While it has long been considered that there are only three types of boundary that enclosethe earths tectonic plates, a new class of plate boundary, amagmatic accretionaryridge segments, has recently been recognized as well as a new class of ocean ridge:ultra-slow spreading. This is the direct consequence of the successful investigationsof the very-slow spreading Southwest Indian and Arctic Ridge systems promoted byInterRidge. Amagmatic accretionary ridge segments are a new class of accretionaryplate boundary distinct from magmatic accretionary ridge segments. Amagmatic accretionarysegments are marked by deep troughs often floored by mantle peridotite,with only thin or scattered basalt flows. They represent plate failure originating nearthe base of the plate following the zone of lithospheric necking unlike magmatic accretionarysegments, where the plate fails from the top. Unlike stable transforms andmagmatic accretionary segments, amagmatic accretionary segments may assume anyangle to the spreading direction and replace both orthogonal segments and transformfaults over large sections of ocean ridge. Magmatic and amagmatic accretionary segmentscan stably coexist, one connected to the other for many millions of years, ormay displace one another as mantle thermal structure, composition, ridge geometryor spreading rate change. Ultra-slow spreading ridges are a new class of ridge thatform where the effective spreading rate for mantle upwelling falls below12 mm/yr.The ESR is the orthogonal component of the spreading rate measured perpendicularto the ridge trend. Magmas erupted along ultra-slow spreading ridges may be alkalineand/or isotopically and incompatible trace element enriched compared to typicalMORB. Ultra-slow spreading ridges consist of linked magmatic and amagmatic accretionarysegments and are as mechanically, morphologically and petrologically distinctfrom slow-spreading ridges, as slow-spreading ridges are from fast. The SouthwestIndian Ridge, and the Arctic Ridge system from Iceland to 3E on the Gakkel aretransitional ridges between slow and ultra-slow, with long sections alternating betweenslow and ultra-slow behavior. This is similar to the situation for intermediate spreadingridges, which have long sections that alternate between fast and slow behavior. Apparently,then, there are two classes of intermediate spreading ridges: intermediate-fastand intermediate-slow, representing the transitions from fast to slow and from slow toultraslow spreading.