Plate tectonics is a particular mode of tectonic activity that characterizes the present-day Earth. It is directly linked to not only tectonic deformation but also magmatic/volcanic activity and all ...aspects of the rock cycle. Other terrestrial planets in our Solar System do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation. This indicates the existence of tectonic modes different from plate tectonics. This article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date. A categorization of tectonic modes relates to the issue of when plate tectonics initiated on Earth as it provides insights into possible pre-plate tectonic behaviour. The final focus of this contribution relates to transitions between tectonic modes. Different transition scenarios are discussed. One follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet. The other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions. The latter scenario allows for the existence of multiple stable tectonic modes under the same physical/chemical conditions. The different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents. Under the classic regime transition view, the tectonic mode of a planet is a state variable (akin to temperature). Under the multiple stable modes view, the tectonic mode of a planet is a process variable. That is, something that flows through the system (akin to heat). The different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on Earth and why does Earth have plate tectonics.
This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.
Paleo‐temperature data indicates that the Earth's mantle has not cooled at a constant rate. The data show slow cooling from 3.8 to 2.5 Ga followed by more rapid cooling until the present. This has ...been argued to indicate a transition from a single plate mode to a plate tectonics. However, a tectonic change may not be necessary. Multistage cooling can result from deep water cycling coupled to mantle convection. Melting and volcanism removes water from the mantle (degassing). Dehydration tends to stiffen the mantle, which slows convective vigor and plate velocities causing mantle heating. Higher mantle temperature tends to lower mantle viscosity and increase plate velocities. If these two tendencies are in balance, then mantle cooling can be weak. Breaking this balance, via a switch to net mantle rehydration, can cool the mantle more rapidly. We use coupled water cycling and mantle convection models to test the viability of this hypothesis. Within model and data uncertainty, the hypothesis that deep water cycling can lead to a multi‐stage Earth cooling is consistent with data constraints. Probability distributions, for successful models, indicate that plate and plate margin strength play a minor role for resisting plate motions relative to the resistance from interior mantle viscosity.
Plain Language Summary
The Earth's internal energy drives geologic and tectonic activity. How the Earth's interior has cooled over time provides constraints on how this energy has been tapped. Observational constraints indicate that Earth cooling was multistaged with slow interior cooling followed by an acceleration in cooling rate. We use thermal history models to test the hypothesis that this transition is due to a change in the nature of water cycling between surface and interior reservoirs. The models indicate that water cycling, coupled to plate tectonics, can lead to multistage Earth cooling that is consistent with observational constraints. This hypothesis requires no change in the tectonic mode of the Earth—it is consistent with the idea that plate tectonics has operated over geologic history.
Key Points
Deep water cycle coupled to thermal convection allows for multistage Earth cooling
Multistage mantle cooling does not require tectonic transitions
A switch from mantle dewatering to rewatering leads to accelerated mantle cooling
A model of coupled mantle convection and planetary tectonics is used to demonstrate that history dependence can outweigh the effects of a planet's energy content and material parameters in ...determining its tectonic state. The mantle convection-surface tectonics system allows multiple tectonic modes to exist for equivalent planetary parameter values. The tectonic mode of the system is then determined by its specific geologic and climatic history. This implies that models of tectonics and mantle convection will not be able to uniquely determine the tectonic mode of a terrestrial planet without the addition of historical data. Historical data exists, to variable degrees, for all four terrestrial planets within our solar system. For the Earth, the planet with the largest amount of observational data, debate does still remain regarding the geologic and climatic history of Earth's deep past but constraints are available. For planets in other solar systems, no such constraints exist at present. The existence of multiple tectonic modes, for equivalent parameter values, points to a reason why different groups have reached different conclusions regarding the tectonic state of extrasolar terrestrial planets larger than Earth ("super-Earths"). The region of multiple stable solutions is predicted to widen in parameter space for more energetic mantle convection (as would be expected for larger planets). This means that different groups can find different solutions, all potentially viable and stable, using identical models and identical system parameter values. At a more practical level, the results argue that the question of whether extrasolar terrestrial planets will have plate tectonics is unanswerable and will remain so until the temporal evolution of extrasolar planets can be constrained.
The discovery of terrestrial‐scale extrasolar planets, and their calculated abundance in the galaxy, has prompted speculation on their surface conditions and thermal structure. Both are dependent on ...the tectonic regime of a planet, which is itself a function of the balance between driving forces, and the resistive strength of the lithosphere. Here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses, and thus super‐sized Earths are likely to be in an episodic or stagnant lid regime. This effect is robust when associated increases in gravity are included, as the more dominant effect is increased fault strength rather than greater buoyancy forces. The thermo‐tectonic evolution of large terrestrial planets is more complex than often assumed, and this has implications for the surface and conditions habitability of such worlds.
SUMMARY
Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle ...viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.
Supercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral ...oceanic‐continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long‐term carbon cycle for a few hundred million years. We propose that the relatively warm and unchanging climate of the Nuna supercontinental epoch (1.8–1.3 Ga) is characteristic of thorough mantle thermal mixing. By contrast, the extreme cooling‐warming climate variability of the Neoproterozoic Rodinia episode (1–0.63 Ga) and the more modest but similar climate change during the Mesozoic Pangea cycle (0.3–0.05 Ga) are characteristic features of the effects of subcontinental mantle thermal isolation with differing longevity. A tectonically modulated carbon cycle model coupled to a one‐dimensional energy balance climate model predicts the qualitative form of Mesozoic climate evolution expressed in tropical sea‐surface temperature and ice sheet proxy data. Applied to the Neoproterozoic, this supercontinental control can drive Earth into, as well as out of, a continuous or intermittently panglacial climate, consistent with aspects of proxy data for the Cryogenian‐Ediacaran period. The timing and magnitude of this cooling‐warming climate variability depends, however, on the detailed character of mantle thermal mixing, which is incompletely constrained. We show also that the predominant modes of chemical weathering and a tectonically paced abiotic methane production at mid‐ocean ridges can modulate the intensity of this climate change. For the Nuna epoch, the model predicts a relatively warm and ice‐free climate related to mantle dynamics potentially consistent with the intense anorogenic magmatism of this period.
Key Points
Supercontinent assembly modulates volcanic sources and weathering sinks for
CO2 through effects on mantle convective thermal mixing
Punctuated mixing during Pangea/Rodinia epochs can induce climate cooling‐warming consistent with Mesozoic and Cryogenian‐Ediacaran proxies
Thorough thermal mixing during Precambrian Nuna assembly and breakup can maintain an ice‐free climate consistent with geological data
We use 3D mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet (a proxy for thermal age), ...planetary surface temperature, and lithosphere strength. At both high and low values of internal heating, for moderate to high lithospheric yield strength, hot and cold stagnant-lid (single plate planet) states prevail. For intermediate values of internal heating, multiple stable tectonic states can exist. In these regions of parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet's thermal evolution). However, this state is sensitive to climate driven changes in surface temperatures. Relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once a stagnant-lid mode is initiated, a return to mobile-lid is not attainable by a reduction of surface temperatures alone. For lower levels of internal heating, the tectonic regime becomes less sensitive to surface temperature changes. Collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.
•We use 3D convection models to explore the evolution of planetary bodies.•Planets migrate through tectonic states as functions of age and surface temperature.•Tectonic regimes are a function of thermal histories and specific parameter values.•Multiple stable tectonic states exist over much of a planet's thermal evolution.
Mantle convection and, by association, plate tectonics is driven by the transport of heat from a planetary interior. This heat comes from the internal energy of the mantle and from heat flowing into ...the base of the mantle from the core. Past investigations of such mixed mode heating have revealed unusual behavior that confounds our intuition based on end‐member cases. In particular, increased internal heating leads to a decrease in convective velocity. We investigate this behavior using a suite of numerical experiments and develop a scaling for velocity in the mixed heating case. We identify a planform transition, as internal heating increases, from sheet‐like to plume‐like downwellings that impacts heat flux and convective velocities. More significantly, we demonstrate that increased internal heating leads not only to a decrease in internal velocities but also a decrease in the velocity of the upper thermal boundary layer (a model analog of the Earth's lithosphere). This behavior is connected to boundary layer interactions and is independent of any particular rheological assumptions. In cases with a temperature‐dependent viscosity and weak plate margin analogs, increased internal heating does not cause an absolute decrease in surface velocity but does cause a decrease relative to purely bottom or internally heated cases as well as a transition to rigid‐lid behavior at high heating rates. The differences between a mixed system and end‐member cases have implications for understanding the connection between plate tectonics and mantle convection and for planetary thermal history modeling.
Key Points
In mixed heated thermal convection, velocities can decrease with increased internal heating
Scaling theory and numerical models show that velocity trends reflect changing levels of boundary layer interaction
The energtics of mixed heating mantle convection differs from bottom and internally heated end‐members
We use simulations of mantle convection with surface yielding to show that multiple tectonic regimes are possible for equivalent system parameter values. Models with the same lithospheric strength ...parameters and the same vigor of convection can display different modes of tectonics. Within the region of multiple solutions, the evolutionary pathway of the system is the dominant factor that determines the tectonic mode (e.g., whether mantle convection operates in a plate tectonic like mode). The extent of the multiple regimes window is found to increase with the temperature‐dependent viscosity contrast across the system. The implication for models that seek to predict the tectonic regimes of planets is that the temporal evolution of the planet needs to be taken into account. A further implication is that modeling studies can lead to different conclusions regarding the tectonic state of a planet, extra‐solar planets in particular, despite the final model parameter values remaining equivalent.
Key Points
Models lead to different conclusions with equivalent final parameter values
Viscosity dependant hysteresis in predications of plate tectonics
The evolutionary history is a dominant factor in determining the tectonic regime
Consideration of the structure of dynamical equilibria in terrestrial planets using simplified descriptions of the relevant heat transport processes (rigid‐lid convection, plate tectonics, and heat ...pipe volcanism) reveals that if the efficiency of plate tectonic heat transport decreases at higher mantle temperature, then it cannot govern quasi‐equilibrium dynamical evolution, and the system is always evolving away from the plate tectonic regime. A planet on which plate tectonics is less efficient at higher temperature stays in heat pipe mode longer, spends less time undergoing plate tectonics, and has a low and ever‐decreasing Urey number during this phase. These conclusions are based solely on the structure of the equilibria in a system with less efficient plate tectonics in the past and are independent of the mechanisms leading to this behavior. Commonly used quasi‐equilibrium approaches to planetary thermal evolution are likely not valid for planets in which heat transport becomes less efficient at higher temperature.
Key Points
Parameterized thermal history models assume quasi‐equilibrium dynamical evolution
Some plate tectonic models preclude quasi‐equilibrium dynamical evolution
Multiple equilibria also require nonequilibrium approaches
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