Page 32 - i1052-5173-31-1

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.

32 GSA Today | January 2021