Rivers through time, as seen in Landsat images

Thanks to the Landsat program and Google Earth Engine, it is possible now to explore how the surface of the Earth has been changing through the last thirty years or so. Besides the obvious issues of interest, like changes in vegetation, the spread of cities, and the melting of glaciers, it is also possible to look at how rivers change their courses through time. You have probably already seen the images of the migrating Ucayali River in Peru, for example here. This river is changing its course with an impressive speed; many – probably most – other rivers don’t show much obvious change during the same 30-year period. What determines the meander migration rate of rivers is an interesting question in fluvial geomorphology.

The data that underlies Google Earth Engine is not accessible to everybody, but the Landsat data is available to anyone who creates a free account with Earth Explorer. It is not that difficult (but fairly time consuming) to download a set of images and create animations like this:

ucayali7

This scene also comes from the Ucayali River (you can view it in Google Earth Engine over here) and it is a nice example of how both neck cutoffs and chute cutoffs form. First a neck cutoff takes place that affects the tight bend in the right side of the image; this is followed by a chute cutoff immediately downstream of the neck cutoff location, as the new course of the river happens to align well with a pre-existing chute channel. The third bend in the upper left corner shows some well-developed counter-point-bar deposits. There is one frame in the movie for each year from 1985 to 2013, with a few years missing (due to low quality of the data).

Where on Google Earth? WoGE #296

Hindered Settling hasn’t hosted a Where-on-Google-Earth in a long time, but WoGE #295 (hosted at Andiwhere’s) had such a range of colors and geological features that I couldn’t refrain from looking for it and, once found it, had to post the solution. So, after a short break in the game (busy week!) here is WoGE #296 — the rules of the game are nicely described over here. I invoke the Schott rule. Posting time is July 8, 2011, 14:00 UTC.

click on image for larger view

Snorkeling and geology in Kealakekua Bay, Big Island, Hawaii

For a long time, I didn’t think it was worth spending more than an hour on a beach, even the most beautiful ones, unless there were some nice cliffs nearby showing some interesting geology. My views in this regard have changed dramatically about three years ago, when I spent a week on The Big Island of Hawaii, and the hotel where we were staying offered free rental of snorkeling gear. I put on the mask and the fins, trying to remember how this was supposed to work (I did a bit of snorkeling in Baja California many years before that), and put my face into the not-too-interesting-looking waters in the front of the hotel.

Kealakekua Bay in Google Earth, with some explanations added

I was in for a surprise. The water was far from crystal clear, but I could still see fantastic coral creations lined up along the bay and lots of fish of so many colors and patterns that it felt unreal. Until then I thought that this kind of scenery was hard to see unless you were a filmmaker working for Discovery Channel or a marine biologist specializing in tropical biodiversity. The next day I spotted a couple of green turtles frolicking in the water, clearly not bothered by the nearby snorkelers, and I already knew that I needed to look into the possibility of buying a simple underwater camera.

Lots of coral, mostly belonging to the genera Lobites (lobe coral) and Pocillopora (cauliflower coral)

Three years later I went back to the Big Island with more excitement about tropical beaches, plus bigger plans and a bit more knowledge about snorkeling. After going through a few well-known snorkeling sites on the west coast, like Kahalu’u Beach in Kona and Two Step near Pu’uhonua o Honaunau park, we got on a nice boat (run by a company called Fair Wind – strongly recommended!) and did some snorkeling in Kealakekua Bay.

Visibility in Kealakekua Bay is usually very good

Old wrinkles of pahoehoe lava getting encrusted by algae and corals and chewed up by sea urchins

Kealakekua Bay is difficult to reach; there is no road and no parking lot nearby. You either have to hike in, paddle through the bay in a kayak, or take a boat. I have heard before that this was the best snorkeling spot in Hawai’i, but I think that is an understatement. Unlike all the other spots we tried during the last few years in Hawaii (and that includes several beaches on Kauai and Hanauma Bay on Oahu, the presidential snorkeling site), the water at Kealakekua Bay was calm and very clear, with fantastic visibility.

