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

What’s in a Volcano’s arsenal?

• Pyroclastic flow (very hot ash) • Lahar (deadly mud flow) • Crater Lake’s deadly gases

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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

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 29

Plutons

Type – II: Massive irregular bodies:  Batholiths  Stocks

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

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Clicker test, do not talk, please .

1-Which Magma is more viscous?

a. Basaltic b. Felsic/Andesitic c. Ultramafic d. None of these 35

2-which magma traps more gases?

a. Mafic/Ultramafic b. Felsic/Andesitic c. None of these d. I am not sure 36

3-Which Magma is Hotter?

a. Mafic/Ultramafic b. Felsic /Andesitic c. None of these d. I am not sure 37

4-Which magma produces more violent eruptions?

a. Mafic/Ultramafic b. Felsic/Andesitic c. None of these d. I am not sure 38

5-Magma viscosity is a function of… a. Temperature b. Silica content c. Gases content d. All of these e. b only 39

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 40

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 41

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 42

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 43

10-Volcanoes with steep cones have a. Fluid lava b. Viscous lava c. No relation whatsoever d. I am not sure 44

11-Volcanoes at divergent boundary erupt mainly…lava a. Felsic to Andesitic b. Mafic c. None of these d. I am not sure 45

12-Volcanoes at convergent margins produce mainly…lava a. Felsic to Andesitic b. Mafic c. None of these d. No relation whatsoever 46

13-Which volcanoes are more explosive?

a. Shield volcanoes b. Composite cones c. Cinder cones d. None of these e. I am not sure 47

14-Volcano explosiveness is influenced by … a. Viscosity b. Temperature c. Amount of gases d. All of the above e. A and c only 48

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 49