8/4/2019 ch 4 10e 6 slide handout

1/8

1/9/200

Essentials of Geology, 9e

Volcanic Activity

Chapter 4

InstructorJennifer BarsonSpokane Falls Community College

Geology 101

The Nature of Volcanic Eruptions

Factors determining the violence orexplosiveness of a volcanic eruption

Composition of the magma Temperature of the magma

Dissolved gases in the magma

The above three factors actually controlthe viscosity of a given magma which inturn controls the nature of an eruption

The Nature of Volcanic Eruptions

Viscosity is a measure of a materials resistance toflow (e.g., Higher viscosity materials flow with greatdifficulty)

3 Factors affecting viscosity

Temperature - Hotter magmas are less viscous

Composition - Silica (SiO2) content Higher silica content = higher viscosity

(e.g., felsic lava such as rhyolite)

Lower silica content = lower viscosity or more fluid-like

behavior (e.g., mafic lava such as basalt)

The Nature of Volcanic Eruptions

Factors affecting viscosity, continued Dissolved Gases

Gas content affects magma mobility

Gases expand within a magma as it nears theEarths surface due to decreasing pressure

The violence of an eruption is related to howeasily gases escape from magma

The Nature of Volcanic Eruptions

Factors affecting viscosity, continued

In Summary

Fluid basaltic lavas generally produce

quiet eruptions

Highly viscous lavas (rhyolite or andesite)produce more explosive eruptions

Materials extruded froma volcano

Lava Flows Basaltic lavas are much more fluid

Types of basaltic flows Pahoehoe lava (resembles a twisted or ropey

texture)

Aa lava (rough, jagged blocky texture)

Dissolved Gases One to six percent of a magma by weight

Mainly water vapor and carbon dioxide

8/4/2019 ch 4 10e 6 slide handout

2/8

1/9/200

aa flow

Figure 4.7 A & B

pahoehoeMaterials extruded from

a volcanoPyroclastic materials Fire fragments

Types of pyroclastic debris

Ash and dust - fine, glassy fragments Pumice - porous rock from frothy lava

Lapilli - walnut-sized material

Cinders - pea-sized material

Particles larger than lapilli

Blocks - hardened or cooled lava

Bombs - ejected as hot lava

Figure 4.9

Volcanoes

General Features

Opening at the summit of a volcano

Crater - steep-walled depression at thesummit, generally less than 1 km in diameter

Caldera - a summit depression typicallygreater than 1 km in diameter, produced bycollapse following a massive eruption

Vent opening connected to the magma

chamber via a pipe

Volcanoes

3 Types of Volcanoes

Shield volcano

Broad, slightly domed-shaped

Composed primarily of basaltic lava

Generally cover large areas

Produced by mild eruptions of large volumesof lava

Mauna Loa on Hawaii is a good example

Mauna Loa a shield volcano

Figure 4.12

8/4/2019 ch 4 10e 6 slide handout

3/8

8/4/2019 ch 4 10e 6 slide handout

4/8

1/9/200

Figure 4.1a

A nueardente

on Mt. St.Helens

Figure 4.20

Figure 4.1b Figure 4.2

Figure 4.22

8/4/2019 ch 4 10e 6 slide handout

5/8

1/9/200

Formation

of Crater

Lake,

Oregon

Caldera

Figure 4.22

A Size Comparison

Copyright 2006 Pearson Prentice Hall, Inc.

Other Volcanic Landforms

Fissure eruptions and lava plateaus

Fluid basaltic lava extruded from crustalfractures called fissures

e.g., Columbia River Plateau

Lava Domes

Bulbous mass of congealed lava

Most are associated with explosive

eruptions of gas-rich magma

The Columbia River basalts

Figure 4.23

The ColumbiaRiver basalts

Figure 4.24

A lava dome on Mt. St. Helens

Copyright 2006 Pearson Prentice Hall, Inc.

Figure 4.25

8/4/2019 ch 4 10e 6 slide handout

6/8

1/9/200

Other Volcanic Landforms

Volcanic pipes and necks

Pipes are short conduits that connect amagma chamber to the surface

Volcanic necks (e.g., Ship Rock, NewMexico) are resistant vents left standingafter erosion has removed the volcaniccone

Formation of a volcanic neck

Figure 4.27

Plutonic Igneous Activity

Most magma is emplaced at depth withinthe Earth

An underground igneous body, oncecooled and solidified, is called a pluton

Classification of plutons Shape

Tabular (sheetlike)

Massive

Orientation with respect to the host(surrounding) rock Discordant cuts across sedimentary rock

units

Concordant parallel to sedimentary rockunits

Plutonic Igneous Activity

Plutonic Igneous Activity

Types of intrusive igneous features

Dike a tabular, discordant pluton

Sill a tabular, concordant pluton (e.g.,

Palisades Sill in New York)

Laccolith

Similar to a sill

Lens or mushroom-shaped mass

Arches overlying strata upward

Batholith Largest intrusive body

Intrusive Igneous Structures

Figure 4.28 A

8/4/2019 ch 4 10e 6 slide handout

7/8

1/9/200

Intrusive igneous structuresexposed by erosion

Figure 4.23 B

Sill - Salt River Canyon, Arizona

Figure4.30

Plutonic Igneous Activity

Intrusive igneous features continued

Batholith

Largest intrusive body

Surface exposure of 100+ square kilometers(smaller bodies are termed stocks)

Frequently form the cores of mountains

Figure 4.28

A batholithexposed by

erosion

Plate Tectonics and Igneous Activity

Global distribution of igneous activity isnot random

Most volcanoes are located within or nearocean basins

Basaltic rocks are common in bothoceanic and continental settings, whereasgranitic rocks are rarely found in theoceans

Active volcanoes are often associatedwith plate boundaries

Plate Tectonics and Igneous Activity

Subduction zone (convergent)

Rising magma can form either An island arc if in the ocean

A volcanic arc if on a continental margin

Spreading centers (divergent) The greatest volume of volcanic rock is produced

along the oceanic ridge system

Intraplate volcanism Activity within a tectonic plate, associated with

plumes of heat in the mantle

Form localized volcanic regions in the overridingplate called a hot spot

8/4/2019 ch 4 10e 6 slide handout

8/8

1/9/200

Distribution of some of theworlds major volcanoes

Figure 4.33Figure4.18

Volcanism on a tectonicplate moving over a hot spot

Figure 4.35

Figure4.34

Figure4.34

End of Chapter 4