At great depths the ascending mantle material behaves as a viscoelastic or viscoplastic solid, depending on loading conditions (Section 2.6), and for time intervals involving hundreds or thousands of years can be considered as a fluid. At shallower depths where the adiabat intersects the solidus (Figure 4.3), partial melting occurs. The melt fraction and the depth at which melting occurs are, however, poorly constrained because of the uncertainty in assessing the material composition (Figures 4.4-4.9) and differentiation processes that can produce many different compositions. Extensive melting and segregation of tholeiitic basalt can begin at a depth of 30 km and most of it may erupt without significant differentiation, whereas most of the melt of more primitive basaltic rocks melting at greater depths may not erupt and differentiates for a long time. This can be seen in Figure 4.19 by comparing melting along two different adiabats, one at low and the other at high temperature. Our knowledge of melt segregation and differentiation below the continental crust is even more rudimentary than below the spreading centers (Sawyer, 1994).
KeywordsHost Rock Rayleigh Number Magma Chamber Magmatic Intrusion Volcanic System
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