Earth Evolution, Emergence, and Uniformitarianism

Robert J. Stern, Geosciences Dept., The University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688, USA; and Taras Gerya, Swiss Federal Institute of Technology (ETH-Zurich), Earth Sciences, Sonneggstrasse 5, Zurich 8092, Switzerland

Emergent phenomenon describes the pro- lithospheres respond by becoming thicker, this to identify times when significant shifts pensity for any high-energy, far-from-equi- denser, and stronger. Strong temperature in mean T/P occurred. The variations in librium system to self-organize in ways that gradients between the cold, rigid exterior Earth’s thermobarometric ratio must reflect cannot be predicted from knowing its indi- and the hot, convecting interior cause den- changes in Earth’s convective and tectonic vidual components (Ablowitz, 1939; Pines, sity inversions coupled to large nonlinear style that can usefully be called emergent. 2014). Emergence is closely related to self- variations of rock strength and viscosity Consistent with this conclusion, numerical organization, complexity, and evolution. that together drive emergent behavior, man- modeling investigation even of very simpli- Animals, ecosystems, spiral galaxies, hydro- ifested in the lithosphere as tectonics. fied mantle convection systems with Earth- thermal systems, hurricanes, and civiliza- Although emergent behavior is today like rheology shows emergent behavior, such tions are some of the many examples of impossible to predict, it can leave evidence as spontaneous appearance and self-organi- emergent phenomena, where low-level rules that allows the history of an emergent sys- zation of various tectonic plate boundaries; give rise to higher-level complexity. Entirely tem to be reconstructed and quantitatively growth, aging, and subduction of oceanic new properties and behaviors “emerge,” understood. This is as true for planets as it plates; and generation of a global plate without direction and with characteristics is for civilization. Is it possible to discern mosaic (e.g., Tackley, 2000). Lenardic (2018) that cannot be predicted from knowledge of emergent behavior in the tectonic behavior explored this point further, arguing that any the constituents alone. The whole is truly of active bodies in the Solar System? Yes, convecting Earth-like silicate body would greater than the sum of its parts. Yes, the sec- but it is easier for smaller, dying planets experience multiple emergent transitions ond law of thermodynamics is real, but it can than for more vigorous, larger ones (Earth, between different planetary tectonic regimes, take a long time for the system to stop dissi- Venus), where evidence for earlier tectonic reflecting changes in lithosphere strength and pating energy. In the case of long-lived, styles is often obliterated by newly emer- planetary internal energy with time. Indeed, high-energy systems like convecting silicate gent ones. Mars is a good example of a numerical models reveal that several different planets with a significant fraction of primor- slowly dying planet, because its small size global geodynamic regimes in Precambrian dial heat trapped inside and with slowly has enhanced cooling of its interior over its time likely preceded modern plate tectonics diminishing contributions from radioactive 4.56 Ga lifetime. Mars’ crustal dichotomy (e.g., Gerya, 2019). Multi-stable behavior decay, entropy may have to wait billions of preserves evidence of three successive allows, in particular, for the possibility that years to shut down the party. emergent tectonic styles: (1) creation of the plate tectonics could emerge, transition to Morowitz (2002) outlines the emergence primitive crust now preserved in the south- another mode, and re-emerge along a planet’s of 28 things, beginning with the Big Bang ern hemisphere; (2) crustal rejuvenation cooling path. and ending with civilization. The self-orga- (best exposed in the northern lowlands) by Because the emerging tectonic regime nization of organic molecules to make life widespread volcanism possibly related to will obliterate much of the evidence for ear- may be the most spectacular example of giant impact and subsequent mantle con- lier regimes, we will have to be clever to fig- emergence. Earth’s climate, hydrosphere, vection (e.g., Golabek et al., 2011); and (3) ure out how plate tectonics evolved on Earth and nutrient cycle all are emergent phenom- strongly focused long-term magmatism and and even more clever to figure out what other ena. These are in fact co-emergent systems, tectonics caused by localized mantle plumes, tectonic styles emerged before this. We have evolving together in ways that presently manifested by large volcanoes in the Tharsis argued elsewhere that the modern episode of cannot be predicted. Those who have tried and Valles Marineris regions. plate tectonics emerged when a very strong to predict the stock market or the course of Plate tectonics—Earth’s unique litho- mantle plume ruptured all-encompassing but the COVID-19 pandemic know the futility spheric manifestation of mantle convec- gravitationally unstable lithosphere (Gerya of trying to foresee what will happen next in tion—is almost certainly an example of et al., 2015), and one of us has repeatedly these emergent systems. The tectonic styles emergent behavior of a still-vigorous con- argued on different lines of evidence that this of convecting silicate bodies in our Solar vecting planet. This conclusion was recently happened in Neoproterozoic time (Stern, System are also examples of emergent highlighted by Brown et al. (2020), who 2018). These ideas are controversial but beg behavior. Such behavior is expected for compiled and analyzed thermobaric ratios the question: why hasn’t the conceptual these high-energy, far-from-equilibrium (temperature/pressure, T/P) for Paleoarchean framework of emergent tectonics gained systems as their interiors cool and their to Cenozoic metamorphic rocks and used more currency in our science?

