chapter 4 test review

• P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so

they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and

unsqueezing earth materials as they travel. This produces a change in volume for the material. P-waves bend

slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid

material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down.

This makes the P-waves arrive later and further away than would be expected. The result is a P-wave shadow

zone. No P-waves are picked up at seismographs

• S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second,

and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of

wave travel. This produces a change in shape for the earth materials they move through. Only solids resist a

change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid.

By tracking seismic waves, scientists have learned what makes up the planet’s interior (Figure 4.2).

• P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle.

• S-waves disappear at the mantle core boundary, so the outer core is liquid.

At the planet’s center lies a dense metallic core. Scientists know that the core is metal because:

1. The density of Earth’s surface layers is much less than the overall density of the planet, as calculated from

the planet’s rotation. If the surface layers are less dense than average, then the interior must be denser than

average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the

remaining 15%.

2. Metallic meteorites are thought to be representative of the core. The 85% iron/15% nickel calculation above

is also seen in metallic meteorites