Encyclopedia of Marine Geosciences

Living Edition
| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Axial Volcanic Ridges

  • Isobel YeoEmail author
Living reference work entry

Latest version View entry history

DOI: https://doi.org/10.1007/978-94-007-6644-0_2-2

Keywords

Eruption Style Reykjanes Ridge Southwest Indian Ridge Axial Valley Lava Type 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Synonyms

Definition

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.

Introduction

The surface expression of volcanic activity is extremely variable at different spreading rates (see “Spreading Rates and Ridge Morphology”), as a result of the differing eruption styles associated with each. Axial volcanic ridges (AVRs), sometimes called neo-volcanic ridges, have been recognized at many slow-spreading Mid-Atlantic Ridge (MAR) spreading segments (e.g., Ballard and Van Andel, 1977; Karson et al., 1987; Smith and Cann, 1992; Parson et al., 1993; Sempere et al., 1993; Lawson et al., 1996; Bideau et al., 1998; Gracia et al., 1998; Navin et al., 1998; Briais et al., 2000; Peirce and Sinha, 2008; Searle et al., 2010) and some ultraslow-spreading segments elsewhere (Mendel et al., 2003). AVRs are elongate, composite volcanoes, typically with a ridge parallel orientation (Fig. 1). They vary in size, although are typically a few kilometers wide, are tens of kilometers long, and reach heights of several hundreds of meters above the surrounding seafloor.
Fig. 1

EM120 bathymetry gridded at 50 m showing the axial volcanic ridge at 45°N on the Mid-Atlantic Ridge as surveyed by Searle et al. (2010). The prominent 22-km-long AVR is shown by the gray-shaded area and lies almost parallel to the ridge strike in the center of the hourglass-shaped inner valley. The spreading direction is shown by the white arrows. The northern end (north of 45°33) appears more tectonized than the southern end. The rough surface texture is a result of the hummocks that cover its surface. Inset: TOBI side-scan mosaic of the hummocky terrain in the area covered by the dashed box labeled SS on the main figure (Data is insonified to the north)

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.

Summary

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.

