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The geology of split butte — A maar of the south-central snake river plain,Idaho |
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| Authors: | M B Womer R Greely J S King |
| Institution: | 1. Department of Geology, State University of N.Y. at Buffalo, 14226, Amherst, N.Y. 3. Department of Geology and Center for Meteorite Studies, Arizona State University, 85281, Tempe, Arizona 4. Space Sciences Division, National Aeronautics and Space Administration, 95035, Moffet Field, California 5. Department of Geology State University of N.Y. at Buffalo, 14226, Amherst, N.Y. |
| Abstract: | Split Butte is a volcanic crater of Quaternary age consisting of a tephra ring which at one time retained a lava lake. The tephra is thinly bedded and is composed of partially palagonitized sideromelane clasts and subordinate lithic fragments. The beds typically dip radially away from the center of the crater, but locally dip toward the crater center. The tephra ring resulted from phreatomagmatic eruptions as a result of interaction of groundwater with rising basaltic magma, evidenced by glassy and granulated pyroclastic debris, the presence of abundant palagonite and other secondary minerals, numerous armored lapilli, and plastically deformed ash layers below ejecta blocks. Statistical analysis of the grain size distribution of the ash also indicates a phreatomagmatic origin of Split Butte tephra. In addition, the analysis reveals that the stratigraphically lowest tephra was deposited primarily by pyroclastic flow mechanisms while the upper tephra layers, comprising the bulk of the deposits, were deposited dominantly by airfall and pyroclastic surge. The lava lake and four en echelon basalt dikes were emplaced when phreatomagmatic activity at the vent ceased. Subsequent collapse caused a broad, shallow pit crater to form in the laval lake, and minor spattering occurred at one point along the pit crater scarp. Partial erosion of the tephra, deposition of aeolian sediments and encroachment of the Butte by later lava flows completed the development of Split Butte. |
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