Recent advancements in sea floor instrumentation have allowed the direct measurement of the acquisition of magnetization of oceanic crust. In the past 4 years, NOAA processing of data from the Navy SOSSUS array has detected two submarine volcanic eruptions off the coast of the western United States. In June, 1993, an eruption was detected on the CoAxial segment of the Juan de Fuca Ridge, and in January, 1996, a second eruption was detected on the northern Gorda Ridge. Both locations have been extensively studied using traditional swath mapping, side-scan sonar, submersible/ROV observations and near-bottom geophysical surveys, including magnetics, gravity and bare rock heat flow. Five newly-developed ocean bottom magnetometers were placed on the Juan de Fuca eruption in September of 1995 for a yearlong deployment. After recovery, four of these magnetometers were refurbished and placed on the Gorda Ridge eruption in September, 1996, and will be retrieved in October, 1997. These ocean bottom magnetometers are equipped with 3-axis fluxgate magnetometers, 2-axis tilt sensors and internal temperature thermistors, and record data at 2-hour intervals over a 12 month deployment period.
Several conclusions can be drawn from the initial data sets: 1.) Sea Floor Magnetometers work well over yearlong deployments, and can measure large-scale motion of bottom water, local tectonic movement, as well as the acquisition of crustal magnetization. 2.) Tiltmeters, when coupled with a 3-axis fluxgate magnetometer, can be used to distinguish real instrument motion and rotation associated with tectonic activity. This tectonic activity can be correlated with related thermal and magnetic events. 3.) The acquisition of crustal magnetization is non-linear, with much of the magnetization of extrusive flows being acquired a few months after eruption. However, the magnetization is still continuing to be acquired episodically 2-3 years post- eruption, due to cracking and penetration of sea water into still-warm regions of rock.