Bulletin of Volcanology

, Volume 70, Issue 8, pp 999–1019 | Cite as

Morphological and textural diversity of the Steens Basalt lava flows, Southeastern Oregon, USA: implications for emplacement style and nature of eruptive episodes

  • Ninad R. Bondre
  • William K. Hart
Research Article


This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.


Steens Basalt Lava flows Morphology Texture Emplacement Pahoehoe A’a 



We thank Dr. Steven Anderson and Dr. Stephen Reidel for their detailed and constructive reviews, which helped improve this manuscript and Dr. Nakada for his assistance. We are grateful to Dr. John Morton for his help with geochemical analyses. We appreciate the assistance of Stephen Pasquale in the field and feedback from Dr. Matt Brueseke and Raymond Duraiswami. Some Steens Basalt sections along the Catlow Rim could be easily approached only by traversing ranch land. We thank Mr. Stacy Davies, manager of the Roaring Springs Ranch for allowing us this access. This investigation was partially funded by NSF grants EAR-0106144 and EAR-0506887 to W.K. Hart.


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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Nature GeoscienceNature Publishing GroupLondonUK
  2. 2.Department of GeologyMiami UniversityOxfordUSA

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