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NW Rota-1 is a submarine volcano in the Mariana volcanic arc that is notable as the site where underwater explosive eruptions were first witnessed in A.D. 2004. After years of continuous low-level eruptive activity, a major landslide occurred at NW Rota-1 in August 2009, triggered by an unusually large eruption that produced 10 times the acoustic energy of the background level of activity. An anomalous earthquake swarm preceded the eruption, suggesting that the sequence started with a magmatic intrusion and associated faulting beneath the volcano. We quantify the size and extent of the landslide using bathymetric resurveys and interpret the timing of events using data from an in situ hydrophone. This is the first instrumental documentation of an earthquake-eruption-landslide sequence at a submarine volcano, and illustrates the close interaction between magmatic activity and mass wasting events in the growth of undersea arc volcanoes.

The results of geodetic monitoring since 2002 at Sierra Negra volcano in the Galapagos Islands show that the filling and pressurization of an ∼2-km-deep sill eventually led to an eruption that began on 22 October 2005. Continuous global positioning system (CGPS) monitoring measured >2 m of accelerating inflation leading up to the eruption and contributed to nearly 5 m of total uplift since 1992, the largest precursory inflation ever recorded at a basaltic caldera. This extraordinary uplift was accommodated in part by repeated trapdoor faulting, and coseismic CGPS data provide strong constraints for improved deformation models. These results highlight the feedbacks between inflation, faulting, and eruption at a basaltic volcano, and demonstrate that faulting above an intruding magma body can relieve accumulated strain and effectively postpone eruption.

The regional impact of transform seismicity on ridge-crest hydrothermal venting and ridge-transform dynamics is investigated using a June 1‐7, 2000, Blanco Transform earthquake sequence. The mainshock (Mw 6.2) and 170 foreshocks and aftershocks were located using T waves recorded on U.S. Navy hydrophones, and indicate that the active transform fault may be farther south than in previous tectonic models. During the earthquake sequence, two temperature probes were deployed in black smoker chimneys at the Vent1 and Plume hydrothermal fields along the southern Juan de Fuca Ridge. These two hydrothermal systems are ;39 km northwest of the mainshock’s acoustic location. Both probes show significant (.5 8C) temperature declines following the mainshock, the Vent1 temperature changes occurring over days to weeks while the Plume changes were coseismic. The Vent1 and Plume fluid temperature decreases are consistent with earthquakeinduced changes to permeability in the upper ocean crust. The evidence suggests that deep-ocean hydrothermal systems can be altered by large earthquakes even at a distance and across tectonic provinces, and ridge-crest seismic and magmatic activity are not the only causes of change to hydrothermal systems.

Lava pillars are commonly observed in volcanic terrains at mid-ocean ridges and are unique to the submarine volcanic environment; they are hollow pipes that extend from the base of a lava flow to remnants of its collapsed upper crust. Pillars can be as tall as 5 m and have outer diameters typically between 0.5 and 2 m. Previous models proposed that they form in thick, fluid, fast-moving sheet flows. We present a new model for pillar formation based on sea-floor observations, and on results from laboratory and numerical models, in which lava pillars form within initially thin (

Geologic mapping and lava sampling were carried out after the discovery of large bursts of hydrothermal fluids (megaplumes) over the southern Juan de Fuca Ridge in 1986 and 1987. Our investigations of the northernmost section of the Cleft segment have discovered: (1) semicontinuous low-temperature venting and one major high-temperature vent site along 17 km of the neovolcanic zone and (2) very glassy, lightly sedimented sheet flows and pillow mounds superimposed on older terrain over about 24 km along the northern-most part. The pillow mounds are documented to have erupted between 1981 and 1987. The occurrence of the megaplumes during this same time period strengthens the hypothesis that megaplumes are caused by sea-floor extension events. Although the basalts from the entire length of the neovolcanic zone of the Cleft segment appear to have been derived from the same mantle source, a systematic northward increase in Mg number along the segment within the neovolcanic zone indicates less shallow-level differentiation to the north, possibly related to the development of new magma chambers during the recent phase of sea-floor spreading that has occurred there.