Heads of cauliflower coral, with yellow tangs for scale

I will not attempt to describe this whole new world; instead I will let the photographs speak for themselves (as always, more photos at Smugmug). Even better, if you go to the Big Island, make sure that you visit this place with some snorkeling gear.

Yellow tangs (Zebrasoma flavescens) often congregate in large schools and it is difficult to stop taking pictures of them

When I was at Kealakekua Bay, I didn’t know much about the local geology. The big cliff bordering the bay toward the northwest, called Pali Kapu o Keoua (see image above), shows a number of layered lava flows that belong to the western flank of Mauna Loa; and I suspected that this must have been a large fault scarp, but that was the end of my geological insight. A couple of hours worth of research after I got home revealed that Pali Kapu o Keoua was a fault indeed: it is called the Kealakekua Fault and it has been mapped, along with the associated submarine geomorphological features, in the 1970s and 1980s by U.S. Geological Survey geoscientists.  It turns out that one of the shipboard scientists and key contributors to these studies was Bill Normark (see also a post about Bill at Clastic Detritus). While in California in the late 1990s, I was lucky to get to know Bill and have some truly inspiring discussions with him about turbidites, geology, and wine, so this was a doubly valuable little discovery to me.

So what is the origin of the Kealakekua Fault? The Hawaiian Islands are far away from any tectonic plate boundaries, so there is not a lot of opportunity here for inverse or strike-slip faults to develop. However, the Hawaiian volcanoes are humongous mountains and their underwater slopes are extremely steep by submarine slope standards: gradients of 15-10˚ are common. [This is in contrast by the way with the relatively gentle slopes of 3-8˚ the subaerial flanks of the volcanoes, a difference that – it just occurred to me – has to do something with the different thermal conductivities of water and air. Water is ~24 times more efficient at cooling lavas, or anything for that matter, than air, so once a volcano sticks its head out of the water, basaltic lava flows are pretty efficient at carrying volcanic material far away from the crater, thus building gently sloping shield volcanoes. The same flows are promptly solidified and stopped by the cool ocean waters as soon as they reach the coast.] Slopes that are this steep are also unstable; the underwater parts of these volcanoes tend to fail from time to time and large volumes of rock rapidly move to deeper waters as giant submarine landslides. Seafloor mapping around the islands revealed that the underwater topography is far from smooth; instead, in many places it consists of huge slide and slump blocks.

Topographic map of the Big Island. Note the location of Kealakekua Fault and the rugged seafloor to the southwest of it, marking the area affected by slides and slumps. This is a map based on higher-resolution bathymetric data collected during a collaborative effort led by JAMSTEC (Japan Marine Science and Technology Center). Source: U.S. Geological Survey Geologic Investigations Series I-2809 

Kealakekua Fault is probably part of the head scarp of one such giant landslide, called the Alika landslide. This explains the steep slopes in the bay itself: after a narrow wave-cut platform, a spectacular wall covered with coral – the continuation of the cliff that you can see onshore – dives into the deep blue of the ocean as you float away from the shore. In contrast with submarine landslides that involve well stratified sediments failing along bedding surfaces and forming relatively thin but extensive slide deposits, the Hawaiian failures affect thick stacks of poorly layered volcanic rock and, as a result, both their volumes and morphologic relief are larger (see the paper by Lipman et al, 1988). The entire volume of the Alika slide is estimated to be 1500-2000 cubic kilometers. That is about a hundred times larger than all the sediment carried by the world’s rivers to the ocean in one year! The slides have moved at highway speeds and generated tsunamis. There is evidence on Lanai island for a wave that carried marine debris to 325 meters above sea level; this tsunami was likely put in motion by the Alika landslide*.

