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OCEAN RIDGES 123

Fig. 6.2 Diagrammatic cross-section of the inner rift valley of the Mid-Atlantic Ridge at 36°50′N in the FAMOUS area
(redrawn from Ballard & van Andel, 1977, with permission from the Geological Society of America).

of kilometers along the axis. At fast rates of spreading the relief. They observed both inward and outward
the high may arise from the buoyancy of hot rock at facing fault scarps that give rise to a horst and graben
shallow depth, but on slowly spreading ridges it is topography. This differs from slower spreading ridges,
clearly formed by the coalescence of small volcanoes where the topography is formed by back-tilted, inward-

1–2 km in width, and hence is known as an axial volcanic facing normal faults. Active faulting is confined to the
ridge (Smith & Cann, 1993). region within 8 km of the ridge axis, and is asymmetric

A detailed study of a median rift valley was made in with the greater intensity on the eastern flank. The half
−1
the Atlantic Ocean between latitudes 36°30′ and 37°N, extension rate due to the faulting is 4.1 mm a , com-
−1
a region known as the FAMOUS (Franco-American pared to 1.6 mm a observed on the Mid-Atlantic Ridge
Mid-Ocean Undersea Study) area, using both surface in the FAMOUS area.
craft and submersibles (Ballard & van Andel, 1977). The Historically, for logistical reasons, the slowest spread-
median rift in this area is some 30 km wide, bounded by ing ridges, the Southwest Indian Ocean Ridge and the
fl anks about 1300 m deep, and reaches depths between Gakkel Ridge of the Arctic Ocean, were the last to be
2500 and 2800 m. In some areas the inner rift valley is studied in detail. In the Arctic the year-round ice cover
1–4 km wide and flanked by a series of fault-controlled necessitated the use of two research icebreakers (Michael

terraces (Fig. 6.2). Elsewhere, however, the inner fl oor et al., 2003). The results of these studies led Dick et al.
is wider with very narrow or no terraces developed. (2003) to suggest that there are three types of ridge as a
The normal faults that control the terracing and walls function of spreading rate: fast, slow, and ultraslow (Fig.
of the inner rift are probably the locations where crustal 6.3). Although the topography of the ultraslow Gakkel
blocks are progressively raised, eventually to become Ridge is analogous to that of slow-spreading ridges,
the walls of the rift and thence ocean fl oor, as they are typically with a well-developed median rift, the distinc-
carried laterally away from the rift by sea fl oor spread- tive crustal thickness (Fig. 6.3), the lack of transform
ing. Karson et al. (1987) described investigations of the faults, and the petrology of this ridge set it apart as a
Mid-Atlantic Ridge at 24°N using a submersible, deep- separate class. Note that there are two additional catego-
towed camera and side-scan sonar. Along a portion of ries of ridge with spreading rates between those of fast
the ridge some 80 km long they found considerable and slow, and slow and ultraslow, termed intermediate
changes in the morphology, tectonic activity, and volca- and very slow respectively. Intermediate spreading rate
nism of the median valley. By incorporating data ridges may exhibit the characteristics of slow or fast-
supplied by investigations of the Mid-Atlantic Ridge spreading ridges, and tend to alternate between the two
elsewhere, they concluded that the development of the with time. Similarly, a very slow-spreading ridge may
style of the median valley may be a cyclic process exhibit the characteristics of a slow or ultraslow ridge.
between phases of tectonic extension and volcanic It is interesting to note that at the present day the East
construction. Pacific Rise is the only example of a fast-spreading ridge

Bicknell et al. (1988) reported on a detailed survey of and the Gakkel Ridge of the Arctic is the only ultraslow-
the East Pacific Rise at 19°30′S. They found that faulting spreading ridge. Differences between the crustal struc-

is more prevalent than on slow-spreading ridges, and ture and petrology of fast, slow and ultraslow ridges are
conclude that faulting accounts for the vast majority of discussed in Sections 6.6–6.9.