Crustal processes at spreading centers
Authors: Macdonald Ken C. 
Published: February 2008

Recent discoveries and research on mid‐ocean ridges (MOR's) in the last four years have greatly increased our knowledge of this fundamental type of plate boundary. The last IUGG review of MOR research was in 1975 [Aumento et al., 1975], and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood. Since that time however, there has been a surfeit of important new observations and exciting research, and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two‐thirds of the earth's crust.

New seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers (AMC) beneath intermediate (total opening rate 5–9 cm/yr) and fast (9–18 cm/yr) spreading centers (Fig. 1). Yet there is controversy as to whether they are steady state, whether they persist beneath slow ridges as well in the steady state, what the dimensions and shape of the chamber are, its relation to oceanic crustal stratigraphy, and its relation to observations in ophiolites. “Black smoker” hydrothermal vents have been discovered in the Pacific, spewing forth 350°C fluids blackened by polymetallic sulfide precipitates. This discovery is embroiled in debate as to how long the vents last, whether they occur at slow spreading rates as well, whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates, their influence on geochemical balances in the ocean, and the value of the sulfide deposits commercially. It has been found that our initial view of MOR'S as two dimensional linear structures has served us well as an initial working model, but must be significantly modified to account for variations in tectonic, volcanic and hydrothermal activity along strike as well as for episodicity of these processes. Deep Sea Drilling Project (DSDP) samples have raised fundamental questions about formation of the magnetic source layer, hydrothermal circulation, and fault mechanisms along the MOR. Spreading centers which penetrate existing lithosphere along strike as “propagating rifts” have modified our views of how plate boundaries reorient themselves and how V‐shaped magnetic and topographic trends are formed. We have also found that transform faults may modify spreading center processes for some distance away from their intersections with MOR's, and that “rigid plate” transform faults may fail to develop at fast spreading centers under certain conditions, resulting inen echelonoverlapping of spreading centers.

This review will focus on these developments and several others. Only example references to these topics will be given in the text, and many important topics will not be discussed directly (e.g. petrology and geochemistry, mantle convection, research on lithosphere older than 1 m.y.). However, I have tried to compile and have included an extensive bibliography for U. S. research on a broader range of MOR topics for mid‐1978 to mid‐1982, which is the main purpose of this report. Pre‐1978 references are occasionally included where appropriate, abstracts are not included.