Geoscience Reference
In-Depth Information
Table 9.3 Volcanic characteristics of mid-ocean ridges
Recurrence
Volume of
Full
Width of
interval for
individual
spreading
neovolcanic
eruptions
eruptions
rate (cm yr −1 )
(10 6 m 3 )
zone (km)
(year)
Fast
8-16
0.1-0.2
5
1-5
Intermediate
4-8
0.2-2.0
500
5-50
Slow
1-4
2-12
5000
50-1000
Source : Perfit and Chadwick (1998).
An AVR can be up to a few hundred metres high and tens of kilometres long. On
either side of the inner valley floor normal faults raise the crust up to the crestal
mountains. Individual faults can be hundreds of metres high. Outward-facing
fault scarps are rare. Results of studies on the Mid-Atlantic Ridge have shown
that most of the plate separation is achieved by magmatic accretion (or some
diffuse deformation within the crust) since only
10%-20% is accommodated
by brittle faulting on visible faults. The zone of active deformation extends to
15 km from the ridge axis but almost all the volcanic activity takes place within
5kmofthe axis (Table 9.3). It is possible that the geometry of bounding faults
may constrain the location of the magmatism with the median valley.
Figure 9.17 illustrates the extent of variations in topography, tectonics and
geology found on the Mid-Atlantic Ridge. The area which has undergone the
most recent volcanic constructional event is shown at the top of Fig. 9.17, and
the area at the bottom has the most developed extensional tectonic features. The
other areas are arranged in order between these two extremes. It seems that this
variability, which can occur over short distances along the axis, is the result
of a cyclic tectono-magmatic process (with a period of perhaps 10 000-50 000
years) with more or less continuous extension (moving apart of the two plates)
but only periodic or intermittent magmatism. The magmatism in neighbouring
ridge segments, or cells (Section 9.5.2), need not be synchronous. The last major
volcanic event would have occurred within the last few hundred years for the
area at the top of Fig. 9.17,whereas those areas at the bottom are in the last
extensional phases of their cycle. The time-scale and length scale of tectono-
magmatic evolution are large for slow-spreading ridges but much less for fast-
spreading ridges.
As is evident from Fig. 2.1, earthquakes of large magnitude are not com-
mon along the mid-ocean-ridge system. Most large earthquakes occur along the
transform faults. Earthquake activity associated with small-scale faulting and the
movement of magma is generally of magnitude too low for detection at teleseis-
mic distances. Local seismic networks do, however, record earthquake swarms.
Unusually there was a major period of earthquake activity that was recorded at
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