Plinian-type Eruptions Research Articles

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

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.

Late Pleistocene Eruptive History of Nevado de Toluca Volcano, Central Mexico

Fieldwork and radiocarbon dating provide evidence of two major explosive eruptions and an intervening minor eruption. The Nevado, a central stratovolcano built up in late Pleistocene time, was eviscerated by a violent Vulcanian-type eruption about 25,000 yr B.P., which deposited thick, cold, lithic lahars around its flanks. After a period of widespread soil formation, dated at 24,500 yr B.P., a relatively minor Vesuvian-type eruption produced small amounts of pumice that fell only on the eastern and northeastern slopes of the volcano. A major Plinian-type eruption took place at 11,600 yr B.P., directed mainly east-northeast, which spread thick dacitic pumice over an area of at least 1,700 km 2 . This eruption probably produced the “tripartite” pumice, which forms a marker for the stratigraphy of early man in the adjoining Valley of Mexico. The vent orifice was subsequently plugged by a dome of vesicular dacite.

Two Plinian-type eruptions in the Azores

The Agua de Pau volcano on the island of Sã o Miguel is blanketed with many trachytic pumice deposits, two of which have been selected for detailed study. One, resulting from an eruption 4600 years B.P., has a volume of 0.6–0.7 km 3 (dense rock equivalent) and is mostly a coarse-grained homogeneous pumice breccia produced by a continuous gas blast plinian eruption of great vigour. Pyroclastic flow deposits form fine-grained beds in this deposit, and diagnostic criteria are proposed for them. Such beds, forming thin layers interstratified with a coarser-grained deposit, are easily misinter-preted as air-fall ashes. The other deposit, that of a.d . 1563, has about one-quarter the volume and is well bedded partly resulting from fluctuations in vigour of the eruption and partly from rain-showers. Field grain-size measurements, and mechanical analyses of 100 samples from the two deposits, have been made to learn the value of such measurements and to understand the factors which influence the grain-size parameters. The Fogo A deposit shows striking variations in the proportions of pumice, crystals and lithic fragments. From simple field measurements it seems possible to determine inter alia the threshold diameter between ballistic fragments and fragments which have settled from the ash cloud.