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Australia: The Land Where Time Began |
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Mid-Ocean Ridges – Small Scale Convection and Plate Motion Control Mixing It is believed that oceanic lavas are derived from
different sources within the mantle, each of which has a distinct
composition (Allègre, Moreira & Staudacher, 1995; Hofmann, 1997;
Ballentine et al., 2002). The
primary mechanism for the mixing of these sources is plat motions on a
large scale, though there can still be very significant geochemical
differences (Allègre, Hamelin & Duprè, 1984; Graham, 2002) in the
composition of the lavas erupted at different mid-ocean ridges. At
mid-ocean ridges where plate spreading rates are high geochemical
variability is low, which is consistent with plate-scale mixing
(Allègre, Hamelin & Duprè, 1984; Graham, 2002). At slow-spreading
centres such as the southwest Indian Ridge, in the Indian Ocean, the
lavas are also geochemically homogeneous, which is not consistent with
plate-scale mixing (Graham, 2002, Georgen et
al.,2003). In this study the
authors1 used mantle flow numerical simulations to study
mantle mixing at mid-ocean ridges, under conditions of variable plate
length and rate of spreading. That small-scale convection in the mantle
makes a significant contribution to mantle mixing at slow spreading
rates was revealed by these simulations and faster plate velocities and
smaller plates inhibit small-scale convection in the mantle. The authors1
concluded that whereas lavas from fast-spreading ridges are well mixed
by flow at plate scale, lavas from slow-spreading ridges are mixed by
small-scale convection.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |