Saturday, October 9, 2021

Maps: India Contours

Once in a while I point readers to interesting and useful web mapping applications. Last week, Raj Bhagat Palanichamy, a specialist in GIS and Remote Sensing,  showcased a contour map on his Twitter feed. Intrigued, I found out that the contour map was from a web based application developed by Axis Maps. A detailed blog post explains how they developed the application and the elevation data sources they used to generate the contours. It is quite easy to use, with some controls to set contour intervals, line color, and to depict relief in color shades. 

I am putting a few examples from different Indian terrains to showcase this utility. It is not meant to be a critical review of the app (map scale is missing!), but a fun exploration of its capabilities, and the insights one can get about topography and other aspects of geomorphology and geology. Each example has a contour map on top and a satellite image of roughly the same area in the lower panel. All contours maps have been created using Contours- Axis Maps.

Western Ghats- Edge of the Deccan Plateau: Ghangad, Tail Baila Mesas

The edge of the Deccan Plateau has been deeply dissected to form some stunning relief. Steep sided ridges, mesas, and pinnacles poke upwards from more gently sloping and flattish surfaces. This step like appearance occurs due to the differing styles in which lava flows of varying hardness weather and erode away.  Towards the left-center of the contour map, where the brown meets the darker green, is what appears to be a sinuous thick brown line. It is really an amalgamation of contours, spaced closely, due to the extremely steep slopes and cliffs that make up the Western Ghat escarpment. There is a sudden fall there from the plateau to the coastal plain. Contour interval is 100 feet. 

Himalaya-Tibetan Plateau


Notice how the closely spaced contours in the lower left of the contour map give way to more openly spaced contours towards the top right of the map. This is the transition from the Gharwal Himalaya in to the Tibetan Plateau. The Himalaya, because it receives more rainfall, has more prominent relief, formed by rivers carving deep valleys, and glaciers gouging out rock faces into steep sided, sharp edged mountains. The Tibetan Plateau, although also standing high at around 4500 meters, is in the rain shadow region. It shows less relief. 

This has impacted the geology too. In the Himalaya, the rapid stripping of rock cover over millions of years has exhumed rocks which were once buried 25 kilometers below the surface. In Tibet, lesser erosion has meant that the surface geology is still dominated by 'supracrustal rocks', either volcanic or sedimentary rocks formed at shallow levels of the crust. Erosion has not dug deep down. Contour interval is 500 feet.

Nallamalai Fold Belt- Andhra Pradesh


This map shows the folded ridges of sedimentary rocks formed in the Cuddapah Basin around 1500-1600 million years ago. A fine example of topographic expression giving away the geologic structure of the rocks. Contour interval is 200 feet. 

I would urge those readers who are on Twitter to follow Raj Bhagat Palanichamy's excellent account (@rajbhagatt). He is a mapper par excellence.

Monday, October 4, 2021

Geology Crossword

A couple of years ago I reconnected with my Florida State University friend Shanker Venkateswaran.

Shanker is quite a geology and nature enthusiast. He has put together this geology crossword for beginners on the Crossword Club website.


Click on the link below for the full set of clues and the interactive version.

Geology Crossword - https://crossword-club.blogspot.com/2021/10/earth-science.html.


Saturday, September 25, 2021

LiveHistory India Videos: I Speak About Deccan Volcanism

LiveHistory India has started a wonderful outreach initiative, highlighting India's geological heritage. They invited me to talk about Deccan Volcanism. I spoke about how it all began, the physiography, places of interest, and the fossil bearing intertrappean sediments and their value in understanding ecology and broader patterns of extinction and recovery spanning the mass extinction that occured 66 million years ago. 

This was recorded a couple of weeks ago, and it is now online. The original recording was about 40 minutes, but it has been edited and the video is 17 minutes long. Subtitles are in Hindi.

Permanent Link- Deccan Volcanism And Its Various Aspects

 
 

LiveHistory has put out more such videos with other Indian geologists. 

1) India's Fossil Heritage- Dr. Sunil Bajpai

2) Markers of Earth's Formation in India- Dr. Pushpendra Ranawat

3) A Panel on Geological Heritage of India- Dr. Pushpendra Ranawat, Bidisha Bayan, Dr. Reddy, and Aliya Babi

Hope you enjoy my talk!

Tuesday, September 14, 2021

Links: Birth Of Species, Children's Book, Rise Of Oxygen

 Some geology and evolution material for your perusal.

1) I came across this well crafted documentary by Niles Eldredge and Stefano Dominici on the history of the development of ideas on the birth of species. It is a paleontologist's perspective with fossils being given the centre stage. A great many personalities who contributed to the early thinking on the origin of species are featured. Among the prominent ones who influenced Darwin were Lamarck, Cuvier, Giambattista Brocchi, and John Herschel to name a few. Completely left out of this film is Alfred Wallace. 

