Thursday, January 10, 2019

Cracks In A Rock And The Western Ghat Escarpment

A friend sent me this picture of a section of the Western Ghat escarpment. It is taken from Jivdhan fort, looking north towards the hook nose of Naneghat. This location is about a hundred odd kilometers west-north-west from Ahmednagar town. Naneghat was a mountain pass for travel between the coastal plain and the plateau.

Photo credit: Rajesh Sarde

The yellow bloom makes a pretty contrast with the grey basalt. My geology eye was drawn towards something else; a suspiciously straight flowing stream, which I have highlighted with an arrow.

I looked at a satellite imagery of this location and the stream is seen following a fracture zone (black arrows)  that cuts across Jivdhan fort as well. The escarpment area is riddled with such fractures. They occur as north-south, northwest-southeast, and northeast-southwest (brown arrows) trending sets.

These fractures are regions of shattered rock. That zone erodes away quicker. Water flowing in the linear depressions that form enhance this topographic difference and eventually cut deep straight valleys.

Large fractures or cracks along slopes causes slabs of rocks to cleave away from mountain sides. Slopes retreat due to such rock falls. A large crack is seen in the picture just a few feet away from where my friend took his photograph. At some point a portion of rock will detach itself and Jivdhan fort will become that much narrower.

Look at the zoomed out satellite imagery of this area. The plateau edge has been fragmented into isolated hillocks, mesas and pinnacles by enhanced erosion along fractures oriented in various directions. You can follow some of these fractures (white arrows) to the straight edges of the escarpment suggesting that slab breakoff has played a role in shaping the morphology of the cliff line.

Such fracture systems not only have formed a landscape of mesas and pinnacles but have caused the Western Ghat escarpment to retreat eastwards for at least tens of kilometers from its original location. The escarpment is a legacy of the breakup of the western margin of India with Seychelles at the end of the eruptions of the Deccan Basalts. At that time in the Paleocene (~60 million years ago), continental stretching caused the formation of a series of north-south oriented faults which sloped (dipped) to the west. The westerly block of each of these fault sets sank, created a staircase like crustal structure descending towards the west, with west facing cliffs. The Western Ghat escarpment would have been the easterly most of these cliffs.

See the schematic below which shows this staircase crustal structure of the western margin of India.

The red portion would have been the original extent of the Deccan plateau. It has retreated eastwards over several millions of years. As a result, the coastal plain became progressively broader. Give a thought to the humongous amount of rock that has been removed by erosion.

Along the west coast the erosional  retreat has not wiped clean all evidence of the original plateau. From the coastal plain rise isolated ranges and mesas. The hill station Matheran, where people go to catch the cool wind and a spectacular view, is a fine example.

See the satellite imagery below.

Matheran was where the plateau edge and escarpment once was. It has now moved eastwards (arrows) leaving behind an erosional remnant,  a splendid outlier of the Deccan plateau rising abruptly from the plains.

Let's end with a 3D view of the escarpment along the Jivdhan-Naneghat area.

If you take a flight out of Pune to Delhi, the plane will fly a northerly route parallel to the plateau edge for the first 20-25 minutes of your journey. The Western Ghat escarpment appears as it does in the tilted perspective above, a sinuous line of majestic black cliffs, testimony to the forces of volcanism, continental breakup, and erosion.

A section of this stunning landform deserves to be included in our National Geological Monuments list.

Monday, January 7, 2019

Human Evolution: Focus On Africa

In a lecture delivered to the American Society for Human Genetics, paleo-anthropologist John Hawks gives a lucid summary of the African record of human evolution.  The divergence of the hominin lineage from other apes took place in Africa between 5 and 10 million years ago. Hominins began dispersing out of Africa in pulses beginning 2 million years ago. The vast majority of hominins though continued to live and evolve in Africa. Yet, popular stories of human evolution focus on people leaving Africa and colonizing the world. What has been happening in Africa all along gets sidelined in this narrative.

The “out of Africa” slogan came from well-intentioned scientists. They thought that by emphasizing the idea of an African origin, they would send a clear message that Africa had an important place in evolutionary narratives. That much is true. Africa was the center of human origins. But “out of Africa” stories focused almost exclusively on dispersal, as if it were an exodus. Africa’s place in these stories was the place that people left.

