Wednesday, May 20, 2015

J Tuzo Wilson's Hawaiian Dream

Again via Twitter I came across this collection of essays on eminent geologists titled Rock Stars, produced by The Geological Society of America History of Geology Division.

I haven't gone through all of them yet, but there is a lovely piece on J. Tuzo Wilson, the geophysicist most famous for his work on the then incipient field of moving continents and oceanic crust aka plate tectonics.

We know Wilson from geology coursework on the "Wilson Cycle", the opening and closing of ocean basins over hundreds of millions of years. But Derek York who has written the essay has more insights. He remembers that Wilson had a knack of solving problems by visualizing rather than working equations. One of his famous insights into the origin of the Hawaiian island chain came to him this way-

Tuzo’s mind had a fascinating way of solving problems. Unlike most physicists, who find their solutions via mathematics, Tuzo solved problems almost entirely with visual images and then presented the solutions in extremely clear prose. He had a remarkable ability to look into the heart of extreme complexity and see simplicity itself. The nearest mind that I can think of to compare with Tuzo’s was that of Michael Faraday who, instead of integrating differential equations to calculate the electric field, imagined a charged particle to be an octopus with tentacle-like lines of force reaching out into the space around it.

To solve the problem of the origin of the Hawaiian Islands, for example, Tuzo imagined someone lying on his back on the bottom of a shallow stream, blowing bubbles to the surface through a straw. The bursting bubbles were the Hawaiian Islands, and they lay in a line because they were swept along the surface by the moving stream. Thirty years later, leading geophysical theorists use supercomputers to solve horrendous equations that Tuzo”solved” in the visualizing region of his brain.


In today's jargon, the moving  stream is a tectonic plate in motion and the bubbles are rising molten material originating ultimately from a fixed heat source in the mantle. 

Another famous Wilson piece of reasoning was the sense of movement on transform faults linking mid ocean ridges. He predicted that for earthquakes occurring underwater in the middle of the ocean "rocks everybody believed had moved right to left during the earthquake had moved left to right, and vice versa".

What did he mean by that? Well, the sense of motion along transform faults which connect strands of mid-oceanic ridges is opposite to what one would expect from a classical strike slip fault. See the image below.



Fig. A is a bed offset by a sinistral strike slip fault with a left lateral motion.  This means that the piece of crust north of the fault  has moved towards left.

Fig.B shows strands of a mid oceanic ridge. The red line linking the ridges is the transform fault. Notice however that the sense of motion along the transform fault is opposite that of a classical strike slip fault  i.e. the piece of  crust north of the transform fault is moving towards the right as it would  since the ridge is producing new oceanic crust and pushing older crust away.

There are other interesting essays, I enjoyed the one on Lawrence Sloss And The Sequence Stratigraphy Revolution.

Dive in..

Thursday, May 14, 2015

Ocean Acidification- What Exactly Happens?

I've started following @Scitable, an education resource from journal Nature. A few days back they tweeted a link to an article on ocean acidification.

Pay attention-
 
When CO2 dissolves in seawater to produce aqueous CO2 (CO2(aq)) it also forms carbonic acid (H2CO3) (Eq. 1; Figure 1). Carbonic acid rapidly dissociates (splits apart) to produce bicarbonate ions (HCO3-, Eq. 2). In turn, bicarbonate ions can also dissociate into carbonate ions (CO32-, Eq. 3). Both of these reactions (Eqs. 2, 3) also produce protons (H+) and therefore lower the pH of the solution (i.e., the water is now more acidic than it was — recall that pH is the negative logarithm of the proton concentration or activity, -log10[H+]. Note, as illustrated in Figure 2, Ocean Acidification does not imply that ocean waters will actually become acidic (i.e., pH < 7.0).

CO2(aq) + H2O ↔ H2CO3 (1)
H2CO3 ↔ HCO3- + H+ (2)
HCO3- ↔ CO32- + H+ (3)
 
However, when CO2 dissoves in seawater it does not fully dissociate into carbonate ions and the number of hydrogen ions produced (and the drop in pH) is therefore smaller than one might expect. This is due to the natural capacity of seawater to buffer against changes in pH, which can be represented simply by:

CO2(aq) + CO32- + H2O → 2HCO3- (4)

where CO2 is effectively neutralized by reaction with CO32- to produce HCO3-. The HCO3- produced by Eq. 4 then partly dissociates (Eq. 3), releasing protons and so decreasing the pH-which is where the ‘ocean acidification' actually comes from-but this drop is much smaller than for an un-buffered system. One can also think of the sequence of events resulting from dissolving CO2 in seawater as firstly the production of HCO3- and H+, but because the equilibrium between HCO3- and CO32- (Eq. 3) has now been unbalanced by excess acidity (H+), Eq. 3 goes to the left to consume some of the excess H+, and in doing so, also consumes CO32-.

