Fire Within: Igneous Activity begins with a description of the catastrophic eruption of Mount St. Helens in May 1980. A discussion of volcanism and the factors that determine the nature of volcanic eruptions (magma composition, temperature, and amount of dissolved gases) is followed by an examination of the materials that can be extruded during an eruption. The types of volcanic cones, their origins, shapes, and compositions, as well as the nature of fissure eruptions and volcanic landforms, are also presented.

An examination of intrusive igneous activity includes the classification and description of the major intrusive igneous bodies–dikes, sills, laccoliths, and batholiths. The chapter closes with a discussion of the relations between igneous activity and plate tectonics.

Learning Objectives

After reading, studying, and discussing the chapter, students should be able to:

· Factors that determine the violence of volcanic eruptions.

· Materials that are extruded from volcanoes.

· Features produced by volcanic activity.

· Intrusive igneous features.

· Relation between igneous activity and plate tectonics.

Chapter Outline___________________________________________________________________

I. Volcanic eruptions

A. Factors that determine violence

1. Composition of magma

2. Temperature of magma

3. Dissolved gases in magma

B. Magma viscosity

1. Resistance to flow

2. Factors affecting viscosity

a. Temperature–hotter–less viscous

b. Composition (silica content)

1. High silica–high viscosity–granitic lavas are very viscous

2. Low silica–more fluid (basaltic lavas)

c. Dissolved gases

1. Mostly steam and carbon dioxide

2. Expand near surface

3. Provide force to extrude lava

4. Violence related to ease of escape

a. Easy from fluid magma

b. Viscous magma more violent

II. Materials associated with volcanic eruptions

A. Lava flows

1. Basaltic lavas more fluid

2. Types of lava

a. Pahoehoe–resembles braids in ropes

b. Aa–rough, jagged blocks

B. Gases

1. One to five percent of magma weight

2. Mostly water vapor and carbon dioxide

C. Pyroclasts

1. "Fire fragments"

2. Types of pyroclastic material

a. Ash–fine, glassy fragments

b. Pumice–from "frothy" lava

c. Lapilli–"walnut" size

d. Cinders–"pea-sized" with voids

e. Particles larger than lapilli

1. Blocks–hardened lava

2. Bombs–ejected as hot lava

D. Nueé Ardente

1. Fiery cloud

2. Hot gases infused with ash

3. Speeds up to 200 km per hour

III. Volcanoes

A. General features

1. Crater–steep-walled depression at summit

2. Vent–connects crater to magma chamber

3. Caldera–a large summit depression, greater than 1 km diameter

B. Types of volcanoes

1. Shield volcano

a. Broad, slightly domed

b. Primarily of basaltic (fluid) lava

c. Generally large

d. Produce large volume of lava

e. i.e. Mauna Loa in Hawaii

2. Cinder cone

a. Built from ejected lava fragments

b. Steep slope angle

c. Rather small

d. Frequently occur in groups

3. Composite cone (or stratovolcano)

a. Most encircle Pacific Ocean

b. Large

c. Interbedded lavas and pyroclastics

d. Most violent type of activity

e. e.g. Mt. Rainer, Mt. Shasta

IV. Volcanic landforms

A. Volcanic necks

1. Resistant vents left standing after erosion

2. i.e. Ship Rock, New Mexico

B. Craters and calderas

C. Fissure eruptions and lava plateaus

1. Material extruded from fractures

2. e.g. Columbia Plateau

D. Pyroclastic flows

1. From silica-rich magma

2. Consist of ash and pumice fragments

3. Propelled from vent at high speed

4. e.g. Yellowstone plateau

V. Intrusive igneous activity

A. Magma emplaced at depth

B. Underground features called plutons

C. Classified according to

1. Shape

a. Tabular (sheetlike)

b. Massive

2. Orientation with respect to host rock

a. Discordant–cut across sedimentary beds

b. Concordant–parallel to sedimentary beds

D. Types of igneous intrusive features

1. Dike–tabular, discordant

2. Sill

a. Tabular, concordant

b. e.g. Palisades Sill, NY

3. Laccolith

a. Forms in same way as sill

b. Lens shaped mass

c. Arches overlying strata upward

4. Batholith

a. Largest intrusive body

b. Surface exposure 100 square km plus (smaller bodies are termed stocks)

c. Frequently form cores of mountains

VI. Igneous activity and plate tectonics

A. Distribution of igneous activity is not random

1. Granitic or andesitic rocks are found on the continents and continental margins

2. Volcanoes in deep-ocean basins extrude basaltic lavas

B. Global patterns of igneous activity

1. Plate margins

a. Oceanic ridge spreading centers

1. Lithosphere pulls apart

2. Less pressure on underlying rocks

3. Partial melting

4. Large amounts of basaltic magma

b. Convergent plate margins

1. Subduction zones (trenches)

2. Descending plates partially melt

3. Magma slowly rises upward

4. Rising magma can form

a. An island arc in the ocean

b. Andesitic-granitic volcanoes on a continent

5. Associated with Pacific Basin

a. Called "Ring of Fire"

2. Intraplate volcanism

a. Activity within a rigid plate

b. Basaltic magma source

1. Partial melting of mantle rock

2. Plumes of hot mantle material

a. Form hot spots on surface

b. One may be below Hawaii

c. Granitic magma source (remelting of continental crust over a mantle plume)