Geoscience Reference
In-Depth Information
Figure 9.20.
Four models of mid-ocean-ridge magma chambers show the
development of ideas about the formation of oceanic crust along the mid-ocean
ridges (left-hand pair, fast spreading; right-hand pair, slow spreading). There is no
vertical exaggeration. The early models, namely the 'infinite-onion' model (Cann
1974; Bryan and Moore 1977) and the 'ophiolite' model (Pallister and Hopson 1981)
had large steady-state magma chambers in the lower crust, to produce the lavas and
dykes (layer 2) and the gabbros and cumulates (layer 3) of the oceanic crust. Seismic
experiments should have been able to detect such chambers. The 'infinite-leek'
model (Nisbet and Fowler 1978) did not require a steady-state magma chamber;
rather, magma was envisaged as rising intermittently in packets from the mantle,
nucleating a crack in the basaltic crust and rising to the surface. Each crack is likely
to be in the same location as the previous crack. A small, shallow magma chamber
could be a temporary feature of this model. The 'East Pacific Rise' model
(Macdonald 1986; 1989) had a mushroom-shaped central magma chamber overlying
a zone of very hot rock, which is at most a few per cent partially molten. The
infinite-leek and East Pacific Rise models approximate fast- and slow-spreading
ridges. The infinite-onion and infinite-leek models could be regarded as extreme end
members of a continuum of models whose character depends on the spreading rate
and degree of inflation.
Pacific Rise and Juan de Fuca Ridge. Figure 9.20 shows some of the variations
of magma-chamber models and their possible settings.
9.4.3 Thermal models
It is easy to see how a large infinite-onion magma chamber could produce the lay-
ered lava, dyke and gabbro cumulate crust observed in ophiolites and presumed to
comprise the oceanic crust. Thermal models of the formation of the oceanic crust
indicate, however, that such an extensive steady-state magma chamber cannot be
present beneath slow-spreading ridges such as the Mid-Atlantic Ridge.
