Frequency Distribution of Rock Types in Oceanic, Orogenic, and Kratogenic Volcanic Associations

  1. Gordon A. Macdonald and
  2. Hisashi Kuno
  1. Hisashi Kuno

Published Online: 21 MAR 2013

DOI: 10.1029/GM006p0135

The Crust of the Pacific Basin

The Crust of the Pacific Basin

How to Cite

Kuno, H. (1962) Frequency Distribution of Rock Types in Oceanic, Orogenic, and Kratogenic Volcanic Associations, in The Crust of the Pacific Basin (eds G. A. Macdonald and H. Kuno), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM006p0135

Author Information

  1. Geological Institute, University of Tokyo, Tokyo, Japan

Publication History

  1. Published Online: 21 MAR 2013
  2. Published Print: 1 JAN 1962

Book Series:

  1. Geophysical Monograph Series

Book Series Editors:

  1. Waldo E. Smith

ISBN Information

Print ISBN: 9780875900063

Online ISBN: 9781118669310

SEARCH

Keywords:

  • Fractionation stages;
  • Frequency distribution of rock types;
  • Pigeonitie rock series;
  • Solidification index;
  • Volcanic rocks

Summary

The solidification index SI (MgO×100/FeO+Fe2O3+MgO+Na2O+K2O) is used as a measure of the fractionation stages of crystallizing basalt magmas. The indices of primary basalt magmas are about 40. The frequency relations of volcanic rocks of different indices, based on the numbers of chemical analyses available, were studied for various rock suites. In the Hawaiian tholeiite series, rocks with indices between 40 and 35 occur most frequently, indicating that the primary magma usually reaches the surface without being modified significantly by fractionation. In the pigeonitic and hypersthenic rock series of the orogenic belts, the tholeiite and alkali rock series of the continental lava plateaus, and the alkali rock series of the oceanic islands, the maximum frequency is found in rocks with indices between 25 and 15. It follows that in these volcanic rock associations, magmas of the middle fractionation stage tend to erupt to the surface most frequently regardless of the tectonic environments. The amount of the liquid fraction should decrease as the crystallization of the magma proceeds, whereas the content of volatile substances in the magma would increase. The net result would be a maximum eruptivity of the magmas of the middle fractionation stage. In central eruptions, only the top of a cupola-shaped magma reservoir would be drained to cause the extrusion, reducing the chance for the less fractionated magma to erupt to the surface