VOLCANOES

TYPES OF VOLCANOES Volcanoes are classified by appearance Steep slopes = Strato/composite Volcanoes Gentle slopes = Shield volcano These are the three main types, but there are other types as well on next picture. 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 volcano Violent(explosive) eruptions 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 T The person who took these photos is Gary Lee Rosenquist. He barely made it. David Johston is the Volcanologist who died. He was pretty muc in front of the blast. That's why there is an observatory named after him. hese are the three main types, but there are other types as well on next picture.

Shield volcano Gentle slopes- Non-violent Formed from fluid (thin) lava Only found over a hot spots Ex: Hawaiian islands These are the three main types, but there are other types as well on next picture.

MAGMA/LAVA →ERUPTIONS Non-Explosive Eruptions: Fluid lava flows easily allows gases to bubble away All magma contains gases that expand and rise buoyantly as the magma rises to the surface (like bubbles in a soda) 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 Fluid lava Flows great distances 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 SUBDUCTION explosive eruptions Sea Floor Spreading quiet eruptions Hot Spots usually quiet eruptions Subduction: if continental: oceanic crusts – continental -> Granitic magma -> explosive Sea Floor: crusts are oceanic (basalt) -> quiet eruptions Hot Spots depends upon crust

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 MAGMA VISCOSITY → 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 ↑