volcanoes ppt.ppt
- 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 Violent(explosive) eruptions destroy the volcano Non-Violent (calm or quiet) eruptions build up 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 easily allows 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 Composition Silica(SiO2)=mineral in magma LOW SILICA (basalt) = thin HIGH SILICA (granite) = thick low gas = flows easily
- 13. WHAT KIND OF LAVA FLOW? Viscous lava • thick (granitic – high silica content) • traps gas • violent eruptions • destroys mountains Fluid lava • thin ; flows far; • allows gas to escape; • quiet(non-violent) eruptions • builds mountain Fluid lava Flows great distances
- 14. FORMATION →ERUPTIONS Volcanoes are formed by 1.SUBDUCTION explosive eruptions 2.Sea Floor Spreading quiet eruptions 3.Hot Spots usually 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 ↑