Megainundaciones: ¿cuánto contribuyen al relieve terrestre?

[Este post fue inicialmente escrito para la revista de divulgación Naukas y está relacionado con dos artículos científicos que hemos publicado recientemente (ver lista de referencias al final)]

Antes de la geología estaba el mito. Catástrofes épicas que explicaban porqué vemos fósiles de seres que no existen y porqué otros fósiles que reconocemos como seres marinos se encuentran en lo alto de las montañas. Esa visión catastrofista tenía respuesta para todo y se convirtió en parte fundamental de las religiones.

Pero cuando la revolución copernicana emergió del renacimiento, esa forma mágica y sobrenatural de entender el mundo dejó de bastar y surgió la necesidad de comprender en base a lo cotidiano, a lo empírico, con un alcance universal.
Nicolas Steno desarrolló en 1669 los principios de la estratigrafía y un siglo después, el concepto trending de la época, el uniformismo, fue incorporado a la geología bajo el nombre de gradualismo (Hutton, 1785). Postulaba que las rocas y sus fósiles han sido formadas por los mismos procesos que observamos hoy en día, actuando lentamente, a velocidades similares a las actuales y durante  larguísimos periodos de tiempo que desafiaban los dogmas religiosos.

Así pues, desde los orígenes de la geología como una ciencia moderna más, el relieve de la Tierra ha sido visto como el resultado de lentos procesos: la erosión de los ríos; el movimiento y la deformación de los continentes. La ciencia geológica se fraguó por tanto en contraposición con aquella visión religiosa de grandes cataclismos. El gradualismo se convirtió en uno de sus más sólidos mantras científicos.

Y todo fue muy bien durante 150 años hasta que, a principios del siglo pasado, un hombre se atrevió a blasfemar contra ese paradigma tan lentamente consolidado. Se llamaba J. Harlen Bretz.

J. Harlen Bretz, 1949.

Bretz estudió el paisaje de la región de los Scablands, que ocupan buena parte del estado de Washington (EEUU). Encontró formas erosivas y acumulaciones de sedimento que sólo podía explicar invocando megainundaciones de una magnitud sin precedentes, hoy bien conocidas como las Inundaciones de Missoula. Inundaciones descomunales ocurridas hace unos 17.000 años y que debían haber excedido en varios órdenes de magnitud las inundaciones que habitualmente, en base a nuestra corta experiencia histórica, consideramos catastróficas.

Pese a su conocido carácter terco, Bretz tardó cuatro décadas en convencer a la comunidad geomorfológica de que su interpretación, por excéntrica que pareciera, era la más sencilla. Se estaba enfrentando a siglos de lucha entre las concepciones geológica y religiosa del mundo, y muchos de sus colegas le consideraban un lunático defensor de la segunda. A su manera, Bretz se convirtió en un hereje de la ciencia.

Todavía hoy en día, la noción de que las inundaciones más excepcionales también contribuyen al modelado del paisaje sigue siendo vastamente ignorada.

Pero ¿cuáles son estos fenómenos? ¿Cuanto contribuyen? ¿Cómo de excepcionales son?


Uno de los mecanismos responsables de estas megainundaciones es el desbordamiento de grandes lagos. El fenómeno es idéntico al que ocurre cuando una avalancha de roca bloquea el valle de un río de montaña y forma un nuevo lago: Cuando el lago rebosa, aunque inicialmente lo haga muy lentamente, la erosión puede desencadenar un aumento exponencial del flujo de agua, hasta producir caudales enormes de agua que pueden causar importantes pérdidas humanas y económicas río abajo.

Para emular el proceso, en este experimento en el USGS de Oregón formamos un pequeño lago tras una barrera de arena compactada:

Experimento de desbordamiento de un lago 

de 23 m2 barrado por arena compactada.

