and make multicellular bodies workable. Nike Lane writes at a more fundamental level of the energy currency of the cell. Feeling very excited about this book. I am sure to learn a lot.

Extract:

But the ubiquitous and ambivalent relationship that the Chinese people have had with water has made it a powerful and versatile metaphor for philosophical thought and artistic expression, and its political connotations can be subverted and manipulated in subtle ways for the purposes and protest and dissent. These meanings of water are more than metaphorical. Because the lives of everyday folk has always depended on water, the river and canals mediate their relationship to the state. Water -too much of it,or too little - has incited the people to rise up and overthrow their governments and emperors. Burgeoning economic growth now places unprecedented pressure on the integrity and sometimes the very existence of China's waterways and lakes. Not only can China's leaders ill afford to ignore this potential brake on economic growth, but the environmental problems are leading to more political pluralism in a nominally one party state.

Sweeping... from the Qin Dynasty (200 B.C.) to the present..

Thursday, June 1, 2017

The Serpents Of Nagling- Granite Intrusions Into Greater Himalayan Sequence Metamorphics

Over chai, elders told us about large serpents invading their village. A curse, they said. Only the correct prayers and purification rituals saved them, forcing the serpents to retreat deep into the forest. Some serpents remain trapped in the rock faces near the village, which was renamed Nagling (Nag means cobra..or more generically serpent).

The picture below are the entombed serpents of Nagling (trekkers for scale).


Geologists recognize them to be granite dykes (intrusions cutting across host rock layering) and sills (intrusions parallel to host rock layering) intruding the high grade metamorphic rocks of the Greater Himalayan Sequence (GHS).

The GHS is a block of the Indian crust bounded between the Main Central Thrust (MCT) at the base and the South Tibetan Detachment System (STDS) at the top. It represents mid crustal material which was metamorphosed and then was extruded and exhumed during Himalayan orogeny between 25 million years ago to about 16 million years ago. These dates vary somewhat along the strike of the Himalaya. Thrusting along the MCT took place earlier in the western Himalaya. Eastern regions like the Sikkim Himalaya record younger dates for the movement of the MCT.

The grade of metamorphic varies within the GHS. The figure below is a schematic section of the Greater Himalayan Sequence. It is from a study on the nature of the MCT by Michael Searle and colleagues from the Nepal Himalaya and is a very useful guide to think about the internal structure of the GHS.


 Source: Searle et. al. 2008

From the base of the MCT the grade of metamorphism increases towards higher structural levels. This is recognized as an "inverted metamorphic gradient", since minerals that are formed at higher and higher temperatures and pressures are occurring at structurally higher and by implication apparently shallower levels of the crust. The inverted gradient is recognized by the successive appearance of  biotite, garnet, sillimanite and finally kyanite. The sillimanite-kyanite zone transitions into the zone of partial melting and granite intrusives. This is the zone where the crust experienced conditions that lead to the formation of in situ melts and their mobilization and intrusion into surrounding rock. Above this zone the grade of metamorphism reduces towards the STDS. In the figure, the granite intrusion zone is directly overlain by the STDS and the Tethyan sequence. However, there is variation in this theme across the Himalaya. In the Kumaon region where I was, the "melt zone" is overlain by a sequence of lower metamorphic grade phyllite rocks.

What caused this melting and production of granitic magma? Many geologist point to the STDS. They suggest that this zone of extentional faulting stretched and thinned the crust, resulting in " decompression-related anatexis". This means that when extentional faulting along the STDS and exhumation reduced the overburden on deeply buried hot rocks, the release in pressure resulted in the lowering of rock melting point. This led to a partial melting of the crust (anatexis). Other geologists disagree with this explanation. They point out that since decompression has a minor effect on melting the likely source rock compositions you would require unreasonably large amounts of denudation along the STDS.  Rather, they suggest that crustal thickening by the continued convergence of India with Asia elevated temperatures in the middle levels of the crust to a range where partial melting began. These melts then moved along weak planes and intruded the surrounding GHS above the sillimanite and kyanite grade gneisses. The main pulses of this magma generation took place between 24 million years and 19 million years ago.

Geologists estimate the temperatures of this melt zone to be around 650 deg C to 750 deg C, corresponding to a  burial depth of about 20-25 km. Yes, the GHS represents crust that has traveled from that depth to the Himalayan heights it now commands by a combination of thrust faulting and erosional unroofing i.e. the stripping away of shallower levels of the crust!

During one of my previous treks in the Kumaon region I had walked across the GHS from the base of the MCT to the sillimanite zone in the Goriganga valley from the town of Munsiari to village Paton. This time, one valley to the east,  we began our trek at village Nagling in the zone of  partially melting. All around us were rock faces intruded by sill complexes and dykes. The picture below shows multiple sills of granite cross cut by dykes.


High up from Nagling village towards Nagling Glacier I saw this granite dyke complex (outlined by red dotted lines ) cutting across metamorphic banding (black lines).


And in the stream near Nagling Glacier I came across this rounded stream boulder showing granite cross-cutting banded migmatitic gneiss.


We traveled north and  reached Duktu. Earlier, somewhere near the village of Baaling, we had crossed the zone of partial melting and were in the uppermost levels of the GHS made up of phyllite grade metamorphic rocks. The phyllites are not intruded by granite.

However, granite was present at Dugtu too, but only in the Dhauliganga river bed. This river emerges from the Panchachuli Glacier. The Panchachuli ranges which fall lower in the GHS are made up of high grade gneiss intruded by granite.

As a result, the Dhauliganga river bed near Duktu village is choked with boulders of granite and migmatite rocks.


This is a very distinctive  biotite-tourmaline granite. The picture below shows blocks of granite with tabular black tourmaline.


Here is a picture of me looking intently at a block of GHS made up of a granite intruding in to a gneiss.


And another close up of light colored granite intruding dark grey banded gneiss and encircling and enclosing rafts of the metamorphic host rock (red arrows).


And finally, from the sheer rock faces near Nagling Glacier, one of my favorite examples of the granite intrusions. A near vertical dyke (red broken outline) cut and displaced by a fault (yellow broken lines). Metamorphic banding shown in black lines.


... Pleistocene-Holocene glacial deposits of the Panchachuli Glacier area.. coming up next!