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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