John Hawks refocuses our attention on the African fossil and genetic record that tells us that Africa always has occupied a central place in our evolutionary story.

He points out that this record has yielded three big insights:

First, modern humans did not originate in a bottleneck after 200,000 years ago. Our origin was much deeper in time than this.

Second, our species originated in Africa from deeply structured ancestral populations. These were much more different from each other than any human populations are today. We do not know how they interacted or which gave rise to living peoples.

Third, some of these deeply divergent populations survived in Africa until recent times. During the time of human origins, “modern” humans were not alone. 

The term bottleneck means that at some time in our past there was a drastic reduction in our population size and genetic variability.

Anyone interested in the topic of human evolution should read this article.

Wednesday, December 26, 2018

Extreme Fieldwork In The Karakoram Mountains

This remarkable passage from Colliding Continents by Mike Searle:

"After two weeks of acclimatizing on the Lobsang Spire and Cathedral granite cliffs above camp and establishing our attack camp full of supplies we were ready to go for the summit. I was keen to climb a line up through the granite cliffs in order to map out and sample a vertical profile through the granite batholith. These Karakoram granite spires provide a unique opportunity to map and sample over 3 kilometers deep into such a batholith. We left once again at 3 a.m. for the dangerous plod through the icefall, and arrived at the ice-shelf camp at about 10 a.m. As soon as the sun came on to the glacier, freezing night-time temperatures soared up to incredibly high hot temperatures above 350C. Frozen icicles dripping off the rock face turned into trickles of water and then into torrents. Huge avalanches of powder snow exploded down the steep granite faces all around us. This was nature in the raw: powerful, frightening, but at the same time immensely beautiful.

Next morning we left at the usual 3 a.m., roped up, and started climbing the steep ice face on the south face of Biale. Very soon the ice petered out and we were on vertical solid granite. Climbing vertical granite walls with a 20 kg rucksack, a rack full of slings and nuts and big plastic double boots was not easy. I was trying to record geological observations in the granites and put those onto a map at the same time; the sample collection was to be done on the way down. After two days of this steep and scary climbing we finally broke out of the cliffs onto a  large snowfield that led up to a knife-edge ridge. As we approached the ridge the most spectacular mountain panorama I have ever seen unfolded in front of us. We were in the middle of the Karakoram, with huge glaciers flowing all around us, separating ridiculously steep cliffs of pure granite".

The Himalaya ranges are the northern edge of the Indian continental crust which was deformed during the India-Asia collision. Similarly, the Karakoram ranges are the southern margin of the Asian continent which was deformed during the India-Asia collision. Mike Searle and his colleagues were trying to work out their deformation, metamorphic and uplift history.

In the satellite imagery below, I have overlain the major lithologic and structural elements of the India-Asia collision zone. The imagery covers the central and eastern Karakoram ranges.

The southern margin of the Asian plate is made up of two amalgamated terrains. In the mid Cretaceous (~110 million years ago) a separate terrain/microplate known as the Kohistan-Ladakh block existed south of the Asian continental mainland. As the Indian plate pushed northwards, oceanic crust which made up its leading edge dove under the Kohistan-Ladakh terrain. As the dense crust subducted, it heated up, releasing water trapped in the sediment cover. This water migrated upwards, lowering the melting point of rocks in the lower part of the Asian plate (mantle) and triggering melting. Magmatism and volcanism formed an intra-oceanic island arc system above the subduction zone. The Dras Arc Volcanics are part of this island arc, which continues westwards into Kohistan. The Ladakh batholith represents the root of this arc system, made up of granitic and granodioritic magma congealed in the deep subsurface. Volcanic rocks at the famous Khardung-La Pass are the surficial expression of the Ladakh magmatic system.

Between 70-80 million years ago, the Kohistan-Ladakh arc collided with the Asian mainland forming a composite terrain. The Main Karakoram Thrust-Shyok Suture marks the zone of collision between these two terrains. The Indian plate continued subducting under the Asian plate. Magmatic growth of the Kohistan-Ladakh batholith continued until about 50 million years ago. By this time, the Indian oceanic crust had been consumed and as a result the Indian continental crust collided with Asia. This collision zone is marked by the Indus-Tsangpo Suture.

