GD
Geodynamics

outstanding questions

What controlled the evolution of Plate Tectonics on Earth?

Great Unconformity - Immensity River, Grand Canyon
Stephan Sobolev

Prof. Dr. Stephan Sobolev. Head of the Geodynamic Modelling section of GFZ Potsdam.

Plate tectonics is a key geological process on Earth, shaping its surface, and making it unique among the planets in the Solar System. Yet, how plate tectonics emerged and which factors controlled its evolution remain controversial. The recently published paper in Nature by Sobolev and Brown suggests new ideas to solve this problem….

What makes plate tectonics possible on contemporary Earth?

It is widely accepted that plate tectonics is driven by mantle convection, but is the presence of said convection sufficient for plate tectonics? The answer is no, otherwise plate tectonics would be present on Mars and Venus and not only on Earth. The geodynamic community recognized that another necessary condition for plate tectonics is low strength at plate boundaries and particularly along the plate interfaces in subduction zones (e.g. Zhong and Gurnis 1992, Tackley 1998, Moresi and Solomatov 1998, and Bercovici 2003). To quantify the required strength at subduction interfaces, we have used global models of plate tectonics (Fig. 1A) that combine a finite element numerical technique employing visco-elasto-plastic rheology to model deformation in the upper 300 km of the Earth (Popov and Sobolev 2008) with a spectral code to model convection in the deeper mantle (Steinberger and Calderwood 2006). The model reproduces well present-day plate velocities if the effective friction at convergent plate boundaries is about 0.03 (Fig.1B). Low strength corresponds to subduction interfaces that are well lubricated by continental sediments (low friction; Lamb and Davis 2003, Sobolev and Babeyko 2005, or low viscosity; Behr and Becker 2018). In case of sediment shortages in the trenches (corresponding to a friction coefficient of 0.07-0.1), plate velocities would first decrease about two times (Fig. 1C) and then even more because of less negatively buoyant material having subducted into the mantle, leading to less convection driving force.

Effects of sediments on contemporary subduction according to global numerical models.

Figure 1. Global numerical model showing the effect of sediments on contemporary subduction. (A) The global model combines two computational domains coupled through continuity of velocities and tractions at 300 km depth. (B) NUVEL 1A plate velocities in a no-net-rotation reference frame (black arrows) versus computed velocities (blue arrows) for the global model with a friction of 0.03 at convergent plate boundaries. (C) Root mean square of computed plate velocities in the global model versus friction coefficient at convergent plate boundaries.

Hypothesis and its testing

Based on the previous discussion, we infer that continental sediments in subduction channels act as a lubricant for subduction. In addition, the presence of these sediments in trenches is a necessary condition for the stable operation of plate tectonics, particularly earlier in Earth’s evolution when the mantle was warmer and slabs were relatively weak. With this hypothesis we challenge the popular view that secular cooling of the Earth was the only major control on the evolution of plate tectonics on Earth since about 3 Ga. The hypothesis predicts that periods of stable plate tectonics should follow widespread surface erosion events, whereas times of diminished surface erosion should be associated with reduced subduction and possibly intermittent plate tectonics.

We test this prediction using geological proxies believed to identify plate tectonics activity (supercontinental cycles) and geochemical proxies that trace the influence of the continental crust on the composition of seawater (Sr isotopes in ocean sediments; Shields 2007) and continental sediments in the source of subduction-related magmas (oxygen and Hf isotopes in zircons; Cawood et al. 2013, Spencer et al. 2017). All three geochemical markers indeed show that just before or in the beginning of supercontinental cycles the influence of sediments is increasing, while it decreases before periods of diminished plate tectonic activity, like the boring billion period between 1.7 and 0.7 Ga (Cawood and Hawkesworth 2014; Fig. 2). The largest surface erosion and subduction lubrication events were likely associated with the global glaciation evens identified in the beginning (2.5-2.2 Ga) and at the end (0.7-0.6 Ga) of the Proterozoic Era (Hoffman and Schrag 2002). The latter snowball Earth glaciation event terminated the boring billion period and kick-started the modern phase of active plate tectonics.

Another prediction of our hypothesis is that in order to start plate tectonics, continents had to rise above sea level and provide sediments to the oceans. This prediction is again consistent with observations: there are many arguments for the beginning of plate tectonics between 3 and 2.5 Ga (see the review of Condie 2018) and, at the same time, this period is most likely when the continents rose above sea level (Korenaga et al. 2017).

Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth.

Figure 2. Cartoon summarizing the factors that control the emergence and evolution of plate tectonics on Earth. Enhanced surface erosion due to the rise of the continents and major glaciations stabilized subduction and plate tectonics for some periods after 3 Ga and particularly after 0.7 Ga after the cooling of the mantle. Blue boxes mark major glaciations; transparent green rectangles show the time intervals when all three geochemical proxies consistently indicate increasing sediment influence (major lubrication events); and, a thick black dashed curve separates hypothetical domains of stable and unstable plate tectonics. The reddish domain shows the number of passive margins (Bradley 2008), here used as a proxy for plate tectonic intensity.

What was before plate tectonics?

The earlier geodynamic regime could have involved episodic lid overturn and resurfacing due to retreating large-scale subduction triggered by mantle plumes (Gerya et al. 2015) or meteoritic impacts (O’Neill et al. 2017). Retreating slabs would bring water into the upwelling hot asthenospheric mantle, generating a large volume of magma that formed protocontinents. Extension of the protocontinental crust could have produced nascent subduction channels (Rey et al. 2014) along the edges of the protocontinents lubricated by the sediments. In this way, a global plate tectonics regime could have evolved from a retreating subduction regime.

What is next?

Despite of the support from existing data, more geochemical information is required to conclusively test our hypothesis about the role of sediments in the evolution of plate tectonics. Additionally, this hypothesis must be fully quantified, which in turn will require coupled modeling of mantle convection and plate tectonics, surface processes and climate.

References
Behr, W. M. and Becker, T. W. Sediment control on subduction plate speeds. Earth Planet. Sci. Lett. 502, 166-173 (2018).

Bercovici, D. The generation of plate tectonics from mantle convection. Earth Planet. Sci. Lett. 205, 107–121 (2003).

Bradley, D. C. Passive margins through earth history. Earth Sci. Rev. 91, 1-26 (2008).

Cawood, P. A., Hawkesworth, C. J. and Dhuime, B. The continental record and the generation of continental crust. Geol. Soc. Amer. Bull. 125, 14-32 (2013).

Cawood, P. A. and Hawkesworth, C. J. Earth's middle age. Geology 42, 503-506 (2014).

Condie, K. C. A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga? Geosci. Front. 9, 51-60 (2018).

Gerya, T.V. et al. Plate tectonics on the Earth triggered by plume-induced subduction initiation, Nature 527, 221-225 (2015).

Hoffman, P. F. and Schrag, D. P. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129–155 (2002).

Korenaga, J., Planavsky, N. J. and Evans, D. A. D. Global water cycle and the coevolution of the Earth's interior and surface environment. Phil. Trans. R. Soc. Am. 375, 20150393 (2017).

Lamb, S. and Davis, P. Cenozoic climate change as a possible cause for the rise of the Andes. Nature 425, 792-797 (2003).

Moresi, L. and Solomatov, V. Mantle convection with a brittle lithosphere: Thoughts on the global tectonic style of the Earth and Venus. Geophys. J. Int. 133, 669-682 (1998).

O’Neill, C. et al. Impact-driven subduction on the Hadean Earth. Nature Geosci. 10, 793-797 (2017).

Popov, A.A. and Sobolev, S. V. SLIM3D: A tool for three-dimensional thermo mechanical modeling of lithospheric deformation with elasto-visco-plastic rheology, Phys. Earth Planet. Inter. 171, 55-75 (2008).

Rey, P. F., Coltice, N. and Flament, N. Spreading continents kick-started plate tectonics. Nature 513, 405–408 (2014).

Shields, G. A. A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth. eEarth 2, 35-42 (2007).

Sobolev, S. V. and Babeyko, A. Y. What drives orogeny in the Andes? Geology 33, 617-620 (2005).

Spencer, C. J., Roberts, N. M. W. and Santosh, M. Growth, destruction, and preservation of Earth's continental crust. Earth. Sci. Rev. 172, 87-106 (2017).

Steinberger, B. and Calderwood, A. Models of large-scale viscous flow in the Earth’s mantle with constraints from mineral physics and surface observations. Geophys. J. Intern., 167 1461–1481 (2006).

Tackley, P. J. Self-consistent generation of tectonic plates in three-dimensional mantle convection. Earth Planet. Sci. Lett. 157, 9-22, (1998).

