More reasons to conclude that coastal ‘chevrons’ are not related to mega-tsunamis

ResearchBlogging.org If there was any doubt left that coastal sand accumulations called ‘chevrons’ are *not* related to gigantic tsunamis (see previous thoughts on the subject here and here; Ole also has a recent blog post, and see a news release here), the May issue of Geology provides additional arguments to show that this is the case. Joanne Bourgeois of University of Washington and Robert Weiss of Texas A&M University, both experts in the sedimentology of tsunami deposits, present two lines of arguments. First they show that the orientation of the Madagascar chevrons is significantly different from what is predicted through modeling the tsunami. While the tsunami wave tends to hit the coast with an overall perpendicular orientation, due to wave refraction, the ‘chevrons’ are oriented at various angles to the coast, angles that are more consistent with predominant wind directions. Second, they look at the sediment transport conditions and suggest that even coarse sand must have been in suspension in flows deep enough to create the chevrons. However, dune-like bedforms cannot develop without sediment being transported as bedload; therefore, the bedforms must have a different origin than mega-tsunamis. The obvious alternative is parabolic dunes; these well-known bedforms show up when vegetation partially covers the dune’s tails and slows down sediment transport. The authors don’t hesitate to draw the conclusion that

The extraordinary claim of “chevron” genesis by mega-tsunamis cannot withstand simple but rigorous testing.

I am far from being a tsunami expert, but I find this subject fascinating. The issue of suspended load vs. bedload and stratified or laminated vs. graded bedding is equally important for deposition from tsunami waves and turbidity currents. It is worth spending a bit of time and blogspace to explore the kind of analysis of sediment transport conditions that this paper presents.

Although I see no reasons to disagree with the paper’s conclusions (as it could be predicted from my previous posts on the subject), at first reading I didn’t fully understand the line of reasoning about suspended load vs. bedload. So here goes my attempt to understand it.

The argument goes as follows. The Rouse number is the ratio between the settling velocity of a certain grain size and the shear velocity of the flow, multiplied by von Karman’s constant (which is ~0.4): Ro = ws/k*u_shear. For a grain of a given size, if the Rouse number is larger than 2.5, the grain’s settling velocity is much larger than the upward-directed component of the turbulence, and the grain tends to stay close to the bottom, in the bedload. [This is equivalent to saying that the settling velocity has to be larger or equal to the shear velocity, a condition also known as the suspension criterion]. If the Rouse number is less than 0.8, the flow is turbulent enough to keep the grain fully suspended. In between these values, there is a zone of transitional behavior. For the flows that might have deposited the chevrons, the Rouse number is always less than 2.5, regardless of how the other parameters like the Froude number, grain diameter, and roughness length are varied. Although the authors state that the flows must have been deeper than 8 m (because most chevrons are higher than 4 m, and the flow must be at least twice as high as the bedform), there seems to be no other constraint on tsunami behavior [note that I did not have access – yet – to the supplementary web material].

So the question is: doesn’t this reasoning apply to other types of flows as well? For example, the Mississippi River is certainly deeper than 8 m in many places — does this mean that it is able to suspend very coarse (2 mm diameter) sand? In other words, what is the difference between flow in a tsunami run-up and the Mississippi River? The answers might be obvious to many, but they are certainly not obvious to me.

One thing we can do is to create a different kind of plot: instead of plotting the Rouse number against flow depth, let’s plot velocity vs. depth. I have a better feeling for what are reasonable velocities for different kind of flows than I do for Rouse numbers. The Rouse number would form the third dimension of the plot; one can visualize that as a contour map of Rouse numbers as a function of flow depth and velocity:


The Rouse numbers shown in this plot are valid for a grain diameter of 2 mm and roughness length of 1 m (using the same equations for settling velocity and shear velocity as in Bourgeois & Weiss 2009). Anything coarser than this cannot be called sand any more. So if this grain size doesn’t stay in the bedload, there is no chance for finer sediment either. It is obvious from the plot that, for flows deeper than 8-10 m, very coarse sand will be part of the bedload unless flow velocity is larger than ~5 m/s. The Mississippi River at New Orleans has velocities on the scale 1.5 m/s, so 1-2 mm sand should definitely stay close to the bottom, and in fact it does.

