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

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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

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Intrusive igneous structures exposed by erosion

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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

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A sill in the Salt River Canyon, Arizona

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A batholith exposed by erosion

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End of Chapter 7

Clicker test, do not talk, please.

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1-Which Magma is more viscous?

a. Basaltic

b. Felsic/Andesitic

c. Ultramafic

d. None of these

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2-which magma traps more gases?

a. Mafic/Ultramafic

b. Felsic/Andesitic

c. None of these

d. I am not sure

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3-Which Magma is Hotter?

a. Mafic/Ultramafic

b. Felsic /Andesitic

c. None of these

d. I am not sure

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4-Which magma produces more violent eruptions?

a. Mafic/Ultramafic

b. Felsic/Andesitic

c. None of these

d. I am not sure

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5-Magma viscosity is a function of…

a. Temperature

b. Silica content

c. Gases content

d. All of these

e. b only

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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

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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

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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

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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

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10-Volcanoes with steep cones have

a. Fluid lava

b. Viscous lava

c. No relation whatsoever

d. I am not sure

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11-Volcanoes at divergent boundary erupt mainly…lava

a. Felsic to Andesitic

b. Mafic

c. None of these

d. I am not sure

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12-Volcanoes at convergent margins produce mainly…lava

a. Felsic to Andesitic

b. Mafic

c. None of these

d. No relation whatsoever

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13-Which volcanoes are more explosive?

a. Shield volcanoes

b. Composite cones

c. Cinder cones

d. None of these

e. I am not sure

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14-Volcano explosiveness is influenced by …

a. Viscosity

b. Temperature

c. Amount of gases

d. All of the above

e. A and c only

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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