Bengal Delta, Africa Rifting, India Sand Mining

A few readings for your perusal- 

1) The future of Bengal Delta. With this succinct title Dipen Bhattacharya has written an informative article on the origin and evolution of the Bengal Delta. The Bay of Bengal was created when India broke away from eastern Antarctica about 130 -120 million years ago in the early mid Cretaceous. The basin expanded as India drifted northwards. From Cretaceous to Oligocene times (120-25 million years ago) rivers from Peninsular India were providing most of the sediment being deposited in the Bay. Himalaya derived sediment started overwhelming Peninsular river input from about 25 million years ago. K.S. Krishna and coworkers have very elegantly demonstrated this in their study of sediment pathways in to the Bay of Bengal.

Dr. Bhattacharya has traced the evolution of the delta into more recent times, explaining the role of the Pleistocene ice ages in delta growth. The delta’s future too is at risk with dam building and ground water extraction amplifying the changes due to global warming induced sea level rise. Well worth reading.

2) Eastern Africa Is Splitting Apart, but Not Where We Expected.  Africa is tearing apart along a north south oriented corridor from the Red Sea to Mozambique. Plate motion has formed the famous rift valleys of Ethiopia, Kenya, and Tanzania, as the crust stretches and subsides along faults. Kimberley Cartier explains the geological set up of the region and the stages in which continents break apart with oceanic basins eventually forming along the initial zones of continental rifts.

Why this region of Africa is rifting is not all that easy to explain. If you look at the plate tectonic map of Eastern Africa and the adjacent Indian Ocean and Arabian Sea you will notice that the oceanic Somalia and Indian Ocean tectonic plates are pushing into East Africa. For continents to split and be pulled apart, there have to be extensional forces generated. These are usually provided at the locus of rifting by the mantle doming up, thereby breaking and pushing the lithosphere away, and by one end of the plate subducting underneath another overriding plate. The subducting oceanic crust becomes denser and heavier as it sinks deeper, pulling the rest of the plate with it.

After the breakup of Gondwanaland, the northerly movement of the India plate through the Cretaceous was sustained by the pull force of the northern edge of the India plate sinking under Asia. On the other hand, Eastern Africa is surrounded by plate spreading zones. There is no pull force available for eastern Africa, only the localized extensional stresses due to mantle upwelling.  Is that providing adequate horizontal traction at the base of the Africa Plate for the crust to break apart and stretch? For a deeper understanding into the mantle forces responsible for this, I will recommend J Micheal Kendall and Carolina Lithgow-Bertelloni ‘s article- Why is Africa Rifting?

3)  India’s rivers bear lasting scars from relentless sand mining. Some years ago I heard a podcast on Planet Money about a Jamaican beach that was stolen. An estimated 500 truckloads of sand was hauled away in the middle of the night. Sand is big business all over the world. Indian river beds too are being plundered for their sand to satisfy the demands of the booming construction industry. Sahana Ghosh explores how scientists are surveying Indian rivers using field observations and satellite data. They are trying to track down the amount of sand being extracted and the environmental impact of sand mining.

Photo: Insect Camouflage

 I nearly missed this insect as I was sipping my evening coffee.

The rust, black, and white pattern of the insect blends into the pink feldspar, biotite, and quartz of the granite table top. When the restaurant introduced these tables in the 1980’s, insects with a passing resemblance to the table colors survived the gaze of bird predators better than individuals of the same species not having that coloration. Granite colored insects reproduced more, and the match between the insect patterning and the table top became more fine tuned over time.

I made this up. It is what is known as a “Just So Story”, named after writer Rudyard Kipling’s Just So Stories for children. Kipling wrote imaginative fantastical explanations for how animals looked the way they do. The term made its way into biology and was especially used, rather derisively, by evolutionary biologist Stephen Jay Gould as a critique of evolutionary psychology. Gould complained that the field tends to come up with imaginative yet unsupported adaptationist explanations for every aspect of human behavior. They are Just So Stories. The criticism has extended to other areas of biology too. Such as one can make about my story of the insect.

How did this particular insect species get this coloration that matches the granite? Likely its historical origins lies in a very different environment. Perhaps adaptation through natural selection in an ecologic setting of leaves and colored pebbles did play a role in the evolution of this pattern. Or perhaps it is a side effect of some other developmental changes in the insect body plan. Whatever the explanation, it is only chance that its coloration matches the table top stone.