TECTONICS/Mid-Ocean Ridges 387 Figure 15 Proposed time sequence of along strike propagation and linkage of near axis faults and grabens, which define the edges of abyssal hills; time averaged propagation rates are approximately 20 60 km Ma (Reprinted from Encyclopedia of Ocean Sciences, Steele J, Thorpe S, and Turekian K (eds.), Macdonald KC, Mid ocean ridge tectonics, volcanism and geomorphology, pp 1798 1813, Copyright (2001), with permission from Elsevier.) facing the spreading axis and volcanic growth faults on the opposing side These faults lengthen at a rate of 20–60 Km/Ma reaching total lengths of 10–70 km to form abyssal hills and intervening grabens (Figure 15) See Also Analytical Methods: Gravity Lava Magnetostratigraphy Plate Tectonics Seamounts Tectonics: Hydrothermal Vents At Mid-Ocean Ridges; Seismic Structure At Mid-Ocean Ridges Further Reading Buck WR, Delaney PT, Karson JA, and Lagabrielle Y (1998) Faulting and Magmatism at Mid Ocean Ridges Geophysical Monograph 106 Washington, DC: American Geophysical Union Fox PJ and Gallo DG (1986) The geology of North Atlantic transform plate boundaries and their aseismic exten sions In: Vogt PR and Tucholke BE (eds.) The Western North Atlantic Region: The Geology of North America, pp 157 172 Boulder: Geological Society of America Humphris SE, Zierenberg RA, Mullineaux LS, and Thompson RE (1995) Seafloor hydrothermal systems: physical, chemical, biological and geological inter actions AGU Geophysical Monograph 91 Washington, DC: American Geophysical Union Langmuir CH, Bender JF, and Batiza R (1986) Petrological and tectonic segmentation of the East Pacific Rise, 5 300 N 14 300 N Nature 322: 422 429 Macdonald KC (1982) Mid ocean ridges: fine scale tec tonic, volcanic and hydrothermal processes within the plate boundary zone Annual Review of Earth and Plan etary Sciences 10: 155 190 Macdonald KC and Fox PJ (1990) The mid ocean ridge Scientific American 262: 72 79 Macdonald KC, Scheirer DS, and Carbotte SM (1991) Mid ocean ridges: discontinuities, segments and giant cracks Science 253: 986 994 Menard H (1986) Ocean of Truth Princeton: Princeton University Press Phipps Morgan J, Blackman DK, and Sinton J (1992) Mantle Flow and Melt Generation at Mid Ocean Ridges Geophysical Monograph 71 Washington, DC: American Geophysical Union Shaw PR (1992) Ridge segmentation, faulting and crustal thickness in the Atlantic Ocean Nature 358: 490 493