This is a wonderfully clear explanation of the chemistry of ocean acidification in terms of changing  concentrations or activity of CO2 (atmospheric),  CO2(aqueous), CO3 (carbonate) and bicarbonate (HCO3) ions, as there is later in the article on the negative and positive feedbacks in terms of the capacity of the ocean to absorb CO2 with rising ocean  temperatures.

The proportion of DIC present as CO2 is also affected by temperature, as illustrated in Figure 2. The consequence of this is that, as the ocean warms, less DIC will be partitioned into the form of CO2 (and more as CO32-), hence enhancing the buffering and providing a ‘negative feedback' on rising atmospheric CO2. Here, a feedback describes a mechanism that dimishes or amplifies an initial change and asribed the sign ‘negative' or ‘positive', respectively. For example, melting polar ice caps through global warming will reduce the amount of solar radiation that is reflected back out to space (the Earth's surface becomes less reflective), so producing more warming, which in turn will melt more ice, and so on — a positive feedback. A well-known positive feedback in the carbon cycle arises due to the decrease in solubility of CO2 gas in seawater at higher temperatures. In fact, this greatly outweighs the negative feedback described above, meaning that as the ocean surface warms, even more of the emitted fossil fuel CO2 will remain in the atmosphere.

And how do saturation levels of CO3 in sea water affect the stability of CaCO3 mineral species Aragonite and Calcite which organisms use to build skeletons? What effect will increase in ocean acidification have on organisms? ...

read on.. don't miss out on this chemistry lesson.

Tuesday, May 5, 2015

Return Times Of Great Himalayan Earthquakes

Bollinger, L., S. N. Sapkota, P. Tapponnier,Y. Klinger, M. Rizza, J. Van der Woerd,D. R. Tiwari, R. Pandey, A. Bitri, and S. Besde Berc (2014), Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust, J. Geophys. Res. Solid Earth, 119, doi:10.1002/2014JB010970.

This is a very detailed study taken up along  two strands of the Main Frontal Thrust in Nepal south east of Kathmandu. Along this fault, the Neogene foreland basin Siwaliks are thrust over the Indo-Gangetic alluvium.

Crustal shortening taking place in the Himalayas as a result of convergence between India and Asia is accommodated along a sequence of south younging thrust faults; The Main Central Thrust, The Main Boundary Thrust and the southernmost Main Frontal  Thrust, which is active today. All these thrusts are interpreted to merge into a single decollment called The Main Himalayan Thrust (a "master fault" along which India subducts underneath Asia). The cross section below shows these major thrust faults flattening at depth and merging into the Main Himalayan Thrust. Earthquake clusters in red dots shows a region of the Indian slab which as it slides under Asia, often (over decadal to millenial times scales) gets locked. Rupture follows, thus releasing that accumulated slip. These ruptures propagate southwards and occasionally break the surface along the Main Frontal Thrust.


Source: Bollinger et al. 2014

So, along this fault in front of the Siwalik  hills, there is evidence in the form of fault scarps and fault traces, offset and deformed strata, uplifted river terraces and stacks of colluvial deposits (sediments eroded from a fault scarp) of past earthquakes. This study examines this record in detail going back several thousand years. Its a long paper and there were sections where the reading is a hard slog and when my eyes glazed over, but it is rewarding to understand the techniques (geomorphic measurements, geochronology and shallow seismic profiling)  that have been applied to reconstruct earthquake history.

Abstract:

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT’stwo overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12–17.5m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years.

And in conclusion:

Certainly, the best path toward fully understanding whether and where great or giant earthquakes are likely to occur along the foothills of the highest mountain range on Earth will be to combine many exhaustive geomorphological and paleoseismological field investigations such as that presented here with extensive, long-term geodetic measurements, capable of narrowing uncertainties in estimates of the full seismic moment deficit.

