Plinian-type eruptions in the medicine lake highland, california, and the nature of the underlying magma

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