La erosión que produce el agua en el canal de salida se retroalimenta con el flujo de agua que dicho canal permite evacuar:

Esquema de la retroalimentación entre flujo de agua y erosión del 
desaguadero de un lago de montaña. Cuanta más erosión, más caudal 
de agua. Cuanto más caudal, más rápida la erosión.

Los datos disponibles sobre el pico de caudal que se alcanza en desbordamientos históricos permiten estimar empíricamente el riesgo en escenarios naturales. Los resultados son bastante intuitivos: cuanto mayor es el tamaño del lago y más débil es la barrera, más intenso será el pico de descarga de agua tras el desbordamiento. Pero estos resultados apenas permiten predecir la intensidad de las inundaciones porque las heterogeneidades de la barrera pueden ser tan determinantes como los factores anteriores: Una sola roca de gran tamaño, por ejemplo, puede retrasar la erosión del desaguadero y evitar la inundación.

Este sigue siendo el video que mejor muestra 
la fuerza a la que puede conducir un desbordamiento 
(en este caso el mar desborda sobre una mina a cielo 
abierto). Se trata de la Pantai Remis landslide, 
que ocurrió en Malasia en 1993.

Time-lapse del desbordamiento de una presa de tierra 

en Oregón (Marmot Dam, Sandy River, Oregon)

Sin embargo, las inundaciones por desbordamiento han sido mucho mayores en el pasado geológico que esos casos históricos, y pese a ello han permanecido mayormente ignoradas.

En un  artículo reciente (la referencia está al final de este post, Abril et al., 2018) hemos modelizado en 3D el flujo de la mayor megainundación de entre las mejor documentadas: El desbordamiento del Lago Bonneville durante el Pleistoceno, hace unos 15.000 años:

Our last publication is already online! - As Lake Bonneville overtopped...
Fluid Dynamics simulations of the Late Pleistocene Lake Bonneville Flood https://t.co/JKTIuptsfl pic.twitter.com/xsSd8BPwer

— ∆(Garcia-Castellanos) (@danigeos) May 5, 2018

El desbordamiento del Lago Bonneville (Jarrett & Malde, 1987) tuvo lugar al sobrepasar su nivel la barrera topográfica formada por un delta fluvial (sedimento consolidado) a unos 1500 m sobre el nivel del mar. Alcanzó un caudal de agua de un millón de metros cúbicos por segundo: el agua que cabe en el Camp Nou, cada 2 segundos. Estos posts dan algo más de contexto: [1], [2].

Desde las primeras exploraciones de Gilbert en el Lago Bonneville (Gilbert, 1890) y las de Bretz, se han acumulado numerosas evidencias de que el desbordamiento de muchos otros lagos ha desencadenado inundaciones de mayor intensidad que las registradas históricamente, que alcanzan los 10^5 m3/s (la mitad del débito medio actual del río Amazonas), como ocurrió p.e. tras el bloqueo del río Yigong por una avalancha en 2000.

Sin embargo, la mayor inundación podría haber sido la Inundación Zancliense, que puso fin a la Crisis de Salinidad Messiniense hace 5.3 millones de años (e.g., Blanc, 2006; Garcia-Castellanos et al., 2009), tras el desbordamiento del Océano Atlántico sobre un Mediterráneo parcialmente desecado. El consenso en este caso no es completo, pero de confirmarse podría haber causado caudales de hasta 100 millones de metros cúbicos por segundo. La compilación más completa de este tipo de eventos puede encontrarse en el material suplementario de nuestro artículo (Garcia-Castellanos & O'Connor, 2018, Scientific Reports) referencia más abajo).

Lo que proponemos en ese segundo artículo es un nuevo método para medir la erodabilidad de la superficie de la Tierra, es decir la facilidad con la que ésta es modificada por la acción mecánica del agua. Y ese método utiliza precisamente la erosión producida en todas estas megainundaciones ocurridas en el pasado reciente de la Tierra.

