Earth’s geology is fashioned to a large degree at lithospheric plate boundaries by the types 
of relative motion between adjacent plates. At divergent plate boundaries, such as mid- 
ocean ridges, mafi c basaltic lavas erupt, forming the seafl oor that underlies most of the 
ocean basins. At convergent boundaries, such as subduction zones, oceanic lithosphere is 
consumed at deep-sea trenches, leading to the eruption of chains of andesitic arc volcanoes 
near the edge of the overriding plate. Back-arc basins are especially diverse geologic set- 
tings because they inherently involve both of these types of plate boundaries. 
Back-arc basins are extensional basins formed behind subduction zones by rifting vol- 
canic arcs and accreting new volcanic seafl oor (Karig, 1970). Most back-arc basins are 
located behind the western limb of the Pacifi c Ocean (Figure 1) along current and past 
convergent boundaries. Many important aspects of back-arc crustal formation are similar 
to those of mid-ocean settings, including seafl oor spreading, hydrothermal activity, and 
vent fauna communities. However, back-arc basins differ from mid-ocean ridges in several 
aspects, mainly due to their proximity to convergent plate boundaries. Although back-arc 
basins make up far less of Earth’s surface compared to open-ocean basins, they show a wide 
variety of spreading styles and lithospheric compositions. This diversity makes them im- 
portant areas for studying crustal-accretion and related processes as well as the biological 
communities that inhabit these remote settings.