We know however that tsunamis are not exactly tranquil flows like the big old Mississippi at New Orleans. The larger ones are fast and furious and Google Earth might need massive updates after they rearrange entire coastal landscapes. [Don’t get me wrong, I am not trying to diminish the power and destructive force of the Mississippi.] In other words, the Froude number of a tsunami run-up is larger than the Froude number of the Mississippi River. The Mississippi is relatively slow and deep; the tsunami is fast and relatively shallow. The Froude number is the ratio between velocity and the square root of gravity multiplied by flow depth:

Fr = u/√(g*d)

This number for the Mississippi is much less than one (these flows are called subcritical flows). On the other hand, tsunamis are waves of very large wavelengths, and they behave even in the open ocean as shallow water waves (wavelength 20 times larger than water depth). For these kinds of waves, the velocity is solely a function of water depth:

u = √(g*d)

If we assume that the tsunami run-up has a comparable velocity to that of the tsunami wave in the nearshore zone, we find that the Froude number of the run-up must be around 1. This is obviously a very back-of-the-envelope argument, but the point is that these flows must have in general relatively large Froude numbers. If we plot the lines for Fr = 1 and Fr =1.5 on the depth-velocity diagram (see above), we can see how different likely tsunami behavior is from that of large rivers. It also becomes evident that even coarse sand would not be part of the bedload in these flows, especially not in flows deep enough to build the ‘chevrons’. Which means that sandy tsunami deposits are likely to be largely unstructured or poorly structured sand sheets rather than several m thick accumulations of cross-bedded sand.

And that ends my Saturday exercise in Fluid Mechanics 101.

Reference
Bourgeois, J., & Weiss, R. (2009). “Chevrons” are not mega-tsunami deposits–A sedimentologic assessment Geology, 37 (5), 403-406 DOI: 10.1130/G25246A.1

Links to this post:
Scientia Pro Publica #4

Normal grading

In sedimentology, the word ‘grading’ has nothing to do with exams and assignments. Instead, it refers to a regularly decreasing or increasing grain size within one sedimentary layer. Because it is much more common than the other alternative, upward decreasing grain size is called ‘normal grading’. Grains that consistently increase in size toward the top of the bed are responsible for ‘inverse grading’. Upward fining and coarsening are related terms that are often used to describe grain-size trends in not one, but multiple beds.

Normal grading in a turbidite from the Talara Basin, Peru

The simplest way to generate normal grading is to put some poorly sorted sand and water in a container, shake it up, and then let it settle. The larger grains will settle faster than the smaller ones (as Stokes’ law tells us) and most of the large grains will end up at the bottom of the deposit. [Note that some fine grains will be at the bottom as well – the ones that were already close to the bottom at the beginning of sedimentation.] This kind of static suspension settling is not how most sediment is deposited on a river bed or a beach; even if a grain is part of the suspended load, it usually goes through a phase of bedload transport, that is, a phase of jumping and rolling and bouncing on the bed, before it comes to rest. The resulting deposit usually has lots of thin layers, laminations, and there are no obvious and gradual upward changes in grain size.

What is needed is a sediment-rich flow that suddenly slows down or spreads out and looses its power to carry most of its sediment load. Grains are getting to the bottom so fast that there is not much time for the flow to keep them rolling and bouncing around; instead they quickly get buried by the other grains that are ready to take a geological break. While this is still quite different from static suspension settling (because the flow did not come to a full stop), it can be thought of as a modified version of static settling: all is needed is a horizontal velocity component, in addition to the vertical one. Of course, the segregation of the coarser grains to the bottom of the flow may have started much earlier. Typically, they never made it to the top in the first place.

Conglomerate bed in the Cretaceous Cerro Toro Formation, Torres del Paine National Park, Southern Chile. There is some inverse grading at the base of this bed, before the size of the clasts starts decreasing

Such large, sediment-laden flows are not very common, certainly not on a human timescale. When they do occur, they tend to show up in the news, especially if human artifacts, or humans themselves, become part of the normally graded deposits. Deposits of snow avalanches, volcanic ash-laden pyroclastic flows, subaerial debris flows, tsunamis, submarine turbidity currents can all show normal grading. The images shown here all come from deposits of large submarine gravity flows. Some of them (like the one below) have a muddy matrix, but the grading is still obvious (the two large clasts at the top of the bed have lower densities).

Normally graded conglomerate layer with a muddy matrix, Cerro Toro Formation, Chile

In recent years, some questions have been raised about the common presence of normal grading, especially in turbidites. The fact is that normal grading is often seen in rocks of all ages, and, in a simple view, it is a reflection of larger grains getting quickly to the bottom.

