Basalt Magma Types and Their Tectonic Associations: Pacific Northwest of the United States

  1. Gordon A. Macdonald and
  2. Hisashi Kuno
  1. Aaron C. Waters

Published Online: 21 MAR 2013

DOI: 10.1029/GM006p0158

The Crust of the Pacific Basin

The Crust of the Pacific Basin

How to Cite

Waters, A. C. (1962) Basalt Magma Types and Their Tectonic Associations: Pacific Northwest of the United States, in The Crust of the Pacific Basin (eds G. A. Macdonald and H. Kuno), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM006p0158

Author Information

  1. Department of Geology, Johns Hopkins University, Baltimore, Maryland

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



  • Basalt Magma Types;
  • Hawaiian volcanoes;
  • Mineralogy;
  • Tectonic processes;
  • Yakima basalt formation


In the Pacific Northwest, during Miocene-Quaternary time, three varieties of tholeiitic basalt and one variety of high-alumina basalt erupted in enormous volume. Each is thought to represent a primary magma developed in the mantle, and not a product of the differentiation of a single parent magma. Differentiation did occur where some of these magmas rose to shallow levels and were stored temporarily in underground reservoirs. The ability to form near-surface reservoirs appears to depend on the tectonic conditions accompanying eruptions. The Yakima basalt, which is the most voluminous of the three tholeiitic varieties, poured from large fissures into a slowly subsiding basin. It shows no tendency to develop into central volcanoes, and consists almost wholly of thick nonporphyritic flows that do not vary in composition. The high-alumina basalt of the Oregon plateaus, on the other hand, was erupted during active block faulting; many of its feeder channels followed the faults. Although much of the early high-alumina magma escaped rapidly in fissure eruptions, simple cinder cones, large shield volcanoes, and even more complex volcanic centers have been built up above some of the fracture zones. These reveal a variety of differentiation products, but undifferentiated high-alumina magma was always available during their production for it continued to break through to the surface in nearby areas. Thus differentiation appears to have occurred only in the shallow chambers directly below volcanoes, not at great depth where the primary magmas were generated.