Today: Chapter 6
Volcanism
1) Movie: Eruption of Mauna Loa and
Kilauea on Hawaii
2) Nature of volcanic eruptions
3) Materials extruded during eruptions
4) Types of volcanoes
5) Other volcanic landforms
2.
1. The nature of volcanic eruptions
What determines if an eruption is
- violent?
- “gentle”?
3 primary factors:
 magma’s composition
more silica
more viscous (chains)
 magma’s temperature
hotter magma
magma
viscosity
less viscous
 amount of gas in magma
Can increase fluidity
Its ability to move
2.
1. The nature of volcanic eruptions
Importance of gas in eruptions
Gas can provide the force to violently
hurl molten rock & ash from volcano
2.
1. The nature of volcanic eruptions
•Fluid basaltic lavas generally
produce quiet eruptions
•Highly viscous lavas (rhyolite or
andesite) produce more explosive
eruptions
2.
1. The nature of volcanic eruptions
80% of all volcanoes are found along convergent
plate boundaries.
Fig. 6.18
3.
2. Materials extruded during an eruption
Volcanoes erupt:
 lava
 gas
 pyroclastic materials
broken rock
ash
dust
3.
2. Materials extruded during an eruption
 lava
 gas
 pyroclastic stuff
Basaltic (low Si content)
pahoehoe flow
flows in thin, broad sheets,
~10-300 meters/hr,
smooth twisted or ropey
texture & wrinkled outer
skin with molten flow
beneath
(Hawaiian type)
3.
2. Materials extruded during an eruption
 lava
 gas
 pyroclastic stuff
Basaltic (low Si content)
aa flow
thicker & slower flows
~5-50 meters/hr,
rough & jagged surface
3.
2. Materials extruded during an eruption
 lava
 gas
 pyroclastic stuff
Magmas contain dissolved gases, held in by pressure
Reduce pressure
gases escape
(decompression)
Gaseous portion
1-6% by weight,
mostly water vapor,
some CO2
3.
2. Materials extruded during an eruption
 lava
 gas
 pyroclastic stuff
Magmas contain dissolved gases, held in by pressure
Phreatic explosion
Hot, gas-charged
magma encounters
water and causes a
superheated steam
explosion
3.
2. Materials extruded during an eruption
 lava
 gas
 pyroclastic stuff
Gas can blow pulverized rock, glass, ash,
magma blobs into the air from a volcano
Pyroclastic materials– “Fire fragments”
Types of pyroclastic debris
•Ash and dust - fine, glassy fragments
•Volcanic bombs - ejected as blobs of lava,
solidify in air
3. Materials extruded during an eruption
A volcanic bomb
Bomb is approximately 10 cm long
 lava
 gas
 pyroclastic stuff
4. Types of Volcanoes
Opening at the summit of a volcano
Crater - Depression at the summit <
1 km
Caldera - Summit depression > 1 km,
produced by collapse following a
massive eruption
Vent – opening connected to the
magma chamber via a pipe
See Fig. 6.9
4. Types of Volcanoes
Key terms: magma chamber
vent/pipe
caldera
flank eruption
4. Types of Volcanoes
Types of volcanoes
Shield volcano
Cinder Cone
Composite Cone (Stratovolcano)
4. Types of Volcanoes
Broad, large area
Large volumes of basaltic lava
Mauna Loa on Hawaii
4. Types of Volcanoes
Built from ejected lava
Steep slope angle
Rather small size
Frequently occur in groups
4. Types of Volcanoes
Sunset Crater – a cinder cone near
Flagstaff, Arizona
4. Types of Volcanoes
Sunset crater, AZ
4. Types of Volcanoes
Composite Volcano
Associated with convergent plate boundaries
(e.g., Fujiyama, Mt. St. Helens)
Classic-shaped volcano (1000’s of ft. high &
several miles wide at base)
Composed of interbedded lava flows and
layers of pyroclastic debris
4. Types of Volcanoes
Mt. St. Helens –
a typical composite volcano
4. Types of Volcanoes
Mt. St. Helens following the
1980 eruption
4. Types of Volcanoes
A size comparison of the three
types of volcanoes
5. Other Volcanic Landforms
a. caldera
 crater greater than 1 km wide
 from collapse of summit
Today, Crater Lake
Also:
Yellowstone!
occupies a
caldera 10km wide
Fig. 6.10
5. Other Volcanic Landforms
b. Basaltic fissure eruption
 Fluid basaltic lava extruded from
crustal fractures called fissures
 Ocean ridges; Columbia River
Plateau
Fig. 6.13
5. Other Volcanic Landforms
Columbia River Basalts
Flood basalt
of the Columbia River
Plateau formed during
extensive volcanic
activity 17 mio yr ago
Fig. 6.2
5. Other Volcanic Landforms
c. lava dome
 Bulbous mass of congealed lava
 usually form after explosive
eruption of gas-rich magma