Knowledge of the composition of oceanic crust is critical for calculating the fluxes of mass, heat, and volatiles from Earth’s interior to its crust, oceans, and atmosphere. In the great debate over continental drift, Alfred Wegener claimed the oceanic crust, when explored, would prove to be nothing like the continental crust. Scientists have been fighting over its nature ever since, although it is clear that Wegener had it right. In his seminal paper, The History of Ocean Basins (Hess, 1962), Harry Hess argued: “The oceanic crust is serpentinized peridotite, hydrated by release of water from the mantle over the rising limb of a current. In other words it is hydrated mantle material.” In his view, the seismic structure and thickness of the oceanic crust was the product of metamorphic isograds, with the Mohorovicic Discontinuity (Moho; the boundary between the crust and upper mantle) an alteration front at the 500°C isotherm. By 1971, however, a consensus arose supporting the Penrose Ophiolite Model (Conference Participants, 1972). This consensus was spurred by dredging basalt of amazingly uniform composition along ocean ridges (Engel et al., 1965) and identifying fossil sections of on-land oceanic crust (known as ophiolites). The latter, often exhibiting a layered stratigraphy of pillow lava, sheeted dikes, gabbro, and mantle peridotite (Figure 1a), matched the seismic character of the oceanic crust and rocks dredged from fracture zones (Bonatti et al., 1971; Engel and Fisher, 1969). Seafloor mapping at slow and ultraslow spreading ridges, and deep drilling, however, are dissolving this consensus in favor of an oceanic crust whose composition, structure, and thickness vary with spreading rate, hot spot proximity, ridge geometry, and mantle temperature and composition.