Tim Thompson
Muse
- Joined
- Dec 2, 2008
- Messages
- 969
Subduction
Initiation of subduction in the laboratory from solutal convection in a complex-rheology fluid
E. de Giuseppe & A.B. Davaille; American Geophysical Union, Fall Meeting 2009 (abstract only)
Abstract: Subduction process operates by bending of the lithosphere at trench zone. But how the initiation occurs is still in debate. Numerical and laboratory models of subduction suggest that the significant deformation at the bending zone is the consequence of the variation (with temperature, stress and depth) of the plate’s rheological properties, from brittle to ductile. Up to now, laboratory experiments used a sandwich of several different materials to simulate the complex rheology of the subducting lithosphere, but no experience using only one single natural material managed to produce convection with subducting plates. We report here new analog models of subduction using a fluid, whose rheology varies from brittle to viscous when its water content changes. So as an analogy to cooling from above, the fluid is dried from above, its surface being kept at a constant humidity. Humidity, temperature, fluid thickness and solution concentration are systematically varied. The entire process is monitored by means of cameras. All the experiments show three main stages. First, the fluid dries at the surface and wavelets form: the fluid begins to show a visco-elastic behavior. Then, the fluid surface thickens and gives rise to instabilities that cause the episodic initiation of the subduction process. Finally, the fluid continues to dry and shows cracks localized on the surface. We can conclude that the variation of the fluid’s rheology, and especially the existence of a brittle state, depending on the experimental parameters such as temperature, humidity rate and concentration, is the key ingredient to exhibit convection with plate-like behavior.
Convection scaling and subduction on Earth and Super-Earths
Diana Valencia & Richard J. O'Connell; Earth and Planetry Science Letters 286(3-4): 492-502
Abstract: Super-Earths are the smallest class of discovered extra-solar planets. Owing to their relatively small mass, some might resemble Earth and perhaps be habitable. Because of the connection to habitability through thermal evolution, we investigate the tectonic regime of massive terrestrial planets. Two independent studies [O'Neill, C., Lenardic, A., Oct 2007. Geological consequences of super-sized earths. GRL 34, L19204, Valencia, D., O'Connell, R.J., Sasselov, D. D., Nov 2007a. Inevitability of plate tectonics on super-earths. ApJL 670, L45-L48.] have reached opposing conclusions about the likelihood of plate tectonics. Here, we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics: deformation on faults, negative buoyancy and energy dissipation during subduction. We show that in general, the ratio of driving force to plate resistance increases with planetary mass. This is a consequence of increasing convective stresses, thinning plates and similar plate structure. We conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets, the convective stress increases even more allowing deformation to take place. Also, despite shorter timescales for plate cooling, rocky super-Earths achieve negative buoyancy at subduction zones. Finally, by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active-lid tectonics. In conclusion, rocky super-Earths have more favorable conditions than Earth for the subduction of plates, and hence, for sustaining plate tectonics.
There are numerous other examples, but these will do. First, both theory & laboratory experiment show that convection is certainly a key ingredient in subduction. It is in fact the key process; without convection there is no subduction.
The above statement is much too stupid to respect. I challenge Bjarne to quantitatively demonstrate that subduction is not caused by convection.
Initiation of subduction in the laboratory from solutal convection in a complex-rheology fluid
E. de Giuseppe & A.B. Davaille; American Geophysical Union, Fall Meeting 2009 (abstract only)
Abstract: Subduction process operates by bending of the lithosphere at trench zone. But how the initiation occurs is still in debate. Numerical and laboratory models of subduction suggest that the significant deformation at the bending zone is the consequence of the variation (with temperature, stress and depth) of the plate’s rheological properties, from brittle to ductile. Up to now, laboratory experiments used a sandwich of several different materials to simulate the complex rheology of the subducting lithosphere, but no experience using only one single natural material managed to produce convection with subducting plates. We report here new analog models of subduction using a fluid, whose rheology varies from brittle to viscous when its water content changes. So as an analogy to cooling from above, the fluid is dried from above, its surface being kept at a constant humidity. Humidity, temperature, fluid thickness and solution concentration are systematically varied. The entire process is monitored by means of cameras. All the experiments show three main stages. First, the fluid dries at the surface and wavelets form: the fluid begins to show a visco-elastic behavior. Then, the fluid surface thickens and gives rise to instabilities that cause the episodic initiation of the subduction process. Finally, the fluid continues to dry and shows cracks localized on the surface. We can conclude that the variation of the fluid’s rheology, and especially the existence of a brittle state, depending on the experimental parameters such as temperature, humidity rate and concentration, is the key ingredient to exhibit convection with plate-like behavior.
Convection scaling and subduction on Earth and Super-Earths
Diana Valencia & Richard J. O'Connell; Earth and Planetry Science Letters 286(3-4): 492-502
Abstract: Super-Earths are the smallest class of discovered extra-solar planets. Owing to their relatively small mass, some might resemble Earth and perhaps be habitable. Because of the connection to habitability through thermal evolution, we investigate the tectonic regime of massive terrestrial planets. Two independent studies [O'Neill, C., Lenardic, A., Oct 2007. Geological consequences of super-sized earths. GRL 34, L19204, Valencia, D., O'Connell, R.J., Sasselov, D. D., Nov 2007a. Inevitability of plate tectonics on super-earths. ApJL 670, L45-L48.] have reached opposing conclusions about the likelihood of plate tectonics. Here, we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics: deformation on faults, negative buoyancy and energy dissipation during subduction. We show that in general, the ratio of driving force to plate resistance increases with planetary mass. This is a consequence of increasing convective stresses, thinning plates and similar plate structure. We conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets, the convective stress increases even more allowing deformation to take place. Also, despite shorter timescales for plate cooling, rocky super-Earths achieve negative buoyancy at subduction zones. Finally, by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active-lid tectonics. In conclusion, rocky super-Earths have more favorable conditions than Earth for the subduction of plates, and hence, for sustaining plate tectonics.
There are numerous other examples, but these will do. First, both theory & laboratory experiment show that convection is certainly a key ingredient in subduction. It is in fact the key process; without convection there is no subduction.
The above statement is much too stupid to respect. I challenge Bjarne to quantitatively demonstrate that subduction is not caused by convection.
Gravity is involved. Note the reference in the 2nd paper to "negative buoyancy" at subduction zones. That means the subducting plate is denser than the surrounding mantle, so it is pulled down by its own weight. That means gravity is involved in the standard description of subduction (negative buoyancy is responsible for the "slab pull" mechanism which, along with convective push & pull, leads to subduction).
but then ....
