The importance of the lithosphere-asthenosphere boundary, or LAB for short, in understanding how plate tectonics works. Basics: Different definitions of LAB depending on the type of measurement: petrological, geochemical, thermal, seismic (velocity and anisotropy), electrical conductivity. Ocean-continent differences. Special case of continental cratons.
In this introduction, we recall the origin of the concepts of "lithosphere" (rigid layer) and "asthenosphere" (soft layer), which were proposed as part of the study of post-glacial rebound in the early 20th century, then adapted as fundamental elements of plate tectonics. We have outlined some of the fundamental questions that motivate the subject of this lecture, including: why is the Earth the only planet with plate tectonics? Has the tectonic regime evolved over geological time? How were continents formed? What explains the longevity of continental cratons? What role does the "low-velocity zone" in the upper mantle play in plate tectonics?
We then reviewed the main mechanical structural units of the Earth's interior and their mineralogical composition, and introduced several notions: that of seismic anisotropy and the main types of anisotropy, that of isostasy and the two types of deformation present in the surface parts of the Earth: elastic and viscous. We conclude by introducing the two main modes of rock deformation at the microscopic scale: dislocation and diffusion of crystalline defects.
