Introduction

The western edge of the Pacific tectonic plate is one of the world's oldest sections of oceanic crust. It disappears underneath the Philippine plate sometimes at such sharp angles that it is almost going straight down. This subduction zone creates the deepest point on the Earth's surface, the 7-mile-deep Marianas Trench.

The collision of plates also breathes fire into a volcanic island chain known as IBM, short for Izu-Bonin-Mariana. The chain begins in the south, in islands that comprise a United States commonwealth and which we visited last year (www.sio.ucsd.edu/marianas), and extends northward to the Izu islands and the Japan mainland of Honshu. Scientists have chosen the 1,500-mile-long chain as one of two MARGINS in the world to study some of the most challenging questions in plate tectonics.