El método consiste en resolver con un código escrito en C un sistema de ecuaciones que calcula la erosión producida por el agua (modelos desarrollados por la comunidad geomorfológica global) y el caudal de agua que se produce en el desaguadero de un lago (relaciones hidrológicas relativamente sencillas). Simulando con este programa el desbordamiento de cada lago buscamos el valor de la erodabilidad de la presa natural correspondiente que permite reproducir los datos del caudal de agua. Estos datos de caudal han sido derivados a lo largo de décadas en numerosos estudios de geomorfología de campo en lagos del Pleistoceno (O'Connor & Beebee, 2009).

Esos estudios previos, junto con experimentos realizados con presas de tierra o arena, nos permiten disponer de datos sobre la descarga de agua y la erosión que se extienden a lo largo de 10 órdenes de magnitud en términos de volumen de agua total evacuada. La figura muestra los escenarios naturales mejor estudiados (los de volúmenes más importantes).

Datos sobre inundaciones debidas al desbordamiento de lagos naturales, compilados por O’Connor & Beebee (2010). Cada punto es una inundación indicando la descarga máxima de agua frente al volumen total de agua del lago. Los datos se extienden a 10 órdenes de magnitud en términos de volumen. 

Estos datos han servido para estimar el riesgo en escenarios naturales, aunque con muy poca precisión, para decidir el desalojo de valles fluviales cuando un río es bloqueado por una avalancha de roca, como ocurrió en el río Hunza (sin consecuencias) o en el desbordamiento e inundación en 1963 del Lago Issyk.

A nosotros, los datos de caudal nos han servido para cuantificar mejor a qué velocidad erosiona el agua el relieve del planeta. La esperanza es que en un futuro seamos capaces de predecir mejor la erosión, y concretamente, la peligrosidad de lagos a punto de ser desbordados.

Resultado de la simulación numérica de dos inundaciones (izquierda:
Lago Bonneville; derecha, experimento del vídeo mostrado más arriba).
Se muestra la evolución de varios parámetros como el caudal de agua Q
o el nivel del agua z_l. La erodabilidad necesitada para reproducir los
datos de caudal (círculos) es mucho menor en el experimento que en
Bonneville.

Relación encontrada entre la erodabilidad del desaguadero 
de los lagos estudiados y el tipo de roca. La correlación 
demuestra que el método permite medir la erodabilidad.

Los resultados indican no sólo que los desbordamientos catastróficos, pese a ser poco frecuentes, pueden cambiar significativamente el relieve, sino que además será importante incluir la periodicidad de las inundaciones (meteorológicas o no) en los futuros modelos, porque su distribución frecuencia-magnitud es también crucial en la evolución del relieve terrestre.

[My conference on this subject at the PAGES meeting, 2017]

Referencias:

• Garcia-Castellanos, D., J. O’Connor, 2018. Outburst floods provide erodability estimates consistent with long-term landscape evolution. Scientific Reports. 8:10573. Doi:10.1038/s41598-018-28981-y [open access]
• Abril-Hernández, J.M., Periáñez, R., O'Connor, J.E., Garcia-Castellanos, D. Computational Fluid Dynamics simulations of the Late Pleistocene Lake Bonneville Flood (2018) Journal of Hydrology, 561, pp. 1-15. DOI: 10.1016/j.jhydrol.2018.03.065

Gilbert, Grove Karl, 1890. Lake Bonneville. 438 p., 51 leaves of plates. Monographs of the United States Geological Survey, v. 1.
O’Connor, J.E., 1993, Hydrology, Hydraulics, and Geomorphology of the Bonneville Flood: Geological Society of America Special Paper 274, 83

Tomanowos - the rock that went through cosmic billiard, megafloods, and idiocy

Present display of the meteorite at the AMNH museum in NY. My photo.

Last week I visited again the rock with the most fascinating story on Earth: 

Tomanowos, meaning the visitor from the sky in the extinct Clackamas language, also known as the Willamette meteorite. 

