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Earth Science, 10e

Edward J. Tarbuck & Frederick K. Lutgens

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Volcanoes and Other Igneous Activity

Chapter 8

Earth Science, 10e Stan Hatfield and Ken Pinzke

Southwestern Illinois College

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Volcanic eruptions

Factors that determine the violence of an eruption • Composition of the magma (% silica content)• Temperature of the magma (Very hot and cool)• Dissolved gases in the magma (Water vapor

etc…)

Viscosity of magma (function of silica content)

• Viscosity is a measure of a material's resistance to flow

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Let’s Review

• What types of Magma are there?...

• How does temperature relate to a type of magma?

• How Does viscosity relate to a type of Magma?

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Volcanic eruptions

Viscosity of magma • Factors affecting viscosity

• Temperature (hotter magmas are less viscous)

• 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

• Hotter magmas have less gases

• Cooler magmas trap more gases

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Volcanic eruptions

Viscosity of magma (continued…)• Factors affecting viscosity

• Dissolved gases (volatiles) thicken the magma

• Provide the force to extrude lava

• Violence of an eruption is related to how easily gases escape from magma

• Easy escape from fluid magma

• Viscous magma produces a more violent eruption

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Fluid Lava: The Columbia River basalts

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Let’s summarize

• Viscous magma: Felsic/Andesitic or Mafic?

• Cooler magma Felsic/Andesitic or Mafic?

• Violent eruptions: Felsic/Andesitic or Mafic?

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Materials associated with volcanic eruptions

Types of Basaltic Lava flows • Basaltic lavas are more fluid• Types of basaltic lava

• Pahoehoe lava (resembles braids in ropes)

• Aa lava (rough, jagged blocks)

Gases • Represent one to 5 percent of magma by weight• Mainly water vapor and carbon dioxide

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A Pahoehoe lava flow

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A typical aa flow

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What is a Hot Spot?

• An intermittent stationary source of magma that originates deep inside the core or the mantle

• Plates riding over a hot spot form a chain of volcanoes. i.e. Hawaiian chain

• The chain changes direction as the plate changes its direction over the hot spot

• Only the volcano over the hot spot is active

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A size comparison of the three types of volcanoes

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Types of Volcanoes

There are three types of volcanoes defined according to their shape and size:

1. Shield volcano• Broad, slightly domed

• Primarily made of basaltic (fluid) lava

• Generally large size

• e.g., Mauna Loa in Hawaii

• Mostly found on the Ocean floor and/or over

hot spots

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A shield volcano

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Types of Volcanoes

Types of volcanoes (continued…) 2. Cinder cone

• Built from ejected lava fragments

• Steep slope angle

• Rather small size

• Frequently occur in groups

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Sunset Crater – a cinder cone near Flagstaff, Arizona

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Types of Volcanoes

Types of volcanoes (continued…)• Composite cone (or stratovolcano)

• Are made of Interbedded lavas and pyroclastics

• Most are adjacent to the Pacific Ocean also called Ring of Fire (e.g., Mt. St. Helens, Mt. Pinatubo,…)

• They are of large size

• They produce the most violent eruptions (i.e. Mt. St Helens May 18, 1980 eruption)

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Volcanoes

Types of volcanoes (continued…) • Composite cone (or stratovolcano)

• Often produce nuée ardente (Pyroclastic flow)

• Fiery pyroclastic flow made of hot gases infused with ash

• Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour

• May produce a Lahar – deadly volcanic mudflow

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A composite volcano (stratovolcano)

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Mt. St. Helens – a typical composite volcano

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Pyroclastic flow on Mt. St. Helens

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What’s on a Volcano’s arsenal?

• Pyroclastic flow (very hot ash)

• Lahar (deadly mud flow)

• Crater Lake’s deadly gases

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A lahar along the Toutle River near Mt. St. Helens

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Other volcanic landforms

Calderas • Steep walled depression at the summit • Formed by collapse of the crater• Nearly circular • Size exceeds one kilometer in diameter• When filled with water calderas form

crater lakes

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Crater Lake, Oregon is a good example of a caldera

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Crater Lake in Oregon

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Materials associated with volcanic eruptions

Pyroclastic materials • "Fire fragments" • Types of pyroclastic material

• Ash and dust – fine, glassy fragments • Pumice – from "frothy" lava • Lapilli – "walnut" size • Cinders – "pea-sized" • Particles larger than lapilli

• Blocks – hardened lava • Bombs – ejected as hot lava

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A volcanic bomb

Bomb is approximately 10 cm long

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Volcanoes

General features • Conduit, or pipe caries gas-rich magma to the

surface • Vent, the surface opening (connected to the

magma chamber via a pipe) • Crater

• Steep-walled depression at the summit

• Caldera (a summit depression greater than 1 km diameter)

