Geologic field relationships observed in the axial valley of the southern Juan de Fuca Ridge suggest that high-effusion-rate sheet-flow volcanism has been active for a maximum of about 16,000 yr. Similar observations in the axial region of the East Pacific Rise near lat 21° N suggest that the duration of the pillow-flow-dominant phase may be 5 × 104 yr. Geochemical evidence from axial and off-axis basalt samples from both the Juan de Fuca Ridge and East Pacific Rise 21° N indicates that surficial geologic cycles of volcanism, tectonism, and hydrothermal activity are an expression of changes in the locus of replenishment within the magma chamber that take place with a periodicity of 104−105+ yr. Anomalously high rates of spreading due to magmatic replenishment and inflation during the first phases of a volcanic cycle may cause the southern Juan de Fuca Ridge to display a morphology more commonly associated with fast-rate spreading centers.

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