You don’t want to be snorkeling in Kealakekua Bay when something like that happens. And it will happen again, it is a matter of (geological) time. Giant underwater landslides are part of the normal life of these mid-ocean, hotspot-related volcanoes.

Reference
Lipman, P., Normark, W., Moore, J., Wilson, J., Gutmacher, C., 1988, The giant submarine Alika debris slide, Mauna Loa, Hawaii. Journal of Geophysical Research, vol. 93, p. 4279-4299.

*tsunamis generated by landslides is a whole new exciting subject that we have no time now to dive or snorkel into.

Morphology of a forced regression

‘Forced regression’ is an important concept in sequence stratigraphy – it occurs when relative sea level falls and the shoreline shifts in a seaward direction, regardless of how much sediment is delivered to the sea. This is in contrast with ‘normal’ regressions, which take place when relative sea level doesn’t change or it is rising, but rivers bring lots of sediment to the coast and are able to push the shoreline seaward. These concepts are commonly illustrated with simple cartoons (like the ones on the SEPM sequence stratigraphy website), showing how beach deposits stack in a dip direction, and how their tops are eroded by rivers as sea level continues to fall.

Unless you live in a horizontally challenged flatland (vertical-land? 2D seismic-land?), real regressions happen in three dimensions, and their morphology is much more complicated, more interesting, and more beautiful than what one can dream up with a few lines in a single cross section. The example below is an airborne lidar image from Finland. The original data has a horizontal resolution of 2 meters and a vertical resolution of 30 centimeters.

Airborne lidar image of uplifted coastal plain in Finland
Image courtesy of Jouko Vanne, Geological Survey of Finland

The two dominant morphologies and deposit types clearly visible in the image are (1) ancient coastlines, formed as sand brought to the sea by rivers was reworked by waves into beach ridges; and (2) an incised river valley that cuts through these shoreline deposits. Note how the river seems to be incising and migrating laterally at the same time, generating a scalloped valley edge. The reason for this forced regression during a time of global sea-level rise is the isostatic rebound of the Scandinavian Peninsula after the retreat of the ice sheet.

Looking at this crystal-clear morphology, it is tempting to think that this area must look very interesting in Google Earth as well. It turns out that it doesn’t; this is actually a pretty heavily vegetated land, not too spectacular on conventional satellite imagery (see figure below). The laser rays of the lidar are able to see through the non-geomorphological ‘noise’ and show stunning geomorphological detail.

Comparison of satellite image from Google Earth with detail of lidar topography

To explore a higher resolution version of this image, and for additional lidar visualizations of similar beauty, check out Jouko Vanne’s Flickr site. The National Land Survey of Finland has started collecting this kind of data in 2008 and they are planning to cover the whole country with high-resolution DEMs within a few years.

A great way to spend taxpayer money, as far as I am concerned.

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

Hurricane sedimentology

Hurricane Ike knocked me off the Internets for a while, but things are slowly getting back to normal. I haven’t been so close to – that is, in the middle of – a hurricane before; I have to say it was quite an adrenaline rush to hear and, to a lesser degree, watch the wind going by our windows with gusts of (probably) more than 100 miles per hour. In Houston, the damage was largely restricted to fallen trees and a few broken windows; fortunately not too exciting (see a few post-Ike photos over here). However, things are very different as you get close to the coast. Along the East Texas coast, the storm surge (unusually large for a category 2 hurricane) has shifted the coastline a few tens of meters landward, deposited lots of washover fans toward the lagoonal sides of the barrier beaches, and destroyed a large number of homes in the meantime.

It is worth playing the before-and-after game with the aerial imagery shot by the NOAA’s Remote Sensing Division and made available in Google Earth. Here are a few screenshots from the Bolivar Peninsula.

Before:

After:

The light-colored lobes in the upper part of the ‘after’ image are the washover fans that in places reach the lagoon. Even more interestingly, there are some beautiful little fans built by the water flowing back toward the sea; for each fan, you can see the erosional ‘drainage’ area and little tributary gullies merging into a single large channel seaward, that turns from erosional to depositional as it widens. This is more clear in the zoomed-in pictures below.

