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A Closer Look

Seismic Tomography
he CAT scan is a common diagnostic on Earth’s surface. Thus, a picture of Earth’s groups—are needed to construct a pic-
Timaging procedure that combines the interior can be made by mapping variations ture of Earth’s interior. A really fast com-
use of X rays with computer technology. in seismic wave speeds. puter may take days to process this much
CAT stands for “computed axial tomog- Suppose an earthquake occurs and a data and construct cross sections through
raphy,” and the word tomography means number of seismic stations record when the some in teresting place, such as a subduc-
“drawing slices.” The CAT scan shows S- and P-waves arrive. From these records, tion zone where an oceanic plate dives into
organs of interest by making X-ray images you could compare the seismic velocity the mantle.
from many different angles as the source data and identify if there were low seismic Seismic tomography has also identi-
of the X rays moves around the patient. velocities between the source and some of fied massive plumes of molten rock rising
CAT scan images are assembled by a the receivers. The late arrival of seismic toward the surface from deep within Earth.
computer into a three-dimensional picture waves could mean some difference in the These superplumes originate from the base
that can show organs, bones, and tissues in structure of Earth between the source and of the mantle, rising to the lithosphere. The
great detail. the observing station. When an earthquake hot material was observed to spread out
The CAT scan is applied to Earth’s interior occurs in a new location, more data will horizontally under the lithosphere toward
in a seismic tomography procedure. It works be collected at observing stations, and this mid-ocean ridges. This may contribute to
somewhat like the medical CAT scan, provides additional information about the tectonic plate movement. Regions above
but seismic tomography uses seismic waves shape and structure of whatever is slowing the superplumes tend to bulge upward, and
instead of X rays. The velocities of S and the seismic waves. Now, imagine repeating other indications of superplumes, such as
P seismic waves vary with depth, changing these mea surements for many new earth- variations in gravity, have been measured.
with density, temperature, pressure, and the quakes until you have enough data to paint Scientists continue to work with new seis-
composition of Earth’s interior. Interior dif- a picture of what is beneath the surface. mic tomography data with more precise
ferences in temperature, pressure, and com- Huge amounts of earthquake data— images and higher resolutions to better
position cause the motion of tectonic plates perhaps 10 million data points in 5 million describe the interior structure of Earth.

lithosphere is also known as the “strong layer” in contrast to the from animals and plants that lived in Pangaea, which later broke
“weak layer” of the asthenosphere. The lithosphere includes the up and split into smaller continents. Wegener’s concept came to
entire crust, the Moho, and the upper part of the mantle. As you be known as continental drift, the idea that individual continents
will see next in the section on the theory of plate tectonics, the could shift positions on Earth’s surface. Some people found the
asthenosphere is one important source of magma that reaches idea of continental drift plausible, but most had difficulty imag-
Earth’s surface. It is also a necessary part of the mechanism ining how a huge and massive continent could “drift” around on
involved in the movement of the crust. The lithosphere is made a solid Earth. Since Wegener had provided no good explanation
up of comparatively rigid plates that are moving, floating in the of why or how continents might do this, most scientists found
upper mantle like giant ice sheets floating in the ocean. the concept un acceptable. The concept of continental drift was
dismissed as an interesting but odd idea. Then new evidence dis-
18.3 THEORY OF PLATE TECTONICS covered in the 1950s and 1960s began to indicate that the conti-
nents have indeed moved. The first of this evidence would come
If you observe the shape of the continents on a world map or from the bottom of the ocean and would lead to a new, broader
a globe, you will notice that some of the shapes look as if they theory about the movement of Earth’s crust.
would fit together as do the pieces of a puzzle. The most obvi-
ous is the eastern edges of North and South America, which
seem to fit the western edge of Europe and Africa in a slight EVIDENCE FROM EARTH’S MAGNETIC FIELD
S-shaped curve. Such patterns between continental shapes seem Earth’s magnetic field is probably created by electric currents
to suggest that the continents were at one time together, break- within the slowly circulating liquid part of the iron core. How-
ing apart and moving to their present positions some time in the ever, there is nothing static about Earth’s magnetic poles. Geo-
past (Figure 18.8). physical studies have found that the magnetic poles are moving
In the early 1900s, a German scientist named Alfred Wegener slowly around the geographic poles. Studies have also found
became enamored with the idea that the continents had shifted that Earth’s magnetic field occasionally undergoes magnetic
positions and published papers on the subject for nearly two reversal. Magnetic reversal is the flipping of polarity of Earth’s
decades. Wegener supposed that at one time there was a single magnetic field. During a magnetic reversal, the north mag-
large landmass that he called Pangaea, which is from the Greek netic pole and the south magnetic pole exchange positions. The
meaning “all lands.” He pointed out that similar fossils found in present magnetic field orientation has persisted for the past
landmasses on both sides of the Atlantic Ocean today must be 700,000 years and, according to the evidence, is now preparing

18-7 CHAPTER 18 Plate Tectonics 461