Supernovas spread throughout space the
iron produced in heavy stars. This ejected iron
ends up in particle nebulas that eventually form
new stars and protoplanets. [Image: NASA] 

When European Americans found it near the Willamette River (Portland, OR) more than a century ago, Tomanowos inevitably went through one of the most hilarious and silly geological stories that I know of, surely driven by the fatal attraction that a rare rock like this exerts on humans. But before going into that let me tell a few things about its origins.

Tomanowos is a rare 15 ton meteorite made of iron and nickel (Fe 91%, Ni 7.6%). As in other metal meteorites, these Fe and Ni atoms formed at the core of stars that shattered the space with the products of nuclear fusion when ending their lives in supernovae explosions. These elements ended up in the nebula that clumped together as protoplanets in our Solar System, and Tomanowos was part of the core of one of these protoplanets, where the heavier metals accumulate. 

Vesta, a surviving protoplanet of the 

early Solar System. Due to their large

 size, protoplanets develop a differenciated 
density distribution with heavier elements like 
iron concentrated in the core. Tomanowos is an 
ejected piece of a protoplanet core like this. 
[EPFL/Jamani Caillet, Harold Clenet]

We also know that later on, about 4 billion years ago, a collision between two of those protoplanets sent our museum piece back to space solitude. Subsequent impacts over billions of years eventually made the orbit of this meteorite cross that of the Earth. As a result of such cosmic billiard, the meteorite entered our atmosphere at a speed of ~60,000 km/h nearly 20,000 years ago and landed on an ice cap in Canada.

Over the following decades, the ice flow slowly transported Tomanowos southwards, towards a glacier lobe that was at the time blocking the Fork River in Montana (USA). The glacial tongue piled ice across the river valley forming a 600-m-high ice-dam that impounded the enormous Lake Missoula. Following the ice flow, Tomanowos happened to reach the dam on the precise year when it collapsed, releasing one of the largest floods ever documented: the #MissoulaFloods that shaped the Scablands in Washington. This process is known as glacial outburst flooding and it still happens every few years in the Perito Moreno glaciar, for example. Except that the water discharge during the Missoula Floods reached the equivalent to a few thousand Niagara Falls. The research of the Missoula floods by Bretz and Pardee in the early 20th century led to one of the most significant paradigm shifts in recent geoscience: the recognition that catastrophic events can significantly contribute to the evolution of  landscape.

Map of the Missoula Floods path, showing Lake Missoula 
(blue), the ice cap where Tomanowos landed (north of the 
lake outlet), and the inundated areas of Washington and 
Oregon (grey).
Source: Washington Univ.

Trapped in ice and rafted down by the flood, Tomanowos crossed Idaho, Washington and Oregon along the overflown Columbia River at speeds sometimes faster than 20 meters per second. While floating up on the flood waters near today's Portland, the ice case broke apart and the meteorite sunk in the flooding waters. Hundreds of other ice-rafted erratics (rocks that do not match the local geology, nor could be transported by rivers or glaciers) have been found along the Columbia River. All are souvenirs from the Missoula floods, but none as rare as Tomanowos.

As the flood ceased, the sunk meteorite became exposed to the atmosphere. Over thousands of years, rain mixed with the iron sulfide inclusions producing sulfuric acid that gradually dissolved the iron of the exposed side of the rock:

These cavities were produced by acid dissolution of iron at the exposed side.

A few thousand years after the flood, the Clackamas arrived to Oregon and named the meteorite as the Visitor of the Sky, a heaven's representative that unified earth, water & sky. Apparently they knew that nickel rocks come from heaven. Were they intrigued by the absence of a crater at the Meteorite site? In any case, the name reminds us that pre-scientific cultures were not idiotic, or not more than us today anyway.