Zhong, S. and Gurnis, M. Viscous flow model of a subduction zone with a faulted lithosphere: long and short wavelength topography, gravity and geoid. Geophys. Res. Lett. 19, 1891–1894 (1992).

 

Searching for future directions in tectonic modelling

Searching for future directions in tectonic modelling

Geoscientists frequently use forward geodynamic simulations to test hypotheses derived from geophysical and geologic observations. While numerical simulations of lithospheric deformation have lead to key advances in our understanding of tectonic processes, in many cases it remains difficult to ascertain whether numerical models reproduce observations for the correct underlying regions.  This week, John Naliboff and Jolante van Wijk discuss this issue, and talk about a White Paper being prepared by the Computational Infrastructure for Geodynamics (CIG) Long-Term Tectonics Working Group on this topic.

John Naliboff. Assistant Research Scientist in the Department of Earth and Planetary Sciences, UC Davis.

In recent years, advances in numerical methodology, high-performance computing and elucidation of complex geologic observations have enabled 3-D simulations of long-term lithospheric deformation at kilometer-scale resolution and with complex non-linear material behavior. The lithosphere models generally include rheological and compositional layering that delineate a brittle upper crust, ductile (viscous) to brittle mid- to lower crust and a brittle to ductile lithospheric mantle overlying a purely viscous asthenosphere. Inherently, the rheological behavior of distinct layers varies temporally through complex feedbacks between temperature, grain-size, strain-rate, phase transitions, flexural stresses and finite deformation. Across a wide range of tectonic settings, numerical investigations incorporating this type of behavior have qualitatively and in some cases quantitatively reproduced key first- and second-order geologic observations.

Jolante van Wijk. Associate Professor in the Department of Earth and Environmental Science, New Mexico Tech.

Despite these successes, numerous significant challenges remain as the computational tectonics community looks toward investigations that account for physical processes acting across a wide range of spatial and temporal scales (Figure 1). Geodynamic model development currently evolves around modifying existing models to include surface processes, thermodynamically consistent melt and volatile transport, metamorphic reactions, and brittle failure to reproduce characteristic features of the seismic cycle.

While many of these processes or features are active areas of research and have been addressed on an individual basis, it often remains unclear at best as to how one should numerically validate even the simplest models of lithospheric deformation. In other words, one can ask whether numerical models of lithospheric processes are reproducing key observations for the correct underlying reasons. Significantly, this question of validation equally applies to observational studies: given that many geologic processes contain significant feedbacks across vast spatial and temporal scales, to what degree can a set of specific observations be interpreted to meaningfully reflect first-order processes?

Continued close collaboration between observational, experimental, and computational Earth scientists is needed to overcome these challenges. At present, the CIG Long-Term Tectonics Working Group is preparing a white paper draft that outlines a 5-10 year vision for collaboration between computational Earth scientists and experimental and observational communities. Given the vast series of topics and disciplines associated with lithospheric dynamics, the White Paper will be organized around Transitional Domains within the lithosphere. The Transitional Domains include Earth’s surface, the brittle-ductile transition, the Moho, the mid-lithosphere discontinuity, and the lithosphere-asthenosphere boundary. For each of these domains we will address the following questions:

  1. How are these domains characterized in the Earth’s lithosphere?
  2. (How) have these domains been modeled previously?
  3. What steps can we take to improve the characterization of these transitional domains in numerical models?
  4. What new methods need to be developed to implement the domains?

 

Figure 1. Spatial and temporal scales associated with distinct lithospheric processes, which was published in Cooper et al., 2015, GSA Today, v. 25(6), pp. 42-43 (Figure 1a).

 

A new generation of geodynamic models will be developed to include the transitional domains. These models will need to be validated, using datasets from the observational and experimental communities, with newly developed techniques.

The White Paper will also include sections on increasing the value of lithospheric models for other scientific communities, and on a pathway toward increasing societal relevance of our modeling efforts.

Please contact John Naliboff (jbnaliboff@ucdavis.edu) and Jolante van Wijk (jolante.vanwijk@nmt.edu) for suggestions or questions. A draft White Paper will be presented at CIG’s annual meeting at AGU 2019 in San Francisco.

To serve Geoscientists

To serve Geoscientists

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our latest ‘Geodynamics 101’ post, Fabio Crameri, postdoctoral researcher at the Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway, joins us again. Continuing from his earlier post on the harmful use of the rainbow colour map, Fabio shares his thoughts on some of the expressions and phrases used in the community that propagate confusion, and how the new “Ocean-Plate Tectonics” concept offers relief for at least some on them. 

