Dynamic topography vs. isostasy: The importance of definitions

Fig. 1. Airy isostatic model: every column of rock above the
compensation level should have the same weight.
High topography is compensated by a mass deficit at
the base of the crust (crustal root) 
The term 'Dynamic Topography' is one of the top trending topics in Solid Earth science. It has now prevailed for more than 2 decades, but still the concept involves significant confusion. Dynamic Topography refers to a part of the elevation of the Earth surface that cannot be accounted by the classical crustal isostatic models (Pratt or Airy). But is the term referring to all mantle-sourced loads? Or only to those forces created by the dynamic flow of the mantle? Let's see first where the current confusion exactly comes from.

The term was actually coined by oceanographers to refer to the deviations of the surface of the ocean relative to the Geoid (eg., Bruce, 1968; Wyrtki, 1975). In principle, the geoid should perfectly fit the surface of the ocean, since it is an equipotential surface of the gravity field, but the flow of water adds a secondary shift of the surface, normally less than a meter. This deviation from the surface predicted for a 'static' ocean (ie., the geoid) can be detected in satellite altimetry data because the signal noise introduced by tides, waves, and wind can be removed by time-filtering. The remaining deviation from the geoid is referred to by oceanographers as 'dynamic topography' and is known to be related to the water currents in the ocean.
Fig. 2. Mean ocean dynamic topography from, updated from Tapley et al., 2003).
 It measures the long-term-averaged strength of ocean currents, the 'steady-state'
In the 80s the term was adopted by solid-earth scientists (Hager et al., 1985, Nature). The authors did not follow the original oceanographic meaning, but instead they included 'static' forces originated within the lithosphere (such as the weight of sinking plates, or slab pull) as well as forces caused by flow in the mantle.

Mantle convection model: Mantle temperature (color shading) and flow (arrows). Lines indicate the calculated dynamic topography (blue line) and the horizontal component of plate motion (red, positive means eastward). From Liu et al., 2008, Science
This made sense at the time because there was a big questionmark (still poorly answered today) about the origin of hidden loads, the enigmatic forces needed to explain the depth of sediment accumulations next to mountain belts (see Allen's book Foreland basins, or this article pdf). Sedimentary basins next to orogens in compressional plate boundaries are formed by the isostatic sinking (subsidence) of the lithosphere due to the weight of the growing orogen. These settings became very attractive in solid-earth science not only for prospection purposes, but also because they provide an opportunity to understand how tectonic processes interact with the erosion and transport of sediment in the surface, since the sedimentary layers in such foreland basins record the tectonic evolution of the mountain belt. After many of these foreland basins were modeled, it became clear that the isostatic load of the orogen was generally insufficient to explain the amount of subsidence of the basin. But linking all the hidden load to dynamic effects is misleading, because of the presence of static forces such as the weight of a sinking plate attached to the surface (a lithospheric slab), well known since plate tectonics became mainstream. Another key to understanding the confusion is that before the widespread development of seismic tomography, everything occurring below crustal levels was far more conjectural than today. As a result, part of the Solid-Earth community used the term dynamic topography to refer to all deep-seated forces (originated below the crust) that had an effect on topography, including for instance changes in the thickness of the lithospheric mantle, or a lithospheric slab.

Fig. 3. Two static forces in balance
(weight of the books and the
counteracting human force)
Dynamic forces are added to the
static force to recover the balance
and avoid the books  from falling.
In physics, static vs. dynamic forces refer to whether the forces are in equilibrium (perfectly compensated) or not, and dynamic physical problems refer to motions involving acceleration. This is an additional source of confusion, since both the ocean flow and the Earth's mantle flow can be under steady-state flow and still inflict a constant deflection of the topographic surface, that we yet call 'dynamic'.

But sticking to the original oceanographic definition (as for example in Braun 2010), the dynamic topography of the solid-earth should restrict to the change in elevation produced by dynamic forces related to mantle viscous flow (and flow can only occur beneath the boundary layer of the mantle, underneath the lithosphere). This definition seems robust because the lithosphere is defined based on its strength relative to the underlying asthenosphere, and hence flow-related stresses are expected to be negligible above the lithosphere-asthenosphere boundary (LAB). The fact that the flow is generated by density contrasts does not mean that the forces can be mistaken for static ones, because those density anomalies are out of the rigid body being deformed (the lithosphere).

