Axial Volcanic Ridges
KeywordsEruption Style Reykjanes Ridge Southwest Indian Ridge Axial Valley Lava Type
Axial volcanic ridges (AVRs). Composite volcanic edifices, comprising an elongate, typically spreading-normal orientated topographic high, produced within the inner valleys of mid-ocean ridges, usually those that are slow spreading.
AVRs are usually found in the middle of spreading segments and are often associated with hourglass-shaped axial valleys. They may extend all the way from the center of the valley to the base of the bounding axial valley wall faults or be surrounded by areas of flatter seafloor. Where present, an AVR usually represents the largest volume magmatic structure on the segment.
AVR Eruption Style and Volcanic Architecture
AVRs themselves are built almost entirely of agglomerations of volcanic hummocks (Smith and Cann, 1990; Yeo et al., 2012), which are circular or subcircular, probably monogenetic, volcanic cones, or domes 50–500 m in diameter with heights of tens to hundreds of meters (Fig. 1 inset). Hummocks are constructed of a combination of pillow, elongate pillow, and lobate lavas (see “Lava Types”), which are erupted from a central vent and flow outwards and down the sides of the hummock. These hummocky structures are responsible for the rough, lumpy surface texture of AVRs in multibeam data (Fig. 1). Several hummocks may be produced in one eruption, often, but not always, distributed along an eruptive fissure forming a hummocky lineament on the seafloor (Searle et al., 2010). The feeder dykes for these eruptions are unlikely to be active for more than one eruption as the typical mid-ocean ridge dyke thickness of less than 2 m (Qin and Buck, 2008) is such that it will solidify before the next predicted dyke emplacement (Head et al., 1996). Such predominantly ridge parallel fissure eruptions are thought to be the dominant eruption style on AVRs. The individual hummocks are formed as a result of point focusing down to a number of individual vents that feed discrete, round edifices (Smith and Cann, 1992; Head et al., 1996; Smith and Cann, 1999). Hummocks may coalesce together to form larger hummocky ridges or mounds (Smith and Cann, 1993; Smith et al., 1995; Head et al., 1996; Lawson et al., 1996; Briais et al., 2000), which are similar to large pillow mound eruptions on intermediate-spreading rate ridges (Yeo et al., 2013). Hummocks commonly collapse down the AVR flanks, converting around 12 % of the lavas erupted on the AVR to talus, which probably also form a component of the AVR structure (Yeo et al., 2012). Rare flat-topped seamounts and small areas of smoother lava flows may also form small parts of the AVR structure.
The dominant rock type found on AVRs is normal mid-ocean ridge basalt (N-MORB), with variations due to differences in the degree of partial melting or heterogeneities in the source (see “Mid-Ocean Ridge Magmatism and Volcanism”).
Formation and Growth
Melt supply at slow-spreading mid-ocean ridges is irregular in space and time, and, at a typical slow-spreading ridge, melt production is too low to sustain large, steady-state magma chambers anywhere along the segment (Forsyth, 1992; Lin and Morgan, 1992; Sinton and Detrick, 1992; Magde et al., 2000). Therefore at slow- and ultraslow-spreading ridges, volcanism must be episodic. AVRs lie entirely within the Brunhes chron and therefore are difficult to date; however, a number of AVR life cycles as a result of such episodic magmatism have been proposed. Estimates of the lengths of these cycles are highly variable, ranging from several tens of thousands of years (e.g., 10 kyr (Bryan and Moore, 1977), 20 kyr (Sinha et al., 1998), and 25 kyr (Ballard and Van Andel, 1977)) to much longer periods (e.g., 600 kyr (Searle et al., 1998)) on the Mid-Atlantic Ridge and 400 kyr–2.4 Myr on the Southwest Indian Ridge (Mendel et al., 2003). Additionally, where available, the ages measured for AVRs – 10 kyr (Sturm et al., 2000) and ~12 kyr (Searle et al., 2010) – are much younger than the age of the crust calculated based on spreading rate. This, combined with the similarity of estimated ages for lava flows all over an AVR (Yeo and Searle, 2013), suggests that AVRs are the product of episodes of higher than normal volcanic activity.
Such a life cycle is probably comprised of at least one volcanic phase, followed by an amagmatic phase in which the AVR is broken apart and possibly rifted off axis by tectonic activity (Parson et al., 1993; Mendel et al., 2003; Peirce and Sinha, 2008). The length of these various phases and the extent to which rejuvenation may occur during periods of predominantly tectonic extension are poorly constrained. In the extreme, this could, if periods of tectonic extension were insufficient to destroy the AVR between rejuvenation episodes, actually result in an almost steady-state AVR, where a bathymetric high is present nearly all the time, maintained by regular episodic volcanism. However, evidence from the RAMASSES experiment conducted on the Reykjanes Ridge (Sinha et al., 1998) suggests that magma chambers may only exist beneath an AVR on a slow-spreading ridge for around 10 % of the cycle.
AVRs are large, constructional, volcanic features formed predominantly of volcanic hummocks that are very commonly found on slow- and ultraslow-spreading ridges. They typically lie in the middle of a segment and are the focus of volcanic activity and therefore probably upper crustal construction. Due to the irregular magma supply to slow- and ultraslow-spreading ridges, volcanism on AVRs is almost certainly episodic although the timings and durations of magmatic episodes are currently poorly constrained.
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