## Friday, June 25, 2010

### Sedimentary Basins Meme

Post a satellite image of your favorite exposed ancient sedimentary basin.

Criteria: Do you have a good example of an ancient basin exhumed and exposed which still retains its original geometry, with its basins margins and paleo-shorelines easily identifiable?

Here is mine. The mid-late Proterozoic Cuddapah Basin of south east India. White arrows mark the eastern margin of the basin.

Where there are sediments there must have been a basin. Over time though the original shape of the depression may get obscured mainly through deformation and formation of sucessor basins which may have a different orientation and shape.

The Cuddapah basin however was an intra-cratonic basin i.e a basin which formed far away from plate margins within thick continental crust. Later what was to become India was situated in the interior of Gondwanaland and this region did not experience any Paleozoic sedimentation. And much later Mesozoic rifting was far away from this region further preserving the mid-late Proterozoic basin shape.

The eastern margin did see deformation in the late Proterozoic. The relatively undeformed western margin is still one of my favorite places to visit with exposures of the prominent unconformity between the Archean granites and metsediments and mid Proterozoic basin margin conglomerates and coarse sands representing the earliest deposits.

Here is a figure which shows the location of the Cuddapah basin along with other sedimentary basins of India.

Source: Geotimes

## Tuesday, June 22, 2010

### A New Book On The Geological Evolution Of India

In the latest issue of Current Science, C.P. Rajendran reviews a new book by K. S. Valdiya on the geological evolution of India titled: The Making of India: Geodynamic Evolution.

Prof. K.S. Valdiya has reached a stage in his career where the term "Grand Old Man of Indian Geology" is an entirely appropriate title for him. His deep expertise lies in the geology of the Himalayan mountain chain, but this book is a broader synthesis from Archean times to recent.

I have to admit I found earlier books which synthesized Indian geology in one volume quite disappointing and the review mentions a couple of them. They rambled on and on about formation names and local lithology types and fossils. Written in the days before plate tectonics and geophysical data about the Indian subsurface, the approach was descriptive or one that relied on concepts and terminology that was no longer part of current thinking.

Plate tectonics started appearing in geology textbooks by the early mid 1970's. I graduated in the late 1980's. That there was no textbook which discussed Indian geology within the framework of global plate tectonics more than a decade on is a telling reminder of how slow educational content has been in catching up with the latest developments in the field. Going by C.P. Rajendran's review there was still until 2010 no textbook incorporating all this essential content!

Forty years on, looks like this book will remove that lacuna.

Price is Rs 242/- ( ~\$ 5/-) for a 816 page book, courtesy a generous subsidy from the Indian government. It is not yet listed on Amazon and likewise other online sellers. You might have to order it from the publishers Macmillan Publishers India Ltd.

## Friday, June 18, 2010

### Geological Map Resources Of Afghanistan

The trillions in mineral wealth made me surf a little for good resources on Afghanistan geology.

The United States Geological Survey has a Afghanistan Projects website with links to geological map quads ( 1:250,000), Landsat images, reports and photo gallery.

The British Geological Survey has a collection of downloadable maps on geology, mineral distribution, tectonics and hydrogeology.

For an interactive experience the OneGeology portal has a map viewer to browse Afghanistan geology. The portal is administered by the British and French Geological Survey.

Nature News has quite a good article on the recent history of geological mapping of Afghanistan and tensions and political conflicts over control of this mineral wealth.

GeoCommunity.. a geospatial news and education resource site has a collection of links to maps, images and viewer applications.

## Wednesday, June 16, 2010

### Matt Ridley On Ocean Acidification and Coral Calcification

Matt Ridley (author of Genome and the Red Queen) on his blog The Rational Optimist clarifies recent thinking about ocean acidification and its predicted impact on coral reefs. It is a strong defense of recent observations and experiments that indicate that the degree of ocean acidification expected through global warming does not pose a catastrophic threat to corals.

Many coral species in experimental conditions show enhanced calcification rates at higher pCO2. That may happen because dissolving more CO2 in water increases the bicarbonate content of sea-water among other dissolved inorganic carbon species providing a source for CO3 to bond with Ca to form the skeletal material.

This is the representation:

CO2 (aq) + H2O $\leftrightarrow$ H2CO3 $\leftrightarrow$ HCO3- + H+ $\leftrightarrow$ CO32- + 2 H+

So as pH increases so does the bicarbonate (HCO3) content of sea-water.

