Because one of our goals is to determine out frequently eruptions happen, we need a tool that will tell us about eruptive timing. At the SEPR, we think that eruptions may happen on the order of every few years to a few hundred years. Unfortunately, typical radiometric dating only works over much longer timescales. Geochemical data has been used to help distinguish flows of different ages, but eruptions can be more frequent than the time it takes for magma composition to change. meaning different eruptions can be geochemically indistinguishable. At other times, a single eruption can vary in chemical composition. Given the difficulties in differentiating flows based on chemistry, another approach is needed. The geomagnetic paleointensity of volcanic glass, or the strength of the magnetism recorded in glass, can be used to age date and distinguish between different lava flows.

Different intensities of Earth’s field over the past 10 thousand years.  From Korte et al. (2011), Earth and Planetary Science Letters, 312, 497-505.

When an eruption occurs underwater, the lava quickly cools and hardens into rock as hot magma makes contact with the cold seawater. The glassy coating that develops on the rock records the intensity of Earth’s magnetic field at the time of eruption. For young flows (up to a few hundred years old) paleointensity can be used to get a calendar date of when the eruption happened. Since Earth’s field strength fluctuates throughout time, we can compare the measured magnetization of the samples to historical records of Earth’s field intensity and estimate a date for when the glass formed. For glass older than our historical records, we can still determine if flows are of a similar age to surrounding flows based on whether or not the flows have similar paleointensities.

Ultimately, we’ll combine lava compositional data with the measured magnetic field strength from the glass to identify and date different eruptions.

We’ll also be collecting magnetic anomaly data with the AUV Sentry.  This data will give us magnetic information on ALL the lava flows – not just the ones at the surface from which we can get samples. The magnetic anomaly data will be combined with the surface sample data and other geophysical data to build statistical models of volcanic eruptions.