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Mt. St. Helens following the 1980 eruption

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Other volcanic landforms

Volcanic pipes and necks • Pipes are short conduits that connect a magma

chamber to the surface • Volcanic necks (e.g., Ship Rock, New Mexico)

are resistant vents left standing after erosion has removed the volcanic cone

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Origin of magma

Magma originates when essentially solid rock, located in the crust and upper mantle, melts

Factors that influence the generation of magma from solid rock • Role of heat

• Earth’s natural temperature increases with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle

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Origin of magma

Factors that influence the generation of magma from solid rock • Role of heat

• Additional heat is generated by

• Friction in subduction zones

• Crustal rocks heated during subduction • Rising, hot mantle rocks

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Origin of magma

Factors that influence the generation of magma from solid rock • Role of pressure

• Increase in confining pressure causes an increase in melting temperature

• Drop in confining pressure can cause decompression melting

• Lowers the melting temperature

• Occurs when rock ascends

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Origin of magma

Factors that influence the generation of magma from solid rock • Role of volatiles

• Primarily water

• Cause rock to melt at a lower temperature

• Play an important role in subducting ocean plates

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Plate tectonics and igneous activity

Global distribution of igneous activity is not random • Most volcanoes are located on the margins of

the ocean basins (intermediate, andesitic composition) called subduction zone

• Second group is confined to the deep ocean basins (basaltic lavas) called Mid Ocean ridges

• Third group includes those found in the interiors of continents at Continental rift valley

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Locations of some of Earth’s major volcanoes

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Plate tectonics and igneous activity

Plate motions provide the mechanism by which mantle rocks melt to form magma • Convergent plate boundaries

• Deep-ocean trenches are generated • Descending plate partially melts • Magma slowly rises upward • Rising felsic to andesitic magma can form

• Volcanic island arcs in an ocean (Aleutian Islands)

• Continental volcanic arcs (Andes Mountains)

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Plate tectonics and igneous activity

Plate motions provide the mechanism by which mantle rocks melt to form magma • Divergent plate boundaries

• The greatest volume of volcanic rock is produced along the oceanic ridge system

• Lithosphere pulls apart

• Less pressure on underlying rocks

• Partial melting occurs

• Large quantities of fluid basaltic magma are produced

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Plate tectonics and igneous activity

Plate motions provide the mechanism by which mantle rocks melt to form magma • Intraplate igneous activity

• Activity within a rigid plate

• Plumes of hot basaltic magma rise

• Form localized volcanic regions called hot spots

• Examples include the Hawaiian Islands and the Columbia River Plateau in the northwestern United States

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Tectonic Settings and volcanic activity

• Divergent margin: Has volcanic activity

• Convergent margin: Has volcanic activity

• Transform fault: Is a margin but has no volcanic activity

• Hot spot: Not a margin but has volcanic activity

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Formation of a volcanic neck

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Intrusive igneous activity

Two types of igneous intrusive features • Sills, Laccoliths, and dikes

• Lens shaped masses

• Batholiths and Stocks• Largest intrusive bodies

• Often occur in groups Surface exposure 100+ square kilometers (smaller bodies are termed stocks)

• Frequently form the cores of mountains

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Intrusive igneous structures exposed by erosion

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A batholith exposed by erosion

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Intrusive igneous activity

Most magma is emplaced at depthAn underground igneous body is called a

plutonPlutons are classified according to

• Shape• Tabular (sheetlike)

• Massive

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Intrusive igneous activity

Plutons are classified according to • Orientation with respect to the host

(surrounding) rock • Discordant – cuts across existing structures

• Concordant – parallel to features such as sedimentary strata

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Intrusive igneous activity

Types of igneous intrusive features • Dike, a tabular, discordant pluton • Sill, a tabular, concordant pluton

• e.g., Palisades Sill, NY

• Resemble buried lava flows

• May exhibit columnar joints

• Laccolith • Similar to a sill

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A sill in the Salt River Canyon, Arizona

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End of Chapter 8

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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

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16 - Quartzite is what type of metamorphic rock?

a. Regional metamorphism

b. Contact metamorphism

c. Not a metamorphic rock

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17 – In what type of metamorphism do pressure and temperature work

together?

a. Contact metamorphism

b. Regional metamorphism

c. All of the above

d. None of the above

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18 - What is the parent rock of limestone?

a. Quartzite

b. Granite

c. Marble

d. Basalt

e. This does not make sense

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19 – Which one of these is a foliated metamorphic rock?

a. Marble

b. Granite

c. Mica-schist

d. Basalt

e. Shale

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20 - Which one is the metamorphic rock?

Do not take

a. SANDSTONE for

b. GRANITE (granted)!

Have a

c. GNEISS (nice) day!

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THE END

THAT’S ALL FOLKS!

THANK YOU!!!