VOLCANOES TYPES OF VOLCANOES Volcanoes are classified by

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

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

TYPES OF ERUPTIONS § VEI: Volcanic Explosivity Index

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(Si. O 2)=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 p. H • 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 p. H↓ , gases ↑