VOLCANOES Chapter 5

A. Introduction

1. Terms: Active, Dormant, Extinct

2. Eruptive Cycles

3. Cross-sectional view showing magma reservoir, magma ducts, vent

B. Types of Materials Involved pp. 85-88, p. 78

1. Pyroclastics: (dust), ash, cinders, bombs (blocks)

2. Lava: composition varies from rhyolitic to basaltic

3. Gases: water vapor by far most abundant dissolved gas

C. Viscosity p. 77 (back in Chap 4)

1. Definition (example of molasses vs. water)

2. Viscosity of magma (increases greatly with silica content)

3. Temperature effect on viscosity (also favors basaltic lava being more “fluid”)

4. Effect on trapping gases, pressure build up and eruptive characteristics

D. Classification of Volcanoes (Based on appearance and eruptive characteristics)

See pp 104-107 (but rely on notes for test information)

1. The 3 Types of Volcanoes

2. Composite Volcano Characteristics

a. Magma tends to be rhyolitic and andesitic (highly viscous)

b. Eruptions tend to be explosive; occasionally catastrophically explosive

c. Extrudes large amounts of both pyroclastics and lava

d. Long periods of dormancy between eruptions

e. Steep slopes

f. Large conical “mountains”

g. Not found in deep ocean basins (but can rest on continental shelf)

3. Shield Volcano Characteristics

a. Magma is always basaltic

b. Eruptions are not violent (explosive)

c. Extrudes mostly lava

d. Short periods of dormancy (frequently erupting)

e. Gentle slope

f. Extremely large due to large volumes of lava

g. Only type of volcano found in the deep ocean basin

4. Cinder Cone Volcano Characteristics

a. “One type event”

b. Steep cinder cone

c. Small (1500 feet or so is typical)

d. Often adjacent to an “apron” of (late stage) lava flows

E. Distribution of Volcanoes---most, but not all, volcanoes are associated with Plate Boundaries (see Fig. 5.17, p. 116 and read p. 115)

1. Circum-Pacific Belt = Ring of Fire (largest group of composite volcanoes)

2. Mediterranean Belt (composite)

3. Mid-Atlantic Ridge including Iceland (shield type)

4. Hawaiian Islands (shield type)

F. Examples of Volcanic Eruptions

1. Specific examples

a. Cinder Cone: Paracutin, Mexico, 1943

b. Shield: Kilauea

c. Composite: Mt. St. Helens 1980

2. Historic Catastrophic Composite Volcano Eruptions

a. Mt. Tamboro 1815---climate effect

b. Krakatoa 1883----tsunami

c. Mt. Pelee 1902----St. Pierre, Martinique

d. Vesuvius, 79 AD---Pompeii, Herculaneum

3. Geologic Evidence of “mega-catastrophic” eruptions

G. Volcanic Hazards (Volcanoes rank well behind earthquakes and floods as threats to life)

1. Major Specific hazards (see Table 5.1, p. 108 for historic eruptions & fatalities)

Composite Volcanoes (potential killers)

a. Ash

b. Pyroclastic Flows (defined a bit differently than the text)

c. Mudflows

d. Tsunami

Shield Volcanoes

e. Lava flows

2. Geographical areas in US subject to volcanic activity

a. Cascades: Major hazard (Mt. St. Helens 1980, Mt Lassen 1915)

b. Hawaiian volcanoes---primarily threat to property, not life

c. Alaska---Mt. Katmai 1912, Aleutians---low population density

d. Mono Lakes, California

e. Yellowstone (low probability)

f. Cinder Cones----SW desert areas of US