We therefore can split the observed topography OT in:
where CIT is the crustal isostatic topography; LMIT is the Lithospheric-mantle isostatic topography (including slabs attached to the Earth's crust, or the thinning of the lithosphere); and MFDT is the sublithospheric mantle-flow dynamic topography. Note that OT-CIT (easy to calculate using global databases of crustal thickness) is often called residual topography (RT).

Following this notation, the confusion can be described as emanating from some authors referring to LMIT+MFDT (which equals RT) as the dynamic topography, instead of MFDT alone. While this RT ranges in the order of +-1 km, there seems to be no consensus yet as to how large can MFDT be, with values ranging between that same value and a few hundred meters.

Fig. 4. Global free-air gravity anomaly from GRACE. The low values (+-40 mGal) in comparison with the +- 300 mGal that are often attained in smaller regional scales shows that the crust is in overall isostatic equilibrium: the mass excess of topography at high-elevation areas is compensated by a mass deficit at the base of the crust. For this reason there is little correlation between anomaly and continents. The Hawaiian, Yellowstone, Iceland hotspots are represented by highs. Subduction zones show an asymmetrical pair of low & high anomaly. The Hudson Bay undergoes a glacial rebound in response to the deglaciation (+ info here).
Support for the smaller MFDT values comes from reasonings like this: Consider a Stokes sphere sinking or rising in a viscous fluid by virtue of its density contrast with the surrounding fluid (the viscous mantle for us). The vertical velocity of this sphere can be analytically solved and the expression obtained for the dynamic topography it produces depends on the radius, the density contrast, the depth, and the distance R of the sphere.
MFDT = ∆h[m] = 2*∆density * a^3 * D * (3D^2+3a^2+5*D^2*a^2/R^2) / (3*fluid_density*R^5) 
Stokes' sphere sinking in a viscous fluid. The gravity anomaly and dynamic topography it geenerates are linearly proportional to each other.
Because the free-air gravity anomaly produced by the same sphere (∆g) follows a similar equation, the relation between gravity and MFDT conveniently depends only on the of fluid density, to a first approach:
∆g[mGal] = 2πG * density[kg m-3] * MFDT[m]

This means that the +-40 mGal anomalies shown in the global map above should correspond to a dynamic topography smaller that 300-400 m (see P. Molnar's talk linked below).

Clearly, if we knew well the two isostatic contributions CIT+LMIT, then we would be able to attribute the rest to the flow in the mantle and learn about what happens at those depths. As Jean Braun puts it: "Mantle dynamics remain poorly constrained, but by linking mantle flow to surface topography (...) we can use the geological record to constrain the dynamics and viscosity of the mantle and the density structure that controls its flow".
The problem is that there are too many unknowns in the equation: computer models of 3D mantle flow that estimate dynamic topography rely on seismic tomographic imaging of the mantle that provide the distribution of seismic velocity anomaly but how to translate this wave velocity into lateral inhomogeneities in density and viscosity is poorly known. So, fitting the computer models to the weak available observations of dynamic topography and plate tectonic reconstructions will provide only hints on a vague combination of the velocity-viscosity and the velocity-density relationships. So, this will remain as an Earthling Challenge (a Reto Terrícola) for quite some time.

For more information, I recommend Peter Molnar's talk on youtube, Philip Allen's blog post, or Braun's paper listed below. PS: Check also this recent talk by Jean Braun on the interaction between erosion and dynamic topography.

  • Allen, 2010, Surface impact of mantle processes, Nature Geoscience.
  • Braun, Jean. "The many surface expressions of mantle dynamics." Nature Geoscience 3.12 (2010): 825-833.
  • Bruce, J. G. "Comparison of near surface dynamic topography during the two monsoons in the western Indian Ocean." Deep Sea Research and Oceanographic Abstracts. Vol. 15. No. 6. Elsevier, 1968.
  • Faccenna, C., Becker, T. W., Auer, L., Billi, A., Boschi, L., Brun, J.-P., Capitanio, F. A., Funiciello, F., Horvath, F., Jolivet, L., Piromallo, C., Royden, L., Rossetti, F., and Serpelloni, E.: Mantle dynamics in the Mediterranean. In press at Rev. Geophys., 2014. PDF
  • Hager, Bradford H., et al. "Lower mantle heterogeneity, dynamic topography and the geoid." Nature 313.6003 (1985): 541-545.
  • Tapley B.D., D.P. Chambers, S. Bettadpur and J.C. Ries, 2003: Large scale ocean circulation from the GRACE GGM01 Geoid. Geophys. Res. Letters 30 (22):doi:10.1029/2003GL018622
  • Wyrtki, Klaus. "Fluctuations of the dynamic topography in the Pacific Ocean."Journal of Physical Oceanography 5.3 (1975): 450-459.