To confirm this latter hypothesis, in 1902 a colonist named Ellis Hughes decided to secretly move the iron rock to his own land and then claim property. Millennia of peaceful rest in the Willamette valley had to come to an end. But because moving a 15-ton rock a distance of 1,200 m without being noticed is not easy, not even in Oregon, Hughes and his son labored for three back-breaking months in secrecy: 

As D. J. Preston hilariously explains, after finally
succeeding with the moving, Hughes built a shack around
the meteorite, announced he had found it on his property
and started charging twenty-five cents admission to view
the heavenly visitor.

It was during this transport that the rock sadly underwent severe mutilations.
Unimpressed by this deployment of idiocy, Hughes' neighbor fabricated a lawsuit contending that the meteorite had, in fact, landed on HIS property. And to buttress his case he showed investigators a huge crater on his land. The case was dismissed when a third neighbor reported a great deal of blasting only the week before the trial.

Ironically, the legitimate owner of the original land of the iron rock turned out to be the Oregon Iron and Steel Company, so far unaware of the meteorite but promptly hiring a twenty-four-hour guard who sat on top of it with a loaded gun while the case was being appealed. They won the case in 1905, selling Tomanowos to the AMNH museum in New York a year later.

Tomanowos in the early 1900s, before being transported to the AMNH.

Today, amazingly enough, the AMNH exhibition does not even mention the Missoula Floods as a key part of Tomanowos' story, in spite of the wide scientific consensus. Same as the meteorite, the Clackamas were also reallocated to a reservation. Their descendants do keep the right to visit Tomanowos in NY and talk to the visitor who brought the Sky, the Water, and the Earth together.

[Update: my conference on outburst floods at the PAGES meeting, 2017]

How to refill the Mediterranean?

Mapping Ignorance

Let me tell a story about serendipity in research, a story that involves abrupt changes in the Earth's landscape and a 5-million-year-old flood of unprecedented scale.


Classical authors such as Aristotle, Galileo, or Leonardo da Vinci, used to describe the birth of the Mediterranean Sea as an enormous flood through the Strait of Gibraltar that filled a desiccated basin. These stories trace back to the oldest known encyclopedia: Plinius' Historia Naturalis (1st century AD). In its 3rd volume, Plinius the Elder recounted a legend from southern Hispania that attributed the formation of the Strait of Gibraltar to Hercules, the god who "allowed the entrance of the ocean where it was before excluded". The Atlantic Ocean flooding a desertic Mediterranean Sea at epic scales. Amazingly enough, the geophysical and geological research carried out in the last decades seems to suggest that this ancient vision may be quite appropriate.

Since the identification of vast salt strata throughout the Mediterranean by Austrian naturalist Karl Mayer (late nineteenth century) we know that the marine connections between this sea and the Atlantic Ocean became small by the end of the Miocene, during a period known as the Messinian age. Modern chronostratigraphy has dated this at 6 to 5.3 million years ago, around the time when our earliest hominin ancestors started walking on two legs in Central Africa. As a result, the Mediterranean became a huge salt pan that accumulated about 10% of the salt dissolved in the world's oceans, during the so-called Messinian salinity crisis. The ongoing tectonic uplift of the Gibraltar Arc region finally emerged the last Atlantic seaway and isolated completely the Mediterranean from the ocean, about 5.6 million years ago. The Mediterranean then became largely evaporated as a result of the dry climate of its watershed. Finally, about 5.3 million years ago the Mediterranean was refilled from the Atlantic through the Strait of Gibraltar. The indications that this occurred geologically very fast (namely, the abrupt change from Miocene to Pliocene sedimentary layers) made this event be known as the Zanclean flood.

Simulation of the refill through the strait of Gibraltar

by Steven N. Ward (Univ. California).

Note the water velocity distribution around 1:27.

Geological map of the Gibraltar strait locating the
erosion channel (red) observed with geophysical
methods.

