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Section 9.2 Volcanic Eruptions “I was walking along a path with two friends – the sun was setting - suddenly the sky turned blood red I paused, feeling exhausted, and leaned on the fence there was blood and tongues of fire above the blue-black fjord and the city my friends walked on, and I stood there trembling with anxiety - and I sensed an infinite scream passing through nature." Edvard Munch An agonized figure wails against a blood red Norwegian skyline in Edvard Munch's “The Scream” (1893) in the National Gallery, Oslo, Norway The 1883 eruption of the volcano Krakatoa in Indonesia put so much ash into the atmosphere, it caused unusually intense sunsets throughout Europe in the winter of 1883-4, which Munch captured in his painting. What FACTORS determine if a volcano erupts violently or quietly? Quiet eruptions Hawaii & Iceland Pahoa Lava Flow https://www.youtube.com/watch?v=fmCJS S2YAP0 Explosive, violent eruption Krakatoa eruption 2007 Destructive landslide associated with 1980 Mt. St. Helen's eruption Before the 1980 eruption After the VIOLENT eruption The summit and side has been blown away. Bell-work Pyroclastics consist of …… Ash Cinders Lapilli Glass Pumice/Obsidian Bombs Blocks Tuff (welded ash) Affiliated with Violent Explosions Lahar: is a type of mudflow or debris dense, destructive mass flow composed of a slurry of pyroclastic material, rocky debris, and of very hot ash, lava water. The material flows down from a fragments, and gases ejected explosively volcano, typically along a river valley. (Volcano peaks often have ice and from a volcano and snow that instantly melt when met typically flowing with the heat of an erupting volcano.) downslope at great speed. PYROCLASTIC FLOW: a Factors that affect volcanic eruptions: Pg 121 Felsic magma Feldspar, silica Mafec* magma magnesium and iron FELSIC magma • • • • Light colored minerals: Quartz, orthoclase Feldspar More than 60% silica Lower melting point= colder temperatures (“thick”) • HIGH viscosity MAFIC magma • Dark colored minerals: • Magnesium and iron Fe • Olivine, pyroxene, Hornblende, augite, biotite • Less than 50% silica • Higher melting point= hotter temperatures (“thin”) • LOW viscosity So thick it acts like a “plug” to the volcano… “Thin” enough that it allows the magma to move more freely… Volatiles • refers to the volatile components of magma [mostly water vapor and carbon dioxide] that affect the appearance and explosivity of volcanoes. • Volatiles in a magma with a high viscosity, generally felsic with a higher silica (SiO2) content, tend to produce eruptions that are explosive. • Volatiles in a magma with a low viscosity, generally mafic with a lower silica content, tend to vent and can give rise to a lava fountain. Volatiles Continued: the effect of trapped gases • How explosive a volcano will be is dependent on the gases present as well – How much gas is present • If a lot of gas is present the volcano is more likely to be more explosive – How easily the gas can escape • If the gases cannot escape easily, the volcano is more likely to be more explosive What happens to bring magma to the surface? • How does the type of magma affect the surrounding pressure? (remember gas is a volatile that affects the reactivity of magma) • Volcanic eruptions and what causes them • Build up of pressure occurs more frequently with thicker magma (felsic.)….. EXPLOSIVE ERUPTION Volcanic Explosivity Index How to measure a volcanic eruption VEI Description Plume Height Volume Classification How often Example daily Kilauea 0 non-explosive <100 m 1000s m3 Hawaiian 1 gentle 100-1000 m 10,000s m3 Haw/Strombolian daily Stromboli 2 explosive 1-5 km 1,000,000s m3 Strom/Vulcanian weekly Galeras, 1992 3 severe 3-15 km 10,000,000s m3 Vulcanian yearly Ruiz, 1985 4 cataclysmic 10-25 km 100,000,000s m3 Vulc/Plinian 10's of years Galunggung, 1982 5 paroxysmal >25 km 1 km3 Plinian 100's of years St. Helens, 1981 6 colossal >25 km 10s km3 Plin/Ultra-Plinian 100's of years Krakatau, 1883 7 super-colossal >25 km 100s km3 Ultra-Plinian 1000's of years Tambora, 1815 8 mega-colossal >25 km 1,000s km3 Ultra-Plinian 10,000's of years Yellowstone, 2 Ma Paroxysmal • a sudden outburst of a violent eruption • an increase of periodic eruptions • An eruption with a (V.E.I.) Volcanic Explosivity Index of 5 (on a scale of 0-8) Lava Vocabulary Quiz 9.2 1. pyroclastic Hot, erupted glass, ash, and rock fragments. ________________ 2. igneous All lava flows are this type of rock. _____________________ 3. pahoehoe A basaltic lava that results in smooth, ropelike surfaces: ____________ 4. silica Magma has a high viscosity if it contains over 60% of ________ 5. explosive If the magma has a high gas content, the eruptions tend to be ________ 6. felsic Because of their high silica content, this rock is light colored. ________ viscosity lava aa pahoehoe pillow lava pyroclastic ash silica cinders igneous metamorphic non-explosive mafic explosive sedimentary felsic