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MgO. Cr-spinel and glass melt inclusions in plagioclase, together with zoned olivine, give evidence for a range of parental melts in the recent shallow magmatic system, with Mg # as high as 0.62. Extensive, shallow magma mixing is required to explain these variations in mineral and inclusion chemistry, indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions. Apparently, Fernandina's shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing. Small variations in lava isotopic and compatible element composition, combined with the need to resupply melt to maintain thermal balance, indicate that replenishment of the magma chamber is semicontinuous, with resupply occurring of the order of a decade or less. Plagioclase-dominated, evolved Fernandina lavas are in sharp contrast to more primitive, olivine-dominated Hawaiian lavas; the total lack of primitive melts in the shallow Fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber, despite the fact that these melts were transported through relatively warm, young, and thin lithosphere adjacent to the Cocos-Nazca ridge. The diffuse nature of Galapagos volcanism, with 20th century eruptions from eight distinct volcanoes over a 2 20,000 km area, further supports this interpretation and suggests that the Galapagos plume is relatively diffuse, weak, and significantly less thermally intense than the vigorous Hawaiian plume. It may in fact be waning.

The seismicity associated with the collapse of a volcanic caldera in the Galapagos Islands during June of 1968 has been studied in detail. The rate of seismic energy release, inferred from some 638 assigned surface wave magnitudes, was constant over a 9‐day period and appears to be consistent with removal of magmatic support at a constant rate. Early in the earthquake sequence, the energy release was periodic. Large earthquakes occurred, only at 6‐hour intervals coinciding with extremes in the ocean and earth tides; however, the mechanism of triggering, if any, is not clear. Alternative periodic mechanisms based on passive withdrawal or expulsion of supporting magma are suggested. Values of energy, moment, and stress drop are computed from the known source geometry and are found to compare favorably with those estimated from seismic data. Assuming a rather low rigidity, the seismic data are consistent with a cylindrical block of 2 × 1016 grams dropping some 300 meters in approximately 75 dislocation stages, each averaging about 4 meters. In support of this model, a cumulative frequency versus magnitude curve suggests that some 75 earthquakes larger than about Ms = 4.5 were of a different genre than those smaller. Other volcanic phenomena, including eruptions and large explosions, preceded the caldera collapse and were accompanied by minor seismicity. However, major seismicity accompanied only the collapse, which apparently was the only volcanic event that included major ground deformation through shear.

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