Normal grading is normal, after all.

Description does not suffice for an explanation

On February 3, 1967, J. R. L. Allen gave the fifth “Geologists’ Association Special Lecture”, entitled “Some Recent Advances in the Physics of Sedimentation”. This is from the introduction:

“Two stages can generally be recognised in the historical growth of a reasonably advanced scientific discipline. There is an early, descriptive stage in which with little guide from theory, an attempt is made to collect, define and analyse phenomena. In the later, explanatory stage we see that efforts are concentrated on the production of generalisations and on the explanation of the reduced phenomena in terms of general laws. Of course, there is never a single point in time at which there is change over the entire scope of a discipline from the descriptive to the explanatory stage. The change is, rather, uneven, taking place earlier in some branches than in others, and more gradually in one branch than in another.

Sedimentology stands today in a period of transition. Its subject matter is sedimentary deposits, and its goal the origin and meaning of these in the context of planetary studies in general. But it is apparent, except to adherents of geological phenomenalism, that sedimentary deposits cannot be explained in terms of themselves. Already we are in possession of major generalisations about these deposits, and our chief task for some years should be to explore and ratify them in terms of general laws in order that our understanding of the sedimentary record can be made sharper. In those parts of the field where major generalisations have already been established, the provision of further descriptive data is of little value, except in so far as light is shed on the problems of particular deposits. These are validly a part of the subject, leading to a refinement of certain planetary laws. But the other and no less important laws in terms of which we should seek to frame our understanding are those of general chemistry, physics and biology. In order to achieve this framework in the case of detrital sediments, it will be necessary to set aside for a while the problems of particular deposits. This will, of course, be unacceptable to those who claim that geology, or sedimentology, is only to do with rocks as conceived in a historico-geographical manner. But they will be proved wrong, provided we keep our major goals in mind, for it is a mistake to suppose that a description will suffice for an explanation. Most of our explanations will probably turn out to be no better than qualitative, so complex are most sedimentary systems, but we should nevertheless attempt them and try to frame them as exactly as possible.”

Forty years after publication of the paper, this seems as timely as ever.

Reference:
Allen, J. R. L., 1969. Some Recent Advances in the Physics of Sedimentation. Proceedings of the Geologists’ Association 80:1-42.

Three photos from Chilean Patagonia

I was lucky to attend a few days ago a field conference in southern Chile, looking at deep-water rocks in an area that includes Torres del Paine National Park. It was good to be back in this place of unbloggable beauty. The conference was well organized (of course! – Brian was one of the conveners) and we were extremely lucky with the weather: no rain at all on the outcrops, beautiful sunshine most of the time. Although I have been to Chilean Patagonia three times before on various geological field trips and even did some field work there, I realized during this conference that it doesn’t matter how many times you have seen some rocks, there is always a chance to rethink what you thought you have already settled in your mind (see blog title). It was also good to see that these field conferences are increasingly not just about the local geology: many if not most presentations and spontaneous discussions compare the local outcrop data with sedimentary systems from other basins, and try to think about how the always-too-small outcrops would look like in seismic sections and volumes.

Brian did not have time to take a lot of photos, so here are three shots (more here). As if anybody needed more shots of the Paine Grande and the Cuernos.

Conference participants examine the turbidites of the Punta Barrosa Formation

The Paine massif (Paine Grande and Cuernos), with Rio Serrano in the foreground

Strong winds on Paine Grande

Update – here is a Gigapan:

function FlashProxy() {}
FlashProxy.callJS = function() {}

http://gigapan.org/viewer/PanoramaViewer.swf?url=http://share.gigapan.org/gigapans0/18452/tiles/&suffix=.jpg&startHideControls=0&width=42349&height=11090&nlevels=9&cleft=0&ctop=0&cright=42349.0&cbottom=11090.0Launch full screen viewer

[it is strongly recommended that you do launch the full screen viewer if you want to do justice to the Gigapan]

The science of espresso, with a dash of geology

Almost every morning, I start the day with an experiment on flow in porous media. First, I generate some fine-grained sediment with a well-defined average grain size and proper sorting; then I use that sediment to fill a little basin of sort and try to mimic compaction. Finally, I use a machine to put water under pressure and force it to flow through this miniature sedimentary basin. Then I sit down to drink the fluid which is not simple water anymore, due to its interaction with the grains; and its taste and consistency tell me whether I got the grain size and the porosity right.

