Saturday, July 22, 2017

Tsunami History Preserved In Indonesian Cave Deposits

How would you know if a coastline had been inundated by a tsunami say 5000 years ago? Well, a tsunami carries sediment stripped from the ocean bed and deposits this material over the flooded coastline, beyond the range of what a regular storm would. The problem is that such deposits have poor preservation potential and over time get eroded away. There are however some environments where such tsunami deposits may get preserved inland. These are estuaries, coastal marshes and lakes. Here, interlayered with normal estuarine, marsh or lacustrine sediment, one may find layers of sand of a distinctly different composition and texture and containing remains of organisms which live in an open marine setting. This implies a sudden incursion of marine waters into these inland coastal settings. The other coastal setting with a good preservation potential are caves. These too get flooded by storm surges and tsunamis and may preserve a record of such events in the form of sand deposits. The picture to the left (Source: Rubin 2017)  shows sand layers deposited by the 2004 tsunami.

In one such cave on the coast of Aceh, Indonesia a record of the 2004 tsunami along with sand layers deposited by 11 older tsunamis going back to 7400 years ago have been preserved.

Highly variable recurrence of tsunamis in the 7,400 years before the 2004 Indian Ocean tsunami-
Charles M. Rubin, Benjamin P. Horton, Kerry Sieh, Jessica E. Pilarczyk, Patrick Daly, Nazli Ismail & Andrew C. Parnell


 We identify coastal caves as a new depositional environment for reconstructing tsunami records and present a 5,000 year record of continuous tsunami deposits from a coastal cave in Sumatra, Indonesia (Fig. 1), which shows the irregular recurrence of 11 tsunamis between 7,400 and 2,900 years BP. The sedimentary record in the cave shows that ruptures of the Sunda megathrust vary between large (which generated the 2004 Indian Ocean tsunami) and smaller slip failures. The chronology of events suggests the recurrence of multiple smaller tsunamis within relatively short time periods, interrupted by long periods of strain accumulation followed by giant tsunamis. The data demonstrates that the 2004 tsunami was just the latest in a sequence of devastating tsunamis stretching back to at least the early Holocene and suggests a high likelihood for future tsunamis in the Indian Ocean. The sediments preserved in the costal cave provide a unique opportunity to refine our understanding of the behaviour of the Sunda megathrust, as well as study in detail the sedimentology and hydrological characteristics of tsunami deposits.

There is one point that cannot be over stressed. The average recurrence time for earthquakes and tsunamis has been estimated to be on the order of several hundred years. However, there is a great variation in the actual occurrence, with several smaller tsunamis occurring just decades apart. While our understanding of earthquake mechanisms and tsunami generation will go on improving, ultimately what will save lives is better preparedness. This includes adherence to structurally appropriate building codes, functioning tsunami warning systems and well drilled and practiced disaster management plans. South East Asia has long neglected these issues and there needs to be a renewed focus on them.

Sunday, July 16, 2017

Olivia Judson On Energy Expansions Of Evolution

Nature Ecology and Evolution has published a fine perspective by evolutionary biologist Olivia Judson on energy availability and evolutionary transitions on earth -

" The history of the life–Earth system can be divided into five ‘energetic’ epochs, each featuring the evolution of life forms that can exploit a new source of energy. These sources are: geochemical energy, sunlight, oxygen, flesh and fire. The first two were present at the start, but oxygen, flesh and fire are all consequences of evolutionary events. Since no category of energy source has disappeared, this has, over time, resulted in an expanding realm of the sources of energy available to living organisms and a concomitant increase in the diversity and complexity of ecosystems. These energy expansions have also mediated the transformation of key aspects of the planetary environment, which have in turn mediated the future course of evolutionary change.Using energy as a lens thus illuminates patterns in the entwined histories of life and Earth, and may also provide a framework for considering the potential trajectories of life–planet systems elsewhere."

Coincidentally, I just finished reading Nick Lane's book The Vital Question, which covers the first three sources of energy discussed in this article. Nick Lane writes about energy currencies of the cell and the constraints it places on the early evolution of life on earth. Why don't bacteria become morphologically larger and more complex?... because there are intrinsic constraints on the energy available for ATP synthesis.  You'll have to read Nick Lane's book for a detailed account but Olivia Judson's essay mentions this and more. The other two, animals and fire, encompass the evolution of complex multicellular life and their impact on evolutionary arms races and ecosystem changes.

..and what about life on other planets?..

"As this is the only life–planet system we currently know of, it is impossible to know how representative it is of life–planet systems in general. But if the development of other life–planet systems requires a similar series of energy expansions, the framework presented here suggests a way to anticipate the paths that such systems might take. For instance, if a planet has only geochemical energy— perhaps because it is far from its star, or because it is a nomad and has no star at all—any life present may have “a limited future in terms of the heights it could achieve”. Or suppose a planet is unable to accumulate oxygen. This could happen if living organisms never evolve a way of splitting water to produce the gas in the first place, but even if they do, the planet itself may have characteristics that prevent oxygen from ever building up. Without oxygen, the geological, ecological and evolutionary potential of a life–planet system is likely to be constrained, even if life forms analogous to eukaryotes in their energy-harnessing power (Box 2) were to evolve. Conversely, some planets might be able to accumulate new forms of energy, and life forms able to take advantage of them, much fasterthan Earth has."