Blog author Fabio Crameri – in a shirt that translates from Tamasheq as “deserts” or “empty spaces.” You can expect no empty spaces in your lunchtime conversations after reading this post.

Do you, after reading the title, still wonder what this blog post is all about?
I’ll give you a hint, it’s about the Earth. No, wait, it’s about Earth, or perhaps earth, or isn’t it? And maybe it is a little bit about Moon, I mean the Moon. But also, it is about the Venus, I mean Venus.

It is not confusing, it is just well mixing.

You’ve got it; it is about confusion in the Geosciences. Confusion caused by symbols, letters, words and phrases through misuse, ambiguity or over-interpretation. So, after all, this is a blog post about geo-semantics rather than about culinary excursions.

The Geodynamics community is a diverse group of people with different backgrounds, native languages and customs. This is an attractive breeding ground for semantic related problems, particularly when you throw in some inherent peculiarities of the English language in which we largely operate.

Some use the symbol “a” for years, for years and years.

In line with a widely used standard definition (Holden et al., 2011) – but against the common convention of the Geosciences – the author of this blog post was using the unit of time “a”, or arguably just its symbol, for “years” (and I mean calendar years, neither financial years nor dog years), for years and years. A distinction between discrete points in time and the duration of time is at the heart of this confusion, and indeed has plagued a sub-selection of discussions, working groups and interpretations of the International System of Units (SI; e.g., Christie-Blick, 2012).

Figure 1. An ambiguously phrased situation near the recent end of the Cretaceous.

The symbol “a” for “annus” [year] (“Ma” being the symbol for 106 years, or “mega-annus”) in the Geosciences is most commonly used for a specific time or date in the past as measured from now. For example, “At 65 Ma (which is 65 Myr ago), the dinosaur looked up the sky.” (see Figure 1). On the other hand, “yr” for “year(s)” is commonly used for a duration of time, as in “The Cretaceous period ran for 79 Myr (from approximately 145-66 Ma).”. Other mutations within the convention of time in the Geosciences include “My”, “Myrs”, “Mya” or “m.y.” for “Millions of years”. Thus, the time unit and symbols for multiples of a “year” are likely amongst the most ambiguous expressions in the Earth Sciences, likely because, in contrast to the “second”, a universally applied scientific definition for the “annus” still remains elusive (Thompson and Taylor, 2008).

Such quibbling over semantics may seem petty.

Amongst other examples to cause geodynamic misunderstandings (e.g., Figure 2) might be the misuse of the phrase “stagnant slabs”? Are slabs ever really stagnant? Or are they just being deflected, slowing down, interrupting their downward motion, not directly entering the lower mantle at the same speed and trajectory as before?

Figure 2. One ambiguously phrased geodynamic explanation.

From the literature, you might be forgiven for having the false impression that slabs either fully stagnate around the upper-mantle transition zone or directly and effortlessly penetrate it; they likely do neither of the two (as explained in e.g., in an earlier Geodynamics101 post here).

When these slabs sink, and not temporally stagnate, they induce flow in the surrounding mantle. “Slab suction” is the downward suction induced by the nearby mantle that is set in motion through its dynamic coupling with the slab [e.g., Conrad and Lithgow-Bertelloni 2002]. Or isn’t it? “Slab suction” is also contrarily used as an upward directed force on the slab itself that is induced by the upper plate and might foster low-dipping shallow-depth slab portions in the uppermost upper mantle (unambiguously speaking of which: see again Figure 2).

The downward directed version of “slab suction” can induce “dynamic topography”. Estimates of the maximum amplitude of “dynamic topography” on Earth range from only a few hundred meters up to a few kilometres (see e.g., Molnar et al., 2015 and references therein). Such unusually large ranges of estimates are, as a general rule, a quite solid indicator for an underlying ambiguous definition, or in this case, rather a mix-up of multiple different definitions for the term “dynamic topography”. 

If you’re not confused, you did not pay attention.

As I keep talking about geodynamics, I hope we are all on the same page about subduction, one of the key players: Let’s assume planet XY has one single active subduction zone. Another subduction zone initiates on the opposite side of the same planet. Did “subduction” start once or twice on that planet?

It started once on that planet. Because “subduction” describes a process and not a physical feature; it is nonetheless easily mistaken for a physical feature.

