Primary crustal types used to construct the global crustal model CRUST 5.1. Each crustal type has seven crustal layers. The crustal layers are: ice, water, soft sediments, hard sediments, and the upper, middle and lower crystalline crust. Vp, Vs, and density are specified in each layer (only Vp is shown here). Values for the upper mantle below the Moho are also given. Each primary crustal type was derived by calculating an average model based on seismic refraction profiles recorded in crust of specific age or tectonic setting. There are many variants of each primary crustal type (not shown) to account for variations in ice and sediment thickness and to ensure close agreement with field measurements. About 140 different crustal models were needed to construct the global model CRUST 5.1.