Man – despite his artistic pretensions, his sophistication, and his many accomplishments – owes his existence to a six inch layer of topsoil and the fact that it rains. – Author Unknown
Yesterday’s post discussed plate tectonics and how the presence of water on earth aids subduction, i.e. the downward movement of oceanic lithosphere beneath continental lithosphere. Subduction is only one aspect of plate tectonic theory while another is rifting, where plates move away from one another. What does rifting require? Geologists have long studied the role of mantle/asthenosphere upwelling to “push” plates apart, what amounts of forces this “conveyor belt” phenomenon requires and if subduction on one end of the plate is enough to “pull” the plate along which aids rift propagation, or vice versa. At lunch yesterday, I brought this topic up with a fellow geophysicist and he, wonder of wonders, did his PhD research on lithosphere strength. In fact, a paper published from that research is entitled The tensile strength of the lithosphere and the localization of extension. What a coincidence!
Continents appear to rift in preference to oceans. Furthermore, some areas of continents appear to be more susceptible to rifting than others.
… Using optimum creep parameters for silicic, mafic and ultramafic rocks, we find that lithospheric strength is inversely related to both crustal thickness and heat flow. By virtue of its thinner crust, oceanic lithosphere is inherently stronger than continental lithosphere. We find that oceanic lithosphere older than about 10 Ma should be able to withstand the lithospheric forces exerted on it by gravity sliding and plate interactions
… Rifting or ridge-jumps are therefore only likely to occur in very young oceanic lithosphere. Low heat flow continental shields should also be able to withstand likely lithospheric forces without significant deformation.
In essence, David’s work suggests that young continental lithosphere is still warm (hasn’t had time to cool) and weakest. This weak continental material is more prone to rifting, weakens and subsides, is filled in with water and eventually becomes seafloor. Examples of this process, at various stages from incipient to full-on, include the Gulf of Mexico, Basin & Range in the western United States, the East African rift and, possibly in the future, the area that is now the Himalayas (when the Indian and Eurasian plates stop butting heads).
Note that these mechanics of continental rifting are well-known to geologists, but to know why and in terms of heat, strength, rock type, chemical composition and age is where the research lies.
The rifting, subduction and collision of inanimate tectonic plates are the stuff of life. That geology, physics, chemistry, biology and time made us never ceases to amaze me.