49 Open Challenges in Earth Science

Mapping Ignorance
What keeps Earth scientists busy? These 49 open scientific questions aim at providing an updated, fully-referenced account of the main current scientific disputes and challenges in Geoscience. It is biased towards the Solid Earth disciplines and the references may not always be the best representative, but the contents are updated and based on a large number of sources (details in this earlier draft). Please send additions/suggestions to

The Early Earth and the Solar System

Advances such as those occurred in the geochemistry of meteorites lead to new exciting hypotheses about the early stages of our planet, but as usual, answers are outnumbered by the new knowledge gaps: 
  1. How did the Earth and other planets form? Were planets formed in situ? Or are orbital changes relatively frequent? What determined the different deep layering of the solar planets? [McKinnon, 2012, Science on Mercury] 
  2. Was there ever a collision of the Earth with another planet Theia, giving birth to our satellite? [Canup, 2013, Science] There is compelling evidence, such as measures of a shorter duration of the Earth's rotation and lunar month in the past, pointing to a Moon much closer to Earth during the early stages of the Solar System. [Williams, CSPG Spec. Pubs., 1991]
  3. What is the long-term heat balance of Earth? How did its internal temperature decay since it formed by accretion of chondrites? How abundant are radiogenic elements in the interior? Did a "faint young sun" ever warm a "snowball Earth"? [WiredMarty et al., 2013, Science]
  4. What made plate tectonics a dominant process only on Earth? [outreach paper] How did the planet cool down before plate tectonics?[Moore & Webb, 2013, Nature] Was the Earth's crust formed during the early stages of its evolution or is it the result of a gradual distillation of the mantle that continues today along with crustal recycling? Is the crust still growing or does its recycling compensate for crust formation at mid-ocean ridges and other volcanic areas?
  5. How inherent to planetary evolution is the development of life conditions? [Zimmer, 2005, ScienceElkins-Tanton, 2013, Nature] Earth-like planets are now known to be abundant in our galaxy (two out of three stars may have one [e.g., Cassan et al., 2012, Nature]), but how many of them develop widespread durable water chemistry? 

Earth’s Interior

Our rock-sampling reach is limited to the upper 12 km of the Earth's crust, but the keys to extend our knowledge often lay far deeper than that. Indirect measurements such as seismic wave tomography, together with geodynamic and petrological modeling, become crucial: 
  1. As planets age and cool off, their internal and surface processes coevolve, chemically and mechanically, shaping the atmospheric composition. What are the chemical composition and mechanical properties of rocks in the Earth’s mantle at the extreme pressure and temperature they undergo? [ref.2]
  2. What are the dynamic processes in the Earth interior that accommodate and fuel plate tectonics? As seismometers spread more evenly over the planet's surface, the seismic imaging of the interior will rapidly improve, providing a detailed distribution of seismic wave velocity. Simultaneously, lab-based mineral physics must better constrain what these mechanical wave velocities tell us about the hot, deep rocks of uncertain composition in the mantle. Only then will computer models be able to test the proposed geodynamic models by trying to fit quantitatively those data and other geophysical observations such as gravity variations. [ref.3]
  3. How does the geomagnetic field link to the iron convection properties at the deep Earth? Or what can we learn about the mechanical behavior of the materials at those depths from the geomagnetic field? [more context in Nature] Geomagnetic changes are recorded in rocks, so they provide a view of the old Earth: Are the magnetic reversals too fast to be related to core dynamics? [example.1] [ex.2] [ex.3] Could their frequency be related to the distribution of tectonic plates? [GRL article]. What causes superchrons (periods without reversals)? Something internal to the core, or induced externally by the mantle/subducting slabs? Was the geomagnetic field always dipolar, or was it more asymmetric in the past? [introduction]
  4. Are intraplate hotspots really made by deep sources of uprising materials (mantle plumes) coming from the deepest Earth's mantle? Or can they be explained by shallower convection? [e.g., Morgan, 1971; see also this recent Geology paper on Yellowstone].
  5. What is the history of and what controls the excursions of the rotation pole relative to the surface geography, known as true polar wander? [ex]
  6. What tells us about the dynamics of the Earth its present heterogeneity in density, seismic wave velocity, and electromagnetic resistivity in the mantle and the lithosphere? [ex.3] How do these measures relate to the mineralogical composition? 
  7. What are the causes for Large Igneous Provinces and massive flood basalts such as the Columbia River Basalts?

