Transcript 3. Materials extruded during an eruption
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
What determines if an eruption is
- violent?
- “gentle”?
3 primary factors:
magma’s composition
more silica more viscous (chains)
magma’s temperature
hotter magma less viscous
amount of gas
Can increase fluidity
in magma magma
viscosity
Its ability to move
Importance of gas in eruptions Gas can provide the force to violently hurl molten rock & ash from volcano
•
Fluid basaltic lavas generally produce quiet eruptions
•
Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
80% of all volcanoes are found along convergent plate boundaries.
Fig. 6.18
Volcanoes erupt:
lava gas pyroclastic materials
broken rock ash dust
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)
lava gas pyroclastic stuff Basaltic (low Si content) aa flow thicker & slower flows ~5-50 meters/hr,
rough & jagged surface
lava gas pyroclastic stuff
Magmas contain dissolved gases, held in by pressure
Reduce pressure Gaseous portion gases escape (decompression) 1-6% by weight,
mostly water vapor
, some CO 2
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
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
lava gas pyroclastic stuff
A volcanic bomb
Bomb is approximately 10 cm long
4. Types of Volcanoes Opening at the summit of a volcano Crater 1 km - Depression at the summit < 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
Sunset crater, AZ
4. Types of Volcanoes
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
Today, Crater Lake occupies a caldera 10km wide
crater greater than 1 km wide
from collapse of summit
Also: Yellowstone!
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