Ron Smith continues his Yale course The Atmosphere, the Ocean and Environmental Change with a lecture on plate tectonics and ocean floor structure. He explains how mid-ocean ridges form where rising mantle material solidifies into new crust, and how seamounts arise from volcanic activity that never breaches the surface. The lecture works through bathymetry region by region, comparing the depth profiles of the Atlantic, Pacific, Indian, and Arctic Oceans, and describes acoustic depth profiling as the main tool for mapping the seafloor. In the second half Smith turns to ocean water properties, covering how temperature and salinity are measured at the surface and in deep water, and what those measurements reveal about circulation. Chapter markers divide the fifty-one minute recording into eight segments, moving from tectonic theory into increasingly specific oceanographic measurement techniques. It is a straightforward classroom lecture with slides and data, aimed at students building a working vocabulary for ocean science.