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More than 95% of the eastern Snake River Plain (ESRP) is covered by basaltic lava flows erupted in the Brunhes Normal-Polarity Chron; thus they are younger than 730 ka. About 13% of the area of the ESRP is covered by lava fields of latest Pleistocene and Holocene age <15 ka. More than 90% of the basalt volume of the ESRP is included in coalesced shield and lava-cone volcanoes made up dominantly of tube- and surface-fed pahoehoe flows. Deposits of fissure-type, tephra-cone, and hydrovolcanic eruptions constitute a minor part of the basalt volume of the ESRP.

Eight latest Pleistocene and Holocene lava fields serve as models of volcanic processes that characterize the basaltic volcanism of the ESRP. The North Robbers, South Robbers, and Kings Bowl lava fields formed in short-duration (a few days), low-volume (each <0.1 km3), fissure-controlled eruptions. The Hells Half Acre, Cerro Grande, Wapi, and Shoshone lava fields formed in long-duration (several months), high-volume (1 to 6 km3), lava cone- and shield-forming eruptions. Each of these seven lava fields represents monogenetic eruptions that were neither preceded nor followed by eruptions at the same or nearby vents. The Craters of the Moon lava field is polygenetic; about 60 flows were erupted from closely spaced vents over a period of 15,000 years.

Most of the basaltic volcanism of the ESRP is localized in volcanic rift zones, which are long, narrow belts of volcanic landforms and structures. Most volcanic rift zones are collinear continuations of basin-and-range-type, range-front faults bordering mountains that adjoin the ESRP. It is not clear whether the faults extend into the ESRP in bedrock beneath the basaltic lava flows.

The great bulk of basaltic flows in the ESRP are olivine basalts of tholeiitic and alkaline affinities. The olivine basalts are remarkably similar in chemical, mineralogical, and textural characteristics. They were derived by partial melting of the lithospheric mantle at 45 to 60 km, and they have been little affected by fractionation or contamination. Evolved magmas having SiO2 contents as high as 65% occur locally in and near the ESRP. The chemical and mineralogical variability of the evolved rocks is due to crystal fractionation in the crust and to contamination by crustal minerals and partial melts of crustal rocks. The trace-element compositions of the olivine basalts and the most primitive evolved basalts do not overlap, suggesting that the evolved rocks were derived from parent magmas that are fundamentally different from the parent magmas of the olivine basalts.

The distribution and character of volcanic rift zones in the ESRP are partly controlled by underlying Neogene rhyolite calderas. Areas that lack basalt vents and have only poorly developed volcanic rift zones overlie calderas or parts of calderas filled by thick, low-density sediments and rocks, which served as density barriers to the buoyant rise of basaltic magma. Volcanic rift zones are locations of concentrated extensional strain; they define regional stress patterns in the ESRP.

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