Chapter Overview

Geology uses the scientific method to explain natural aspects of the Earth - for example, how mountains form or why oil resources are concentrated in some rocks and not in others. This chapter briefly explains how and why Earth's surface, and its interior, are constantly changing. It relates this constant change to the major geological topics of interaction of the atmosphere, water and rock, the modern theory of plate tectonics, and geologic time. These concepts form a framework for the rest of the book. Understanding the "big picture" presented here will aid you in comprehending the chapters that follow.

Learning Objectives

1. Geology is the scientific study of the earth. Physical geology is that division of geology concerned with earth materials, changes in the surface and interior of the earth, and the dynamic forces that cause those changes.

2. Earthquakes, like Northridge, reflect the sudden release of energy along faults that respond to plate motions.

3. Not all geologic hazards are immediately apparent. For example, most of the deaths associated with the 1985 eruption of the Nevado del Ruiz volcano in Columbia were caused by a mudflow.

4. Western economic systems depend on abundant supplies of cheap energy.

5. Most geological resources are nonrenewable and their extraction poses potential ecologic damage.

6. Knowledge of geology can enhance appreciation of one's surroundings, such as the scenery produced as mountains are eroded.

7. Earth processes are driven by two heat engines: one that is internal powered by heat from the earth's core; one that is external powered by the sun.

8. The earth's interior comprises three concentric zones: crust (thin, oceanic crust and thicker, continental crust), mantle (solid and thickest zone), and core. Lithosphere is the crust and upper mantle that is broken into plates. Asthenosphere is the soft, "lubricating" layer beneath lithosphere upon which the plates move. Tectonic forces cause vertical and horizontal deformation from forces within the earth.

9. Plate tectonics is a theory that views the earth's lithosphere as broken into plates that are in motion. At mid-oceanic-ridges, plate boundaries are diverging because magma rises from the asthenosphere, pushes the ridge crests apart, and solidifies in the fissures created. Ridges spread at a rate of 1-18 centimeters per year. Transform boundaries occur where plates slide past each other, such as the San Andreas fault. Converging boundaries reflect subduction, where oceanic plates descend into the mantle creating either extrusive or intrusive igneous rocks from melting at depth. Metamorphic rocks may be formed from high-temperature and pressure at subduction zones, if melting does not occur.

10. Rocks formed within the earth are pushed to the surface by tectonic forces. They are unstable and reach equilibrium as new materials formed by the affects of water, solar heating and other surficial processes. Removal of this material, called sediment, by agents of erosion transports it to a site of deposition where it may become sedimentary rock, when lithified.

11. Geology follows the scientific method (see Box 1.3).

12. Geology involves deep time, vastly greater than human lifetimes or even human contemplation. The earth is about 4.5 billion years old. Most geological processes are slow and take place over many million years. Fast, to a geologist, is an event or process completed in a million years or less. Plate motions are relatively fast. Complex life forms have existed on the earth for the past 545 million years. Humans have only been on earth for about 3 million years. Geology follows the scientific method (see Box 1.3).

Related Readings

Drake, E. T., and W. M. Jordan, eds. 1985. Geologists and Ideas: A History of North American Geology. Boulder, CO: Geological Society of America.

McPhee, J. 1981. Basin and Range. New York: Farrar, Straus and Giroux.

This is one of several outstanding books about geology written for the layman by John McPhee.

Moores, E. M., ed. 1990. Shaping the Earth: Tectonics of Continents and Oceans. New York: W. H. Freeman and Company.

Nuhfer, E. B., R. J. Proctor, and P.H. Moser. 1993. The Citizens Guide to Geologic Hazards. Arvada, CO: American Institute of Professional Geologists.

Officer, C. B. and J. Page. 1993. Tales of the Earth: Paroxysms and Perturbations of the Blue Planet. New York: Oxford University Press.

Pirsig, R. M. 1974. Zen and the Art of Motorcycle Maintenance. New York: Bantam Books (paperback).

This book contains an exceptionally good exposition of the scientific method as well as considerable insight into philosophy of science.

Rhodes, F. H. T., and R.O. Stone, eds. 1981. Language of the Earth. New York: Pergamon Press.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 1:

8.C, 9.C, 10.D, 11.A, 12.G, 13.B, 14.D, 15.C, 16.D, 17.D, 18.C, 19.A,B

Boxed Readings