Observations of oceanic ridges from the Mantle Electromagnetic and Tomography Experiment (MELT), the largest coordinated marine geophysical field program ever attempted, promise to distinguish between two competing models of magma generation. Funded by the National Science Foundation as part of the Ridge Interdisciplinary Global Experiments (RIDGE) program, the experiment will increase current understanding of where melt is formed and how it is transported to the ridge crest to form a new crust of seafloor.
The lack of direct, subsurface observations of the melt production region has led to the development of two classes of models that describe the nature of upwelling and melting beneath spreading centers (Figure 1). In passive flow models, viscous drag from the separating plates induces a broad zone of upwelling, with melt produced over a region that may be 100 km or more across. From this melt area, the magma migrates horizontally to a narrow zone at the ridge axis. In models of dynamic flow, in contrast, the buoyancy from retained melt within the mantle matrix, depletion of the mantle, and lower viscosity within the upwelling zone combine to focus upwelling and melting into a narrow zone perhaps only a few kilometers across. In dynamic models, melt transport is primarily vertical, without the horizontal component required by passive models.
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