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To understand the processes that form oceanic crust as well as the role of submarine volcanoes in exchanging heat and chemicals with the ocean and in supporting chemosynthetic biological communities, it is essential to study underwater eruptions. The world's most advanced underwater volcano observatory--the Ocean Observatories Initiative Cabled Array at Axial Seamount--builds upon ~30 years of sustained geophysical monitoring at this site with autonomous and remote systems. In April 2015, only months after the Cabled Array's installation, it recorded an eruption at Axial Seamount, adding to the records of two prior eruptions in 1998 and 2011. Between eruptions, magma recharge is focused beneath the southeast part of the summit caldera, leading to steady inflation and increasing rates of seismicity. During each eruption, the volcano deflates over days to weeks, coincident with high levels of seismicity as a dike is emplaced along one of the volcano's rifts and lava erupts on the seafloor. Cabled Array seismic data show that motions on an outward-dipping ring fault beneath the caldera accommodate the inflation and deflation. Eruptions appear to occur at a predictable level of inflation; hence, it should be possible to time deployments of additional cabled and autonomous instrumentation to further enhance observations of the next eruption. [ABSTRACT FROM AUTHOR]

Seismic observations in volcanically active calderas are challenging. A new cabled observatory atop Axial Seamount on the Juan de Fuca ridge allows unprecedented real-time monitoring of a submarine caldera. Beginning on 24 April 2015, the seismic network captured an eruption that culminated in explosive acoustic signals where lava erupted on the seafloor. Extensive seismic activity preceding the eruption shows that inflation is accommodated by the reactivation of an outward-dipping caldera ring fault, with strong tidal triggering indicating a critically stressed system.The ring fault accommodated deflation during the eruption and provided a pathway for a dike that propagated south and north beneath the caldera’s east wall. Once north of the caldera, the eruption stepped westward, and a dike propagated along the extensional north rift. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge volcanism generates two-thirds of the surface of our planet and plays an important role in chemical exchange with the overlying ocean, yet little is known about the dynamic processes involved in mid-ocean ridge eruptions. This is largely due to the costs and challenges of deploying long-term instrumentation on the seafloor, particularly those that transmit data to shore in real time and would allow the scientific community to respond to and coalesce around a particular event. The 2015 eruption at Axial Seamount, which lies along the Juan de Fuca Ridge in the Northeast Pacific Ocean, resulted in the first in situ, real-time geophysical data collected during a mid-ocean ridge eruption. The results provided insights into the caldera fault structure and response to a seafloor-spreading episode, and also confirmed the origin of seismically recorded impulsive signals that are associated with fresh lava erupting onto the seafloor. This confirmation of a seismic signal associated with erupting lava led to revisiting data from an eruption almost a decade earlier and a fundamental new view of seafloor spreading at fast-spreading ridges thousands of kilometers from Axial Seamount. This example illustrates the point that even though cabled observatories are necessarily bound to a specific location, their results can have significant implications for understanding systems that are quite different, in far reaches of the globe. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge volcanism forms two-thirds of the Earth's surface yet our knowledge of the dynamics of mid-ocean ridge eruptions is limited due to the difficulty of long-term monitoring on the seafloor. A recently-established cabled observatory atop Axial Seamount on the Juan de Fuca ridge allows unprecedented monitoring of a ridge-based hotspot volcano. Integrating observed patterns of seismicity, seismic velocity change and deformation, we infer that the volcano's 2015 eruption was preceded by an increased rate of magma influx into a laterally-discontinuous shallow magma reservoir. Our results highlight that the shallow magma reservoir can be relatively complex with multiple segments of eruptible melt, and that the different segments can interact during the same diking-eruptive sequence. [ABSTRACT FROM AUTHOR]