1. Today: Chapter 6 Volcanism
Movie: Eruption of Mauna Loa and Kilauea on Hawaii
Nature of volcanic eruptions
3) Materials extruded during eruptions
4) Types of volcanoes
5) Other volcanic landforms

2. 2. The nature of volcanic eruptions
- violent?
- “gentle”?
What determines if an eruption is
3 primary factors:
magma’s composition
magma’s temperature
amount of gas in magma
magma
viscosity
more silica more viscous (chains)
hotter magma less viscous
Its ability to move
Can increase fluidity

3. 2. The nature of volcanic eruptions
Importance of gas in eruptions
Gas can provide the force to violently
hurl molten rock & ash from volcano

4. Fluid basaltic lavas generally produce quiet eruptions
2. The nature of volcanic eruptions
1. The nature of volcanic eruptions
Fluid basaltic lavas generally produce quiet eruptions
Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

5. 2. The nature of volcanic eruptions
80% of all volcanoes are found along convergent plate boundaries.
Fig. 6.18

6. broken rock ash dust 3. Materials extruded during an eruption
Volcanoes erupt:
lava
gas
pyroclastic materials
broken rock
ash
dust

7. Basaltic (low Si content)
lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
2. Materials extruded during an eruption
Basaltic (low Si content)
pahoehoe flow
flows in thin, broad sheets, ~ meters/hr, smooth twisted or ropey texture & wrinkled outer skin with molten flow beneath
(Hawaiian type)

8. Basaltic (low Si content)
lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
2. Materials extruded during an eruption
Basaltic (low Si content)
aa flow
thicker & slower flows
~5-50 meters/hr,
rough & jagged surface

9. mostly water vapor, some CO2
lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
2. Materials extruded during an eruption
Magmas contain dissolved gases, held in by pressure
Reduce pressure
gases escape
(decompression)
Gaseous portion
1-6% by weight,
mostly water vapor, some CO2

10. lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
2. Materials extruded during an eruption
Magmas contain dissolved gases, held in by pressure
Phreatic explosion
Hot, gas-charged magma encounters water and causes a superheated steam explosion

11. Types of pyroclastic debris Ash and dust - fine, glassy fragments
lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
2. Materials extruded during an eruption
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

12. A volcanic bomb 3. Materials extruded during an eruption
lava
gas
pyroclastic stuff
3. Materials extruded during an eruption
A volcanic bomb
Bomb is approximately 10 cm long

13. Opening at the summit of a volcano
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

14. 4. Types of Volcanoes vent/pipe caldera flank eruption
Key terms: magma chamber
vent/pipe
caldera
flank eruption

15. Types of volcanoes Shield volcano Cinder Cone
Composite Cone (Stratovolcano)

16. 4. Types of Volcanoes
Broad, large area
Large volumes of basaltic lava
Mauna Loa on Hawaii

17. 4. Types of Volcanoes
Built from ejected lava
Steep slope angle
Rather small size
Frequently occur in groups

18. Sunset Crater – a cinder cone near Flagstaff, Arizona
4. Types of Volcanoes
Sunset Crater – a cinder cone near Flagstaff, Arizona

19. 4. Types of Volcanoes
Sunset crater, AZ

20. Composite Volcano 4. Types of Volcanoes
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

21. Mt. St. Helens – a typical composite volcano
4. Types of Volcanoes
Mt. St. Helens – a typical composite volcano

22. Mt. St. Helens following the 1980 eruption
4. Types of Volcanoes
Mt. St. Helens following the eruption

23. A size comparison of the three types of volcanoes

24. 5. Other Volcanic Landforms
a. caldera
crater greater than 1 km wide
from collapse of summit
Also:
Yellowstone!
Today, Crater Lake
occupies a
caldera 10km wide
Fig. 6.10

25. b. Basaltic fissure eruption
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

26. Columbia River Basalts
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

27. 5. Other Volcanic Landforms
c. lava dome
Bulbous mass of congealed lava
usually form after explosive eruption of gas-rich magma