Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth's evolution. The theory explains observations of magnetic lineations on the seafloor, linear volcanic island chains, large transform fault systems, and deep earthquakes near deep sea trenches. These features occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle, which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm-adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric (MT) imaging are consistent with general lithospheric thickening with age, which suggests that temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere-asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: in small or large melt triangles beneath spreading ridges, in channels, in layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small-scale convection and the dynamics of melt generation and migration. A melt-defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic style convection, allow multiple scales of convection, and dictate the driving forces of the system. A better understanding of plate tectonics has broad implications for life on Earth. These include mitigating natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo-oceans and potentially affecting climate change estimates through geologic time. Finally, plate tectonic processes shape the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free-surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favorable for life.
