Stagnant Lid Tectonics: Perspectives from Silicate Planets, Dwarf Planets, Large Moons, and Large Asteroids
Robert J. Stern1, Taras Gerya2, Paul J. Tackley2
1Geosciences Dept., U. Texas at Dallas, Richardson TX 75083-0688 USA, email@example.com
2Department of Earth Sciences, ETH-Zurich, 8092 Zurich, Switzerland, firstname.lastname@example.org ; email@example.com
This webpage is a summary of: Stern, R. J., T. Gerya & P. J. Tackley (2018), Stagnant lid tectonics: Perspectives from silicate planets, dwarf planets, large moons, and large asteroids, Geoscience Frontiers, 9, 103-119.
To improve our understanding of Earth’s present tectonic style – plate tectonics – and how it may have evolved from single-plate (stagnant lid) tectonics, it is instructive to define it and then consider how common it is among similar bodies in the Solar System. Plate tectonics is a style of convection for an active planetoid where lid fragment (plate) motions reflect sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and accompanied by focused upwellings, or mantle plumes. Any other tectonic style is usefully called “stagnant lid” or “fragmented lid”. In 2015 humanity completed a 50+ year effort to survey the 30 largest objects in our Solar System (not including the Sun): planets, asteroids, satellites, and inner Kuiper Belt objects. We call these “planetoids” and use images of these bodies to infer their tectonic activity.
The lower size limit for the planetoids of interest is determined by the smallest nearly-spherical body which is ~500 km in diameter. The four largest planetoids are enveloped in gas and ice (Jupiter, Saturn, Uranus, and Neptune) and are normally thought to have solid rock interiors. The other 26 range in mass over 5 orders of magnitude and in diameter over 2 orders of magnitude, from massive Earth down to tiny Proteus. These bodies also range widely in density, from 1000 to 5500 kg/m3 (Figure 1). There is a well-defined density gap separating 8 silicate or rocky planetoids with density = 3000 kg/m3 or greater from 20 icy planetoids (including the gaseous and icy giant planets) with density = 2200 kg/m3 or less.
Figure 1: Spectrum of densities for the 30 planetoids studied. These bodies can be made of 4 types of matter: gas, water and other ices, silicates, and iron. Gas is only an important component in the 4 giant planets: Jupiter, Saturn, Uranus, and Neptune, which are identified with bold letters. Note the “density gap” between the 8 silicate-dominated planetoids with density > 3000 kg/m3 and the 18 “icy” planetoids with density between 1000 and 2200 kg/m3. Note which planetoids even have the possibility of being active silicate bodies and even have the potential to have plate tectonics.
We define the “Tectonic Activity Index” (TAI), scoring each body from 0 to 3 based on evidence for recent volcanism, deformation, and resurfacing (inferred from impact crater density). Planetoids with TAI = 2 or greater are interpreted to be tectonically and convectively active whereas those with TAI <2 are inferred to be tectonically dead. On this basis, 9 planetoids are tectonically active and 17 are dead.
We further infer that active planetoids have lithospheres or icy shells overlying asthenosphere or water/weak ice. Our results indicate that plate tectonics is unusual and that stagnant lid tectonics is the dominant mode for active planetoids. Focused upwellings (FU) of deeper material – sometimes called mantle plumes on Earth – are characteristic of active silicate planetoids. Something like FUs are also seen on some active icy bodies, for example the icy plumes of southern Enceladus and the coranae of Uranus’ moon Miranda. The TAI of silicate (rocky) planetoids correlates positively with their inferred Rayleigh number such that the largest bodies with strong gravity and thick silicate mantle (Earth, Venus) convect more vigorously and therefore have the strongest tectono-magmatic activity at the surface (Figure 2).
Figure 2: Possible evolution of magmatotectonic styles for a large silicate body like Earth. Plate tectonics requires certain conditions of lithospheric density and strength to evolve and is likely to be presaged and followed by stagnant lid tectonics.
In contrast, icy planetoids do not show a similar trend and their surface activity is thus likely regulated by more complex dependencies such as tidal dissipation (Figures 3 - 5). Even if we restrict consideration to active rocky planetoids, we conclude that some type of stagnant lid tectonics is the dominant mode of heat loss and that plate tectonics is unusual. We have detailed observations of how plate tectonics works but not active stagnant lid tectonics, which probably characterized the pre-plate tectonic Earth. To make progress understanding Earth’s tectonic history and the tectonic style of active exoplanets, we need to understand better the range and controls of active stagnant lid tectonics.
Figure 3: Simplified compositional and tectonic summary for the 30 large planetoids of the solar system discussed here. Three compositional types are identified, based on the nature of the innermost visible layer: gas, rocky material, or ice. Tectonically active planetoids require one or more strong outer layers (lithosphere or ice shell; above dashed line) underlain by an equal number of weak zones (asthenosphere or ocean; below dashed lines).
Figure 4: Relationship between the planetary Tectonic Activity Index and Rayleigh number (Ra) for rocky (silicate) and icy planetoids.
Figure 5: Silicate planet plate tectonics (A, B) vs. ice-floe tectonics (C, D, E, F). A: Schematic section through the upper 140 km of a subduction zone, showing the principal crustal and upper-mantle components and their interactions. The location of the “mantle wedge” (unlabeled) is that part of the mantle beneath the overriding plate and between the trench and the most distal part of the arc where subduction-related igneous or fluid activity is found. MF: Magmatic Front (Stern, 2002). (B) Schematic section through the center of Earth, which shows better the scale of subduction zones. Subducted lithosphere is shown both penetrating the 660-km discontinuity (right) and stagnating above the discontinuity (left). A mantle plume is shown ascending from the site of an ancient subducted slab. Dashed box shows the approximate dimensions of the shallow subduction. C: zone of Figure 1b (Stern, 2002). Weddell Sea, Antarctica D: Conamara Chaos, Europa. This landscape was produced by repeated disruption of the icy crust of Europa. Conamara Chaos contains two main terrain types: fragmented and dislocated ice blocks up to 20 km across with 100-200-m-high margins set in a matrix of finer ice lying at lower elevations (Carr et al., 1998) E: Ice compression, Vermillion River, Michigan USA Jan. 2014. F: Hypothetical interaction between two floes, resulting in a pressure ridge–a linear pile-up of sea ice (Wikipedia “Pressure Ridge”). Click here or on Figure for enlargement.
last updated 3rd May, 2018