Tectonic plate motion and deformation 

The successful adoption of the plate tectonics paradigm has lead to a myriad of new questions about its limits and about the lessons for risk mitigation.
Velocity of the earth's surface at the Indian-Asian collision,
from GPS data. Blue star indicates the 2008 China earthquake.
  1. What is the relative importance of the forces driving plate tectonics: slab pull, slab suction, mantle drag, and ridge push? [e.g., Conrad & Lithgow-Bertelloni, 2004; Negredo et al., GRL, 2004, vs. van Benthem & Govers, JGR, 2010]. What is the force balance and the geochemical cycle in subduction zones? [Emry et al., 2014, JGR] How much water (and how deep) penetrates into the mantle? [Ranero et al., 2003, Nature] How much subcontinental erosion takes place under subduction areas? [Ranero et al., 2000, Nature]
  2. What happens after the collision of two continents? Does continental collision diminish the rate of plate subduction, as suggested by the slab-pull paradigm? [Alvarez, EPSL, 2010How frequent are the processes of mantle delamination and slab break-off? What determines their occurrence? [Magni et al., GRL, 2013; Durezt & Gerya, Tectonoph., 2013]
  3. Why are orogens curved when seen from space? [Weil & Sussman, 200, GSASP 383]
  4. How does the long-term deformation derived from paleomagnetism and structural geology link quantitatively to the present-day motions derived from GPS and from neotectonic patterns of crustal deformation? [Calais et al., EPSL, 2003] How do these last two relate to each other? [ref] Can we learn from regional structure of the crust/lithosphere from that link (or viceversa)? 
  5. Are plate interiors moving in steady-state linear motion? How rigid are these and why/when did they deform? [Davis et al., (2005) doi:10.1038/nature04781, and Wernicke & Davis, (2010) doi:10.1785/gssrl.81.5.694]
  6. How is the relative motion between continents accommodated in diffuse plate boundaries? (eg., the Iberian/African plate boundary). What determines the (a)seismicity of a plate contact? 
  7. How/when does deformation propagate from the plate boundaries into plate interiors? [e.g., Cloetingh et al., 2005, QSR] 
  8. What is the rheological stratification of the lithosphere: like a jelly sandwich? Or rather like a creme brulée? [Burov & Watts, 2006]. Is the lower crust ductile? Is strength concentrated at the uppermost mantle? Or just the other way around? [e.g., McKenzie et al., 2000; Jackson, 2002; Handy & Brun, 2004; and a nice recent post]
  9. Does the climate-controlled erosion and surface transport of sediment modify the patterns of tectonic deformation? Does vigorous erosion cause localized deformation in the core of mountain belts and prevent the propagation of tectonic shortening into the undeformed forelands? Does the deposition of sediment on the flank of mountains stop the frontal advance of the orogen? Is there any field evidence for these effects predicted from computer models? [Philip Allen's blog] [Willett, 1999; Whipple, 2009Garcia-Castellanos, EPSL, 2007]
  10. Can earthquakes be predicted? [Heki, 2011, GRLFreed, 2012, Nat.Geosc.]. How far can they be mechanically triggered? [Tibi et al., 2003, Nature]. Little is known about how faults form and when do they reactivate [ex.6], and even worse, there seems to be no clear pathway as to solve this problem in the near future. Unexpected breakthroughs needed. 
  11. How can the prediction of volcanic eruptions be improved? What determines the rates of magma accumulation in the chamber and what mechanisms make magmas eruptible? [ex.7][ex.7b]
  12. In many regions, the elevation of the continents does not match the predictions from the classical principle of isostasy for the Earth's outer rigid layer (the lithosphere). This deviation is known as dynamic topography, by opposition to isostatic topography. But what are the mechanisms responsible? Can we learn about the mantle dynamics by estimating dynamic topography? [ref.1Can the hidden loads needed to explain the accumulation of sediment next to orogens (foreland basins) be linked to these dynamic forces? [Busby & Azor, 2012]
  13. How do land-forming processes react to climate change at a variety of scales, ranging from the Milankovitch cycles to the late Cenozoic cooling of the Earth? Is there a feedback from erosion into climate at these time scales, through the Carbon cycle and the weathering of silicates, for example? What is the role of the surface uplift and erosion of Tibet on the drawdown of atmospheric CO2 over the Cenozoic? [Garzione, 2008, Geology]

