volcanoes and other igneous activity chapter 7 (page 207)
TRANSCRIPT
Volcanoes and Other Igneous Activity
Chapter 7 (page 207)
Volcanic eruptions
Factors that determine the violence of an eruption • Composition of the magma (silica content)• Temperature of the magma (hot or cool)• Dissolved gases in the magma (volatiles) • Viscosity of magma (Viscosity is a measure of
a material's resistance to flow; it is determined by the amount of silica content in the magma)
Volcanic eruptions
Factors affecting magma viscosity• Temperature (hotter magmas are less viscous)
• Chemical composition (silica content)
- High silica – high viscosity (e.g., rhyolitic lava)
- Low silica – more fluid (e.g., basaltic lava)
• Dissolved gases (volatiles)
- Mainly water vapor and carbon dioxide
- Gases expand near the surface → explosiveness
Controls of Volcanic eruptions
Mafic lava → low silica →low viscosity (more fluid) → hotter → less gases → less explosive → flatter cones
Example: Hawaiian volcanoes, ocean floor volcanoes
Felsic andesitic lava → high silica → high viscosity (stiffer) → cooler→ more gases → more explosive → steeper cones
Example: Mt St. Helen (cascade volcanoes), subduction zone volcanoes
Types of Hawaiian lava flows
Types of lava
• Pahoehoe lava (gas filled, resembles braids in ropes)
• Aa lava (gas free, rough, jagged blocks)
Go to Youtube to watch videos
A Pahoehoe lava flow
A typical aa flow
A size comparison of the three types of volcanoes
A volcanic bomb
Bomb is approximately 10 cm long
Types of Volcanoes
Shield volcano
• Broad, slightly domed
• Primarily made of basaltic (fluid) lava
• Generally large size
• Associated with ocean floor magmatism
• e.g., Mauna Loa in Hawaii
Types of Volcanoes
Cinder cone
• Built from ejected lava fragments (cinder)
• Steep slope angle
• Rather small size
• Frequently occur in groups
• Associated with subduction zones
Sunset Crater – a cinder cone near Flagstaff, Arizona
Types of Volcanoes
Composite cone (or stratovolcano)
• Large size
• Steep slope
• Interbedded lava flows and pyroclastic material
• Most violent type of eruptions
• Associated with subduction zones
• Most are adjacent to the Pacific Ocean (e.g., Cascade, Andes volcanoes)
A composite volcano (stratovolcano)
Features of Volcanoes
General features • Conduit, or pipe• Crater • Vent• Crater• Fumaroles• Caldera• Crater Lake
16
What’s in a Volcano’s arsenal?
• Pyroclastic flow (very hot ash)
• Lahar (deadly mud flow)
• Crater Lake’s deadly gases
Mt. St. Helens – a typical composite volcano
Mt. Hood, Oregon
Mt. St. Helens Pyroclastic flow
A lahar along the Toutle River near Mt. St. Helens
Formation of a caldera
Crater Lake in Oregon
Formation of a volcanic neck
Plate Tectonics and Igneous Activity
Volcanism at Divergent plate margins:• Type of magma: Basaltic
• Example: Mid Ocean ridge
Volcanism at Convergent plate margins• Type of magma: Andesitic/felsic
• Example Island arcs; Andes Mountains
Plate Tectonics and Igneous Activity
Intraplate Volcanism (Hot Spot) not a plate margin
• Form over a stationary pocket of magma;
type of lava: Basaltic
• Form a chain of volcanoes Example: Hawaiian volcanic chain
• The only active volcano is over the hot spot
Locations of some of Earth’s major volcanoes
Igneous Bodies
Are bodies formed by the solidification of magma or lava
28
Intrusive igneous structures exposed by erosion
29
Plutons
Type – I: tabular sheet like bodies
Concordant bodies (parallel):• Sills (tabular) • Laccoliths (irregular)
Discordant bodies (cut through):• dikes
Plutons
Type – II: Massive irregular bodies:
BatholithsStocks
31
A sill in the Salt River Canyon, Arizona
32
A batholith exposed by erosion
33
End of Chapter 7
Clicker test, do not talk, please.
35
1-Which Magma is more viscous?
a. Basaltic
b. Felsic/Andesitic
c. Ultramafic
d. None of these
36
2-which magma traps more gases?
a. Mafic/Ultramafic
b. Felsic/Andesitic
c. None of these
d. I am not sure
37
3-Which Magma is Hotter?
a. Mafic/Ultramafic
b. Felsic /Andesitic
c. None of these
d. I am not sure
38
4-Which magma produces more violent eruptions?
a. Mafic/Ultramafic
b. Felsic/Andesitic
c. None of these
d. I am not sure
39
5-Magma viscosity is a function of…
a. Temperature
b. Silica content
c. Gases content
d. All of these
e. b only
40
6-Which volcano is over a hot spot?
a. Mt St Helens (Washington State)
b. Mt Rainier (Washington State)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
41
7-Which one is a Shield Volcano?
a. Mt St Helens (Washington State)
b. Mt Rainier (Washington State)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
42
8-Which one is a Composite cone?
a. Mt St Helens (Washington State)
b. Sunset Crater (Arizona)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
43
9-Which volcano type produces lava + pyroclastic flows
a. Mt St Helens (Washington State)
b. Kula (Hawaii)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
44
10-Volcanoes with steep cones have
a. Fluid lava
b. Viscous lava
c. No relation whatsoever
d. I am not sure
45
11-Volcanoes at divergent boundary erupt mainly…lava
a. Felsic to Andesitic
b. Mafic
c. None of these
d. I am not sure
46
12-Volcanoes at convergent margins produce mainly…lava
a. Felsic to Andesitic
b. Mafic
c. None of these
d. No relation whatsoever
47
13-Which volcanoes are more explosive?
a. Shield volcanoes
b. Composite cones
c. Cinder cones
d. None of these
e. I am not sure
48
14-Volcano explosiveness is influenced by …
a. Viscosity
b. Temperature
c. Amount of gases
d. All of the above
e. A and c only
49
15-A Pahoehoe lava is
a. Gases-free with jagged surface lava
b. Wrinkled, ropy, and gaseous lava
c. Explosive ash and fragments flow
d. None of these
e. I am not sure