Emergent phenomenon describes the propensity for any high-energy, far-from-equilibrium system
to self-organize in ways that cannot be predicted from knowing its individual components
(Ablowitz, 1939; Pines, 2014). Emergence is closely related to self-organization,
complexity, and evolution. Animals, ecosystems, spiral galaxies, hydrothermal systems,
hurricanes, and civilizations are some of the many examples of emergent phenomena, where
low-level rules give rise to higher-level complexity. Entirely new properties and behaviors
“emerge,” without direction and with characteristics that cannot be predicted from knowledge
of the constituents alone. The whole is truly greater than the sum of its parts. Yes, the
second law of thermodynamics is real, but it can take a long time for the system to stop
dissipating energy. In the case of long-lived, high-energy systems like convecting silicate
planets with a significant fraction of primordial heat trapped inside and with slowly
diminishing contributions from radioactive decay, entropy may have to wait billions of years
to shut down the party.
Morowitz (2002) outlines the emergence of 28 things, beginning with the Big Bang and ending
with civilization. The self-organization of organic molecules to make life may be the most
spectacular example of emergence. Earth’s climate, hydrosphere, and nutrient cycle all are
emergent phenomena. These are in fact co-emergent systems, evolving together in ways that
presently cannot be predicted. Those who have tried to predict the stock market or the
course of the COVID-19 pandemic know the futility of trying to foresee what will happen next
in these emergent systems. The tectonic styles of convecting silicate bodies in our Solar
System are also examples of emergent behavior. Such behavior is expected for these
high-energy, far-from-equilibrium systems as their interiors cool and their lithospheres
respond by becoming thicker, denser, and stronger. Strong temperature gradients between the
cold, rigid exterior and the hot, convecting interior cause density inversions coupled to
large nonlinear variations of rock strength and viscosity that together drive emergent
behavior, manifested in the lithosphere as tectonics.
Although emergent behavior is today impossible to predict, it can leave evidence that allows
the history of an emergent system to be reconstructed and quantitatively understood. This is
as true for planets as it is for civilization. Is it possible to discern emergent behavior
in the tectonic behavior of active bodies in the Solar System? Yes, but it is easier for
smaller, dying planets than for more vigorous, larger ones (Earth, Venus), where evidence
for earlier tectonic styles is often obliterated by newly emergent ones. Mars is a good
example of a slowly dying planet, because its small size has enhanced cooling of its
interior over its 4.56 Ga lifetime. Mars’ crustal dichotomy preserves evidence of three
successive emergent tectonic styles: (1) creation of the primitive crust now preserved in
the southern hemisphere; (2) crustal rejuvenation (best exposed in the northern lowlands) by
widespread volcanism possibly related to giant impact and subsequent mantle convection
(e.g., Golabek et al., 2011); and (3) strongly focused long-term magmatism and tectonics
caused by localized mantle plumes, manifested by large volcanoes in the Tharsis and Valles
Plate tectonics—Earth’s unique lithospheric manifestation of mantle convection—is almost
certainly an example of emergent behavior of a still-vigorous convecting planet. This
conclusion was recently highlighted by Brown et al. (2020), who compiled and analyzed
thermobaric ratios (temperature/pressure, T/P) for Paleoarchean to Cenozoic
metamorphic rocks and used this to identify times when significant shifts in mean
T/P occurred. The variations in Earth’s thermobarometric ratio must reflect changes
in Earth’s convective and tectonic style that can usefully be called emergent. Consistent
with this conclusion, numerical modeling investigation even of very simplified mantle
convection systems with Earth-like rheology shows emergent behavior, such as spontaneous
appearance and self-organization of various tectonic plate boundaries; growth, aging, and
subduction of oceanic plates; and generation of a global plate mosaic (e.g., Tackley, 2000).
Lenardic (2018) explored this point further, arguing that any convecting Earth-like silicate
body would experience multiple emergent transitions between different planetary tectonic
regimes, reflecting changes in lithosphere strength and planetary internal energy with time.
Indeed, numerical models reveal that several different global geodynamic regimes in
Precambrian time likely preceded modern plate tectonics (e.g., Gerya, 2019). Multi-stable
behavior allows, in particular, for the possibility that plate tectonics could emerge,
transition to another mode, and re-emerge along a planet’s cooling path.
Because the emerging tectonic regime will obliterate much of the evidence for earlier
regimes, we will have to be clever to figure out how plate tectonics evolved on Earth and
even more clever to figure out what other tectonic styles emerged before this. We have
argued elsewhere that the modern episode of plate tectonics emerged when a very strong
mantle plume ruptured all-encompassing but gravitationally unstable lithosphere (Gerya et
al., 2015), and one of us has repeatedly argued on different lines of evidence that this
happened in Neoproterozoic time (Stern, 2018). These ideas are controversial but beg the
question: why hasn’t the conceptual framework of emergent tectonics gained more currency in
One problem may be our (mostly implicit but still pervasive) attachment to the principle of
uniformitarianism,“The present is the key to the past” and its offspring, actualism “The
present, punctuated by occasional catastrophes, like bolide impacts and snowball Earth, is
the key to the past” (Windley, 1993). Uniformitarianism was very useful when eighteenth- and
nineteenth-century geologists were debating the age of the Earth with clergy claiming it was
6,000 years old, but that was then, and this is now. Does our allegiance to the old
philosophy stop us from addressing questions that need to be asked?
