FY 1999

Patterns of event and chronic hydrothermal venting following a magmatic intrusion: New perspectives from the 1996 Gorda Ridge eruption

Baker, E.T.

Deep-Sea Res. Pt. II, 45(12), 2599–2618 (1998)

The remote detection of a seismic swarm on the northern Gorda Ridge on 28 February 1996 prompted a three-cruise response effort to investigate event and chronic hydrothermal discharge associated with a dike intrusion. The GR1 cruise reached the northern Gorda only 10 days after seismicity began and discovered a 15 km diameter event plume, EP96A, centered between depths 1800 and 2800 m above the shallowest portion of the axial valley axis (~3100 m). One month later, GR2 returned and found only a weak, near-bottom chronic plume at the EP96A site. A few kilometers to the south, however, GR2 mapped a distinctly different chronic plume (~2500-2900 m depth) as well as the edge of a second event plume, EP96B (~1800-2400-m depth), above the western wall of the axial valley. EP96B was seeded with a neutrally buoyant float, which traveled 10 net km to the northwest before surfacing on 10 June at the start of GR3. Mapping around the float location fully revealed EP96B, a 10 km diameter plume with a heat content ~25% that of EP96A. Extensive observations within the axial valley determined that chronic venting was effectively exhausted within three months. Models seeking to explain the perturbation of hydrothermal venting by a dike intrusion and eruption must satisfy several criteria generalized from this and previous events: (1) venting begins (or increases) with the intrusion/eruption and declines exponentially afterwards, (2) the time scale of the post-intrusion decline varies within and among sites, (3) the discharge of multiple event plumes is common, (4) an existing high-temperature vent field may not be necessary or even conducive to event plume formation, and (5) the ratio of total chronic to event discharge varies among intrusion events.