Earth's landscape history and present environment

The shape of the planet's solid surface, its topography, is the key feature that connects many of the disciplines within Earth science, probably because it is the feature that most affects our daily life. It is today common wisdom that landscape forms from a complex interplay between tectonics, climate, and a list of mechanical, chemical, and biological processes acting at the surface of the Solid Earth. Topographic data is becoming now available at resolutions finer than a few meters, and the sedimentary record is also being archived at unprecedented rates. But:
Drainage patterns in Yarlung Tsangpo River, China (NASA)

  1. Can we use these data to derive past tectonic and climatic conditions? Will we ever know enough about the erosion and transport processes? Was also the stocasticity of meteorological and tectonic events relevant in the resulting landscape? And how much has life contributed to shape the Earth's surface? 
  2. Can classical geomorphological concepts such as 'peneplanation' or 'retrogressive erosion' be understood quantitatively? Old mountain ranges such as the Appalachian or the Urals seem to retain relief for > 10^8 years, while fluvial valleys under the Antarctica are preserved under moving ice of kilometric thickness since the Neogene. What controls the time-scale of topographic decay? [Egholm, 2013, Nature]
  3. What are the erosion and transport laws governing the evolution of the Earth’s Surface?[Willenbring et al., Geology, 2013] Rivers transport sediment particles that are at the same time the tools for erosion but also the shield protecting the bedrock. How important is this double role of sediment for the evolution of landscapes? [Sklar & Dietrich, Geology, 2001 (tools and cover effect); Cowie et al., Geology, 2008 (a field example)].
  4. Can we predict sediment production and transport for hazard and scientific purposes? [NAS SP report, 2010Geology, 2013] 
  5. What do the preserved 4D patterns of sediment flow tell us from the past of the Earth? Is it possible to quantitatively link past climatic and tectonic records to the present landforms? Is it possible to separate the signals of both processes? [e.g., Nature Geosc]. 
  6. Can we differentiate changes in the tectonic and climate regimes as recorded in sediment stratigraphy? Some think both signals are indeed distinguishable [Armitage et al., 2011]. Others, as Jerolmack & Paola [2010, GRL], argue that the dynamics intrinsic to the sediment transport system can be 'noisy' enough to drown out any signal of an external forcing. 
  7. Does surface erosion draw hot rock towards the Earth’s surface? Do tectonic folds grow preferentially where rivers cut down through them, causing them to look like up-turned boats with a deep transverse incision? [Simpson, 2004, Geology]
  8. How resilient is the ocean to chemical perturbations? What caused the huge salt deposition in the Mediterranean known as the Messinian Salinity Crisis? Was the Mediterranean truly desiccated? What were the effects on climate and biology, and what can we learn from extreme salt giants like this? [e.g., Hsu, 1983; Clauzon et al., Geology, 1996; Krijgsman et al, 1999; Garcia-Castellanos & Villaseñor, Nature, 2011]. Were the normal marine conditions reestablished by the largest flood documented on Earth, 5.3 million years ago? [G-C et al., 2009]
  9. How do the patterns of river networks form? [eg. Devauchelle et al., 2012, PNAS; Perron et al., 2012, Nature]. And what information about the past do these patterns contain? Can we quantitatively reconstruct past ecology or climate from old river patterns? [e.g., Hartley et al., 2010, J. Sedim. Res.]
  10. Do we need a new geological epoch called Anthropocene? When do the Homo Sapiens start to have a significant impact on the Earth System? 8000 BP? [Ruddiman, 2003]; 2000 BP? [Scalenghe, 2011]; 1850 AD? [Crutzen & Steffen, 2003]

Climate, Life, and Earth

The geological record shows that climate is relatively stable over tectonic time-scales while undergoing abrupt changes in periods as short as decades or millenia. Past periods when the planet underwent extreme climate conditions may help to understand the mechanisms behind that behavior.