The flood along the Gibraltar threshold may have been caused by its subsidence below the Atlantic level, or by faulting, or by erosion (or a combination of these three proposed mechanisms). But beyond the causes for the flood, another key unknown is the abruptness and evolution of the flood itself: From the sharp transition in the sedimentary layer record, it is widely thought (though not unanimously) that the event was very fast. But in geology fast can mean a hundred thousand years. Because little was known about its dynamics, and perhaps because for geologists rapid major events are rare and challenge the principle of uniformitarianism, the flood duration underwent a wide range of estimations from tens to tens of thousands of years.

Before knowing anything about the Messinian Mediterranean, I used to model the evolution of landscape over geological time scales, particularly interested on the role of lakes in controlling the long-term evolution of topography of large continental regions.

Lakes are those water bodies collecting precipitation in local topographic minima (yes, this sounds a bit Sheldon-like). Lakes are usually ephemeral over geologic timescales: Unless there are vertical tectonic motions enlarging the topographic basin, they soon fill up with sediment, overspilling their banks. When the water finds a way out, it incises along the outlet, drawing the lake's level down, and propagating this erosion upstream into the lake. In our landscape evolution models this transition was systematically very fast, but this result was not convincing enough for two reasons: First, lake data to compare with were scarce, and we were in the need of a large case scenario where traces of erosion were more evident. Secondly, our models where not accounting for transitory water flow, but instead it was calculating a steady flow (i.e., the water precipitation equals the water losses through evaporation at each time step of the simulation).

2D simulation of the evolution of a tectonic lake 

formed in front of a growing tectonic barrier. 

The lake evaporates the water collected from 

the left side. When the barrier stops growing, 

its erosion leads to a sudden capture of the lake.

(Garcia-Castellanos, 2006, GSA spec. pub.)

Then I accidentally learned about the Messinian salinity crisis, about its impact in the Mediterranean evolution, and about the megaflood hypothesis for its ending. It struck me that the feedback between water flow and incision we envisaged for lakes should be similar during the Zanclean flood, taking the global ocean as a huge lake on the verge of overtopping towards the dry Mediterranean. Combining the formulation of river incision with the proper hydrodynamic equations, we built a simple but robust mathematical formulation for overtopping outburst floods. We used then erosional parameters derived from the study or mountain river incision, and then incorporated a reconstruction for the Mediterranean seafloor geometry. Then we started running virtual floods. 

The first results were so surprising that we thought something was probably wrong with the code. Things were happening much faster than in those lake scenarios we were used to. Because in the Zanclean flood the source is virtually infinite, the Mediterranean was filling in only a few years along a large erosion channel excavated across the Strait of Gibraltar, some hundred meters deep. Unfortunately, the results were strongly dependent on a parameter that is badly constrained: the erodibility of rocks. But if that was correct, we should be able to find traces of the flood erosion preserved under the sedimentary layers in the strait. 

Dry Mediterranean, by Roger Pibernat.

So we turned towards previously published research, finding two other pieces of evidence: The first was a vintage seismic image showing a cross-section of the sedimentary layers near the strait area (Campillo et al., 1992). It showed a clear channel running west to east from the strait into the Alborán Sea, carved in the pre-Messinian sediments. The channel had previously been thought to be the result of river erosion of the dried-up strait, but there’s no obvious large river that could have produced that erosion. The second piece of evidence came from cores of rock drilled from the strait area as part of the exploration for the Africa-Europe tunnel project that would build a train connection between Spain and Morocco (Blanc, 2002). These cores also showed a channel deeper than 200 m, wider than 3 km, and filled with post-Messinian sediment. Altogether, the documented erosion valley connecting the Eastern Atlantic to the Western Mediterranean is more than 200-kilometre long. If this were a result of fluvial erosion, it would be strange to find erosion on both sides of the present water divide between Atlantic and Mediterranean. Furthermore, rather than a waterfall over the Gibraltar Strait as previously suggested, the seismic data show a huge ramp, several kilometres wide descending rather gradually from the Atlantic to the Mediterranean.