The pre-collision tectonic setting of the Kohistan-Ladakh arc is depicted in the graphic posted below.

Source: Searle et. al. 1999

There is another model which proposes that the Kohistan-Ladakh arc first collided with the Indian plate around 85 million years ago. Collision of this combined plate with Asia then took place around 50 million years ago. I am ignoring this debate in this post.

Continental collision lead to crustal thickening on both sides of the suture zone. In the Karakoram region, high temperatures and pressures in the lower parts of the thickened crust resulted in metamorphism of buried rocks (Karakoram Metamorphic Complex). Eventually, temperatures in the buried crust exceeded the melting point of rocks, triggering magma generation. These post-collisional crustal melts intruded the surrounding metamorphic rocks forming sills (intrusions parallel to the layering) and dikes (intrusions cutting across the layering). Granites formed in this manner are most conspicuously exposed in the region around the Baltoro Glacier. These granites form many of the jagged spires and pinnacles that Mike Searle describes so vividly in his book. They are part of the Karakoram batholith.

Crustal melting and granite magma formation occurred in the thickened Indian plate as well, south of the Indus-Tsangpo suture zone. These granites are exposed along the crest of the High Himalaya ranges. The Himalayan post-collisional granites are beautiful to look at.  Faceted crystals of black tourmaline and red garnet, along with gleaming flakes of white mica, are set in a white to pink colored matrix made up of quartz and feldspar. The picture to the left shows a tourmaline bearing leucogranite from the Greater Himalaya Sequence making up the Panchachuli range in Kumaon, Uttarakhand. I collected it from the moraines of the Panchachuli glacier.

The Karakoram Batholith is a composite body made up of magmas formed during different times. Older granitic rocks (150-70 million years ago) in this batholith formed in an Andean type margin setting wherein the Tethyan ocean crust was subducting under the Asian continental plate.

Certain metamorphic minerals like kyanite, sillimanite and garnet are thermobarometers. Their elemental ratios tell us about the temperature and pressure prevalent during mineral growth. Geologists estimate that the sillimanite and kyanite bearing Karakoram metamorphic rocks were formed at 35 km depth. These metamorphic rocks, along with the earlier formed granites of the Karakoram batholith, eventually partially melted to form the Baltoro granites.

Geologists have also been able to work out when metamorphism and melting took place. Using radiogenic uranium isotopes trapped in zircon crystals they find that peak metamorphism took place in several pulses. The oldest metamorphic event occurred around as early as 60-65 million years ago, with the heat source likely being the collison of the Kohistan Arc. Subsequent metamorphic pulses, driven by crustal thickening,  have been dated to about 45 million yeas ago and an even younger event to about 16 million years ago.

Magmas which form the Baltoro granites intruded and crystallized between 20 million to 13 years ago, similar in age to the post collisional leucogranites of the High Himalaya which are 24 million to 15 million years old . Crustal melting and granite formation in the Karakoram continued sporadically until around 9 million years ago.

The deep crustal processes occurring in the Karakoram region were also taking place eastwards in the thickened crust below Tibet. In the Karakoram, metamorphic rocks and granites formed in the lower levels of the crust now lie spectacularly exposed along the steep mountain sides. Erosion over the past 30 million years has removed over 35 km of overburden, exhuming these deep crustal layers. The Tibetan plateau however is made up of sedimentary and volcanic rocks. Only the uppermost parts of the crust are exposed here. Karakoram equivalent high grade metamorphic rocks and crustal melt granites, which undoubtedly exist in this region, still lie deeply buried in the subsurface.

Why is there such a difference between Karakoram and the Tibetan Plateau in the level of crustal exposure?  Take a look at the satellite imagery. Karakoram is covered by snow, while much of the Tibetan Plateau lies bare. Karakoram receives rain and snowfall from both the India summer monsoon as well as the north westerlies bringing winter moisture from the Mediterranean and Caspian areas. Tibet lies in the rain shadow of both these systems.  High rates of stream incision and the enormous erosive power of glaciers has resulted in high rates of exhumation in the Karakoram, eventually exposing deeply buried crust. Low erosion rates in the arid Tibetan region has failed to cut deeply into the crust, resulting in exposure of only the uppermost levels of the crust. Both Karakoram and Tibet have an average elevation of about 5000 meters. In the Karakoram though there is prolific relief, with peaks in the 7000-8000 m range and valleys at 2000-3000 m elevation. Relief in Tibet is subdued.

