Ex yellowstone hotspot track chapter 4 volcanism and

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Ex: Yellowstone hotspot track Chapter 4: Volcanism and Extrusive Rocks Volcano Mt. St. Helens Eruption 1980 video Why do some volcanoes erupt violently? - Dissolved gases - Ease/difficulty of gases escaping to the atmosphere (related to velocity) Silicate Tetrahedra the greater amount of silicate tetrahedral in felsic and intermediate magmas causes a higher viscosity - Mafic = ~ 50% - Felsic = > 65% High viscosity = more resistance to flow = “thicker” ^^ More Si content, less gasses, cooler temperature Low viscosity = less resistance to flow = “thinner” ^^ Less Si content, more gasses, hotter temperature Volcanic (Extrusive) Rock Textures: Vesicular contains holes in the rock; lava very gaseous - Lighter rock – pumus , felsic - Darker rock – scoria, mafic Glassy Obsidian; lava cooled so quickly, they can’t form crystallites // felsic (disregarding color) Porphyritic rate of cooling differs, developing fine-grained and course-grained minerals Aphanitic fine-grained; cools quickly near or above the surface Pyroclast - Dust and ash (<2 mm) - Cinders (2 – 64 mm) - Blocks and bombs (>64mm) - Comes out of eruptive sequence - Pyroclastic flow ” moves from volcano a mixture of gas and pyroclastic debris that is so dense that it hugs the ground as it flows rapidly into low areas When pyroclasts accumulates and is cemented together: - Tuff = fine – grained (dust and ash) - Volcanic breccia = larger material (cinder, blocks, bombs) Types of Volcanoes
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Shield Volcano gentle slopes – btw 2 and 10 degrees - Basalt, layers of solidified lava forms - In Hawaii - Pahoehoe (rope texture) and Aa (jagged crumbling ashes) < 2 types of flow - Associated with volcanic domes and pyroclast glows - Less violent more mafic > start to see basalt less viscous Cinder cone short lived make of volcanic debris - Not soley felsic or mafic - Steep slopes - 33 degrees - Smallest of the three types - Angle of repose normally loose fragments - Pyroclastic fragments of any composition - Basalt is most common - Ex: Sunset Crater, Arizona Composite Volcano (“stratovolcano”) violent and deadly - Slopes less than 33 degrees - Larger than cinder cones - Pyroclastic flow lands below/under lava layering effect - Mostly andesite - Ex: Mt Shasta, CA Flows associated with volcanic / extrusive eruptions Pyroclastic flows mixture of gas and pyroclastic debris Mudflow mixtures of water and volcanic debris Volcanic domes a rebuilding of an eruption - Steep-sided dome – shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic tent Caldera violent eruption in which the top blows up - Large feature after violent eruption - Ex: Crater lake or Yellowstone - Forms a huge depression - “Wizard island” - Can be created when a volcano’s summit is blown off by exploding gases or when a volcano collapses into a partially emptied magma chamber Lava Floods not all extrusive rocks are associated with eruptions of volcanoes - Very low viscosity (basalt) (mafic) - Extremely large in volume - May be related to lava plumes
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