And what about “plate tectonics”, the 50 yr old overarching concept that fascinates us, and for so many of us has become the foundation of our professional lives. Let’s approach this by considering the big question: When did “plate tectonics” start? Serious opinions in the plate tectonics community range from around 850 Ma (Hamilton 2011) all the way back to 4.3 Ga (Hopkins et al., 2008). – Remember what unusually large estimate ranges often indicate? – It is not surprising that the only commonly accepted specific answer everyone seems to agree on currently is that it depends on the very definition of plate tectonics.

So, what is the definition of “plate tectonics”? According to its original formulation, “plate tectonics” is the horizontal relative movement of several discrete and mostly-rigid surface-plate segments (Hess, 1962; see the corresponding visual representation in Figure 3). A generous interpretation of the original formulation might additionally define the plate-interface nature, but that is all.

Figure 3. As long as it is not overinterpreted, there is nothing wrong with the original definition of plate tectonics that solely describes the horizontal motion of several discrete surface plates: It does not discriminate the oceanic from the continental plate, does not consider the important framework of mantle convection, and does not specify the underlying key driver of the surface motion.

Considering the knowledge we have gained about the moving surface plates and their underlying causes and consequences during the past 50 yr, this is an extremely broad definition: As of today, we know that (A) the surface plates with their relative motion are an integral part of whole mantle convection (Turcotte and Oxburgh, 1972), that (B) Earth’s surface has a characteristic bimodal nature due to the partitioning into long-lived continental plates and short-lived oceanic plates (e.g., Wilson, 1966), and that (C) the latter are mainly driven by their very own subducted portions (i.e., all or parts of their slabs; Forsyth and Uyeda, 1975; Conrad and Lithgow-Bertelloni, 2002).

A clear, unambiguous and up-to-date definition for such a crucially important, wide-reaching concept is imperative. It is therefore not surprising that less ambiguous re-definitions have been suggested recently. To avoid propagating confusion, the introduction of alternative phases of plate tectonics that describe the various different possible modes of mantle convection during Earth’s evolution have been cast into the arena (e.g., Sobolev 2016). These include “plate-tectonics phase 1”, in short “PT1”, describing regional, plume-induced plate tectonics (e.g., until 3.0 Ga), “PT2” describing episodic, global plate tectonics (e.g., between 2.5-1.0 Ga), and finally “PT3” describing stable, global plate tectonics (e.g., 1.0-0.0 Ga). Other efforts result in different naming conventions, such as “modern plate tectonics”. However, apart from the fact that “modern” is a time dependent term, “modern plate tectonics” might be a somewhat unfortunate expression, as other planets like Venus might have undergone different, modern styles of plate tectonics than present-day Earth.

Stern and Gerya (2017) then actually suggests an entire update to the definition of “plate tectonics”:

“A theory of global tectonics powered by subduction in which the lithosphere is divided into a mosaic of strong lithospheric plates, which move on and sink into weaker ductile asthenosphere. Three types of localised plate boundaries form the interconnected global network: new oceanic plate material is created by seafloor spreading at mid-ocean ridges, old oceanic lithosphere sinks at subduction zones, and two plates slide past each other along transform faults. The negative buoyancy of old dense oceanic lithosphere, which sinks in subduction zones, mostly powers plate movements.”

Unfortunately, such a re-definition of the same old phrase makes it impossible to know which version of the definition (i.e., the original or the updated one) an author of a subsequent study should be applying and referring to.

In an effort to prevent all of the above problems, we recently introduced an entirely new concept; one that can coexist in harmony with the original definition; one that fully captures the dynamics of the oceanic plate according to our current knowledge. The concept is called “Ocean-Plate Tectonics” or, if you really like the term, “OPT”.

“Ocean-Plate Tectonics is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process.” – Crameri et al. (2018).

“Ocean-Plate Tectonics” captures not only the relative horizontal surface motion of plates, but crucially also accounts for (A) the importance of the whole mantle framework, (B) the bimodal nature of Earth’s surface plates, and (C) the underlying engine of the surface-plate motion (see Figure 4).

Figure 4. “Ocean-Plate Tectonics”, the unambiguous up-to-date definition describing the dynamics of the oceanic plate that crucially incorporates the bimodal nature of Earth’s surface, the convecting-mantle framework, and the key driver of surface-plate motion (after Crameri et al., 2018).

“Ocean-Plate Tectonics” is here to serve Geoscientists.