Cross-References

Bibliography

  1. Ballard, R. D., and Van Andel, T. H., 1977. Morphology and tectonics of the inner rift valley at lat 36°50′N on the Mid-Atlantic Ridge. Geological Society of America Bulletin, 88(4), 507–530, doi:10.1130/0016-7606.CrossRefGoogle Scholar
  2. Bideau, D., Roger, H., Sichler, B., Bollinger, C., and Guivel, C., 1998. Contrasting volcanic-tectonic processes during the past 2 Ma on the Mid-Atlantic Ridge: submersible mapping, petrological and magnetic results at lat. 34°52 N and 33°55 N. Marine Geophysical Researches, 20(5), 425–458, doi:10.1023/A:1004760111160.CrossRefGoogle Scholar
  3. Briais, A., Sloan, H., Parson, L. M., and Murton, B. J., 2000. Accretionary processes in the axial valley of the Mid-Atlantic Ridge 27 degrees N – 30 degrees N from TOBI side-scan sonar images. Marine Geophysical Researches, 21, 87–119, doi:10.1023/A:1004722213652.CrossRefGoogle Scholar
  4. Bryan, W. B., and Moore, J. G., 1977. Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N. Geological Society of America Bulletin, 88(4), 556–570, doi:10.1130/0016-7606(1977)88<556.CrossRefGoogle Scholar
  5. Forsyth, D. W., 1992. Geophysical constrains on mantle flow and melt generation beneath Mid-Ocean Ridges. In Morgan, J. P., Blackman, D. K., and Sinton, J. M. (eds.), Mantle Flow and Melt Generation and Mid-Ocean Ridges. Washington, DC: American Geophysical Union, pp. 1–65.CrossRefGoogle Scholar
  6. Gracia, E., Parson, L., Bideau, D., and Hekinian, R., 1998. Volcano-tectonic variability along segments of the Mid-Atlantic Ridge between Azores Platform and the Hayes Fracture zone: evidence from submersible and high resolution sidescan data. Special Publication Geological Society of London, 148, 1–15, doi:10.1016/0040-1951(91)90352-S.CrossRefGoogle Scholar
  7. Head, W., Wilson, L., and Smith, D. K., 1996. Mid-ocean ridge eruptive vent morphology and substructure: evidence for dike widths, eruption rates, and evolution of eruptions and axial volcanic ridges. Journal of Geophysical Research, 101(B12), 28265–28280, doi:10.1029/96JB02275.CrossRefGoogle Scholar
  8. Karson, J. A., Thompson, G., Humphris, S. E., Edmond, J. M., Bryan, W. B., Brown, J. R., Winters, A. T., Pockalny, R. A., Casey, J. F., Campbell, A. C., Klinkhammer, G., Palmer, M. R., Kinzler, R. J., and Sulanowska, M. M., 1987. Along-axis variations in seafloor spreading in the MARK area. Nature, 328, 681–685, doi:10.1038/328681a0.CrossRefGoogle Scholar
  9. Lawson, K., Searle, R. C., Pearce, J. A., Browning, P., and Kempton, P., 1996. Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform. Geological Society, London, Special Publications, 118, 61–102, doi:10.1144/GSL.SP.1996.118.01.05.CrossRefGoogle Scholar
  10. Lin, J., and Morgan, J. P., 1992. The spreading rate dependence of three-dimensional mid-ocean ridge gravity structure. Geophysical Research Letters, 19(1), 13–16, doi:10.1029/91GL03041.CrossRefGoogle Scholar
  11. Magde, L. S., Barclay, A. H., Toomey, D. R., Detrick, R. S., and Collins, J. A., 2000. Crustal magma plumbing within a segment of the Mid-Atlantic Ridge 35°N. Earth and Planetary Science Letters, 175(1–2), 55–67, doi:10.1016/S0012-821X(99)00281-2.CrossRefGoogle Scholar
  12. Mendel, V., Sauter, D., Rommevaux-Jestin, C., Patriat, P., Lefebvre, F., and Parson, L. M., 2003. Magmato-tectonic cyclicity at the ultra-slow spreading Southwest Indian Ridge: evidence from variations of axial volcanic ridge morphology and abyssal hills pattern. Geochemistry, Geophysics, Geosystems, 4(5), 1–23, doi:10.1029/2002GC000417.CrossRefGoogle Scholar
  13. Navin, D. A., Peirce, C., and Sinha, M. C., 1998. The RAMESSES experiment – II. Evidence for accumulated melt beneath a slow spreading ridge from wide-angle refraction and multichannel reflection seismic profiles. Geophysical Journal International, 135(3), 746–772, doi:10.1046/j.1365-246X.1998.00709.x.CrossRefGoogle Scholar
  14. Parson, L. M., Murton, B. J., Searle, R. C., Booth, D., Keeton, J., Laughton, A., Mcallister, E., Millard, N., Redbourne, L., Rouse, I., Shor, A., Smith, D., Spencer, S., Summerhayes, C., et al., 1993. En echelon axial volcanic ridges at the Reykjanes Ridge: a life cycle of volcanism and tectonics. Earth and Planetary Science Letters, 117, 73–87, doi:10.1016/0012-821X(93)90118-S.CrossRefGoogle Scholar
