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Chapter 18 Volcanism

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8/2/2019 Chapter 18 Volcanism

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Chapter 

18 Volcanism

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18.1: Volcanoes (p. 500)

• Volcanism: describes all the processesassociated with the discharge of magma,hot fluids, and gases

• 20 volcanoes are erupting right now

• Over a year, eruptions occur at 60 places

• Most form at plate boundaries

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Origin of Magma Geologists conclude that magma originates

when essentially solid rock, located in the

crust and upper mantle, partially melts.

18.1 Intrusive Igneous Activity 

The most obvious way to generate magmafrom solid rock is to raise the temperatureabove the level at which the rock begins tomelt.

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Origin of Magma Role of Heat

18.1 Intrusive Igneous Activity 

• Additional heat is generated by 

• The geothermal gradient—Earth’s natural

temperature increases with depth but is notsufficient to melt rock in the lower crustand upper mantle

- friction in subduction zones- crustal rocks heated during subduction

- rising, hot mantle rocks

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Origin of Magma Role of Water

18.1 Intrusive Igneous Activity 

• Causes rock to melt at a lower temperature 

• Plays an important role in subducting

ocean plates

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Convergent Plate Boundaries Subduction zones: oceanic crust is pushed

down into the mantle

As the crust descends, magma forms Magma moves upward because it is less

dense Usually oceanic continental boundaries Explosive erruptions

18.1 Volcanoes 

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Convergent Boundary Volcano

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Convergent Boundary

Volcanoes: 2 Major Belts• See pages 500 and 501

• Circum-Pacific Belt (ring of fire)

• Mediterranean Belt (Mt. Etna, Mt.Vesuvius-Eurasian, African, and Arabianplates)

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Divergent Plate Boundaries

Magma rises through the ridge createdwhen the lithosphere pulls apart.

18.1 Volcanoes 

-not explosive

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Hot Spot Volcanoes

occur within a tectonic plate away fromplate boundaries.

Unusually hot regions of Earth’s mantlewhere plumes of magma rise to the surface

Many under the ocean: Hawaiian islands punch holes through crust as the plate

moves, creating chains of volcanoes (seep. 503)

18.1 Volcanoes 

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Kilauea, a Hot Spot Volcano

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Types of Volcanoes

18.1 Volcanoes 

Anatomy of a Volcano (p. 505) Volcano: mountain formed of solidified

layers of lava Describe each of the following parts of

a volcano and label them on a drawing. Conduit

Vent Crater CalderaAlso, label lava and pyroclastic material

(ejected lava that has hardened)

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Anatomy of a “Typical” Volcano 

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18.1 Volcanoes

• 3 Types of Volcanoes (p. 506-507)

 – Shield

 – Cinder cone

 – Composite

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Types of Volcanoes

18.1 Volcanoes 

Shield Volcanoes 

-broad, gently sloping volcanoes built from fluid

basaltic lavas-non-explosive eruptions create gentle slope

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Shield Volcanoes

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18.1 Volcanoes

• Cinder Cone Volcanoes

 – Eruptions eject small pieces of magma intothe air called tephra

 – The tephra piles up around the vent

 – Steep sides and small

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Cinder Cones

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Types of Volcanoes

10.1  The Nature of Volcanic Eruptions 

Composite Volcanoes • Composite cones are volcanoes composed of

both lava flows and pyroclastic material.- Most are adjacent to the Pacific Ocean: Mt.

Rainier, Mt. St. Helens

- Large size

- Interbedded lavas and pyroclastics

- Most violent type of activity: dangerous

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Composite Cones

M S H l B f d

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Mount St. Helens Before andAfter the May 18, 1980, Eruption

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Profiles of Volcanic Landforms

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18.2 Eruptions (p. 508)

• Making Magma

 – Read intro on p. 508

 – Need high temperature to melt rock

 – Magma composition determines explosivity

• As amount of gas increases, explosivity increases

• As viscosity (resistance to flow) increases,

explosivity decreases – High silica=viscous which leads to explosive eruptions

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Types of Magma (p. 510)

• Basaltic: quiet eruptions (least silica)

• Andesitic: middle level (intermediateeruptions-middle amount of silica)

• Rhyolitic: very explosive (most silica)

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Basaltic Magma at the Surface

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Factors Affecting Eruptions

18.2 Volcanoes 

 Factors that determine the violence of aneruption

• Composition of the magma 

• Temperature of the magma 

• Dissolved gases in the magma 

 Viscosity• Viscosity is the measure of a material's

resistance to flow.