The concept of “Ocean-Plate Tectonics” is intended to bring together the extremely diverse research communities, but also the general public, to meet on common, fruitful ground in order to discuss and further develop our understanding of the fascinating dynamics involved in Earth’s plate-mantle system; the unambiguous “Ocean-Plate Tectonics” is here to serve us.

 

Christie-Blick, N., (2011), Geological Time Conventions and Symbols, GSA Today, 22(2), 28-29, doi: 10.1130/G132GW.1

Conrad, C. P., and C. Lithgow-Bertelloni (2002), How mantle slabs drive plate tectonics, Science, 298 (5591), 207–209, doi:10.1126/science.1074161.

Crameri, F., C.P. Conrad, L. Montési, and C.R. Lithgow-Bertelloni (2018), The life of an oceanic plate, Tectonophysics, (in press), doi:10.1016/j.tecto.2018.03.016 .

Forsyth, D., and S. Uyeda (1975), On the relative importance of the driving forces of plate motion*, Geophysical Journal of the Royal Astronomical Society, 43(1), 163–200, doi:10.1111/j.1365-246X.1975.tb00631.x.

Hamilton, W.B. (2011), Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated, Lithos, 123, 1–20, doi:10.1016/j.lithos.2010.12.007.

Hess, H.H. (1962), History of ocean basins, Petrologic studies, 4, 599–620.

Holden N.E., M.L. Bonardi, P. De Bièvre, P.R. Renne and I.M. Villa (2011), IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011, Pure Appl. Chem., Vol. 83, No. 5, pp. 1159–1162, 2011. doi:10.1351/PAC-REC-09-01-22

Hopkins M., T.M. Harrison, C.E. Manning (2008), Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions, Nature, 456, 493–96, doi:10.1038/nature07465.

Molnar, P., P. C. England, and C. H. Jones (2015), Mantle dynamics, isostasy, and the support of high terrain. J. Geophys. Res. Solid Earth, 120, 1932–1957. doi: 10.1002/2014JB011724.

Sobolev, S.V. (2016), Plate Tectonics Initiation as Running Hurdles, Workshop on the Origin and Evolution of Plate Tectonics abstract, Ascona, Switzerland, http://jupiter.ethz.ch/~plates/.

Stern, R.J. and T.V. Gerya (2017), Subduction initiation in nature and models: A review, Tectonophysics, doi:10.1016/j.tecto.2017.10.014

Thompson, A., and B.N. Taylor (2008), Guide for the Use of the International System of Units (SI) NIST Special Publication 811, 2008 Edition (version 3.2). [Online] Available: http://physics.nist.gov/SP811 [2018, 05 02]. National Institute of Standards and Technology, Gaithersburg, MD.

Turcotte, D. L., and E. Oxburgh (1972), Mantle convection and the new global tectonics, Annual Review of Fluid Mechanics, 4 (1), 33–66.

Wilson, T. (1966), Did the Atlantic close and then re-open?, Nature, 211(5050), 676–681, doi:http://dx.doi.org/10.1038/211676a0

The Venus enigma: new insights into ‘Earth 2’

The Venus enigma: new insights into ‘Earth 2’

Apart from Earth, there are a lot of Peculiar Planets out there! Every 8 weeks, we look at a planetary body worthy of our geodynamic attention. This week Richard Ghail, lecturer in Engineering Geology at Imperial College London in the United Kingdom, discusses Earth’s sister: Venus.

Geologists have long held the view that they only have the results of one experiment: Earth. The growing list of Earth-like planets around other stars (exo-Earths) means that such a view is no longer valid, even if we have limited knowledge of those worlds. Surprisingly, perhaps, our own Solar System boasts two exo-Earths: and the other one is not Mars, as you might think, but Venus. Our nearest planetary neighbour is also the most similar to Earth: at about 80% of its mass and 95% of its radius, and orbiting arguably within our Sun’s habitable zone, Venus would be recognised as an Earth-like exoplanet if it were in orbit around another star. Yet the results of these two experiments could not be more different: Earth may not quite be Eden, but Venus is certainly the closest place we know to hell. Its dense global shroud of sulphuric acid clouds hides a surface on which a person would be simultaneously roasted (at 450°C), crushed (at 90 atmospheres pressure), poisoned and asphyxiated (its atmosphere is 95% CO₂), and corroded (not by sulphuric acid, which decomposes under the extreme conditions, but by HCl and even HF!). The armoured Soviet Venera landers survived only a couple of hours on the surface, but still managed to return tantalising pictures of a barren rocky landscape bathed in an orange light.