  15. Peirce, C., and Sinha, M. C., 2008. Life and death of axial volcanic ridges: segmentation and crustal accretion at the Reykjanes Ridge. Earth and Planetary Science Letters, 274(1–2), 112–120, doi:10.1016/j.epsl.2008.07.011.CrossRefGoogle Scholar
  16. Qin, R., and Buck, W. R., 2008. Why meter-wide dikes at oceanic spreading centers? Earth and Planetary Science Letters, 265, 466–474, doi:10.1016/j.epsl.2007.10.044.CrossRefGoogle Scholar
  17. Searle, R. C., Keeton, J. A., Owens, R. B., White, R. S., Mecklenburgh, R., Parsons, B., and Lee, S. M., 1998. The Reykjanes Ridge: structure and tectonics of a hot-spot-influenced, slow-spreading ridge, from multibeam bathymetry, gravity and magnetic investigations. Earth and Planetary Science Letters, 160(3–4), 463–478, doi:10.1016/S0012-821X(98)00104-6.CrossRefGoogle Scholar
  18. Searle, R. C., Murton, B. J., Achenbach, K., LeBas, T., Tivey, M., Yeo, I., Cormier, M. H., Carlut, J., Ferreira, P., Mallows, C., Morris, K., Schroth, N., van Calsteren, P., and Waters, C., 2010. Structure and development of an axial volcanic ridge: Mid-Atlantic Ridge, 45°N. Earth and Planetary Science Letters, 299, 228–241, doi:10.1016/j.epsl.2010.09.003.CrossRefGoogle Scholar
  19. Sempere, J. C., Lin, J., Brown, H. S., Schouten, H., Purdy, G. M., and Oceanography, I., 1993. Segmentation and morphotectonic variations along a slow-spreading center: the Mid-Atlantic Ridge (24°00′N-30°40′N). Marine Geophysical Researches, 15(3), 61–102, doi:10.1007/BF01204232.CrossRefGoogle Scholar
  20. Sinha, M. C., Constable, S. C., Peirce, C., White, A., Heinson, G., MacGregor, L. M., and Navin, D. A., 1998. Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57° 45′N on the Mid-Atlantic Ridge. Geophysical Journal International, 135(3), 731–745, doi:10.1046/j.1365-246X.1998.00704.x.CrossRefGoogle Scholar
  21. Sinton, J. M., and Detrick, R. S., 1992. Mid-Ocean Ridge magma chambers. Journal of Geophysical Research, 97(B1), 197–216, doi:10.1029/91JB02508.CrossRefGoogle Scholar
  22. Smith, D. K., and Cann, J. R., 1990. Hundreds of small volcanoes on the median valley floor of the Mid-Atlantic Ridge. Nature, 348, 152–155, doi:10.1038/348152a0.CrossRefGoogle Scholar
  23. Smith, D. K., and Cann, J. R., 1992. The role of seamount volcanism in crustal construction and the Mid-Atlantic Ridge. Journal of Geophysical Research, 97(B2), 152–155, doi:10.1029/91JB02507.CrossRefGoogle Scholar
  24. Smith, D. K., and Cann, J. R., 1993. Building the crust at the Mid-Atlantic Ridge. Nature, 365, 707–715, doi:10.1038/365707a0.CrossRefGoogle Scholar
  25. Smith, D. K., and Cann, J. R., 1999. Constructing the upper crust of the Mid-Atlantic Ridge: a reinterpretation based on the Puna Ridge, Kilauea Volcano. Journal of Geophysical Research, 104(B11), 25379–25399, doi:10.1029/1999JB900177.CrossRefGoogle Scholar
  26. Smith, D. K., Cann, J. R., Dougherty, M. E., Lin, J., Spencer, S., Macleod, C., Keeton, J., Mcallister, E., Brooks, B., Pascoe, R., and Robertson, W., 1995. Mid-Atlantic Ridge volcanism from deep-towed side-scan sonar images, 25 degrees-29 degrees-N. Journal of Volcanology and Geothermal Research, 67, 233–262, doi:10.1016/0377-0273(94)00086-V.CrossRefGoogle Scholar
  27. Sturm, M. E., Goldstein, S. J., Klein, E. M., Karson, J. A., and Murrell, M. T., 2000. Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area. Earth and Planetary Science Letters, 181(1–2), 61–70, doi:10.1016/S0012-821X(00)00177-1.CrossRefGoogle Scholar
  28. Yeo, I. A., and Searle, R. C., 2013. High resolution ROV mapping of a slow-spreading Ridge: Mid-Atlantic Ridge 45°N. Geochemistry, Geophysics, Geosystems, 14(6), 1693–1702, doi:10.1002/ggge.20082.CrossRefGoogle Scholar
  29. Yeo, I., Searle, R. C., Achenbach, K. L., Bas, L., Tim, P., and Murton, B. J., 2012. Eruptive hummocks: building blocks of the upper ocean crust. Geology, 40(1), 91–94, doi:10.1130/G31892.1.CrossRefGoogle Scholar
  30. Yeo, I. A., Clague, D. A., Martin, J. F., Paduan, J. B., and Caress, D. W., 2013. Pre-eruptive flow focusing in dikes feeding historical pillow Ridges on the Juan de Fuca and Gorda Ridges. Geochemistry, Geophysics, Geosystems, 14(9), 3586–3599, doi:10.1002/ggge.20210.CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.GEOMAR Helmholtz Institute for Ocean Research KielKielGermany