A half century of investigations are summarized here on the youngest Hawaiian volcano, L?`ihi Seamount. It was discovered in 1952 following an earthquake swarm. Surveying in 1954 determined it has an elongate shape, which is the meaning of its Hawaiian name. L?`ihi was
mostly forgotten until two earthquake swarms in the 1970’s led to a dredging expedition in 1978, which recovered young lavas. This led to numerous expeditions to investigate the geology, geophysics, and geochemistry of this active volcano. Geophysical monitoring, including a realtime
submarine observatory that continuously monitored L?`ihi’s seismic activity for three
months, captured some of the volcano’s earthquake swarms. The 1996 swarm, the largest
recorded in Hawai`i, was preceded by at least one eruption and accompanied by the formation of
a ~300-m deep pit crater, renewing interest in this submarine volcano. Seismic and petrologic
data indicate that magma was stored in a ~8-9 km deep reservoir prior to the 1996 eruption.
Studies on L?`ihi have altered conceptual models for the growth of Hawaiian and other
oceanic island volcanoes and led to a refined understanding of mantle plumes. Petrologic and
geochemical studies of L?`ihi lavas showed that the volcano taps a relatively primitive part of
the Hawaiian plume, producing a wide range of magma compositions. These compositions have
become progressively more silica-saturated with time reflecting higher degrees of partial melting
as the volcano drifts towards the center of the hotspot. Seismic and bathymetric data have
highlighted the importance of landsliding in the early formation of an ocean island volcano.
L?`ihi’s internal structure and eruptive behavior, however, cannot be fully understood without
installing monitoring equipment directly on the volcano.
The presence of hydrothermal activity at L?`ihi was initially proposed based on nontronite
deposits on dredged samples that indicated elevated temperatures (31oC), and on the detection of water temperature, methane and 3He anomalies, and clumps of benthic micro-organisms in the
water column over the volcano in 1982. Submersible observations in 1987 confirmed a low
temperature system (15-30oC) prior to the 1996 formation of Pele’s Pit. The sulfide mineral
assemblage (wurtzite, pyrrhotite, and chalcopyrite) deposited after the pit crater collapsed are
consistent with hydrothermal fluids >250oC. Vent temperatures have decreased to ~60oC during
the 2004 dive season indicating the current phase of hydrothermal activity may be waning.