Before:

After:

Note how some of the streets and roads that were perpendicular to the coast have become sites of preferential water flow and therefore locations for these channels.

Of course, all this cool sedimentology (I cannot wait to be able to get out there and have a closer look) also means that many-many homes have just become part of the stratigraphic record. These beaches and islands are the product of the interaction between storms like Ike, when large amounts of sand is eroded from the beach and transported offshore, and fairweather conditions, when sand has some chance to be deposited on the beach (if there is a large enough source of sand somewhere – usually not the case along most of the present-day Gulf coast).

The message should be already boring, but apparently it is not: these beaches and the barrier islands they create are geologically extremely active creatures, and in general it is not a good idea to build homes on them, certainly not right next to the beach. Hurricanes will be around for a while (and some experts say they are getting larger and stronger); and they are very good at creating large storm surges that are highly destructive on shallow shelves with low gradients, such as the continental shelf in the northern Gulf of Mexico.

Where on (Google) Earth? #138

It must be obvious by now that I am a lazy blogger – it looks like I settled down to a comfortable and boring average of one post per month.

But. I got back my WoGE-mojo yesterday, and bumped into Peter’s WOGE screenshot while gliding over the karstified limestones of the Dinarides in Montenegro.

So here is WoGE number 138. No rules. Have fun.

Geologic misconceptions: 2D vs. 3D

[This is a contribution to Accretionary Wedge #5]

One of the problems that a geologist is often faced with is the difficulty of reconstructing a complex three-dimensional geometry and history from limited information that is often one-dimensional (e.g., well data, cores) or two-dimensional (outcrops, 2D seismic sections). Humans in general, and geologists in particular tend to look for evidence where the light is better, and we are tempted to think that the beautiful core we have described, the one good outcrop face we have, the one textbook-quality seismic line on our wall is a good representation of the geology and stratigraphy of a much broader area, and that one can build a coherent story without knowing much about the third dimension.

That, of course, may well be true of ‘layercake’ stratigraphy: after all, a single thickness value can be used to fully characterize the geometry of a layer that has the same thickness over a large area. But, as Brian points out, ‘layercake stratigraphy’ should be considered an oxymoron: every sedimentary layer shows some thickness variations if traced for a long enough distance, even if some layers change their thickness more slowly than others. Stratigraphy is only layercake-like for human observers; subtle but persistent variations in thickness and relief can become striking geometries with some vertical exaggeration. Again, if this variation only occurred in one direction, a two-dimensional section along the same direction would summarize very well the whole story.

However, complex three-dimensionality is the rule rather than the exception in geology. Take for example a meandering river: its geometry is complex enough as it is, a single snapshot of a snaky morphology in time. But try imagining what happens as point bars and levees are deposited and cutbanks are cut; the channel changes its position over time and, over thousands and hundreds of thousands of years, it leaves behind an extremely complicated stack of deposits that would probably be difficult to fully understand even if you somehow could see and describe everything at the greatest detail in 3D. Obviously, a nice outcrop or a number of cores through such a deposit can provide a wealth of information, but we would be fooling ourselves if we thought that a single fining-upward sequence with some cross-bedding (that is, the classic point-bar facies model) was enough to understand a fluvial system.

But strong three-dimensionality is not restricted to fluvial deposits; look at any present-day depositional system in Google Earth and you will find that alluvial fans, deltas, barrier islands and tidal inlets, wind-blown dune fields are all intricate patterns, usually with lines running in more than one direction. Yet many of the classic facies and stratigraphic models are either one- or two-dimensional. Maybe, probably, these are necessary and useful simplifications and conceptual models, but they can only be useful if one is also aware how far they are from capturing the full 3D complexity of nature.

