Why Japan?

The Izu-Bonin-Mariana (IBM) arc system is the prime example of intra-oceanic convergent margin: in other words, oceanic crust of the Pacific Plate is being subducted beneath oceanic crust of the Philippines Plate. In addition, the sediments on the down-going plate are relatively simple — 500 meter sequences of chert (silica) and deep-sea clays — and they are completely subducted. For these (and other) reasons, NFS's MARGINS program has targeted the IBM system for detailed inter-disciplinary study.

One advantage of studying intra-oceanic arcs is that more confident estimates of volcanic fluxes can be obtained, without the complication of volcanoes being built on continental crust – as in the case of the Andes for example. Because we are interested in quantifying these fluxes - to assess the state of mass balance (link to mass balance) of subduction systems, our first reason for working in Japan is that the Izu Seven Islands allow us to estimate volatile fluxes from volcanoes which are unmodified by the potentially complicating factors associated with thick (continental-type) crust.

However, as part of our studies, we are actually interested in the influence of such crust. It can be thought of as a 'forcing function' i.e. a factor which has an influence whose effect we are trying to quantify. For this (second) reason, we are interested in making the comparison of volatiles from the purely intra-oceanic Izu volcanoes with gases released from Fuji and Hakone which are built on thicker crust of the Japanese mainland.

Our third and final reason for targeting Japan is that it is part of the 1500-mile IBM system and there are some important differences between the southern end of the chain (the Marianas) and the northern part – the Izu Islands of Japan. For example, subduction in the northern section of the IBM system occurs at a slower convergence rate (3 cm/yr vs. 6 cm/yr) then in the south. The down-going plate dips more gently into the mantle in the north (40 degrees versus 80 degrees) and it is much older (mid-Jurassic vs. mid-Cretaceous). We want to compare our results from the two extremities of the IBM chain to see what effect (if any) these various forcing functions have on the volatile mass balance.