Monday, May 30, 2022

Articles: Earthquake Temperatures, Marshes, Ocean Drilling

Sharing some good stuff I came across recently.

1) Taking the temperature of faults. From AGU News. Rocks get hot as they slide past each other during faulting. Chemical changes in organic molecules and helium content in zircon crystals are sensitive to temperature changes. Scientists have used these to estimate short lived temperature rise during faulting. Understanding patterns and magnitude of earthquake generated heat informs us about earthquake intensity, heat dissipation, and fault movement history. 

2) Why a Marsh: "Neither land nor water, maybe both, a marsh is a balancing act, a collaboration between changing elements, uncertain by its very nature, in flux"

A beautiful long essay by Daniel Wolff and Dorothy Peteet on marshes and wetlands and the vital role they play in ecosystem functioning. Especially fascinating is the description of Piermont Marsh, along the banks of the Hudson river, north of New York City. Sediment cores going back a thousand years preserve a record of climate change and human modification of the landscape. The marsh was first exploited by Native Americans and then more extensively by European settlers. It is a detailed look at the wealth of information a wetland can yield about climate, ecology, and human disturbance of the environment.

3) Drilling Into the Sea Bed: This is a really interesting summary of scientific ocean drilling by Neena Notman . They are attempts going on to reach the earth's mantle by drilling into the sea bed. Although drilling in the deep ocean seems daunting, this makes more sense than trying to reach the mantle from land. The crust making up the continents is much thicker. Ocean bed drilling offers a short cut to the earth's mantle. We know about the mantle mostly through geophysical data. Actual samples of the mantle are rare. Recovering pristine pieces of the mantle will allow us to validate what we've gleaned from geophysics. Drilling is also going on along a subduction zone to understand the nature of fault interface and rock properties.


Tuesday, May 24, 2022

Book: Up The Mountains Of India

Finally it arrived! The author Mala Kumar connected with me a couple of years ago. She was writing a book on the mountains of India aimed primarily at young readers and wanted a geologist to help with fact checking some of the contents. We have had quite a few email exchanges since then, and my curiosity about this book has been growing even since. Mala has written over 40 books for children and this one is sure to enthrall them. But why just children? Anyone interested in India's varied physiography, geology, and wildlife, should read this book. 

I loved the way Mala has used everyday analogies to describe complex geology. One of my favorite examples is using a stuck piano key to explain about fault block mountains. The stuck key is a horst or a block that has moved up relative to the adjacent depressed keys which are called grabens. India's mountains are geological varied. The Himalaya, the Satpura, and the Western Ghats, for example, have formed by very different processes. Mala has used clear language and everyday familiar examples to simplify explanations of their origins. 

But this book is much more than about geology. Forests, wildlife, and human communities living in these rugged provinces really are the heart of the book. There are evocative descriptions of the animal and plant life of the different mountains ranges. Tibetan gazelles and mysterious snow leopards of the Himalaya, hoolock gibbons of the northeastern hills, majestic tigers of the Satpura and the Aravalli, and the bewildering varieties of frogs and reptiles of the Western Ghats, make us proud of our magnificent wildlife. There are lovely photographs scattered through the book. I could not stop gazing at the clouded leopard sitting elegantly on a branch deep in forests of the Mizo Hills. 

People too have lived in these mountains and forests since times immemorial. Their lives and life practices are being threatened as more mountain and forest land is gobbled up for dams, mines and expanding townships. In the northern Aravallis, villagers fought for years to save the sacred grove of Mangar Bani. In the Nallamali hills of Andhra Pradesh, members of the Chenchu tribe along with other organizations protested uranium mining that would have transgressed on the Amrabad Tiger Reserve. The book tells many stories from across the country, from the Aravallis, to the Eastern Ghats, to the Himalaya, of people struggling to conserve forests and water sources. 

Sketches and insets provide a welcome addition to the text. There are fun stories about the people of the mountains, about their crafts, about intrepid explorers, along with short quizzes, crammed in the insets that make the mountains come alive. 

Mountain ranges form the most important watersheds of our big rivers. Today, rampant development of mountain slopes coupled with climate change are becoming big threats to our water security. Mala Kumar's urgent call for conservation and sustainable development emerges again and again throughout the book and she has chosen her audience well. Whom better to inculcate a love and awareness for nature than in our children. These young citizens will be living in an increasingly challenging world and the hope of changing it for the better lies in a deeper understanding of how nature works. 

Go out for a trek in the mountains. Keep a lookout for that bright bluetail, and that curious langur who is peering at you half hidden from behind the thick leaves. Pick up pebbles from the stream bed and ask yourself about its geological secrets. Take a mountain train to the scented Nilgiri heights.  If you are hesitating, this book is an ideal place to begin.

Up the Mountains of India: A Fun, Fact-Filled Trek across the Country's Major Ranges - Mala Kumar. 

Tuesday, May 10, 2022

Field Photos: Dikes And Gneiss

My friends have been traveling across India and sending me pictures of landscapes and rocks. I am doing field work vicariously.

Posting below a few pictures that I have received.

Dikes Intruding Bundelkhand Gneiss. Pictures by Rajesh Sarde.

These two photos were taken at the Ken River gorge in Madhya Pradesh, near a gharial sanctuary. 

The dark rock making up the floor of the gorge is a dike. It has intruded the pink colored gneiss rock. Notice that the gneiss is fractured. Intrusions follow major weak zones in the gneiss.  Being softer than the gneiss, erosion over time has removed much of the dike, forming a narrow valley.

And in this picture, an arm of the dike known as an apophysis can been seen. It is almost at right angles to the gorge.

The Bundelkhand Gneiss ranges in age between 3.2 billion to about 2.5 billion years ago. The mafic dikes, igneous rocks rich in iron and magnesium silicate minerals, intruded later into the granite gneiss. Recent geochronological work on the dikes suggest two distinct events of dike emplacement, an early episode dated to about 2 billion years ago, and a later one at 1.1 billion years ago. Interestingly, the magnetic signatures frozen in these dikes have been used to make inferences about paleogeography. The results indicate that the north and south Indian crustal blocks which had independent origins were in close proximity by about 2.5 billion years ago. 

The magnetic signatures of the 1.1 billion year old dikes throw up a puzzle. They match those preserved in the Upper Vindhyan strata and intrusive rocks, seemingly constraining the age of the Upper Vindhyan sequence to around 1 billion years. However, recent fossil finds which I wrote about in a recent article for Nature India point to the Upper Vindhyans being much younger, about 550 million years old!

Dikes Intruding the Deccan Traps. Pictures by Rajesh Sarde.

These two photos were taken at the base of Tamhini Ghat, west of Pune, near a popular trekking spot known as Plus Valley. The rocks are about 66-65 million years old.

As in the previous example, the dike has eroded away faster than the host rock forming a narrow depression. Notice the closely spaced jointing pattern or cracks in the dike. 

 In this photo, the sharp boundary between the dike and the basalt rock can be clearly seen.

 Tonalite Trondhjemite Gneiss, Palolem Beach, Goa. Picture by Aneeha

These rocks, abbreviated as TTG, are relicts of early continental crust. They are about 3.4-3.2 billion years old. They represent Archaean age magmatism that formed the lighter continental crust. Such TTG's  are found all across India. They are the oldest component of cratons, the nucleus of the first continents. These magmas are generally granodiorites, rich in sodium and calcium feldspars and poor in potassium feldspars. They were deformed and metamorphosed subsequently in to a gneiss, in the process acquiring a characteristic banding. 

Next time hopefully pictures from my own field trips!