That’s a geologist’s view of making espresso. Unless you have a fully automated and ultra-expensive espresso machine, creating a high-quality caffeine concoction is not trivial, because the water must have the right temperature and has to spend the right amount of time in contact with the coffee grains that have the right size. The right temperature is 85–95 °C (185–203 °F), and, at least with our simple machine, the trick is to start the brewing at the right time. Better espresso machines do not use steam to generate pressure because that makes the water too hot; instead, they have a motor-driven pump that generates the ~9 bars of pressure. The correct grain size is easily achieved with a burr grinder (as opposed to a simple blender); a good espresso grind is a fine grind, because the water spends relatively little time in contact with the grains.

The duration of this contact is the most difficult bit to get right. To get a good shot with lots of crema, it cannot be less or more than 20 to 30 seconds. Not just grain size, but grain sorting as well play a role. If the coffee grinder produces a poorly sorted ‘sediment’ (and that’s what a blender does), the coffee will not be porous and permeable enough. Another factor is how well the sediment is compacted; that is, how much pressure do you apply to the coffee during tamping. This affects permeability again. Finally, it matters how much coffee you put in the coffee holder; the thicker the layer that the water has to go through, the longer the trip becomes for the same amount of water.

After using the machine hundreds of times, I still manage from time to time to produce something undrinkable. The art and science of espresso making started to make more sense once I started to think of it in terms of Darcy’s Law.

Henry Darcy was a French engineer who initially had made a name for himself by designing an enclosed and gravity-driven water-supply system for the town of Dijon. Later he had time and opportunity to do experiments of his own interest. In 1855 he measured the discharge of water under variable hydraulic heads through sand columns of different heights, and found that the discharge was directly proportional with the hydraulic head and inversely related to the height of the sand column:

Q = AK(H1-H2)/L,
where A is the cross sectional area of the sand column, L is the height of the sand column, K is the hydraulic conductivity (which is constant for the same granular material and same fluid), and H1-H2 is the hydraulic head. This is Darcy’s drawing of his experimental setup:

The hydraulic conductivity depends on both the properties of the fluids and of the granular material; these properties are the viscosity and density of the fluid, and the permeability of the sediment:

K = kρg/μ,

where K = hydraulic conductivity, k = permeability, ρ = fluid density, and μ is the dynamic viscosity.

In coffee speak, the hydraulic head is given by the pressure generated by the machine, and is fixed; one cannot change the density and viscosity of water either. The most important variable is coffee permeability, which is influenced by size, sorting, and packing (compaction) of the coffee grains. Also, it helps if you get the value of L right, that is, you shouldn’t try to save coffee.

Darcy’s Law was established with some simple experiments, and it has since then been generalized and derived from the Navier-Stokes equations, but it has a huge range of applicability, from ground-water hydrology to soil physics and petroleum engineering.

Add to that list everyday espresso making.

ps. Fantastic resource on Darcy’s work and his law here.

Earth, water, wind, and fire: ‘Lava viewing’ in Hawaii

Our Christmas gift to ourselves was a little trip to the Big Island of Hawaii, something we were thinking (dreaming) about for a long time. There are many great posts about the Hawaiian volcanoes on the geoblogosphere (see for example the ones here and here); I will try to add a few notes and pictures without being too repetitive (and will try to seem less ignorant in volcanic and hard-rock matters than I actually am).

Probably the most memorable experience we had was the lava viewing at Kalapana. This is where ‘officially’ you can get relatively close to the place where the lava from Pu`u `Ō`ō enters the ocean. The USGS has a nice website with updates on what’s going on. I was so anxious to see this place that we had to go there on our first day in Hawaii, that is, on December 22. You have to drive all the way to the end of road 130; there are some big ‘No trespassing’ signs at one point, but everybody seems to ignore them, and there is an official parking lot at the end of the road, way beyond the ‘no trespassing’ signs. It is best to get there 30-60 minutes before sunset, and to stay until it’s completely dark, to see the potential show both in daylight and in nighttime darkness. Unfortunately, on December 22 we didn’t see much, apart from a beautiful sunset and a few small puffs of steam:

Sunset at the Kalapana viewing site on December 22, 2008

That was a bit of a disappointment, but I knew I had to give it another try. After talking to a ranger from Volcanoes National Park, we drove back to Kalapana five days later. This time, the show was definitely on. More than that, it was spectacular. A huge column of steam formed where the active lava tube spills the lava into the sea, and repeated explosions painted red the lower part of the column. From time to time, several tornado-like funnels formed and connected the steam cloud to the ocean.

