29. Mechanics of Benioff Zone Magmatism

  1. George H. Sutton,
  2. Murli H. Manghnani,
  3. Ralph Moberly and
  4. Ethel U. Mcafee
  1. Bruce D. Marsh

Published Online: 17 MAR 2013

DOI: 10.1029/GM019p0337

The Geophysics of the Pacific Ocean Basin and Its Margin

The Geophysics of the Pacific Ocean Basin and Its Margin

How to Cite

Marsh, B. D. (1976) Mechanics of Benioff Zone Magmatism, in The Geophysics of the Pacific Ocean Basin and Its Margin (eds G. H. Sutton, M. H. Manghnani, R. Moberly and E. U. Mcafee), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM019p0337

Author Information

  1. Department of Earth and Planetary Sciences, the Johns Hopkins University, Baltimore, Maryland 21218

Publication History

  1. Published Online: 17 MAR 2013
  2. Published Print: 1 JAN 1976

ISBN Information

Print ISBN: 9780875900193

Online ISBN: 9781118663592



  • Geophysics—Pacific area—Congresses;
  • Woollard, George Prior, 1908


By deducing that andesitic magmas are strongly undersaturated with regard to water in the near-surface environment, the hypothesis that andesitic magmas arise from partial melting of peridotitic upper mantle material under hydrous conditions is questioned. Andesitic lavas are more likely to have formed by partial melting of subducted oceanic crust, quartz eclogite;under nearly anhydrous conditions. The distinctive K2O, SiO2, TiO2, and REE concentrations displayed by andesitic lavas can be explained by the mineralogy of the eclogite source. In particular. the characteristic low TiO2 content of andesites is probably a result of the highly refractory nature of rutile (TiO2) in quartz eclogite. The zone of melting along the slab-mantle interface probably extends from a depth of about 100 to 150 km downdip, although in some instances it may extend to a depth of 200 km. The temperature of the magma at a depth of about 110 km is ∼1400°C. The low density layer of andesitic magma is gravitationally unstable and as it strives to stabilize itself blobs of magma ascend to the surface at regularly spaced distances along the layer, parallel to the island arc. The surface manifestation of this instability is the regular spacing of island arc volcanic centers, which is about 70 km in Tonga and the Aleutian Islands. Cooling curves have been calculated for an ascending body of magma by assuming a uniform ascent velocity and a heat conduction approximation. These curves, along with boundary conditions on the initial depth of crystallization deduced from lavas, suggest that andesitic magmas travel to within 25 km, or less, of the surface in a superheated state. Thus differentiation through crystal settling is probably ruled out as an agent in chemically modifying the ascending magma. The shear stress needed along the slab-mantle interface to cause melting is shown to be about 1 kb. The thickness of the shear zone is also calculated by modelling it as a Couette flow; the thickness is inversely proportional to the shear stress. For a Newtonian material the shear zone thickness is less than 10 km with a 1-kb shear stress. For a non-Newtonian material, the shear zone thickness is about 1 m for a 1-kb shear stress. Thus, at any place and time, a layer of subducted oceanic crust perhaps only a few tens of meters thick is involved in partial melting and andesitic magma production.