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Culebra - subaerial basaltic flows

Culebra - subaerial basaltic flows. FIELD TRIP - Viernes, Oct 12th Andesitic sequence (breccias, falls & eroded lava dome) Meet in Dept car-park at 08:30am. PYROCLASTIC FALL DEPOSITS. In some cases, the volume of pyroclastics erupted is far greater than the amount of lava erupted

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Culebra - subaerial basaltic flows

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  1. Culebra - subaerial basaltic flows FIELD TRIP - Viernes, Oct 12thAndesitic sequence (breccias, falls & eroded lava dome)Meet in Dept car-park at 08:30am

  2. PYROCLASTIC FALL DEPOSITS In some cases, the volume of pyroclastics erupted is far greater than the amount of lava erupted DEFINITION - material falling to Earth from eruption column Fresh air-fall mantles landscape (even thickness) Often thin film BUT covers 1000’s of km’s high volume Deposits < 2cm rarely preserved (rainfall!) Construct Isopach Map contour lines of equal ash thickness

  3. FALL DEPOSITS • Important parameter Grain size NOTE: Best general term for fall material Tephra Wide range of clasts present Important parameter Maximum clast size Collect 10 largest visible clasts in field For finer deposits Sieving (degree of ‘sorting’) Fall deposits generally well-sorted From granulometry construct Isopleth map lines of ‘equal clast size’

  4. Typical fall deposit - narrow size range, strong central peak (1mm) - v. well sorted • Typical PDC (pf) deposit - v.poorly sorted * Samples from Bandelier Tuff, Valles Caldera

  5. Bandelier Tuff (aka ignimbrite)

  6. PYROCLASTIC FALLS: • 3 other useful parameters measured 1/ Vesicularity of clasts volume fraction of vesicles ‘Dry’ magmatic eruptions vesicularity 70 to 80% Magma-water eruptions vesicularity highly variable 2/ Crystal concentrations distinctive distribution patterns in some ignimbrites where crystals blasted out of glassy froth. NOTE: Crystals are denser than glass. 3/ Lithic content most pyroclastic deposits contain lithic fragments ripped off wall of vent + conduit Note the content + size variation (important for interpretation!)

  7. PYROCLASTIC FALLS: • Distinguish eruptive activity sieving deposits • Technique of Pyle plot dec. in thickness from vent + decrease in fragment size • These 2 parameters measure of column height eruptive intensity Parameters can be calculated from Isopleth Maps

  8. ‘Pylogram’ for classifying pyroclastic fall deposits • Bcis the clast half-distance + HT is eruption column height

  9. Hawaiian Deposits • Hawaiian eruptions small volume of tephra • Typical event gas-rich magma spray liquid lava into air as ‘fire-fountains” • Clasts ejected at high speed & fall to ground as liquid • Globs of lava ‘weld’ together spatter cones (10’s of meters high) • Large globs thrown further & ‘splat’ onto ground as ‘cow-pat bombs’ • Molten basaltic lava v. low viscosity (ie 100 Pa s) • Ejected lava shaped by surface tension Pelee’s tears (teardrop shaped) + Pelee’s Hair (fine strands) stretched out by the wind

  10. Mauna Ulu, 1969

  11. Reticulite - extremely vesiculated basaltic ‘froth’ IMPORTANT: Plasticity of basalt magma = minor volume of ash ! Pelee’s Hair - strands of basaltic glass

  12. Strombolian Eruptions • Also basaltic different flow regime • Typical products scoria & cinders Clasts are vesiculated + v. angular + grey-black • Lava fragments easily not much ash! • Erupted volumes small (<0.01 km3) • Eruption column rarely exceeds few 100 meters above vent • Scoria cones develop large blocks + bombs + regular layering formed by pulsations in eruption

  13. Spatter around scoria cone, Mt Etna Cinder cone along rift on Mount Cameroon

  14. Strombolian Eruptions • Volcanic Bombsglobs of viscous lavaaerodynamically shaped during fallout (spindle bombs)

  15. Strombolian Eruptions Rhythmic layering in scoria cone, Costa Rica Roadcut through scoria cone, note large bombs

  16. Isopach Isopleth Circular tephra maps typical of scoria cone eruptions Scoria inundating house in Heimaey, Iceland 1973

  17. VULCANIAN DEPOSITS • Vulcanian events variable, small cannon-like blasts to sustained explosions (many hours) • Eruption columns < 10 km high Large ballistics near-vent (< 1 km) + fine-grained further away • Generally small volume deposit with limited dispersal Famous vulcanian events isle of Vulcano, Italy 1880s ~2 meters of fine ash + meter size lava bombs Characteristic Breadcrust bombs (cooled exterior with molten, expanding interior)

  18. VULCANIAN DEPOSITS Crater rim of Vulcano Dark layers from 1888 eruption Breadcrust bomb

  19. VULCANIAN DEPOSITS Vulcanian plume from Lascar volcano, 1986 c.15 km high Vulcanian explosion, Monty 1997

  20. VULCANIAN DEPOSITS common in Pacific ‘Ring of Fire’ • Vulcanian events typically andesite & dacite magmas Eg. Soufrière Hills Volcano - 88 Vulcanian explosions in 2 months (1997), cyclic activity (8 hours ) Deposits of ‘finger-shaped’ pumice flows (pumice levèes) + lapilli + ash

  21. PLINIAN DEPOSITS Extensive + mantle landscape evenly • Eruption column > 30 km high • Plinian eruptions ‘sheet-forming’ deposits Well sorted + angular Plinian fall - angular pumices Roadcut near Arequipa, Peru

  22. PLINIAN DEPOSITS • Plinian deposits thicken towards vent • Vent may be negative topographic feature caldera Caldera location not always obvious (erosion of walls) Pastos Grandes caldera, SW Bolivia

  23. PLINIAN ERUPTIONS • Type example - AD 79 Vesuvius eruption • Witnessed & first described by Pliny the Younger • Several distinct horizons in stratigraphy (on Roman soil) • Initially, thin layer of fine ash small explosions • Overlain by main tephra fall thick Plinian pumice fall grey pumice Pumice color changes from white to grey Compostional zonation in magma chamber white pumice

  24. ISOPLETH maps: Eg. 15 cm isopleth: White pumice much closer to vent indicates column height increased as eruption progressed Calculations suggest column height was ~27 km in white pumice phase + increased to 33 km during grey pumice phase

  25. Bay of Naples

  26. VESUVIUS ERUPTION: • Increasing size of lithics as eruption continued further evidence for increase in intensity All isopleths elongate to SE Strong NW wind

  27. VESUVIUS ERUPTION • In Pompeii, 1.3 m of white pumice fell - phonolitic magma • Overlain by 1.2 m of grey pumice layer (more mafic) • Researchers measured variations in Max. pumice + lithics in exposures around the volcano • Magma volumes White pumice 2.5 km3 (~1km3 DRE) Grey pumice 6.4 km3 (~2.6 km3) Total = 3.6 km3 magma

  28. VESUVIUS ERUPTION CHRONOLOGY Phonolitic magma chamber at 3 to 5 km depth Chamber was compositionally stratified + volatile-rich overlying more mafic magma (less volatiles) Early Stages - magma interacted with groundwater (phreatics) Conduit opening explosive decompression + Plinian activity, eruption plume as high as 27 km, white pumice fall Widening of vent higher mass eruption rates of more mafic magma (grey pumice). Eruption column ~ 33 km Column collapse pyroclastic flows + major devastation eruption waned + stopped

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