TECTONICS/Seismic Structure At Mid-Ocean Ridges 411 Figure Crustal structure along the intermediate spreading Galapagos Spreading Centre (A) Seafloor depth along the ridge axis (B) Two way travel times to the base of layer 2A (top line) and the axial magma chamber (AMC) reflection (bottom line) (C) Crustal thickness derived from two way travel time to Moho on the basis of velocities derived from refraction data Black line shows best fit polynomial to crustal thickness data (Reproduced from Detrick RS, Sinton JM, Ito G, et al (2002) Correlated geophysical, geochemical, and volcanological manifestations of plume ridge interaction along the Gala´pagos Spreading Centre Geochemistry Geophysics Geosystems 3: 8501; DOI 10.1029/2002GC000350.) Figure Seismic velocity structure of a fast spreading ridge, showing the region of low velocities associated with melt and hot rock at the ridge axis The velocity model is derived from a tomography experiment at 9 300 N on the East Pacific Rise (Re produced from Dunn RA, Toomey DR, and Solomon SC (2000) Three dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9 300 N Journal of Geophysical Research 105: 23 537 23 555.) small steps or offsets, which may be the boundaries between individual dyke swarms In many places, the magma-chamber reflector does not disappear beneath these offsets (Figure 3) However, changes in the depth and width of the reflector are often seen Seismic-tomography studies show that the broader region of low velocities associated with the crustal magmatic system pinches and narrows beneath these small offsets These results suggest that segmentation of the axial magma chamber is associated with the full range of offsets observed along the ridge axis Migration of seismic profiles shot perpendicular to the ridge axis reveals that the magma-chamber reflection arises from a narrow feature that is typically less than km wide (total range from 200 m to km; Figure 4) Refraction data and waveform studies of the magma-chamber reflection suggest that it arises from a thin body of magma a few hundred to perhaps a few tens of metres thick, leading to the notion of a magma lens or sill Initial studies assumed that this lens contained pure melt However, recent research suggests that much of the magma lens may have a significant crystal content (more than 25%), with regions of pure melt limited to pockets only a few kilometres or less in length along the axis Magma-lens reflections similar to those imaged beneath the East Pacific Rise have been imaged along intermediate-spreading ridges, including the Galapagos Spreading Centre (Figure 6), south-east Indian Ridge, and Juan de Fuca Ridge, and at the back-arc spreading centre in the Lau Basin In these areas,