The 210Po-210Pb dating technique provides ages of eruptions having occurred within about 2 years of sample collection. With this method one monitors the rate of change of 210Po with time in a lava once it is sampled, to determine the date of eruption. Upon eruption, 210Po is degassed by the lava while its radioactive grandparent 210Pb is not significantly volatilized. Following eruption, this Po isotope "grows back" toward radioactive equilibrium with 210Pb with a 138.4 day half-life. This phenomenon is illustrated in the image to the left; repeat measurments of 210Po in a single lava were regressed to the ingrowth curve. A time is calculated from this and the date of sample collection.
In the figure below, the colored bars plotted versus calendar time represent our best-estimate of the time of the eruption for the various submarine eruptions we have studied.
	Figure 1. 210Po-210Pb dating results from 3 eruptions on the EPR and Gorda Ridges. Ages are given as eruption windows (black bars), which represent unavoidable uncertainty in the extent of initial Po degassing. Heavy dashed lines show likely eruption intervals. Po is completely degassed upon eruption at subaerial and shallow submarine volcanoes (see review by Rubin, 1997). We are not certain this occurs at oceanic depths of 2-3 km, so we report an “eruption window” based on a conservative minimum degassing estimate and 100% degassing. The 2-month window we currently report is based on observations at 9°50'N EPR (panel A), although the still preliminary data from 10°44'N EPR (panel c) may allow us to narrow eruption window widths because 3 of 4 samples were >75% degassed of Po upon sample collection. Errors in maximum age (gray horizontal bars) reflect data regression and analytical errors (see the eruption window inset). Sample collection dates are depicted by a --|, coinciding with cruises to those areas (vertical gray bars). Absolute and relative times are given at the base of the plot. Note the difference in error on maximum ages for Gorda samples taken in April and August 1996 (ages overlap for all samples in panel B but early sampling led to much higher resolution ages). The young lava flow image is a video frame grab from Alvin dive 3935 of the FIELD expedition (provided by Robert Zierenberg). An example ingrowth curve and the seismic record that led to the Gorda event response are also shown, tied to calendar time and sample ages. figure from the NSF Ridge2000 program newsletter (Spring 2005, pdf-format)
We developed this dating method after divers in the manned submersible Alvin appeared on the sea soon after a submarine eruption at 9°50'N on the East Pacific Rise (EPR) back in 1991, so that we could determine when the volcanic event had occured. See this link for details and results of this inaugural application of this method.     We have also recently applied this technique to lavas collected after suspected submarine eruptions at 10 deg 44'N on the EPR, on the Gorda Ridge, Boomerang Caldera (near South East Indian Ridge), and to lavas from the Loihi Seismic Event response expedition in Aug 1996.
	Figure 2: 1996 was a busy year: At Loihi, sesimic activity detected at shore-based stations led researchers on "rapid response" oceanographic expeditions to the areas of suspected volcanism. Young-appearing lavas were sampled and shipped to our lab for radiometric dating. Dates from Loihi and Gorda eruptions in 1997 (as well as the seimicity of the volcanoes) are shown in the figure to the right     Interestingly, our dating results at from Loihi and Gorda have shown that seismic-event detection does not necessarily correspond directly with eruptive events on the seafloor, but that they never the less provide very stong evidence of heightened states of activity at submarine volcanoes. In fact, while seismic detection is a highly effective means of submarine event detection, direct visual observation of the sea floor followed by age determination on young-appearing lavas is necessary to provide a detailed geochronolgy of lava effusion. The latter results in high spatial resolution temporal information needed to address subsequent hydrological and ecological changes.