Ultraslow-spreading ridges (< 20 mm yr–1 full rate) represent a major departure from the style of crustal accretion seen in the rest of the ocean basins. Since the 1960s, observations of fast- and slow-spreading mid-ocean ridges of the Pacific and Atlantic Oceans, combined with those of ophiolites (pieces of oceanic crust that have been thrust onto land through plate-tectonic processes), were used to define the conceptual structural, tectonic, and hydrothermal architecture of oceanic crust. Over the last 15 years, studies of ultraslow-spreading ridges have identified several anomalies that cannot be explained by the standard model of oceanic crustal formation. Thus, in recent years, ultraslow-spreading ridges have become recognized as a class unto themselves. Their “anomalous” characteristics in fact provide key information about many of the underlying processes that govern crustal accretion at all spreading rates. Ultraslow ridges include the Southwest Indian Ridge (between Africa and Antarctica), the Gakkel Ridge (which bisects the Arctic Ocean), and several smaller ridges (Figure 1).