Magnetic anomalies with unusually large amplitudes are found along the Galapagos spreading center near the Galapagos Islands from long. 85° to 95° W. On either end of the Galapagos spreading center, however, normal amplitudes occur. Modeling of these magnetic anomalies, which were measured by both surface and deep-tow methods, indicates that the amplitude differences are probably caused by variations in the magnetic intensity of the rocks in the area. Tholeiitic basalt samples that were recovered from the ridge crests involved showed the expected variation in magnetic intensity.

Basalt samples from the ridge crests with normal-amplitude anomalies contained 9 percent total Fe (calculated as FeO) and 1 percent TiO2, whereas basalt samples from the ridge crests with large-magnetic amplitude contained 14 percent total Fe and 2 percent TiO2. Variations in MgO, CaO, P2O5, K2O, and Al2O3, as well as in FeO and TiO2, are consistent with the theory of Fe enrichment by fractional crystallization as a mechanism for the formation along ridge crests of tholeiite with high magnetic intensity. This phenomenon may be related to the proximity of those ridge segments with large magnetic amplitudes to the Galapagos melting anomaly.

Furthermore, the transition from large magnetic amplitudes at the ridge crest near the Galapagos Islands to normal magnetic amplitudes on the flanks suggests that the Galapagos melting anomaly may have formed 3 to 4 m.y. ago.

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