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Rhyolitic Explosive Eruptions of the Central Snake River Plain, Idaho: Investigations of the Lower Cassia Mountains Succession and Surrounding Areas

By Benjamin Stephen Ellis


The Snake River Plain of north-western U.S.A. was the site of voluminous, bimodal, hotspot volcanism in the Miocene. Between c. 12.7-6 Ma silicic volcanism produced an association of deposits so different to typical Plinian and ignimbrite deposits elsewhere it has been termed Snake River (SR)-type. The Cassia Mountains of southern Idaho contain SR-type ignimbrites produced from complex and dynamic magmatic plumbing systems involving multiple magma chambers which gave rise to multiple compositional populations of clinopyroxene that mixed during eruption and were deposited together.\ud The Cassia Mountain ignimbrites become progressively more mafic up-succession in terms of whole rock, glass, feldspar and clinopyroxene compositions, reflecting decreasing time available for fractional crystallisation, as supported by geochronology.\ud Two Cassia Mountain ignimbrites are among three newly discovered ‘super-eruptions’ defined on the basis of phenocryst, glass and whole rock compositions; magnetic polarity; 40Ar/39Ar geochronology; oxygen isotopes; and field data. Erupted volumes range between 640 and 1200 km3, amongst the largest recorded. Intercalated within the Cassia Mountain succession is a newly discovered deposit representing the first recorded explosive, rhyolitic phreatomagmatic eruption from the central Snake River Plain. The fine-grained, non-welded deposit has similar whole rock, glass, oxygen isotope and magmatic temperature characteristics to the surrounding welded ignimbrites, so the unusual deposit facies are interpreted as representing interaction of rising rhyolitic magma with near-surface water. During SR-type volcanism, lavas and ignimbrites of similar chemistry were erupted within a short time. Water contents of melt inclusions were low in both ignimbrites and lavas, consistent with the anhydrous mineralogy and high inferred magmatic temperature. Volatile contents of the magmas (as recorded by the melt inclusions) did not control eruptive style. The intense rheomorphism which characterises SR-type ignimbrites appears to be due to high emplacement temperatures rather than enhanced halogen contents

Publisher: University of Leicester
Year: 2009
OAI identifier:

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  1. (1960). Ash flows. Geol Soc Am Bull 71:795-842 doi
  2. (1972). Cenozoic volcanism and plate tectonic evolution of the Western United States . Early and Middle Cenozoic. Phil Trans Roy Soc London A 271:217-248 doi
  3. (1996). Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events. doi
  4. (1996). Chapter 5: Volatiles in Snake River‐type magmas Chapter 5: Volatiles in Snake River‐type magmas
  5. (2007). Compositional zoning of the Bishop Tuff. doi
  6. (1992). Distinguishing strongly rheomorphic tuffs from extensive silicic lavas.
  7. (1975). Emory cauldron, Black Range, New Mexico, source of the Kneeling Nun Tuff. Field Conf Guide NM Geol Soc 26:283–292
  8. (1993). Eruptive processes and caldera formation in a nested downsag collapse caldera: Cerro Panizos, central Andes Mountains. doi
  9. (2004). Event stratigraphy of a caldera-forming ignimbrite eruption on Tenerife: the 273 ka Poris Formation. Bull Volcanol 66:392-416 doi
  10. (1995). Fallout tuffs of Trapper Creek Idaho – a record of Miocene explosive volcanism in the Snake River Plains volcanic province. doi
  11. (1971). flow sheets from southern Nevada. Geol Soc Am doi
  12. (1970). Geol Soc Am Abstracts with Programs,
  13. (1997). Geological field trips in southern Idaho, eastern Oregon and northern Nevada. USGS Open File Report 2004-1222
  14. (1979). K-Ar geochronology of Oligocene volcanic rocks, David and Barrilla Mountains, Texas. Geol Soc Am Bull 90:1100–1110 doi
  15. (1991). Melt inclusions in volcanic systems: methods, applications and problems. Elsevier Webster JD, doi
  16. (1987). Physical features of rhyolite lava flows in the Snake River Plain volcanic province, Southwestern Idaho. Geol Soc Am Spec Pap 212:119-145 doi
  17. (2008). Primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of Jeju Island, Korea. Sedimentology 55:899-924 doi
  18. (1973). Properties of Some Common Igneous Rocks and Their Melts at High Temperatures. Geol Soc Am doi
  19. (1981). Rheomorphism of welded tuffs. doi
  20. (2006). Silicic lava dome growth in the 1934–1935 Showa Iwo-jima eruption, Kikai caldera, south of Kyushu, doi
  21. (1995). Single pyroclastic beds deposited by simultaneous fallout and surge processes: Roccamonfina volcano, doi
  22. (1993). Solubilities of carbon dioxide and water in rhyolitic melt at 850 C and 750 bars. doi
  23. (1994). SOLVCALC: an interactive graphics program package for calculating the ternary feldspar solvus and for two-feldspar geothermometry. doi
  24. (2003). Structural control and plate-tectonic origin of the Yellowstone melting anomaly. In: The hotspot handbook, proceedings of the Penrose Conference Plume IV: beyond the plume hypothesis, Hveragerdi, Iceland (abst) Christiansen RL, Foulger GR,
  25. (1983). The compositionally zoned eruption of 1912 in the Valley of Ten Thousand Smokes, Katmai National Park, doi
  26. (1981). The pyroclastic deposits of the 1875 eruption of Askja, doi
  27. (1994). The Yellowstone hotspot. doi
  28. (1989). Volatile compositions of glass inclusions from the 75Ka Toba Tuff Sumatra.
  29. (2000). Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks,

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