Reading 1

EARTH’S INTERIOR

1                                 Earth’s immense interior has never been explored directly, and its mysteries have long been a subject of imagination and conjecture. One seventeenth–century map depicted the planet’s interior as a cavern with numerous chambers, each filled with air, water, or fire. Two centuries later, in the novel Journey to the Center of the Earth, Jules Verne described the subterranean world as a yawning abyss filled with giant sea serpents and other horrifying creatures. Many prominent scientists proposed hypotheses about Earth’s structure, including Edmond Halley, who in 1692 described the interior as mostly hollow, with three concentric shells rotating around a core. According to Halley, each shell had its own magnetic poles, and gaseous atmospheres separated the shells.

2                                 Today, scientists believe that Earth has a solid iron core, approximately the size of the moon, with an outer core of liquid iron and nickel, surrounded by a solid mantle and a crust of tectonic plates. Most knowledge about the interior comes from analysis of variations in the speed of seismic waves recorded during earthquakes. The invention of the seismograph in 1875 was critical in the development of a detailed picture of the planet’s structure. By the early twentieth century, a global network of seismographs allowed scientists to record earthquake activity around the world. When an earthquake occurs, it releases seismic waves that race through the planet’s body. These waves are measured by the thousands of seismographs worldwide. Seismic waves tend to travel in a straight line and at an unchanging velocity as long as they pass through a homogeneous medium at constant temperature and pressure. Changes in speed indicate that the waves are passing through materials of varied composition and structure at different temperatures and pressures.

3                                 During an earthquake, some seismic waves move across the surface, causing great damage. Other, less destructive waves travel through the body of the planet. Those traveling through the body are classified as either primary waves (P–waves) or secondary waves (S–waves). The speeds and paths of both types of body waves vary with the density and elasticity of the materials they encounter. P–waves travel very quickly and are the first waves to reach seismographs. Solid, liquid, or gaseous materials conduct P–waves at different rates. When P–waves pass through solid rock, an elastic medium, the rock is compressed and then quickly returns to its original volume, which causes the waves to accelerate in velocity. In contrast, when P–waves enter a liquid or a gas, which are not elastic, the waves decrease in speed. S–waves move at about half the speed of P–waves and are slower in reaching seismographs. As S–waves undulate through the planet, they create shear forces that tear rocks apart. S–waves can pass only through solid materials. Liquids are not elastic, and hence cannot deform and then return to their original shape; they simply flow away from the shearing stress.

4                                 Earth’s interior is filled with layers of rocks and metals, which are subjected to increasing heat and pressure at deeper levels. By analyzing the data from seismic waves, scientists have been able to identify the materials that compose each layer. When seismic waves move between layers, their paths suddenly change. When they meet a boundary between regions differing in density, the waves are deflected from their paths. As waves travel deeper through the interior, they increase in velocity, and the deeper segment of a wave front travels faster than segments that are less deep. Any given layer becomes more rigid with increasing depth, and the velocity of seismic waves increases with the rigidity of the materials composing the layer. Seismic waves increase in speed as they pass through colder regions, where the rocks are more rigid than surrounding rocks. Conversely, waves slow down when they pass through warmer regions, where the rocks are less rigid than surrounding rocks.

5                                 Through the process of seismic tomography, which uses seismic waves to provide three– dimensional images, scientists now have a detailed map of Earth’s interior. By synthesizing data from hundreds of thousands of earthquakes, computers have generated a sequence of cross-sectional views, which together form a three–dimensional image of the planet. Seismic tomography reveals that the outer surface of Earth’s core is not smooth and featureless, as scientists once believed. In some places the outer core projects upward into the mantle, while in other places the mantle extends downward into the liquid outer core.