White Paper Title: 
Linkages and decoupling in the magmatic system between mantle and seafloor

The “type” spreading ridges represented by the ISS’s exhibit distinctive terrains among the ISS’s and within single ISS sites.  Integrated studies at the ISS's and a few other ridges reveal a relatively uniform progression from low magma supply rate to higher.  The general progression is as follows: Large volcanic cones; complexes of smaller cones; smaller pillow ridges and isolated hummocks; low-relief lava flows and volcanic collapse troughs.  Also, as spreading rates increase (seemingly independent of magma supply) ridge morphology transitions from high relief -- conical volcanic peaks -- to low-relief -- lava flow fields.

Combining detailed seafloor geological observations and previous geophysical imaging of the lower crust, possible connections between seafloor features and underlying melt bodies can be explored. The wide variability in the depth and shape of the melt bodies under the 9N OSC improves our ability to decipher linkages between surface volcanism and sub-surface melt. We find geomorphic boundaries are associated with changes in the mid- crustal melt sill, but appear form more in response to the dynamic crustal stress pattern at the OSC.

The locations of the youngest lavas based on surface appearance (A. Nunnery, MS thesis) and U-series dates (C. Waters & K. Sims, pers. comm.) fall into three different provinces.  One is on-axis over a wide, robust melt sill.  The second is clearly off-axis, where a ridge flank fault may be tapping the distal edge of a wide melt sill.  The third is on a bathymetric ridge interpreted as the axis but has no underlying melt sill.  However, most of this area has melt sills that are appearing to be uneruptible.

• No recent volcanism is observed over the plunging melt sill beneath the southern portion of the East Limb.  Rather, recent volcanism occurs along the bathymetric ridge nearest the overlap basin.

• An unusual abundance of andesites and dacites erupt on-axis on the northern portion of the East Limb.

The study of the 9N OSC offers two lessons applicable to the study of other ridges. First as a cautionary example on over-interpreting the meaning of sub-surface melt bodies from geophysics alone.  Much more must be learned about the properties of the melt bodies before their effects on seafloor processes can be predicted.  Second, is that crustal deformation seems to play the dominant role in creating the seafloor geology even in many magma-rich places. For example, analysis of the eruption patterns on the EPR in 1991 and 2005-06 suggests pre-existing segmentation controls subsequent eruptions.