Pioneering work in subaerially exposed submarine volcanics has contributed much, as have submersible studies, but recent high resolution side-scan images of spreading centres have shown a great variety of morphology at a scale not previously accessible to view. By great good fortune, two seafloor eruptions have recently been studied as they happened, at 9 30'N on the East Pacific Rise and at the Coaxial Segment of the Juan de Fuca Ridge. In both cases it has been possible to define the overall geometry of the erupted unit, which has been done only once before for a single eruption. In other places, such as further south on the East Pacific Rise and at 25 N on the Mid-Atlantic Ridge, there is evidence for much larger eruptive episodes. Perhaps even at slow-spreading ridges, crustal construction periodically takes place in major eruptive events.
Hydrothermal systems present important opportunities and also a rich variety of dynamics for study.
Understanding of hydrothermal systems requires estimates of the total fluxes of heat and chemicals from vent fields, and also observation of variability at a wide range of time scales, from that of the life time of the vent field down to that of the growth and collapse of individual sulphide chimneys. In between these limits are a rich variety of time variable behaviours, including hydrothermal brecciation events, megaplume generation, and periods of rapidly changing temperatures. The recent Coaxial eruptive event showed that short-lived hydrothermal systems can be generated above dykes intruded laterally beneath the surface. Associated with hydrothermal systems are biological systems which have their own dynamics. Organisms must have developed a range of strategies to cope with the constantly changing environment in which they live.