Just a reminder that decreasing ocean pH and calling it acidification does not mean that the oceans will becoming technically an acid, that is having pH below 7. In fact they will becoming less alkaline from the current pH of 8.1 to about 7.9 or so projected several decades on. This is within the range of variation in pH of seawater inside the coral body cavity and at the interface of the tissue and calcification sites. In other words organisms have already built in mechanisms to handle fluctuations in pH within certain limits.

Off course if pH keeps dropping, at some point the dynamics will change and calcification will be affected. That is because at the site of calcification bicarbonate (HCO3) is stripped of its proton (H+) and the carbonate used to bond with Ca. The proton has to be transported away from this site of lower pH (higher H+ activity) into the coral body cavity and beyond which is at relatively higher pH (lower H+ activity). If sea-water pH is lowered so will eventually the pH in the coral body cavity as there is an easy exchange between the two. The gradient in pH between the body cavity and the calcification site will decrease and that will hinder the transport of H+ away from the calcification site. With H+ building up at that site it becomes harder to strip more protons from HCO3 to form CO3. What that pH range is and whether such pH conditions will ever be reached in the natural oceans due to global warming is not very well understood.

Still there are other dangers to corals from global warming and the most important seems to be coral bleaching caused by the expulsion of symbiotic algae. This may happen as the temperature of water rises causing some internal mechanism within the coral to expel algae or via invasion of the coral by other parasitic microbes that may expel the algae.

Coral metabolism is tightly coupled to the health of this photosynthetic algae and CO2 released through coral metabolism contributes according to some estimates about 70% of the carbon for building skeletons. The other 30% comes from the sea-water HCO3 I mentioned in the earlier para. So removing the algae and degrading the metabolism of the coral would mean cutting off a significant supply of carbon for skeletons.

Again the effect of all these parameters have been demonstrated in experiments that have run from a few tens of hours to a few weeks. For example how important this metabolic source of carbon is in the long run and whether different coral species may be able to harvest more and more carbon directly from sea-water bicarbonate in case the supply of metabolic carbon is shut off is an interesting question.

Likely there will be coral species who will have flexible mechanisms to harvesting carbon. They will flourish and others who have no physiological flexibility might die off.

Nature is resilient due to its variability. The community structure of coral reefs may change over the next few hundred years. But corals as a group will live on.

Meanwhile there are many local dangers to coral reefs in the form of overfishing and destructive fishing methods, turbidity due to increased coastal runoff, nutrient overdose leading to algal blooms that have choke off the corals. As Matt Ridley's article reminds us, these have the potential of wiping out many coral reefs faster than global warming might.

## Monday, June 14, 2010

### Sinking Deeper And Deeper For Groundwater In Pune

This is a sight becoming common in Pune.

A drilling rig is often the first order of business for many new constructions on commercial and residential premises. It reflects both an increased consumption of water due to affluent lifestyles and a growing uncertainty about water security often due to lack of piped water supply especially in the suburbs and despite claims from the Pune Municipal Corporation that surface water reservoirs have enough capacity to provide for future population growth and increased use.

I guess people want to ensure that they have a water supply under their control in case of future inability of the city to provide them with piped water. I talked recently with a geologist with years of experience in the groundwater situation in Pune and I heard the same ominous story that is reverberating all over India.

The deeper aquifer in Pune which means drilling to depths of 150-200 feet seems to be experiencing overdraft as unregulated bore-wells extract more than natural recharge. The city is trying to address the problem by proposing that it will limit the number of bore-wells based on surveys of aquifer capacity. This kind of aggregate limit however will not regulate current and future use by individual wells already in operation. Indian groundwater law allows the owner of a property unlimited use of the groundwater and increased reliance on groundwater in the future will keep depleting the resource if not managed scientifically.

And therein lies the problem because just how much water lies underneath is at best only sketchily understood. Any blanket limit going by watersheds  imposed on extraction makes little sense because the deeper aquifers in Deccan basalts are not congruent with the surface water contours. They show extreme laterally variability and are compartmentalized. So for proper groundwater management the underlying aquifers need to be mapped, monitored and a scientific database on aquifer capacity needs to be built before any allocation plan that is sustainable is finalized.

