The project involves dynamic effects that occur at the base of the lithosphere or equivalently at the base of tectonic plates. The lack of tectonics in cratonal continental crust (older than ~2.5 billion years) is a salient feature of the Earth. Much of the Canadian Shield is one example. Data from xenoliths erupted from diamond pipes provide hard constraints on physical models. Cratonal lithosphere approached its present thickness, about 200 kilometers, soon after it formed before 2.5 billion years ago and has persisted since then. The second key observation from xenoliths is that the rheologically active boundary layer (the zone of active flow) at the base of the lithosphere has a temperature range of less than 200 K. These observations allow application of the fluid-dynamic formalism of stagnant-lid convection to deduce the physical properties of the convecting region. Cratonal lithosphere is chemically less dense than normal mantle and much more viscous. It acts as a lid on the underlying convection. The project concentrates on geological implications of the chemically buoyant lithosphere. The first objective is to obtain the scaling relationships for the temperature range across convection beneath a chemically stable layer. A second more protracted implication involves basin formation within continental regions. Platform lithosphere (younger than about 2.5 billion years) that lacks chemically buoyant mantle is now about as thick as cratonal lithosphere. This situation did not prevail in the past when the interior of the Earth was hotter. Stagnant-lid convection was more vigorous beneath platforms and the platform lithosphere thinner than now. The cooling over time caused platforms to subside relative to cratons and produced the tendency for sedimentary basins to overlie platforms. A more subtle effect is that the difference in mechanical strength between platforms and cratons has waned with time. Continental break-up now sometimes occurs through cratons, but a systematic effect is not yet evident. The project involves the effects of mantle plumes beneath basins, platforms, and ocean basin. One implication is whether plume material ponded beneath interior basins, like Michigan. Overall plume material is a natural probe for the base of the lithosphere. Plume material flows laterally from thick lithosphere to thin. This flow is a widespread but poorly recognized and understood process. Much of the project is directed toward real examples, which effect surface geology. The project has broader implications to comparative planetology. Much of the evolution of animals occurred when the sea went in and out over platforms. This situation requires the right amount of water in the oceans so that erosion-deposition and tectonics tune the continents to the same long-term base level. It is fundamental to the rise of oxygen in our air. Too much water precludes continental erosion and there is no source of phosphorus in the ocean. Too little water leaves the ridge axis exposed where ferrous iron in mid-oceanic ridge basalt is a sink for atmospheric oxygen.