Climate plays an enormous role in shaping the topography and evolution of mountains.

Tuesday, December 18, 2018

Interviews: Meteorite Researcher And A Palaeontologist

Came across these two interesting interviews with a meteorite researcher and a paleontologist.

Meenakshi Wadhwa grew up in Chandigarh, North India. She wanted to study architecture. She ended up being a meteorite researcher. Quanta Magazine highlights her path from college to Director of the Center for Meteorite Studies at Arizona State University.

I totally related to this!

Applying from India, at a time when there was no internet, I had the Barron’s guide to graduate schools in the U.S., which was outdated by like 10 years at that point. I didn’t care about geography or any of that. I didn’t care if it was East Coast or West Coast or the Midwest. It was all half a world away.

.. and this was pretty amazing-

We get something like 100 tons of stuff falling on the Earth every single day. Spread over the entire planet, it’s not all that much if you think about it. Most of that is sand-size particles — tiny, tiny particles. Things that are about the size of a car, or van-size bolides, they hit a few times a year. Something the size of the Chelyabinsk meteor [which exploded over Russia in 2013], that’s a few times a year.

It's a terrific interview.

Dr. Lisa White is a paleontologist. Her specialty is Diatoms. These are single celled algae. They have a lot to tell us about past ecology and climate.  African Americans are poorly represented in the geosciences, and Dr. White as the director of education and outreach at the University of California Museum of Paleontology is actively working to increase diversity in the geosciences.

An excerpt:

I work nationally on a number of boards and with working groups and communities that are constantly examining the diversity in geosciences. We know our numbers don’t compare to engineering and the biological sciences. African American students are more likely to know about those fields and see the direct link to jobs. So we do have a bit of an image problem.

[It can be] difficult for students to have access to information about geosciences careers. There aren’t often a lot of standalone courses in high school. But there are a lot of interdisciplinary connections between all the fields, especially geoscience engineering, chemistry, water science, even agriculture…soil science.

Black Enterprise has the full interview.

Its always fun to read about how people arrive at a particular career trajectory.  A casual conversation, a book read during a holiday, or a trip taken with friends or for some other work can lead someone down  a career path they never thought they would take.

Thursday, December 13, 2018

Books: Colliding Continents And Reading The Rocks

These two beauties came by mail!

Readers know that I have been traveling and writing about the Himalaya the past few years. I had gone to Delhi in 2010 to attend a wedding and casually asked my cousin whether he could recommend a trip to the Himalaya. After the wedding I ended up in Mukteshwar, Uttarakhand for a short stay. I was hooked and have been going regularly since. I realized that I knew almost nothing about Himalaya geology. My Masters course in Pune had barely touched the surface. There were no Himalaya geology experts among the faculty and that showed in the minimal attention it was given in the syllabus. After all these years,  I decided to use my trekking trips to observe the local geology and teach myself about the geological architecture of the Himalaya.  I have been reading from the research literature too. After 6 years of trekking, field observations and reading I can say that I do have a broad understanding of the lithology and structure of the Uttarakhand Himalaya. Mike Searle's book, based on his 30 years of field and lab work in the High Himalaya and Tibet, covering almost the entire mountain chain from the western extremity in Pakistan to Bhutan and the Indo-Myanmar ranges in the east, is going to add enormously to my understanding of the details of the geological processes in operation at the zone of collision between India and Asia and how they formed this enormous mountain belt.

Marcia Bjornerud's book comes highly recommended from my Twitter friends and colleagues. It tells the story of the earth by delving in to and elucidating the basic geological processes in operation on the surface and in the interior of the planet. I have been actively pursuing geology outreach for over a decade now, through my blog mainly, but more recently by taking people out in the field and conversing with them about the rocks we see around us and their place in the geologic history of the earth. I have been trying hard to improve my ability to explain basic concepts in an easy to understand language. I have a feeling this fine book will help me refine that skill.

I will be posting my thoughts and some excerpts from these two books from time to time.