Eyewitness Account Of A Great Nepal Earthquake

From 1934 - as reported by military officer Bhrama Shumsher Rana in his book  Mahabhukamp (The Great Earthquake) -

“The trees were moving as if they were agitated by a storm and it seemed that the tree-tops would touch the ground … Pillars and walls of houses were cracking,doors and windows slamming. With movements up and down, houses collapsed. Statues and decorations placed on top of temples and houses fell to the ground. The noise made by the houses collapsing was reminiscent of canon fire, as…during festivities. Because of the dust, it was darker and no one was able to see at more than a distance of 8 to 10 hands apart. This cloud of dust came from the city itself, invading open areas such as the Thundikhel—a large open and unconstructed space in the centre of Kathmandu—which was such as lost in a fog. People rushed to all these open spaces. Those who could not move themselves were seizing pillars, while others were searching to hide in shelters or ran to the fields. People would run on all fours like animals…”

“Cracks opened in the fields and roads. Water spurted from these cracks. There was flooding in all streams.Rivers like the Bagmati and Bishnumati were invaded by black muddy water. At some places, the water rose 8to 10 hands above the cracks. Many fields were flooded with water. Warm water and sand spurted from some of the cracks. The roads toward Balaju and Shankhamul were affected by a subsidence as large as one or both hands in height. There were few roads that were not cracked.”

via Bollinger et al 2014 

Kathmandu has been leveled and rebuilt several times over the past 1000 years or so. There are a series of posts on Dot Earth about damage done by the recent 7.8 mag Nepal earthquake, on building codes and earthquake preparedness.

Wednesday, April 22, 2015

On Rock Classification

Two interesting articles:

1) In the Journal of Sedimentary Research (behind paywall) Kitty Milliken proposes a tripartite classification of fine grained sedimentary rocks, those with grain assemblages with greater than 50% of particles by weight or volume less than 62.5 µm (4 Phi). There are a number of names for these types of rock; mudstone, claystone, pelite, argillite to name a few. This classification categorizes the rocks according to the composition, thereby indicating the source of the grains. Composition in turn controls to a large measure bulk rock properties upon burial and interaction with fluids, thus enabling general predictions about their economic and engineering qualities.

Abstract:

A tripartite compositional classification is proposed for sediments and sedimentary rocks that have grain assemblages with greater than 50 percent of a weight or volume of particles smaller than 62.5 µm (4 Phi). Tarl (terrigenous–argillaceous) contains a grain assemblage dominated by more than 75 percent of particles of extrabasinal derivation, including grains derived from continental weathering and also volcanogenic debris. Carl (calcareous–argillaceous) contains less than 75 percent of particles of extrabasinal derivation debris and among its intrabasinal grains contains a preponderance of biogenic carbonate particles including carbonate aggregates. Sarl (siliceous–argillaceous) contains less than 75 percent of particles of extrabasinal derivation and contains a preponderance of biogenic siliceous particles over carbonate grains.

These three classes of fine-grained particulate sediments and rocks effectively separate materials that have distinct depositional settings and systematic contrasts in organic-matter content and minor grain types. In the subsurface the grain assemblages that define these classes follow contrasting and predictable diagenetic pathways that have significant implications for the evolution of bulk rock properties, and thus, assigning a fine-grained rock to one of these classes is an important first step for predicting its economic and engineering qualities. For purposes of description these three class names can be joined to modifier terms denoting rock texture, more precise compositional divisions, specific grain types of notable importance, and diagenetic features. 


2) In Earth Magazine, a delightful article (open access) titled Geologic Column: The Rumpelstiltskin Factor by Ward Chesworth, professor emeritus at the University of Guelph, Canada.

Chesworth muses on the importance of naming objects and whether it is better to be a "lumper" or a "splitter" i.e. whether it is better to organize variation in to as few groups as possible or whether it is better to draw finer and finer distinctions and place a smaller range of variation into its own distinct cubicle.

An excerpt:

Excessive splitting can lead to problems, though. If an overly meticulous taxonomist kept on splitting hairs ad absurdum, we would wind up with a classification resembling an advanced case of logorrhea, the kind of thing guaranteed to drive working geologists to the brink. C.B. Hunt staged his own rebellion against this tendency when he considered the plethora of names invented for minor igneous intrusions. He expressed his displeasure by sarcastically concocting one more, cactolith, which he described as “a quasi-​horizontal chronolith composed of anastomosing ductoliths whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an atmolith or ethmolith.” He insinuated it into his 1953 U.S. Geological Survey professional paper on the Henry Mountains of Utah, and from there it crept under the radar into the first edition of AGI’s very own “Glossary of Geology.” Unfortunately, some humorless jobsworth banned it from all subsequent editions.

Sprinkled with more anecdotes, this is a fun read.