GSA Today, v. 31, https://doi.org/10.1130/GSATG479GW.1. CC-BY-NC. One problem may be our (mostly implicit This emergent trend in earth sciences REFERENCES CITED but still pervasive) attachment to the princi- reflects the maturing of the discipline from Ablowitz, R., 1939, The Theory of emergence: Phi- ple of uniformitarianism,“The present is the a descriptive qualitative to a predictive losophy of Science, v. 6, p. 1–16, https://doi​.org/​ key to the past” and its offspring, actualism quantitative science and opens the door to 10.1086/286529. Brown, M., Kirkland, C.L., and Johnson, T.E., 2020, “The present, punctuated by occasional clearer thinking about emergent phenom- Evolution of geodynamics since the Archean: Sig- catastrophes, like bolide impacts and snow- ena on Earth. In this respect, modeling nificant change at the dawn of the Phanerozoic: ball Earth, is the key to the past” (Windley, combined with observations offer a good Geology, v. 48, p. 488–492, https://doi​.org/​ 1993). Uniformitarianism was very useful way to better calibrate our intuition for 10.1130/G47417.1. when eighteenth- and nineteenth-century emergence, as well as to test if a geological Gerya, T., 2019, Geodynamics of the early Earth: geologists were debating the age of the Earth system of interest is prone to emergent Quest for the missing paradigm: Geology, v. 47, with clergy claiming it was 6,000 years old, behavior and what are the main physical p. 1006–1007, https://doi.org/10.1130/focus102019.1. but that was then, and this is now. Does our parameters controlling it. Gerya, T., Stern, R.J., Baes, M., Sobolev, S., and Whattam, S., 2015, Plume-induced subduction allegiance to the old philosophy stop us from We think that encouraging thinking about initiation triggered plate tectonics on Earth: Na- addressing questions that need to be asked? the role of emergence in all earth systems ture, v. 527, p. 221–225, https://doi.org/10.1038/ Gould (1965) distinguished substantive ​ should be part of the way for the geosciences nature15752. and methodological uniformitarianism. Sub- to advance in the twenty-first century. The Golabek, G.J., Keller, T., Gerya, T.V., Zhu, G., stantive uniformitarianism considers that field of emergence is much broader than the Tackley, P.J., and Connolly, J.A.D., 2011, Origin ancient Earth processes (e.g., orogeny, sedi- earth sciences, with entire institutes study- of the Martian dichotomy and Tharsis from a mentation, erosion) were the same as now ing a wide range of emergent phenomena; for giant impact causing massive magmatism: Ica- operating. In contrast, methodological unifor- example, the Santa Fe Institute, https://www​ rus, v. 215, p. 346–357, https://doi.org/10.1016/​ mitarianism states the obvious: that the laws .santafe.edu/about. At present, the emergence j.icarus.2011.06.012.​ Gould, S.J., 1965, Is uniformitarianism necessary?: of physics and chemistry pertain to all of of planetary tectonic styles is not being con- American Journal of Science, v. 263, p. 223–228, Earth’s history. Gould (1965) concluded that sidered by these researchers, and it should https://doi.org/10.2475/ajs.263.3.223. substantive uniformitarianism was “…false be. How can we help make this happen? A Lenardic, A., 2018, The diversity of tectonic modes and stifling to hypothesis formation…” and is good first step would be for more geoscien- and thoughts about transitions between them: “…an incorrect theory [that] should be aban- tists to learn about emergence; the Wikipedia Philosophical Transactions of the Royal Society doned” (p. 223). There is still an important entry “emergence” is a good place to start. A, v. 376, https://doi.org/10.1098/rsta.2017.0416. role for substantive uniformitarianism in our Second steps include teaching about emer- Morowitz, H.L., 2002, How the World Became Com- efforts to reach and teach students and the gence in our classes and considering it in plex: The Emergence of Everything: Oxford, UK, Oxford University Press, 209 p. public. Perhaps in 1965 it appeared that the our research. Pines, D., 2014, Emergence: A unifying theme for battle with creation pseudoscience was over, Embracing emergence for understanding 21st century science: Foundations and Frontiers of but not in 2020, at least in the United States. Earth’s history not only can inject excite- Complexity, v. 28, no. 2, https://medium.com/​sfi​ Substantive uniformitarianism is still useful ment into our science, the philosophy can -30-foundations-frontiers/emergence-a-unifying​ for teaching lower-division undergraduates pay psychic benefits. We are facing a very -theme-for-21st-century-science-4324ac0f951e and in battles with creationists, for example, uncertain future, but thinking about emer- (accessed 10 Sept. 2020). to show why and how the Grand Canyon was gence can perhaps reassure us that all Stern, R.J., 2018, The evolution of plate tectonics: carved in a few million years by the Colorado futures are uncertain except for low-energy Philosophical Transactions of the Royal Society A, v. 376, https://doi.org/10.1098/rsta.2017.0406. River flowing through a plateau lifted up by systems (e.g., dead planets and dead peo- Tackley, P.J., 2000, Self-consistent generation of mantle convection, not in a few days by ple). Which would you rather be part of, a tectonic plates in time-dependent, three dimen- Noah’s flood. But within the scientific com- low-energy system with a certain future or sional mantle convection simulations Part 1: munity, substantive uniformitarianism poi- a high-energy system with an unpredictable Pseudo-plastic yielding: Geochemistry, Geo- sons scientific discussions about how plate future but with the promise that something physics, Geosystems, v. 1, no. 8, https://doi.org/​ tectonics came to be Earth’s dominant con- will emerge, some time in the future? We 10.1029/​2000GC000036. vective mode. know which planet we want to be on! Windley, B.F., 1993, Uniformitarianism today: Modern earth sciences use methodologi- Plate tectonics is the key to the past: Journal of the Geological Society, v. 150, p. 7–19, https:// cal uniformitarianistic approaches for both ACKNOWLEDGMENTS doi.org/10.1144/gsjgs.150.1.0007. discovering and understanding emergence Thanks to Pete DeCelles and an anony- based on numerical modeling that uses fun- mous referee for constructive comments Manuscript received 23 July 2020 damental physical laws for investigating and suggestions. This is University of Texas Revised manuscript received 20 Aug. 2020 behavior of complex geological systems. at Dallas geosciences contribution #1373. Manuscript accepted 6 Sept. 2020