S U M M A R YSeismic refraction velocity data from the acoustic basement (called layer 2A) have been compiled for different mid-ocean ridges. The data from post-1970 studies show a strong correlation between velocity and basement age. Importantly, velocities double in less than 10 Myr for all ridges, but for older crust, up to 160 Myr in age, velocities do not increase significantly.

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A wide‐angle reflection/refraction survey was conducted along a flow line on the eastern flank of the East Pacific Rise at 14°S. The data were acquired with a single 32‐litre airgun and ocean‐bottom hydrophones (OBH) and consist of six profiles on 0.5–8.3 Myr old seafloor. In total, 17 OBHs provided seismic records of excellent quality. To generate 2‐D velocity models, traveltimes and amplitudes have been calculated using asymptotic ray theory. A comparison of the traveltimes and amplitudes of P‐wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust. The uppermost crust is composed of a 0.4–0.7 km thick low‐velocity (2.9–4.6 km s−1) surficial layer (layer 2A) and is bounded at its base by a sharp velocity increase to ∼5.5 km s−1 at the top of layer 2B. Overall, layer 2 is 1.4–1.8 km thick. Small seamounts, frequently superposed on fast‐spreading crust, are characterized by velocities lower than within the adjacent upper crust. Velocities <6.0 km s−1 exist throughout the edifices, suggesting that the seamounts are dominantly extrusive. Decreasing amplitudes beyond 8–10 km offset constrain a moderate‐gradient lower crust (layer 3) with velocities of 6.8–7.2 km s−1. Crustal thickness is found to be remarkably uniform, at 6.11±0.14 km. Nevertheless, the seismic data provide evidence for age‐dependent variations in both the low‐velocity surficial layer and the upper mantle. We find that velocities in layer 2A increase from 2.9 to 4.3 km s−1. Velocities in the middle and lower crust show uniform values, while velocities within the uppermost mantle increase gradually from <7.5 km s−1 to 8.0 km s−1. The most viable explanation for increasing velocities in the upper crust is that off‐axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces, thereby decreasing the porosity and increasing the velocity. Low mantle velocities beneath young crust are suggested to be a temperature‐controlled phenomenon and may indicate the presence of partial melt in the upper mantle.