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VOLCANOS 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 TYPES OF ERUPTIONS ERUPTIONS FALL INTO 2 CATEGORIES Non-Violent (calm or quiet) eruptions build up the Violent(explosive) eruptions volcano destroy the volcano 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 SHIELD VOLCANO Gentle slopes- Non-violent Formed from fluid (thin) lava Only found over a hot spots Ex: Hawaiian islands 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 ERUPTION HAZARDS LAHAR Water, mud & ash that flow like a river 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 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 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 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 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 FORMATION →ERUPTIONS Volcanoes are formed by 1. SUBDUCTION explosive eruptions 2. Sea Floor Spreading quiet eruptions 3. Hot Spots usually quiet eruptions 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” 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 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 ↑ Plate Tectonics and Mountain Building: Terrain Accretion and Continental Shields Plate Tectonics and Mountains - Convergent Boundaries - Fault Block Mountains - Isostatic Mountains Continental Shields Older nucleuses that younger crust (objects like Japan) adheres to are called continental shields and the process is called terrain accretion Plate Convergence Mountains Isostatic Mountains - Superstitions Continents behave like wooden blocks in water… Mountains Uplift as they are eroded… Basins depress as they fill… Isostatic Mountains Old volcanic center erupted 18 million years ago, and collapsed on itself, and then rebounded into the current mountain range Superstition Mountains http://discoverourearth.org/student/isostasy.html Fault Block Mountains Nevada, US