Plinian-type eruptions in the medicine lake highland, california, and the nature of the underlying magma |
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| Authors: | Grant Heiken |
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| Abstract: | Contemporaneous Plinian eruptions of rhyolite pumice from Glass Mountain and Little Glass Mountain during the last 1100 years B.P. were followed by extrusion of lava flows. 1.2 km of material was erupted and 10% by volume is tephra. All of the tephra deposits consist of very poorly sorted coarse ash and lapilli that are mostly pumice pyroclasts.Eruptive sequences, chemical composition and petrographic character of the rhyolites at Little Glass Mountain and Glass Mountain suggest that they came from the same magma body. The 1:9 ratio of tephra to lavas is typical of small silicic magma chambers. Eruption from a small chamber, 4–6 km deep, at vents 15 km apart is possible if magma rose along cone sheets with dips of 45–60°. The caldera rim and arcuate lines of vents near it may represent the surface expression of several concentric cone sheets.Pumice pyroclasts erupted at Glass Mountain and Little Glass Mountain may have formed in the following manner: (1) vesicle growth and coalescence beginning at 1–2 km depths; (2) elongation of the vesicles by flow within the cone sheets; (3) disruption of the vesiculated magma when it reached the surface by an expansion wave passing down through it; and (4) eruption of comminution products as pumice pyroclasts. Plinian activity at Little Glass Mountain and Glass Mountain continued until the volatile-rich top of the magma chamber had been depleted. |
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