Steam cloud with mini-tornadoes on December 27, 2008; lava-viewing boat on the left for scale

As the sun goes down, the explosions become more colorful and more obvious

S-shaped funnel between the steamy sky and cool hot ocean

This was such a uniquely beautiful scene. I wish we went there more than two times, because the whole spectacle changes as a function of the activity of lava flow, weather conditions, the direction and nature of lighting.

I have also learned that it is not easy to take good photographs of fast-moving and rapidly changing distant things in the dark. Here is the proof:

Zoom, baby, zoom*

For a few months now, I have been spending (wasting?) some time with a gadget called Gigapan, a robot that can take hundreds of shots of the same scene with a simple point-and-shoot camera. The pictures are taken in a well-defined rectangular grid pattern so that there is the right amount of overlap between all neighbors. Later the photos can be stitched into a gigantic photograph on a computer and shared with the world through the Gigapan.org website and, even better, through Google Earth. [If you are a tiny bit familiar with geoblogs, you must have seen some of the gigapans that Ron Schott has put together; he is one of the earliest and most enthusiastic adopters of the technology and has assembled an impressive set of panoramas on the gigapan site.]

I have to confess that I had to actually buy this thing and start playing with it to realize how different gigapixel panoramas are from the usual few-megapixel digital photographs. The idea is simple: a ten megapixel camera takes photos that contain ten million pixels; if you put together a 10×10 grid of such photographs into one image, you end up with a gigapixel panorama. Because some overlap is needed between the photographs, more than 100 pictures are necessary to exceed the gigapixel limit. But the point is that the more pixels there are in a photograph, the more information it contains and the more sense it makes to zoom in and see the details – details that are usually non-existent in a conventional digital picture. The other side of the coin is that it is only worth taking gigapans of scenes with plenty of small-scale and variable detail (although I am getting to the point that I see a potential gigapan everywhere).

I do not think that gigapixel images will replace conventional (that is, megapixel) photography. There is only a limited number of things that the human eye can see at one time; and often the value of a good photograph comes not from the pixels it captures, but from the ones it consciously ignores. Beauty and the message an image can hold are scale-dependent; and zooming in to see the irrelevant detail could be a distraction.

That being said, I am all for taking home as many pixels as possible from outcrops and landscapes in general. The gigapan system is simple and works surprisingly well, and it *is* exciting to explore big outcrop panels from the scale of entire depositional systems to the laminae of single ripples or even grains.

No photos or panoramas posted/embedded this time; but here is a link to my giga-experiments.

* title is courtesy of Kilgore661

Images from South Africa: Patterns

A few more photos from the same trip that I already posted photographic highlights from. To be more factual and fair, the title should be “Images from the Western Cape”, because I have only seen a few places in South Africa, and all of those places are in the Western Cape province. Anyway, here are three photos of… well, not much, just some visually interesting patterns.

Halophytic (salt-loving) vegetation in the supratidal zone of the Langebaan Lagoon, West Coast National Park

Old tree trunk at Groot Constantia winery, Cape Town, the first winery in South Africa, created in 1685

A look at the pebble (beach near Cape of Good Hope)

Liesegang bands in sandstone

Liesegang bands are poorly understood chemical structures often seen in rocks, especially sandstones. They were discovered more than a hundred years ago by the German chemist Raphael E. Liesegang, when he accidentally dropped a drop of silver nitrate solution on a layer of gel containing potassium dichromate, and concentric rings of silver dichromate started to form.

In sedimentary rocks, Liesegang bands appear well after the sediment has become a rock (that is, it got compacted and cemented). Stratification and lamination within the sansdtone are typically cross-cut by the Liesegang bands; fractures usually have a more obvious effect on the distribution and orientation of these.

The rocks shown here are turbidites of the Permian Skoorstenberg Formation, in the Karoo desert of South Africa. This Liesegang banding developed in the neighborhood of a small thrust and consists of brown bands of iron oxide that entirely ‘ignore’ the original lamination of the sandstone (not visible in the photos), but clearly like to precipitate along some of the fractures in the rock.