TECTONICS/Mid-Ocean Ridges 375 saw was completely unexpected: white bacterial matting billowing out of the seafloor, creating a scene much like a mid-winter blizzard in Iceland, covering the freshly erupted glassy black lava with a thick blanket of white bacterial ‘snow’ Ridge Segmentation The most recognizable segmentation of mid-ocean ridges is that defined by transform faults These plate boundaries are usually perpendicular to the ridge segments they offset and are tens to hundreds of kilometres long, although some exceed 1000 km in length (e.g the Romanche and San Andreas faults) In plate tectonics, a transform fault traces a small circle about the Euler pole of opening between any pair of plates Thus the transform fault and its off-axis fracture zone traces may be used to determine the pole of opening as well as changes in the pole of opening At a ridge–transform intersection, normal spreading processes are truncated Normal faulting predominates on mid-ocean ridges, while strike-slip faulting dominates along transform faults The transition can be very complex, with normal faults and strike-slip faults occurring along trends that are affected by shear stresses on the transform fault Crustal accretionary processes are also affected by the juxtaposition of thick cold lithosphere against the end of a spreading segment This effect increases with the age and thickness of the lithosphere that is sliding past the ridge–transform intersection Transverse ridges occur along the length of some of the largest transform faults; some of these ridges have been elevated above sea-level for part of their history Between major transform faults, the axial depth profile of mid-ocean ridges undulates up and down with a wavelength of tens of kilometres and an amplitude of tens to hundreds of metres at fast-spreading and intermediate-spreading ridges This pattern is also observed for slow-spreading ridges, but the wavelength of undulation is shorter and the amplitude is larger (Figure 3) In most cases, ridge-axis discontinuities occur at local maxima of the axial depth profile These discontinuities include transform faults, as discussed above (first order), overlapping spreading centres (second order), and higher order (third and fourth order) discontinuities, which are increasingly short-lived, mobile, and associated with smaller offsets of the ridge (see Table and Figure 4) A much-debated hypothesis is that the axial depth profile (Figures and 5) reflects the magma supply along a ridge segment According to this idea, the magma supply is enhanced along shallow portions of ridge segments and is relatively starved at segment ends (discontinuities) In support of this hypothesis Figure Axial depth profiles for (A) slow spreading, (B) fast spreading, and (C) superfast spreading ridges Discontinuities of orders and typically occur at local depth maxima (discontinu ities of orders and are not labelled here) The segments at faster spreading ridges are longer and have smoother lower amplitude axial depth profiles These depth variations may reflect the pattern of mantle upwelling (Reprinted from Encyclopedia of Ocean Sciences, Steele J, Thorpe S, and Turekian K (eds.), Macdonald KC, Mid ocean ridge tectonics, volcanism and geo morphology, pp 1798 1813, Copyright (2001), with permission from Elsevier.) is the observation at ridges with an axial high (fastspreading ridges) that the cross-sectional area or axial volume varies directly with depth (Figure 6) Maxima in the cross-sectional area (more than 2.5 km2) occur at minima along the axial depth profile (generally not near ridge-axis discontinuities) and are thought to correlate with regions where magma supply is robust Conversely, small cross-sectional areas (less than 1.5 km2) occur at local depth maxima and are interpreted to reflect minima in the magma-supply rate along a given ridge segment On slow-spreading ridges characterized by an axial rift valley, the crosssectional area of the valley is at a minimum in the midsegment regions, where the depth is at a minimum In addition, there are more volcanoes in the shallow midsegment area, and fewer volcanoes near the segment ends Studies of crustal magnetization show that very highly magnetized zones occur near segment ends; these are most easily explained by a local restriction of magma supply resulting in the eruption of highly fractionated lavas that are rich in iron Multichannel seismic and gravity data support the axial volume–magma supply–segmentation hypothesis (Figure 6) A bright reflector, which is phasereversed in many places, occurs commonly (>60%