Hess' ideas related to the tectonic significance of ultramafic rocks, ultramafic magmas, evolution of oceanic crust, and H2O outgassing, all emanating from his abiding interest in serpentinite. Hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional (Alpine-type) mountain belts. They are not magmatic intrusions, as he proposed, but tectonic remnants of fossil plate boundaries. Hess was the formulator of the concept of sea-floor spreading. His ideas on the nature of oceanic crust changed through the years, but he repeatedly argued that oceanic crust was partly or predominantly serpentinite. Despite the lack of subsequent popularity of this view, he was at least partly right. Serpentinite is a part of oceanic crust, notably in tectonically thinned regions along rifted ridges, near transform faults, and along obliquely rifted continental margins, such as the Mesozoic western Alpine continental margin.