That being said, I have to add that 3D is not always better than 2D. Nowadays, some of the best three-dimensional geological datasets are 3D seismic surveys, and, with the increasing availability of such gold-mines of stratigraphic beauty (there are other uses as well, but let’s focus on one thing for now :) ) it is easy to fall victim to the temptations of colorful three-dimensional displays. Despite claims like ‘3D interpretation and visualization are the future’, the truth is that a good set of old-fashioned maps and cross sections are more valuable in the long term than some glossy presentation slides with no exact spatial location.

Unless, of course, you can visualize and share your data relying on an easy-to-use and truly three-dimensional viewer. Like Google Earth. Even William Smith would be excited about that.


Detail from “Geological view and section through Dorsetshire and Somersetshire to Taunton, on the road through Yeovil toWimborn[e] Minster, &c.”, by William Smith, 1819. Source: Oxford Digital Library

Catastrophic flooding of the Black Sea and the expansion of agriculture in Europe

ResearchBlogging.orgBoth Ole and Chris have blogged about a new paper, published in Quaternary Science Reviews, that discusses the link between the catastrophic inundation of the Black Sea and the expansion of agriculture in Europe during the Neolithic. I am not going to repeat what they already summarized well; I only want to expand a bit on what I see as the weak points of the Black Sea flood story. [Disclaimer: I am not an expert in the geology and stratigraphy of the Black Sea, and even less of an expert in archaeological matters].

While the recently published Turney & Brown paper presents some nice data and argues convincingly that the start of Neolithic expansion in Europe roughly coincides with the ~8300 yr BP age estimate for the catastrophic flooding of the Black Sea, this correlation does not necessarily suggest a cause-and-effect relationship. The question is still open: yes, the flooding might have caused the migration, but it is also possible that the two events are independently related to the same climatic changes. Very few of the radiocarbon dates, representing the earliest Neolithic sites in Europe, come from the territory of present-day Romania, yet one would expect that the low-lying areas along the lower Danube River would be the first places to be colonized by the population forced out from the inundated shelves of the Black Sea that are the widest east of the Danube Delta. Why are early Neolithic settlements so scarce in this area, which is good for agriculture? In their 1998 book, Ryan and Pitman suggest that Vinca farmers showed up abruptly along the Danube valley, soon after the flooding occurred. The earliest Vinca settlements however are dated at 7500 yr BP, so there is a gap of a few hundred years between the flood and these first settlements. That seems too long; in addition, even if the initial displacement of people living near the Black Sea resulted in the expansion of agriculture into Southeastern Europe, it is questionable how much effect this flooding had on the spread of Neolithic people into Northwestern Europe. It is unlikely that the main motivation to cross the English Channel for people living in today’s Northern France was that their ancestors were scared off the shores of the Black Sea many hundreds of years before.

The image below is a screenshot from Google Earth, showing the data published by Turney and Brown. The Neolithic locations are color-coded according to their age, red being the oldest, and dark blue being the youngest. The KMZ file (made with GPS Visualizer and some help from Matlab) is available here.


Regarding the link between the Black Sea flood and Noah’s story — I think it is an interesting idea, but not much more. There were and there will be numerous large floods that affect human lives and human history, and the one featured in the Bible is so generic that it will be difficult to unequivocally link it to any specific event. In addition, the catastrophic flooding of the Black Sea might have been catastrophic only in a geological sense; calculations suggest that it took 34 years to erase the 155 m difference in water level between the Black Sea and the Mediterranean. The slow and relentless rise of the sea must have been disquieting and annoying to people living close to the shore, but, unless they built their huts in the middle of the Bosporus, it probably was not as traumatic for most of them as Hurricane Katrina was for lots of Gulf Coast residents.

The recent (I mean geologically recent) history of the Black Sea region is certainly fascinating and the controversy surrounding the exact sequence and nature of the events can only result in more top-notch oceanographic and archeologic research. And that is always exciting.