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.

Himalaya Overview, African Population History, Iceland Volcano

 From past couple of weeks:

1) This is a fine synthesis of geological, geophysical, seismic and geodetic data of the growing Himalaya mountains. The review examines the interplay and feedbacks between seismic cycles and tectonic deformation. Earthquakes result in rock deformation and faulting. Tectonic structures developed this way over million of years, in turn, influence stress accumulation and the extent and location of earthquakes.

Building the Himalaya from tectonic to earthquake scales.

2) Holocene-age ancient DNA and genetics of extant populations is increasing our understanding of African population history.

The deep population history in Africa. 

3) The remarkable drone footage of the ongoing eruption of Geldingadalir volcano in Iceland.

Email subscribers who can't see the embedded video can view it here- Iceland Volcanic Eruption.

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.