Australia: The Land Where Time Began

A biography of the Australian continent 

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.

Sources & Further reading

1. Samuel, H. and S. D. King (2014). "Mixing at mid-ocean ridges controlled by small-scale convection and plate motion." Nature Geosci 7(8): 602-605.