A geodynamic surprise: Catastrophe or not?
NASA’s Magellan mission (1989-1994) revealed that geodynamically too, Venus was a surprise. A wealth of volcanoes, rifts and mountains cover its surface but there is little evidence for the spreading ridges and deep trenches that characterise plate tectonics on Earth. More perplexing was the realisation that the 950 or so impact craters – implying a youthful 500 Ma average age – were distributed apparently at random. Had the whole planet been somehow catastrophically resurfaced in one go, half a billion years ago? As strange as it might sound, there appeared to be good reasons to think so: that 450°C surface temperature is enough to stop the crust subducting, effectively shutting down plate tectonics. Without that safety valve, the interior of Venus must be getting ever hotter at the same time that exterior cools and thickens the lithosphere. Such a situation is inherently unstable and calculations showed that Venus should ‘blow its top’ every 500 Ma or so – explaining both the lack of plate tectonic features and the crater distribution. The Venus enigma was solved.

Or was it? The theory divided the community into bitterly opposed sides for the next decade or more. One group could see a global sequence of events in its geological features that seemed to confirm the theory; while the other could see an array of geological complexity at odds with it. ESA’s Venus Express mission (2005-2012) focussed on the planet’s atmosphere but it revealed a remarkably dynamic and changeable system that must somehow reflect geodynamic activity below. It even found tantalising hints of recent volcanic activity. Understanding both the geological evidence and the crater distribution turns out to depend on the very thing that set Venus apart from Earth: its extreme surface conditions. The high temperature not only makes the crust buoyant, but weak, especially so at about 10 km depth, where it is able to shear relative to the mantle below. In this new geodynamic view, plate tectonics does operate on Venus much as it does on Earth, but under 10 km of crust, not 5 km of ocean (Ghail, 2015). As well as explaining the large-scale features of Venus, including its geoid, calculations show that this subcrustal rejuvenation, as it is called, is able to maintain the heat balance on Venus). No catastrophic events are required.

If the crater distribution is not the result of a global catastrophe, what caused it? Mechanically, the basaltic crust of Venus behaves much like Earth’s granitic continental crust, and is similarly broken into many small plates, or terranes, on the order of 500 to 1500 km across, characterised by low strain interiors and highly deformed margins, similar to terrestrial continental blocks. On Earth these terranes are driven by far-field plate tectonic stresses but on Venus they are driven by subcrustal rejuvenation stresses that jostle the terranes against one other but do not move them far across the surface. Impact craters are preserved in terrane interiors but rapidly destroyed at their margins, so that the average crater spacing is similar to the size of terranes. The preserved terrane-interior craters are only destroyed when the terrane is itself destroyed, most likely by interaction with a subcrustal plate boundary (rift or collision), which by inference is something that occurs on average every half billion years. This new geodynamic understanding refines our appreciation of how the geochemistry and geomechanics of the outer few kilometres of the planet profoundly influence stagnant-lid behaviour, promising new insights into early-Earth geodynamics and the nature of newly-discovered exo-Earths.

Welcome to hell Venus. I want you to explore it!
Credit: Pixabay

Exploring Venus
So our views of Venus have changed; our Solar System’s second experiment is, geodynamically, rather more similar to Earth than once imagined. Even so, these ideas have yet to be tested, and our nearest neighbour retains many secrets. Almost nothing is known about its interior, its rates of activity, or even how Venus maintains such a hostile atmosphere. A new phase in Venus exploration is called for, and within Europe the most promising is the proposed EnVision mission, which is currently undergoing evaluation by ESA. EnVision will use an advanced Earth Observation heritage interferometric radar to measure and monitor geological activity over a 5-year period and obtain images at up to 1 m resolution – sufficient to locate and track the Venera landers, providing the precise geodetic control needed to measure terrane deformation. A radar sounder will probe the near subsurface and an IR/UV emission spectrometer will map geochemistry and follow volcanic gases from their source to the upper atmosphere. NASA has proposed landers that could probe interior seismicity, and in the future balloons may directly sense cloud chemistry and dynamics. Unlike the Moon and Mars, these missions will be exploring a world that is – in a geodynamic sense, at least – very much alive.

References
Ghail, R. (2015). Rheological and petrological implications for a stagnant lid regime on Venus. Planetary and Space Science, 113, 2-9.