Peek into the earth

Drilling and mining provide information about the Earth's crust, but it's the movement of Earth's crust that reveals most about the layers below. The crust is made up of continent-size plates that float on a layer of molten rock beneath. Where these "tectonic plates" meet, earthquakes and volcanoes are most likely to occur. Earthquakes help scientists "see" into the Earth.

Alan Kafka, a professor at Boston College, studies the movements of these plates. He conducts research at the Weston (Mass.) Observatory near Boston. Some people call him "Dr. Quake" or "Seismo-man." "Seismos" is Greek for "shock." The study of earthquakes is called "seismology." How does seismology help us see inside the Earth?

To explain, Dr. Kafka leads a reporter to a display case in a hallway outside his campus office. Inside is a seismograph, a scientific instrument which ever-so-slowly records vibrations it detects in the Earth. The vibrations are recorded by a zigzag line on the recording paper mounted on a drum.

When the ground shakes hard during a big earthquake, the recording needle of the seismograph moves way up and down, drawing dramatic spikes. Milder, unfelt quakes occur more often. They look as if the line hiccuped.

Students who see this seismograph sometimes are surprised to learn that it may be recording vibrations that occurred far from Boston, in New York, El Salvador, or the other side of the globe.

When an earthquake occurs, two kinds of seismic waves fan out through the ground. One kind (longitudinal P-waves, for "primary" or "pressure") travels through rock or soil at 3.8 miles per second, and through water much more slowly. Shear or S-waves travel at only 1.9 miles per second.

Knowing this, scientists can make an X-ray of sorts using data collected by seismographs around the globe. By comparing when waves are received at various points, they can determine the thicknesses and densities of the material the waves pass through.

The waves go faster through hard dense rock, and slow down in softer rock. Where the layers change (lighter-density rocks atop more dense ones, for example), seismic waves may bounce back or change direction.

By studying seismic waves' direction and speed, scientists have figured out about how deep Earth's layers are. (See diagram in PDF.) As scientists continue to study the Earth, they may someday be better able to predict earthquakes, volcanic eruptions, even a switch in the magnetic poles' charges – which, they say, has happened before.

Earth, by Susanna Van Rose (Dorling Kindersley, 2000). This 'Eyewitness' book is richly illustrated and well-organized, offering a host of topics including the Earth's composition and age.

The Kingfisher Young People's Book of Planet Earth, by Martin Redfern (Kingfisher Books, 1999). Though written for 9-to 12-year-olds, the text and wonderful illustrations attract older readers, too. It's organized into seven major topics and includes a glossary and list of facts.

Earthquakes, by Sally Walker (Carolrhoda Books, 1996), ages 9 to 12. Where do earthquakes occur? How are they measured? How can you prepare for them?

Journey to the Center of the Earth, by Jules Verne (various publishers). A science-fiction classic, first published in 1864. A professor and his geologist nephew explore a dormant volcano in Iceland.

www.exploratorium.edu/faultline

San Francisco's Exploratorium science museum presents 'Life Along the Faultline.' This section explains the how and why of earthquakes and provides a week's worth of webcasts by a research team traveling along California's San Andreas Fault.

interactive2.usgs.gov/learningweb

The 'Explorers' section of this United States Geological Survey webpage has a wealth of student-friendly project ideas and homework aids.

SOURCE: Wright Center for Innovative Science Education, Tufts University

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