It does center around Darwin, and, later towards the end, on Niles Eldredge's work on Punctuated Equilibrium, which he published in collaboration with Stephen Jay Gould. I have often wondered whether there was any tension between Eldredge and Gould regarding proprietorship over Punctuated Equilibrium given Gould's very bombastic advocacy of this idea. The last section of this film is revealing! 

Beautifully compiled. Do watch. 

 

If  you are unable to access the embedded video, view it at this permanent link - The Birth of Species

2) Zircon (zirconium silicate) is a remarkable mineral. Born in the cauldron of magma chambers, it is henceforth virtually indestructible unless it is melted down again. This makes it a witness to geological processes affecting and shaping terrains over hundreds of millions of  years. What a story it has to tell us. And that is precisely what geochronologist Matthew Fox has done. He has written a children's book titled Jane's Geological Adventure, which follows Jane, the zircon grain, from her birth in a magma chamber to a life lasting 400 million years. Alka Tripathy-Lang reviews the book.

Meet Jane, the Zircon Grain—Geochronology’s New Mascot.

3) Elizabeth Pennisi writes about a new paper published in Nature Geosciences on the link between the increase in the length of the day and the rise of atmospheric oxygen on early earth. 

‘Totally new’ idea suggests longer days on early Earth set stage for complex life.

Wednesday, August 25, 2021

In Praise Of The Estuary

Be sure to take some time out and watch his lovely documentary on the Mulki Estuary by Dakshina Kannada Wildlife team.

Email subscribers who can't see the embedded video can watch it here - The Mulki Estuary.


This estuary has formed at the confluence of the Sambhavi and Nandini rivers (the documentary wrongly calls it the Gurpur river) on the Karnataka coast. Estuaries are particularly rich ecosystems. The confluence of fresh and saline waters, changes in water temperature and turbidity, nutrient supply by the river and coastal upwellings, the rise and fall of tides, and a meeting and interference of shore parallel and river currents create conditions suitable for a thriving biosphere. The documentary brings these aspects out beautifully in its capture of coastal landforms and varied wildlife.

Estuaries are of great interest to geologists too. The present configuration of our coastline has developed relatively recently in the geologic past. Before 15,000 years ago, during the Ice Age, sea levels were  about 100 meters lower than present. The coastline would have been tens of kilometers to the west of today's shore, and rivers like the Sambhavi and Nandini would have carved a valley across this stretch of the continental shelf and would have met the sea at a far westward location. As the Ice Age ended and the earth warmed, the rising seas progressively inundated the continental shelf. The estuaries along the Indian coast as with other coastlines are really drowned river valleys.

Coastal environments like beaches, mud flats, sand bars, mangroves, lagoons, and marshes, are in a state of constant adjustment to the movement of tides and currents, sediment supply from rivers and their distribution and deposition, and the impact of vegetation in stabilizing these features. Shore currents and sediment supply have an outsize effect on the evolution of the coast. One interesting example is found at Mangaluru, just south of Mulki, where another estuary has formed at the confluence of the Gurpur and Netravati rivers. The map shows this coastal setting. 

A geological investigation by B. R. Manjunatha and K. Balakrishna has shown that the Gurpur river once flowed a shorter route to the sea (paleo-course outlined in blue). Starting a few thousand years ago, excessive sediment deposition blocked this channel and strong southerly currents began building a barrier spit, forcing the Gurpur river to turn south and flow parallel to the coast for a good 8 kilometers before eventually joining the Netravati river in a joint exit to the sea. The Netravati river too has shifted slightly northwards over time. A quick survey along the Karnataka and Maharashtra coast show many similar situations where the river makes an abrupt turn and flows parallel to long sand spits before entering the sea. Do these rivers too have a similar history of course change? Its an intriguing observation.

Besides such adjustments to the coast due to currents and sediment deposition,  fluctuations in sea level too will change the prevailing geography. A sea level rise will push beaches and lagoons landwards, while a fall will cause rivers and deltas to extend seawards and bury past shorelines.

The estuary being a low lying region and a sediment trap preserves this history of sea level change. Geologists love to drill and take out sediment samples from the estuary bed and associated marshes and mangroves, because they contain, in its changing sediment and biological composition, clues to earlier environmental shifts. Understanding how past sea level change affected the coastal system is of great value in tuning our expectations and our preparedness of how the ongoing sea level rise will impact our present day coastlines.

I must confess that this post was inspired not just by this documentary but by my own memories of an estuary. During my student days, I had visited the coastal town of Malvan at the end of a small trek along the west coast of Maharashtra. A friend suggested we spend the day boating and exploring an estuary south of town. It turned out to be a most relaxing and enjoyable experience.

Taking inspiration from these children, we too rowed vigorously and covered quite a bit of distance. 

Malvan then was a small sleepy place and this meeting of river with the sea even more so. Not a soul was in sight, as hour after hour the waters of the estuary gently lapped against our boat. A salty sea breeze kept us refreshed. As the sun beat down upon us we finally decided to take a break. In search of something to eat and drink we scoured the shore for a small settlement. Up ahead in the middle of the estuary was our savior. 


 A luscious looking coconut island beckoned.