  1. What caused the largest carbon isotope changes in Earth? [Nat.Geo review] How does Earth’s climate system respond to elevated levels of atmospheric CO2? 
  2. Was there ever a snow-ball Earth during the earliest stages of Life on Earth? 
  3. Were there also rivers and lakes on Mars? Were there large outburst floods similar to those on Earth?
  4. What were the causes and what shaped the recovery from mass extinctions as those at the K-T boundary, the Permian-Triassic or the Late Triassic? Massive volcanism? Meteorites? Microbes? [recent papers: ex.8ex.9ex.10, Rothman et al., 2014, PNAS]
  5. What triggered the extreme climatic variability during the Quaternary and the roughly coeval acceleration in continental erosion and sediment delivery to the margins? [Peizhen, Molnar et al., 2001, Nature; Herman et al., 2013, Nature] Was this related to the tectonic closure of the Central American Seaway? How do these climate events translate quantitatively into sea level changes?
  6. How do climate changes translate quantitatively into sea level changes? How do ice sheets and sea level respond to a warming climate? What controls regional patterns of precipitation, such as those associated with monsoons or El Niño?
  7. What caused the Quaternary extinction(s)? Human expansion? Climate Change? How sensitive are ecosystems and biodiversity to environmental change? Was the large fauna extinction ~13,000 yr ago a result of the Younger Dryas climatic event? Was this caused by an extraterrestrial impact? [ex.11ex.12] Or may it be linked to the outburst of Lake Agassiz
  8. How relevant are subsurface microorganisms to earth dynamics by controlling soil formation and the methane cycle? What are the origin, composition, and global significance of deep subseafloor communities? What are the limits of life in the subseafloor realm?  
  9. The atmosphere is shaped by the presence of life, a powerful chemical force. Earth’s evolution has clearly affected the evolution of life [see the Cambrian explosion of animal life, for instance; plus this recent paper on that]. To what extent is evolution determined by geology? How much control has life on climate? [another recent one]. Is it possible to quantify these links to make reliable predictions that allow filling the data gaps or assessing the chances of extraterrestrial life?
  10. How much of the present climate change is anthropogenic? How will growing emissions from a growing global population (and with a growing consumption) impact on climate? 

Broader open questions

  1. Many of the questions above are related to the extreme diversity of spatial scales of Earth processes. Direct observation (by sampling or remotely) is mostly limited to a thin layer around the solid surface of the Earth, and physical experimentation is limited to the pressures of the uppermost layers of the planet. Many processes including plate tectonics are known to be driven by the nature of the materials that make up the planet interiors, down to the smallest atomic scales, as thought for instance for the trigger of earthquakes. Answers may arrive via new devices and analytical tools working at the high pressures and temperatures of Earth’s interior.
  2. Time scales also pose a problem to know the mechanical and chemical properties of Earth's materials. Partly because we deal with time scales in very different orders of magnitude while we are limited to make observations from the present. But also because scaling the rates of lab experiments (e.g., mineral physics) or analogue models (e.g., sandbox experiments) with the corresponding geological scenarios is not always convincing. 
  3. Implementing Episodicity in Gradualism: For historical reasons, geology has generally underestimated the role of episodicity in nature. However, there is a growing view that exceptional events and stochasticity have a relevant role in many of Earth's subsystems. An example for this is the preeminence of flooding events (larger than average) in erosion and surface sediment transport and during the evolution of landscape, and the importance of upscaling flood stochasticity into sediment transport models [eg., Lague, 2010, JGR]. Climate variability at all time-scales has been already mentioned above. Even plate tectonics may have been episodic (during the Archean at least, [ref]).  4D hyperscale data sets in geomorphology are increasingly showing the limits of smooth-process approaches. Future understanding of the Earth will benefit from incorporating the full frequency spectrum (the episodicity) in modeling natural phenomena, rather than systematically approaching these as gradual processes. 
  4. Computer models tell whether the complexity of nature can be explained by the interplay between simple processes, but: how can we further model the Earth as a complex system of complex systems? And when can we expect ‘compact’ explanations? 

