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, steadystate magma chambers anywhere along the segment
(Forsyth, 1992; Lin and Morgan, 1992; Sinton and
Detrick, 1992; Magde et al., 2000). Therefore at slowand 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.
Bibliography
Ballard, R. D., and Van Andel, T. H., 1977. Morphology and tectonics of the inner rift valley at lat 36
50
0 N on the Mid-Atlantic
Ridge. Geological Society of America Bulletin, 88(4),
507–530, doi:10.1130/0016-7606.
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.
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.
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
0 N. Geological Society of America Bulletin,
88(4), 556–570, doi:10.1130/0016-7606(1977)88<556.
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.
Gracia, E., Parson, L., Bideau, D., and Hekinian, R., 1998. Volcanotectonic 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.
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.
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.
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.
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.
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/S0012821X(99)00281-2.
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
38
AXIAL VOLCANIC RIDGES
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, steadystate magma chambers anywhere along the segment
(Forsyth, 1992; Lin and Morgan, 1992; Sinton and
Detrick, 1992; Magde et al., 2000). Therefore at slowand 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.
Bibliography
Ballard, R. D., and Van Andel, T. H., 1977. Morphology and tectonics of the inner rift valley at lat 36
50
0 N on the Mid-Atlantic
Ridge. Geological Society of America Bulletin, 88(4),
507–530, doi:10.1130/0016-7606.
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.
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.
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
0 N. Geological Society of America Bulletin,
88(4), 556–570, doi:10.1130/0016-7606(1977)88<556.
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.
Gracia, E., Parson, L., Bideau, D., and Hekinian, R., 1998. Volcanotectonic 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.
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.
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.
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.
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.
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/S0012821X(99)00281-2.
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
38
AXIAL VOLCANIC RIDGES