With these data, we turned again to the models. Using the observed erosion depth and width as a constrain, the model estimated now that the flood may have began slowly, taking up to several thousand years before a significant rise in Mediterranean level occurred. But importantly they also show that 90% of the water must have entered in a period shorter than two years, and that at the peak discharge, water poured in at a rate of 100 million cubic meters per second, about a thousand times the largest river on Earth today. If 'harder' erosion parameters were used, then the refill of the Mediterranean would be predicted to be slower, but the calculated erosion at the end of the flood would be insufficient to reproduce the geophysical images. To fit the observations, the flooding channel had to cut down into the bedrock almost half a meter per day, leading to a large inlet flow that would increase the Mediterranean sea level by more than 10 meters per day.

The technique does not allow constraining the speed of the initial stages, nor the mechanisms involved in triggering the flood. This means that the initial trigger may have been a geologically modest event such a large storm, a tsunami, or a partial collapse of the dividing barrier. What the results do ensure is that in order to account for the final size of the erosion channel, 90% of the water must have been rapidly transferred in a period ranging from a few months to two years.

Possible evolution of the flood derived from the model. The lower panel shows the evolution of the seaway depth as it is eroded by the increasing flow of water (black lines, left scale) and the rising Mediterranean level (red lines). From our article in Nature. 

If these observations and calculations are independently confirmed, the Zanclean flood would become the largest known flood on Earth's history. The Zanclean flood involved an order of magnitude more water flow than the megafloods that we know took place during the last deglaciation (e.g., the Missoula floods or the Bonneville flood). 

The implications of such a rapid flooding are inevitably big, as a large number of multidisciplinary studies have documented: Global flora and fauna had to adapt to the new environmental conditions rapidly. Marine species colonized a huge new realm rapidly whereas for land species, particularly in islands, the flooded Mediterranean became a sudden barrier triggering speciation. Had the land connection remained, it could have facilitated an earlier arrival of early humans in western Europe. Instead early humans had to take a circuitous route to Western Europe and didn't arrive until 1.5 million years ago. The Messinian salinity crisis also highlights the importance of seaways in understanding the geological record: straits limit the mix with the global ocean and their size is the key parameter modulating the chemical registry found in sediment. The flood may also have had tectonic implications: The weight of the flooding waters is such that it should have modified the rotation of the Earth, and it should have made the entire Mediterranean region sink by at least one kilometer in the mantle, according to the principle of Isostasy. Also global climate surely must have been impacted by the Messinian salinity crisis and its rapid ending, but so far this is perhaps the most elusive aspect of the crisis, something remarkable since I am not aware of other scenarios in geological history where the climatic response to such a large environmental change can be better tested.

So there are plenty of open questions on the Zanclean Flood that need an answer. More pending Retos Terrícolas.

Video on the Atlantropa Project, showing the collapse of a 

projected dam across the Strait of Gibraltar.

[This is related to research of our own group here at CSIC, as published in this article]
[This post has been published in Mapping Ignorance]

References:

• Blanc, P.-L. The opening of the Plio-Quaternary Gibraltar Strait: assessing the size of a cataclysm, Geodin. Acta 15, 303—317 (2002).
• Campillo, A., Maldonado, A. & Mauffret, A., Stratigraphic and tectonic evolution of the western Alboran sea: Late Miocene to recent, Geo Mar. Lett., 12, 165– 172 (1992).
• Garcia-Castellanos, D., 2006. Long-term evolution of tectonic lakes: Climatic controls on the development of internally drained basins. In: Tectonics, Climate, and Landscape evolution. Eds.: S.D. Willett, N. Hovius, M.T. Brandon & D.M. Fisher. GSA Special Paper 398. 283-294. doi:10.1130/2006.2398(17) [pdf]
• Garcia-Castellanos, D., F. Estrada, I. Jiménez-Munt, C. Gorini, M. Fernàndez, J. Vergés, R. De Vicente, 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature 462, 778-781 doi:10.1038/nature08555 [pdf]