Gould (1965) distinguished substantive and methodological uniformitarianism. Substantive
uniformitarianism considers that ancient Earth processes (e.g., orogeny, sedimentation,
erosion) were the same as now operating. In contrast, methodological uniformitarianism
states the obvious: that the laws of physics and chemistry pertain to all of Earth’s
history. Gould (1965) concluded that substantive uniformitarianism was “…false and stifling
to hypothesis formation…” and is “…an incorrect theory [that] should be abandoned” (p. 223).
There is still an important role for substantive uniformitarianism in our efforts to reach
and teach students and the public. Perhaps in 1965 it appeared that the battle with creation
pseudoscience was over, but not in 2020, at least in the United States. Substantive
uniformitarianism is still useful for teaching lower-division undergraduates and in battles
with creationists, for example, to show why and how the Grand Canyon was carved in a few
million years by the Colorado River flowing through a plateau lifted up by mantle
convection, not in a few days by Noah’s flood. But within the scientific community,
substantive uniformitarianism poisons scientific discussions about how plate tectonics came
to be Earth’s dominant convective mode.
Modern earth sciences use methodological uniformitarianistic approaches for both discovering
and understanding emergence based on numerical modeling that uses fundamental physical laws
for investigating behavior of complex geological systems. This emergent trend in earth
sciences reflects the maturing of the discipline from a descriptive qualitative to a
predictive quantitative science and opens the door to clearer thinking about emergent
phenomena on Earth. In this respect, modeling combined with observations offer a good way to
better calibrate our intuition for emergence, as well as to test if a geological system of
interest is prone to emergent behavior and what are the main physical parameters controlling
We think that encouraging thinking about the role of emergence in all earth systems should be
part of the way for the geosciences to advance in the twenty-first century. The field of
emergence is much broader than the earth sciences, with entire institutes studying a wide
range of emergent phenomena; for example, the Santa Fe Institute,
https://www.santafe.edu/about. At present, the emergence
of planetary tectonic styles is not
being considered by these researchers, and it should be. How can we help make this happen? A
good first step would be for more geoscientists to learn about emergence; the Wikipedia
entry “emergence” is a good place to start. Second steps include teaching about emergence in
our classes and considering it in our research.
Embracing emergence for understanding Earth’s history not only can inject excitement into our
science, the philosophy can pay psychic benefits. We are facing a very uncertain future, but
thinking about emergence can perhaps reassure us that all futures are uncertain except for
low-energy systems (e.g., dead planets and dead people). Which would you rather be part of,
a low-energy system with a certain future or a high-energy system with an unpredictable
future but with the promise that something will emerge, some time in the future? We know
which planet we want to be on!
Thanks to Pete DeCelles and an anonymous referee for constructive comments and suggestions.
This is University of Texas at Dallas geosciences contribution #1373.
- Ablowitz, R., 1939, The Theory of emergence: Philosophy of Science, v. 6,
p. 1–16, https://doi.org/10.1086/286529.
- Brown, M., Kirkland, C.L., and Johnson, T.E., 2020, Evolution of geodynamics since the
Archean: Significant change at the dawn of the Phanerozoic: Geology, v. 48,
p. 488–492, https://doi.org/10.1130/G47417.1.
- Gerya, T., 2019, Geodynamics of the early Earth: Quest for the missing paradigm:
Geology, v. 47, p. 1006–1007, https://doi.org/10.1130/focus102019.1.
- Gerya, T., Stern, R.J., Baes, M., Sobolev, S., and Whattam, S., 2015, Plume-induced
subduction initiation triggered plate tectonics on Earth: Nature, v. 527,
p. 221–225, https://doi.org/10.1038/nature15752.
- Golabek, G.J., Keller, T., Gerya, T.V., Zhu, G., Tackley, P.J., and Connolly, J.A.D.,
2011, Origin of the Martian dichotomy and Tharsis from a giant impact causing massive
magmatism: Icarus, v. 215, p. 346–357,
- Gould, S.J., 1965, Is uniformitarianism necessary?: American Journal of Science,
v. 263, p. 223–228, https://doi.org/10.2475/ajs.263.3.223.
- Lenardic, A., 2018, The diversity of tectonic modes and thoughts about transitions
between them: Philosophical Transactions of the Royal Society A, v. 376,
- Morowitz, H.L., 2002, How the World Became Complex: The Emergence of Everything: Oxford,
UK, Oxford University Press, 209 p.
- Pines, D., 2014, Emergence: A unifying theme for 21st century science: Foundations and
Frontiers of Complexity, v. 28, no. 2,
(accessed 10 Sept. 2020).
- Stern, R.J., 2018, The evolution of plate tectonics: Philosophical Transactions of the
Royal Society A, v. 376, https://doi.org/10.1098/rsta.2017.0406.
- Tackley, P.J., 2000, Self-consistent generation of tectonic plates in time-dependent,
three dimensional mantle convection simulations Part 1: Pseudo-plastic yielding:
Geochemistry, Geophysics, Geosystems, v. 1, no. 8, https://doi.org/10.1029/2000GC000036.
- Windley, B.F., 1993, Uniformitarianism today: Plate tectonics is the key to the past:
Journal of the Geological Society, v. 150, p. 7–19,
Manuscript received 23 July 2020.
Revised manuscript received 20 Aug. 2020.
Manuscript accepted 6 Sept. 2020.
Posted 25 Sept. 2020.
© 2020, The Geological Society of America. CC-BY-NC.