Monday, August 21, 2023

Darwin's Earthworms, Ocean Currents, Geology Heritage Lost

My latest set of readings.

1) Why Darwin Admired the Humble Earthworm. A delightful essay by Philip Ball on Darwin's work on earthworms. Published towards the end of his career, this book apparently sold more copies than the Origin of Species! As Philip Ball wittily observes, that tells us something about the English passion for gardening. Darwin's research on earthworms consisted of detailed observations and cleverly designed experiments, often carried out with the help of family members. 

His powers of observation and analysis remained undimmed - "Darwin reported that 80 percent of leaves he removed from worm burrows had been inserted tip first—a far from random distribution".

2) No, the Gulf Stream isn't going to shut down. The premise of the movie The Day After Tomorrow is that of a catastrophic cold snap engulfing north America and Europe, triggered by the shutting down of the Gulf Stream. This massive ocean current forms in the subtropics in the western side of the Atlantic and transports heat from the lower latitudes to northern Europe, moderating the temperatures in these northern regions. Media reports claim that recent work might be pointing to a collapse of the Gulf Stream, but as Frank Jacobs explains, people are conflating two very different current systems. 

Some studies are suggesting that the Atlantic Meridional Overturning Circulation, a much smaller and restricted circulation system, might be slowing down and might even collapse by 2050. This will result in some cooling in the Greenland and Norwegian seas, but will not affect the larger Gulf Stream. The article has a nice animation of global ocean currents which I found informative.

3) They Have Put Geology in Coffins. For long, geologists have been complaining about the utter indifference shown by successive Indian governments to our natural heritage. Here is one more example from Himachal Pradesh. Along the Kalka-Shimla highway, on the stretch between Parwanoo and Solan lay a treasure. This was a section of sedimentary rocks recording the retreat of the Tethys Sea which began after the collision between India and Asia started creating high topography. Along this stratigraphic section, marine sediments give way to freshwater deposits. The outcrop was a natural outdoor laboratory for students and researchers. Now it is gone. The National Highway Authorities of India has covered it with concrete and stone walls. Science is the big loser again. 

Arundeep Ahluwalia expresses the anguish of geologists who knew and loved this part of the Himalaya- "It forever denies coming generations any chance to study the long stretches of such highways and to the nature lovers in society the excitement of the history and grandeur of the earth".  

Read and weep. 

Monday, August 14, 2023

Field Photos: Boudinage In Sandstone

Boudinage is a structure that is developed when rock layers are being pulled and stretched. The term is derived from the French word for sausage. A rock layer is deformed by necking and is segmented into a string of sausages. The structure is best developed when there is contrast in competence or strength in a rock pile. The stronger layer is broken up in boudins, while the weaker layer 'flows' around and fills the gaps created in the neighboring boudin layer.

I wanted to showcase examples of boudins from igneous, sedimentary, and metamorphic rocks. 

The first example has been recently documented in a paper on silica rich magmatism from Sausrashtra by Anmol Naik and colleagues. The photograph shows a rhyolite with boudinage (black arrow) developed in a lava flow band. This deformation is due to layer parallel stretching during flow of a viscous lava.

 Source: Anmol Naik Geological Magazine 2023

The second instance is from Darma Valley, Kumaon,  near the village of Philum, not so far from the Panchachuli Glacier. It is a remarkable instance of boudins developing in a sandstone. Black arrows highlight elliptical and oval knobs, fragments of a once continuous sand rich layer. Notice how the surrounding thinly layered material has flown into the gaps between these boudins. The more competent sand layer broke up, while the softer clay rich material got wrinkled and warped but did not break. 

And lastly, here is an example of the more common variety of boudinage encountered in the field. This has developed in a high grade metamorphic rock. The lighter more competent layer made up of quartz and feldspar has been deformed into a series of boudins, while the mica rich grey layers are ductile. This outcrop is also from Darma Valley, Kumaon, near the village of Baaling. 

The three examples showcase deformation which produced similar structures in very different settings. Internal forces generated during flow of a viscous lava can result in folding and boudins. In sedimentary basins, similar forces may  affect a sand rich slurry moving down slope as a gravity flow or on semi consolidated sediment shaken during an earthquake. And forces acting on high grade metamorphic rocks  produce some spectacular boudins at high temperatures during episodes of mountain building.

Documenting rock deformation in the field is fun!