TECTONICS/Seismic Structure At Mid-Ocean Ridges 405 formation of some transient microplates that appear to be the modern analogues of large-scale spreadingcentre jumps This hypothesis provides a mechanistic explanation for the way in which many (if not all) spreading-centre jumps occur, why they occur in systematic patterns, and how spreading centres reorient when the direction of seafloor spreading changes It also explains the origin of large areas of petrologically diverse seafloor, including the major abyssal ferrobasalt provinces The common occurrence of rift propagation over a wide range of tectonic environments and spreading rates indicates that it represents an efficient mechanism of adjustment of extensional plate boundaries to the forces driving plate motions See Also Geomorphology Plate Tectonics Tectonics: Seismic Structure At Mid-Ocean Ridges Volcanoes Further Reading Bird RT and Naar DF (1994) Intratransform origins of mid ocean ridge microplates Geology 22: 987 990 Hey RN (1977) A new class of pseudofaults and their bearing on plate tectonics: A propagating rift model Earth and Planetary Science Letters 37: 321 325 Hey RN, Duennebier FK, and Morgan WJ (1980) Propa gating rifts on mid ocean ridges Journal of Geophysical Research 85: 3647 3658 Hey RN, Sinton JM, and Duennebier FK (1989) Propagat ing rifts and spreading centers In: Winters EL, Hussong DM, and Decker RW (eds.) Decade of North American Geology: The Eastern Pacific Ocean and Hawaii, pp 161 176 Boulder, CO: Geological Society of America Hey RN, Johnson PD, Martinez F, et al (1995) Plate bound ary reorganization at a large offset, rapidly propagating rift Nature 378: 167 170 Kleinrock MC and Hey RN (1989) Migrating transform zone and lithospheric transfer at the Galapagos 95.5 W propagator Journal of Geophysical Research 94: 13 859 13 878 Manighetti I, Tapponnier P, Gillot PY, et al (1998) Propa gation of rifting along the Arabia Somalia plate bound ary: into Afar Journal of Geophysical Research 103: 4947 4974 McKenzie D and Jackson J (1986) A block model of distrib uted deformation by faulting Journal of the Geological Society, London 143: 349 353 Naar DF and Hey RN (1991) Tectonic evolution of the Easter microplate Journal of Geophysical Research 96: 7961 7993 Schouten H, Klitgord KD, and Gallo DG (1993) Edge driven microplate kinematics Journal of Geophysical Research 98: 6689 6701 Searle RC, Bird RT, Rusby RI, and Naar DF (1993) The development of two oceanic microplates: Easter and Juan Fernandez microplates, East Pacific Rise Journal of the Geological Society 150: 965 976 Tapponnier P, Armijo R, Manighetti I, and Courtillot V (1990) Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar Geophysical Research Letters 17: Vink GE (1982) Continental rifting and the implications for plate tectonic reconstructions Journal of Geophysical Research 87: 10 677 10 688 Seismic Structure At Mid-Ocean Ridges S M Carbotte, Columbia University, New York, NY, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Oceanic crust is created at mid-ocean ridges (see Tectonics: Mid-Ocean Ridges) as mantle material upwells and undergoes pressure-release melting in response to ongoing seafloor spreading As mantle melts rise to the surface and freeze, they form an internally stratified crust of extrusive basalts and sheeted dykes underlain by layered and massive gabbros Spreading rate has long been recognized as a fundamental variable governing crustal accretion at ridges, with first-order differences observed in a wide range of ridge properties However, significant changes in ridge properties are also observed along the ridge axis at any given spreading rate, which suggests that factors other than the rate of plate separation contribute to the local supply and distribution of magma from the mantle Seismic methods permit imaging of structures within the crust that result from magmatic processes at midocean ridges and provide important insights into the role of spreading rate and magma supply in crustal creation Since the early days of seafloor exploration, seismic studies, which rely on the propagation of sound waves through rocks, have been the primary tool used to investigate the internal structure of the oceanic crust (see Seismic Surveys) These studies reveal two primary seismic layers, which are generally believed to correspond to lithological structures in the crust: seismic layer corresponds to the dykes and basaltic lava