Open Access.

Saturday, July 8, 2017

Field Photo: A Bend In The Rocks

I saw this textbook example of a fold in the Lassar Yankti valley, about 2 kilometers south of Tidang village in the Kumaon Himalaya.

Consider how rocks bend and deform in response to stress. Blue arrows denote the direction of maximum compressive stress perpendicular to the fold axis. As rocks fold, the convex portion of the fold will experience tensile forces and fractures parallel to the axial plane develop. Notice also conjugate stress fractures (black arrow). Since this is a loose boulder I cannot assign actual directions to the stress field.

The graphic below summarizes the typical fracture patterns found in folded rocks. How many of these can you identify in the fold above?

Source: Applied Hydrogeology of Fractured Rocks

My Himalayan treks over the past few years have taken me on a walk across almost the entire thickness of the Greater Himalayan Sequence. As I mentioned in an earlier post, the GHS is bounded at its base by the Main Central Thrust and at the top by the South Tibetan Detachment. It shows an "inverted" metamorphic sequence. This means that the grade of metamorphism increases as one climbs to higher structural levels. Finally, sillimanite and kyanite grade metamorphic rocks transition into a zone of partial melting and leucogranite intrusions. Above this level the grade of metamorphism decreases to biotite grade and then to a finer grained phyllite grade. One conspicuous structural feature of the GHS is that large folds are very rare. Instead, from the base right up to the zone of partial melting the GHS exhibits a homoclinal northerly dip as seen in the picture below.

Within these northerly dipping slabs, small scale ductile folding in high grade gneiss and migmatites can be seen (picture below), but the slabs themselves are not contorted into mountain face scale folds.

Large isoclinal and recumbent folding is present only in the uppermost structural levels of the GHS in the phyllite grade rocks above the zone of partial melting. The picture below shows tightly folded phyllite grade metamorphic rocks north of the village of Baaling in the Darma Valley.

And this splendid recumbent fold is exposed at village Dantu.

Why is large scale folding rare to absent over much of the thickness of the GHS? Could the movement of the South Tibetan Detachment cause folding in the underlying phyllites?

These are some of the niggling questions I am struggling with. I still have much to learn about Himalayan geology. I need to go there with a structural geologist!

Finally, a view of the outcrops from which was derived the textbook quality folded phyllite.

Tuesday, June 20, 2017

Quiz: Spot The Granite Intrusion

I came across this glacially transported boulder in the Duktu village valley near the Panchachuli Glacier in the Kumaon Himalaya.

It is a block of high grade gneiss intruded by a granite. Without scrolling beyond the first photograph, try to work out the contact between the gneiss and the granite.


The boulder is encrusted by moss. There is some mineral staining too. And sunlight falling on the rock gives it a speckled appearance.. All this reduces the contrast in color between the gneiss and the granite.

But there is a vital clue in the orientation of structures. Both the gneiss and the granite have a planar fabric imprinted on them.

The fabric of the gneiss is due to the orientation of platy minerals like micas stacked in layers, alternating with layers richer in quartz and feldspars. Assume this is the original disposition of the rock as well. The gneiss layering you see is due to the trace of horizontal planes of separation of different mineral layers. I have outlined some of this planar fabric in brown lines.

The granite has a planar fabric too, but this is due to near vertical fractures. The rock has been broken in to thin slabs  by fractures (red lines) which may have formed during the cooling of the magma. These fractures don't pass into the surrounding host gneiss. Two arms of the granite have penetrated between the gneiss layers forming mini sills.

You can see the contact (black line) between the gneiss and the granite roughly where my wallet is. Here, the horizontal planar fabric of the gneiss abruptly juxtaposes against the vertical planar fabric of the granite.

Thursday, June 15, 2017

Field Photo: Glacial Erratic

Inspired by this xkcd comic:

I saw quite a few of these glacial erratics in the Dhauliganga river valley around the villages of Duktu and Dantu. Here is my friend sitting on one of them.

This boulder is a high grade gneiss. It is an erratic because the surrounding bedrock is all low grade phyllite and slate. The source of the high grade gneiss boulder is the snow capped range you see in the background. These are the Panchachuli peaks and the Panchachuli glacier has eroded, transported and deposited gneiss rocks all the way down the valley onto a different bedrock.

The photo below shows another erratic from this valley. If you look closely it is a mixed rock made up of high grade gneiss intruded by light colored granite. A big patch of dark grey banded gneiss is visible in the lower right corner of the boulder. The cliffs in the background and the substrate on which the boulder rests is low grade phyllite.

And a long view of village Duktu with glacial erratics strewn all over the hill slope (blue arrows).

I have been promising a post on the glacial deposits of the Dhauliganga river valley. That post will come soon. Meanwhile, here is a view of some of the moraines I saw near village Duktu.  Photo taken from near the snout of the glacier facing downstream.

The linear ridge in the center of the photo made up of rust, brown and light colored boulders is a medial moraine. It was formed when two glacial streams carrying debris along their edges joined. As these glaciers receded the debris along their edges (lateral moraines) coalesced and formed a ridge in the center of the valley. You can see the milky white colored Dhauliganga river flowing to the right of the ridge. The blue arrows to the right of the picture high up along the mountain slopes point to older lateral moraines deposited when the Panchachuli glacier was thicker and extended further down in the valley...

more on these deposits later..