Peek into the earth

Some of the greatest mysteries of the world lie right beneath your feet. Think about it: No one has really explored inside the Earth.

Oh, some deep holes have been dug. But the deepest is only about 7-1/2 miles. That's not much, considering it's about 4,000 miles to the center of the Earth.

We've landed on the moon 240,000 miles away. Why have we only scratched the surface of the Earth? Because it's too difficult, too hot, and too expensive. But that doesn't mean people don't wonder what's down there.

In the 16th century, English astronomer Edmund Halley (of Halley's Comet fame) theorized that the Earth was hollow and had a luminous core at least partly filled with gas.

His vision inspired some fascinating fictional accounts. The most famous, "Journey to the Center of the Earth," was published by Jules Verne in 1864. The book chronicles the adventures of a geologist and his nephew as they descend into a volcanic crater. They encounter prehistoric beasts, spewing lava – even the lost city of Atlantis.

Even into the 1900s, there were some pretty kooky theories about the Earth's interior. One envisioned a place where billions of people might live. Another anticipated a space with a small sun, which could be reached through openings at the Earth's poles. (The explorer, Adm. Richard Byrd, disproved this when he flew over both poles in the 1920s and found no large holes.)

Today scientists largely agree on the basic structure of the Earth. It does not include alien hideouts or underworlds of any kind. Some diagrams compare the layers of the Earth to an egg (a round egg, mind you). The yolk represents the core, the egg white the mantle (a semisolid layer of rock). The shell of the egg is the crust, the thin brittle "skin" on which we live.

That's the theory, anyhow. It's based on scientists' observations of earthquakes and volcanoes. It's also based on experiments using small amounts of material and subjecting them to high heat and pressure. That's the only way researchers can get an idea of the effect of conditions deep underground.

Eric Muller, who works for the Exploratorium, a science museum in San Francisco, says the inner core may be as hot as the surface of the sun – about 6,000 degrees Celsius (11,000 degrees F.). The inner core is also under extreme pressure, an estimated 3.6 million times as much pressure as on the surface of the Earth. Because of this pressure, the core of the Earth is thought to be a solid metal ball surrounded by a slightly cooler molten outer core.

Scientists theorize that after the Earth formed billions of years ago it began to cool slowly. Denser materials – iron and nickel, common to our Solar System – sank into the core. Lighter elements floated near the top.

If the Earth were an apple, the crust would be as thin as the peel. So far, no one has even drilled through the crust. The most ambitious attempt to do so began in 1958 and ended in 1966. "Project Mohole" was to be Earth sciences' answer to the space program.

Project Mohole's goal was to reach the point where the crust and upper mantle meet. This region is called the Mohorovicic Discontinuity, after Croatian geologist Andrija Mohorovicic, the first to suggest its existence.

Because the crust is thinnest on the ocean floor, Project Mohole began drilling off the coast of Mexico, 11,700 feet underwater. The drill reached 601 feet below the seafloor. It stopped short of the "Moho." Congress ended the project.

Deeper holes have been drilled on land, including oil wells of 20,000 feet. A research borehole on the Kola Peninsula in northeastern Russia was twice that depth.

Miners in South Africa routinely go deeper into Earth than any other humans. The deepest gold mine there is 2-1/8 miles below the surface. Temperatures can be unpleasant at that depth, reaching 120 degrees F.

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