One of the most important geologic discoveries of the 1960s was the remarkable uniformity of the basalts pro-duced at mid-ocean ridges. These basalts have a marked-ly different tholeiitic composition compared to the al-kalic basalts from ocean islands (Engel et al., 1965). The major element compositions of ocean-floor basalts and the compositions of the phenocrysts when present (pla-gioclase, olivine, and, more rarely, clinopyroxene) were considered so similar in all of the oceans that the acro-nym MORB (mid-ocean ridge basalt) was used to char-acterize basalts formed at seafloor spreading ridges. Just as remarkable, considering the homogeneity of mid-ocean ridge basalts, is the presence of sections of ridges where basalts of significantly different character-istics are found. For example, along the Mid-Atlanti
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