Geodesy is the science of mathematically determining the size, shape and position of the earth and the nature of the earth’s gravity field. Geodynamics is a subfield of geophysics dealing with dynamics of the Earth that applies physics, chemistry and mathematics to the understanding of how mantle, convection leads to plate tectonics and geologic phenomena such as seafloor speeding, mountain building, volcanoes, earthquakes, faulting and so on.