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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
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(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 ↑