Volcanoes are classified based on their slope and type of eruption. Steep sloped stratovolcanoes are prone to explosive, violent eruptions due to thick, viscous lava. In contrast, gently sloped shield volcanoes have non-violent eruptions as their fluid lava allows gases to easily escape. The viscosity of magma determines whether an eruption will be explosive or non-explosive, with thick, silica-rich magma causing violent eruptions and thin, basaltic magma resulting in quiet flows. Volcano formation also influences eruption type, with subduction zones generally producing explosive eruptions and hot spots commonly resulting in non-explosive

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 ↑