Jpg explosive volcanism pyroclastic flows flows of

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Unformatted text preview: – “fire fragments” Pyroclasts www.gc.maricopa.edu/.../bomb_big.jpg www.indiana.edu/~geol116/week4/cinder.jpg Explosive Volcanism • Pyroclastic flows – flows of hot rock, gas and ash hot • Flow down hill • Often massive – no layers • Poorly sorted – mixture of fine & coarse material coarse Mayan Volcano, Phillipines Explosive Volcanism • Nueé ardente: A deadly Pyroclastic Flow • Hot gases infused with incandescent ash and larger rock fragments • Race down the slopes of a volcano at speeds up to 200 km per hour Explosive Volcanism *Highly vesicular ejecta classified by composition Pumice – int. to felsic Scoria – mafic *Can be ejected as large blocks or pulverized into particles as fine as dust *Deposited as tephra in layers *Distribution a function of column height (up to 35 km) *Well sorted once deposited Pumice Scoria Explosive Volcanism Types of pyroclastic debris classified by size Ash and dust - fine (2 mm), glassy fragments Lapilli or cinders - 2-64 mm diameter Blocks - ejected as hard or cooled lava Bombs - ejected as hot lava Pyroclastic surges • • • • Pyroclastic surges– flows of gas, H2O, hot rock (a lot of water) Flow down hill and up-hill Turbulent (up to 350 km/h) Well sorted, bedded, finer grained volcanoes.usgs.gov/.../TreesGround_large.JPG www.bbc.co.uk/.../features/features_cutaway.gif www.terjenet.com/.../Bigpics/IMG_2084.jpg Explosive volcanism Solidification of Solidification Pyroclastics Pyroclastics – Quenching: Produce Quenching: glass glass – Freeze fragments Freeze into shape Deposits -tuff -lapilli tuff -breccia Lahars • Heat melts snow and ice on Heat volcano = mudflow of meltwater and volcanic ash and Mount St. Helens Hotspot Volcanoes (intraplate environments) • • • • Magma formation thought to be due to localized heating and decompression Magma melting over mantle plume (not directly associated with plate boundaries) melting Volcanic chain forms as plate passes over hotspot Simplest examples are in oceanic settings where magma is not contaminated by Simplest melting of continental...
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