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VOLCANOES

VOLCANOES. TYPES OF VOLCANOES. Volcanoes are classified by appearance Steep slopes = Strato/composite Volcanoes Gentle slopes = Shield volcano. Appearance predicts type of eruption Steep slopes → explosive (violent) eruptions Gentle slopes → non-violent (quiet) eruptions.

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VOLCANOES

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  1. VOLCANOES

  2. TYPES OF VOLCANOES • Volcanoes are classified by appearance • Steep slopes = Strato/composite Volcanoes • Gentle slopes = Shield volcano • Appearance predicts type of eruption • Steep slopes → explosive (violent) eruptions • Gentle slopes → non-violent (quiet) eruptions

  3. TYPES OF ERUPTIONS ERUPTIONS FALL INTO 2 CATEGORIES Non-Violent (calm or quiet) eruptions build up the volcano Violent(explosive) eruptions destroy the volcano

  4. TYPES OF ERUPTIONS VEI: Volcanic Explosivity Index

  5. Strato/Composite Volcanoes Steep slopes – Explosive • Formed from viscous (thick) lava • Found in subduction zones & some hot spots Ex/: Casades, Mt St Helens, 2, 3

  6. Shield volcano Gentle slopes- Non-violent • Formed from fluid (thin) lava • Only found over a hot spots Ex: Hawaiian islands

  7. MAGMA/LAVA →ERUPTIONS Non-Explosive Eruptions: Fluid lava flows easilyallows gases to bubble away • Explosive Eruptions: • Viscous lava traps the gases until large pressures build up & the system explodes • Pyroclastic flow (ash, rock fragments) flow out of vent

  8. ERUPTION HAZARDS LAHAR Water, mud & ash that flow like a river

  9. EXPLOSIVE HAZARDS PYROCLASTIC FLOW Travels over 200 mph Tephra = all ash & rock fragments • ash: pieces smaller than 2 mm; travel farthest • lapelli: small pieces between 2-66 mm • volcanic bomb: pieces larger than 64 mm Burns EVERYTHING in its path

  10. EXPLOSIVE HAZARDS VISCOUS LAVA (High Viscosity) • Cool temperature • Compositon: silica-rich (granitic) • Thick & gooey→Erupt violently; scattering ash and fragments widely • Does not flow very far;builds steep-sides; often destroys volcanoes Ex. rhyolithic & andesitic lava→ strato volcanoes

  11. NON-EXPLOSIVE HAZARD FLUID LAVA (Low viscosity) Higher temperatures Composition: Low silica (basaltic) Thin → Erupt “quietly” Great flows of lava that build mountains Ex. Basaltic lava→ shield volcanoes

  12. VISCOSITY→MAGMA/LAVA • Viscosity is resistance to flow • Viscosity determined by • Temperature HOT= thin, flows easily COOL= thick • CompositionSilica(SiO2)=mineral in magmaLOW SILICA (basalt) = thinHIGH SILICA (granite) = thicklow gas = flows easily

  13. WHAT KIND OF LAVA FLOW? • Fluid lava • thin ; flows far; • allows gas to escape; • quiet(non-violent) eruptions • builds mountain • Fluid lava • Flows great distances • Viscous lava • thick (granitic – high silica content) • traps gas • violent eruptions • destroys mountains

  14. FORMATION →ERUPTIONS • Volcanoes are formed by • SUBDUCTION explosive eruptions • Sea Floor Spreading quiet eruptions • Hot Spotsusually quiet eruptions

  15. VOLCANO FORMATION: HOT SPOTS • A fixed source of magma rising beneath a plate forming volcanic islands • Magma can be basaltic or granitic –so eruptions can be explosive or “quiet”

  16. PREDICTING ERUPTIONS: VOLCANO MONITORING • Predict Eruptions by • Study and comparison of past eruptions • Warning Signs- • Small quakes (seismicity) • Swelling of slopes • Changes in water pH • Changes in gases emitted

  17. VOLCANO SUMMARY APPEARANCE → ERUPTIONS • Steep slopes (Strato volcano) → explosive (violent) • Gentle slopes (Shield volcano)→ non-explosive (quiet) MAGMA VISCOSITY → ERUPTIONS • Viscous (thick) magma → explosive (violent - Strato) • Fluid (thin) magma → non-explosive (quiet - Shield) FORMATION →MAGMA/LAVA →ERUPTIONS • Subduction zones → explosive (violent - Strato) • Sea Floor spreading zones → non-explosive (quiet- shield) • Hot spot → usually non-explosive, but can be explosive MONITORING→PREDICTION OF ERUPTIONS • Slope changes, quakes↑, water pH↓ , gases↑

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