Chapter Overview

Chapters 3 and 4 cover igneous activity. Either may be read before the other. Chapter 3 emphasizes intrusive activity, but it also covers igneous rock classification and the origin of magmas, which are applicable both to volcanic and intrusive phenomena. Chaper 4 concentrates on volcnoes and related extrusive activity.

Volcanic eruptions, while awesome natural spectacles, also provide important information on the workings of the Earth's interior. Volcanic eruptions vary in nature and in degree of explosvie violence. A srong correlation exists between the chemical composition of magma (or lava) and the violence of an eruption. The size and shape of volcanoes and lava flows and their pattern of distribution on the Earth's surface also correspond to the composition of their lavas.

Understanding volcanism provides a background for theories relating to mountain building, the development and evolution of continental and oceanic crust, and how the crust is deformed. Our observations of volcanic activity fit nicely into plate-tectonic theory as described in chapter 3.

HOT! How did you spend your summer vacation? HOT!
Click on the link below to share the experiences of Geoscience students from the University of Arkansas and their professor, Dr. Glen Mattioli, as they survive an explosive volcanic eruption on the Caribbean island of Montserrat during July 2003.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::Montserrat, October 1997::http://comp.uark.edu/~mattioli/redo_plume3.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif">Montserrat, October 1997 (33574K)</a>Montserrat, October 1997

View Dr. Mattioli's photographs of the eruption and its aftermath here, here, or here.

Read a press release about the eruption here.

Read another press release about the eruption here.

Learn more about the Soufriere Hills volcano here.

Learn about the CALIPSO project here.

Contact the University of Arkansas Department of Geosciences here.

Learning Objectives

1. There is a strong correlation between magmatic (lava) chemistry and the violence of an eruption. Size and shape of volcanoes and associated features also reflect the composition of lavas. Lava is magma that has reached the earth's surface.

2. Volcanic activity can have a beneficial or catastrophic effect on humans. Weathered lavas produce fertile soils, lava fields may provide geothermal energy, eruptions may produce global cooling by reducing solar radiation.

3. Catastrophic eruptions have killed thousands in places such as Pompeii and Krakatoa, and threaten the Cascade region. Fatalities have increased in recent centuries because of increased population. Pyroclastic flows represent the greatest threat causing buildings to collapse or hitting people with fragments. Famines may follow eruptions from destruction of crops and animals.

4. Viscosity of lava is its resistance to flow. Viscosity reflects gas content and its ability to escape the molten rock. Temperature at extrusion, silica content, and amount of dissolved gas also control viscosity. Felsic lavas are very viscous, and are associated with the most violent eruptions. Mafic lavas have low viscosity and produce flows. Observation of active volcanism provides insight into past events.

5. Explosive volcanic eruptions produce great quantities of pyroclastic material, that may be released in dangerous flows. Water vapor is the most common gas released by volcanoes. Flows form from either outward exploding froth of gas and magma or gravitational collapse of a vertical column of gas and pyroclastic debris.

6. Igneous rocks are composed primarily of silicate minerals. Felsic rocks form from high viscosity magmas that are silica-rich, light and high in potassium, sodium and aluminum; rhyolite is the most common example. Mafic rocks form from low viscosity magmas that are silica-poor, dark, and exhibit high abundance of magnesium, iron and calcium; basalt is the most common example. Intermediate rocks are, of course, intermediate; andesite is the most common example.

7. Texture (size, shape and arrangement of grains). Grain size is the most important textural characteristic and it is controlled by cooling history and viscosity (Table 4.1). Fast cooling = fine-grained texture, and distinguishes extrusive rocks. Obsidian is volcanic glass that is not composed of minerals and reflects extremely rapid cooling of very viscous lavas. Porphyritic textures exhibit phenocrysts from the intrusive, slow-cooling magmatic stage, and matrix from the extrusive, rapid-cooling eruptive stage. Extrusive rocks are typically vesicular because decreased pressure releases gas from solution within the magma. Explosive eruptions produce significant pyroclastic material in the form of dust, ash, cinders, bombs and blocks (increasing size) that can form rocks tuff and volcanic breccia.

8. Volcanoes have a characteristic geomorphology including the cone, vent, and crater. Flank eruptions and caldera formation may occur. The three major types of volcanoes - shield, cinder cone, composite - also reflect composition of the lava (Table 4.2).

9. Shield volcanoes have low flank slopes that reflect low viscosity, quiet eruptions of basaltic lavas. Aa and pahoehoe are typical expressions of the basaltic composition of flows forming these cones.

10. Cinder cones have very high flank slopes that reflect pyroclastic debris formed because of the high gas content in magmas of any composition. Composite cones are constructed of alternating pyroclastic layers and lava. Most are composed of andesite and reflect the circum-Pacific belt. Their eruptions can be very violent. Volcanic domes may form from felsic lavas that are very viscous and are preceded by violent eruptions (e.g. Mt. St. Helens).

11. Not all volcanic eruptions result in cones. Plateau basalts form from very low viscosity lava floods and may exhibit columnar jointing. Submarine eruptions produce pillow basalts, particularly along mid-oceanic ridges.

Related Readings

Bullard, F. M. 1984. Volcanoes of the Earth. 2d ed. Austin: University of Texas Press.

Decker, R. W., and B. B. Decker. 1991. Mountains of Fire: The Nature of Volcanoes. New York: Cambridge University Press.

Fisher, R. V., G. Heiken, and J. B. Hulen. 1997. Volcanoes: Crucibles of Change. Princeton, NJ: Princeton University Press.

Francis, P. 1993. Volcanoes: A Planetary Perspective. New York: Oxford University Press.

Harris, S. L. 1989. Fire Mountains of the West: The Cascade and Mono Lake Volcanoes. Missoula, MT: Mountain Press.

Krafft, M. 1993. Volcanoes: Fire from the Earth. New York: Harry N. Abrams.

Maurice Krafft and his wife Katia were the world's foremost photographers of volcanoes before they were killed during the June 1991 eruption of Unzen Volcano, Japan.

Simpkin, T., and L. Siebert. 1994. Volcanoes of the World. 2d ed. Tucson, AZ: Geoscience Press.

Wright, T. L., and T. C. Pierson. 1992. Living with Volcanoes. U. S. Geological Survey Circular 1073.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 4:

Boxed Readings