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Factors Affecting Eruptions

18.2  The Nature of Volcanic Eruptions 

 Viscosity

• Factors affecting viscosity 

- Temperature (hotter magmas are less viscous)

- Composition (silica content)

1. High silica—high viscosity

(e.g., rhyolitic lava-explosive)2. Low silica—more fluid, less viscous (e.g.,basaltic lava)

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Factors Affecting Eruptions

18.2 The Nature of Volcanic Eruptions 

Dissolved gases• Mainly water vapor and carbon dioxide 

• Gases expand near the surface 

• A vent is an opening in the surface of Earththrough which molten rock and gases arereleased.

• Provide the force to extrude lava 

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Factors Affecting Eruptions

18.2 The Nature of Volcanic Eruptions 

Dissolved gases• Violence of an eruption is related to how easily

gases escape from magma- Gases escape easily from fluid magma.

- Viscous magma produces a more violenteruption.

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Magma Composition

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Volcanic Material

18.2 The Nature of Volcanic Eruptions 

Lava Flows• Basaltic lavas are more fluid. 

- Pahoehoe lava (resembles braids in ropes)

- Aa lava (rough, jagged blocks)

• Types of lava 

Gases• One to 5 percent of magma by weight 

• Mainly water vapor and carbon dioxide 

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Pahoehoe (Ropy) Lava Flow

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Slow-Moving Lava Flow

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Volcanic Material

18.2  The Nature of Volcanic Eruptions 

Pyroclastic Materials • Pyroclastic materials is the name given to

particles produced in volcanic eruptions.• The fragments ejected during eruptions range in

size from very fine duct and volcanic ash (lessthan 2 millimeters) to pieces that weigh several

tons.

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Volcanic Material

18.2  The Nature of Volcanic Eruptions 

Pyroclastic Materials • Types of pyroclastic material 

- Ash and dust—fine, glassy fragments- Pumice—frothy, air-filled lava

- Lapilli—walnut-sized particles

- Cinders—pea-sized particles

• Particles larger than lapilli - Blocks—hardened lava

- Bombs—ejected as hot lava

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Other Volcanic Landforms

18.2  The Nature of Volcanic Eruptions 

Calderas 

• Calderas are large depressions in volcanoes.

• Formed by collapse

• Nearly circular 

• Size exceeds one kilometer in diameter  

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Plutons (p. 514) Most volcanism is beneath Earth’s

surface

Plutons are intrusive igneous structuresthat result from the cooling and hardeningof magma beneath the surface of Earth.

18.3 Intrusive Igneous Activity 

• Intrusive igneous bodies, or  plutons, are

classified according to their shape, size, andrelationship to the surrounding rock layers.

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Plutons Sills and Laccoliths (see p. 515)

18.3 Intrusive Igneous Activity 

• Sills and laccoliths are plutons that form when

magma is intruded close to the surface.- Sills resemble buried lava flows and may

exhibit columnar joints. Parallel to layers ofrock.

- Laccoliths are lens-shaped masses that archoverlying strata upward. Rounded top and flatbottom.

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Sills

Sill 

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Plutons Dikes (see p. 516)

18.3 Intrusive Igneous Activity 

• Many dikes form when magma from a largemagma chamber enters fractures and cracks inthe surrounding rocks.

• Dikes are intrusive igneous features that cut

across preexisting rock layers.

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Plutons Batholiths

18.3 Intrusive Igneous Activity 

• Batholiths largest, masses of igneous rock that

formed when magma deep in Earth becamecrystallized and eventually was exposed byerosion.

• An intrusive igneous body must have a surface

exposure greater than 100 square kilometers tobe considered a batholith. 

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Batholiths

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Types of Igneous Plutons