General background:
    Note that the specific references given above for each open question are sometimes just examples and may not always be the best representative. Furthermore, the list is surely biased towards Solid Earth Science, my own field. The following general references can give you an alternative perspective on the subject.
    For various inputs/criticisms to this list: Brian Romans, Umberto Lombardo, Jean Braun, Mikael Attal, Alexandra Witze, Michael Klaas, Matt Hall, Chris Rowan.
    A previous version of this post has been published in Mapping Ignorance.


    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. All such stories can be traced back to the oldest known encyclopedia: Historia Naturalis (1st century AD). In the 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". Amazingly enough, the geophysical and geological research carried out in the last decades seems to support this ancient vision about the origins of the Mediterranean Sea.

    Since the identification in Mediterranean strata of the Messinian age 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. Modern chronostratigraphy has dated this at 6 million years ago, 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
    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.

    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 to previously published research and found two 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 results 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: 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]

    • 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]


    How old is Earth Science?

    Many geoscientists think of their field as not much older than Lyell or Hutton; as a science that bloomed sometime during the eighteenth century.

    The term geology was in fact first used by Ulisse Aldrovandi in 1603 (Vai and Cavazza, 2003) and introduced as a regular term by Horace de Saussure in 1779. Short after, the danish scholar Nicolas Steno (1638-1686) set the bases of stratigraphy: the law of superposition and the principle of original horizontality. But the scientific interest for the history of the Earth has roots even deeper than that.

    Ortelius' world map, inspiring his own anticipation of
    continental drift
    In 1596, the geographer for the Spanish crown Abraham Ortelius wrote: "America (...) was (...) torn away from Europe and Africa, by earthquakes and flood" "the vestiges of the rupture reveal themselves: if someone brings forward a map of the world and considers carefully the coasts of the three aforementioned parts of the earth, where they face each other". This quotation extraordinarily anticipated the theory of continental drift, but it remained forgotten and was rediscovered only in 1994.
    Ortelius' idea was in turn a direct result of the vast exploration discoveries that took place in the previous decades. In the words of Alvarez & Leitao (2012, Geology): "The Iberian Voyages of Discovery of the fifteenth and sixteenth centuries marked a major advance in the understanding of the Earth—the greatest advance since antiquity, and comparable in scope and importance with the Geological Revolution, the Darwinian Revolution, and the Plate Tectonic Revolution, and we encourage geologists and other Earth scientists to embrace the Voyages as part of our geological scientific heritage."
    Edmund Halley’s map of geomagnetic declination (~1700 AD), 
    came about two centuries later than similar studies by Portuguese
    explorers, who used these magnetic anomalies for the long-lived 

    An example of the practical drive behind those first geoscientific questions: In the 16th century, "the Portuguese began to make systematic surveys of magnetic declination by comparing the direction of a compass needle with the line of shadow of the Sun at local noon". Magnetic declination could in this way be used to estimate the geographical longitude, a fundamental navigation problem at the time which final solution had to wait for the eighteenth century, when accurate chronometers were first developed.
    In 1736, a franco-spanish expedition is set to south America to determine the size and shape of the globe, whether it is flattened or elongated at its poles, a long standing scientific question at the time. The expedition included Charles de La Condamine, Jussieu, Pierre Bouguer, Jorge Juan & Antonio de Ulloa. Several books were published out of these expeditions that had great novelty and impact at the time. Juan & Ulloa's (Relacion historica del viage a la America Meridional hecho de orden de S. Mag. para medir algunos grados de meridiano terrestre y venir por ellos en conocimiento de la verdadera figura y magnitud de la tierra, con otras observaciones astronomicas y phisicas), was published 3 years after La Condamine's but showed far more detail, maps and illustrations.
    Juan & Ulloa's cover, 1748

    In summary, the systematic study of the Earth has a history behind as long as in any other scientific field. Quoting again A&L: geoscientists can "trace their intellectual ancestry back to the Copernican Revolution of the 16th and 17th centuries, just as astronomers and physicists do"

    • Alvarez & Leitao, 2010, Geology, 38, 231–234, doi: 10.1130/G30602.1
    • Romm, James, Nature 367, 407-408, 1994, A new forerunner for continental drift. doi:10.1038/367407a0
    • G. B. Vai et W. Cavazza, ed, Four centuries of the word 'Geology', Ulisse Aldrovandi 1603 in Bologna, Minerva Edizioni, Bologna, 2003