SUMMIT OVERFLOW PERIODS, CALDERA FORMATION, EXPLOSIVE ERUPTION PERIODS, AND VARYING MAGMA FLUX AT BASALTIC VOLCANOES

Alternating periods of voluminous effusive and small-volume phreatomagmatic explosive eruptions are seen at subaerial and submarine basaltic volcanoes. This pattern is seen at Kilauea, submarine near-ridge seamounts, Loihi, and Axial Seamount. Loihi, for example, is capped by ~3.3-5.8 ka volcaniclastic deposits partly formed during phreatomagmatic eruptions. At these volcanoes, periods of low summit flux may reflect magma diversion from 1) summit reservoirs to deep rift zones (Axial Seamount), 2) to subsequent volcanoes in a sequence (near-ridge seamounts), or 3) to adjacent active volcanoes (Kilauea and Loihi). A decrease in volume of stored magma beneath the summit leads to caldera collapse and explosive activity until magma supply increases again. Stored magma volume may decrease by diversion and eruption elsewhere or because bubbles escape over time in the absence of recharge, causing the volume loss that contributes to caldera collapse.

Segments of the mid-ocean ridge system also undergo cycles of high and low flux that produce alternating axial highs and graben. At Endeavour Segment of the Juan de Fuca Ridge, an axial high formed during a robust magmatic phase that ended ~4.3 ka. A tectonic phase ~2.3 ka-long followed when an axial graben formed. Since ~2 ka, a slow return to magmatism defines a phase with robust hydrothermal activity and small eruptions in the axial graben. The flux changes do not correlate with climate or sea level cycles, but have been correlated with mantle source characteristics with the high flux periods having a greater pyroxenite component.. The explosive eruptions, characteristic on shallower or subaerial volcanoes during the tectonic (caldera-forming) phase, are missing. Their absence during the tectonic phase along global mid-ocean ridges is likely related to the restricted volume change of seawater to steam at the greater pressure of the ridge axis than at the summits of Loihi, Axial, or near-ridge seamounts. Other mid-ocean ridges, such as the Alarcon Rise on the northernmost East Pacific Rise also display similar alternation of axial highs and graben that reflect cyclical changes in magma flux to the ridge. Source heterogeneity through time might account for the observed flux changes at basaltic volcanoes in general, although only at the ridges are alternative mechanisms absent.