Besides understanding the limits of extraction, just how the city is going to regulate individual use of groundwater remains a thorny issue. There is extreme reluctance to price the use of groundwater by metering bore-wells. The reasons are manifold. It will be an "enforcement headache" (read we are too lazy) is one excuse I have heard. Besides, there is a cultural tendency in India to look at groundwater as a free resource. The focus for long on groundwater use as been on rural consumption for agriculture. Making farmers pay a price for electricity and water is considered political Harakiri in India. So politicians keep tip-toeing around making any serious amendments to groundwater law.

Indian cities however are growing and reliance on groundwater by urban users is bound to increase. There needs to be a sensible re-look at groundwater laws for the urban consumer. Water allocation for well owners based on the aquifer capacity, the need to limit new well permits, mandating artificial recharge structures to augment refilling, and getting urban consumers to pay for the water they are pulling out of the aquifer are all ideas that need to be debated and thrashed out if we want to move towards a more sustainable use of this resource.

## Tuesday, June 8, 2010

### Budding Geologists On The Rugby Pitch

Every weekend one of the coaches gives a word quiz to our soccer and rugby kids, ages 8 to 10. Last Saturday it was my turn and I asked about Gondwanaland. .. tell me all about it.

Come Monday and I got three correct answers... the kids had read up and got the right ideas about continents being in different places and attached to other continents in the past.. they also told me that the present arrangement of continents will change in the future.. impressive for 10 year old's

Here are the winners (they do it for the chocolate bars):

Meanwhile we had our first pre-monsoon thunderstorms last week. It was just a massive cloud burst turning the ground into a swimming pool.

All set for rugby season over here.

## Friday, June 4, 2010

### Mineralogy And Deep Subduction Of Indian Continental Crust

Science Daily describes some work on eclogites found in the Ladakh region of the Himalayas and its implications on the subduction of the Indian continental crust underneath the Asian plate.

The paper appeared in last month's Geology and uses the mineral majorite - a high pressure variety of garnet - to estimate the depth of mineral formation. The pattern in which atoms are arranged in a lattice structure and the ratio of certain elements in a mineral are often sensitive to changes in temperature and pressure and can be used to estimate and develop a history of change of these parameters as plate subduction drives crust deeper and mineral growth takes place at higher and higher temperatures and pressures . The results indicate a depth of around 200 km for the formation of the mineral majorite.

So the study throws light not on just on how deep the Indian crust sank during continental collision with Asia but also how quickly it has been exhumed during the formation of the Himalayan mountains. The mineral and its chemical ratios which were stable at 200 km depth have been essentially frozen in that state as the enclosing rock was brought to the surface geologically quickly along thrust faults and through rapid erosion and removal of overlying crust preventing majorite from completely altering to a lower temperature and pressure analog.

Rocks at depths of 200 km formed tens of millions of years ago are today on the surface at great heights of about fourteen thousand odd feet. These ecologites are exposed in the Lake Tso Moriri area of Ladakh in the high Himalayas. It is difficult terrain. Field seasons are restricted to a couple of months in summer. The altitude takes some getting used to.

Lake Tso Moriri / Source: Wolken Berge Wasser Wiese   Author: Jochen Westermann

A faculty friend has a graduate student working on this very same eclogite complex. I hope this publication by a different group does not mean a setback to her work. Finding that you have been scooped after you are in your third year of PhD can be demoralizing but my friend is sure that there are parallel stories of mineral genesis to be wrung out of these ecologites.

Last month's Accretionary Wedge threw open the diverse interests of the geology community... from the microscopic to the continental scale. Discoveries like the one on the Tso Morari ecologites makes you reflect on the fact that these geological features and processes that encompass such vast differences in scale don't occur as independent events not affected by or having no relation to or having no effect on processes larger or smaller.

The secrets of plate tectonics are sometimes hidden in one tiny mineral. I find it fascinating that the chemical ratios found in one mineral reveal a story of continents colliding, the subduction and burial of crust to great depths and its later ascent to icy heights, part of the geological saga of the great Himalayan mountains.

### Gulf Spill: Hear What You Want To Hear

Worst case scenarios and political fallouts:

Source: xkcd comics

## Tuesday, June 1, 2010

### Me And Tafoni

A most healthy obsession with rocks:

Location- Korlai village, west coast India, south of Mumbai. Exposures of Tafoni in the intertidal wave cut basalt benches.

Photo courtesy Daniel MEGE.