Since the discovery of “event (mega) plumes” in 1986 on the Juan de Fuca Ridge, three additional ridge-crest eruptions on the JdF and Gorda Ridges have produced six separate event plumes with volumes ranging from ~15-120 km³. In addition, five additional plumes with some characteristics similar to event plumes have been observed worldwide, at spreading rates from 11 (Gakkel Ridge) to 140 (Manus Basin) mm/yr. In 2008, an eruption on the Northeast Lau Spreading Center created a unique set of multiple event plumes, smaller but chemically similar to all previous event plumes.

The uniformity of event plumes and their unfailing association with seafloor eruptions testifies that the tectonic/volcanic processes that accompany eruptions include fundamental physical and chemical processes that are not yet understood. Candidate hypotheses must satisfy five principal characteristics of event plumes: (1) rapid (<~1 d) formation of plumes, (2) unique and consistent chemistry (low ratios of ³He and Mn to heat), (3) the occasional production of multiple event plumes, (4) lava-seawater interaction (basalt shards, high H₂), and (5) lava and plume heat content correlated (~2:1, respectively) over two orders of magnitude.

Current hypotheses are divided between those that require the sudden release of hydrothermal fluids from the crust or melt, and those that require the rapid transfer of heat from cooling lava to seawater. Hypotheses requiring a combination of these processes are also possible, and perhaps likely. The fluid-release hypotheses require (1) large volumes (as much as ~0.1 km³) of hydrothermal fluid stored along, or created within, a few linear km of ridge crest, (2) fluids with a worldwide chemical uniformity greater than, and distinct from, black smoker discharge, and (3) very high bulk permeability in both the discharge and recharge zones. Lava-cooling hypotheses require (1) transfer of heat from lava to seawater much faster than possible by simple conductive cooling of a solid flow, and (2) consistently uniform (as ratioed to heat) extraction of ³He and Mn from both pillow and sheet lava flows.

An improved understanding of events that create new ocean crust will require the integration of existing field observations of events (perhaps reinterpreted) with new geological, chemical, and hydrodynamic models. Some specific issues include crustal permeability, storage of hot fluids in the crust, the role of magmatic volatiles, and lava-seawater interaction during an eruption.