Just as a child may shake an unopened present in an attempt to discover the contents of a gift, so man must listen to the ring and vibration of our Earth in an attempt to discover its content. This is accomplished through seismology, which has become the principle method used in studying Earth's interior. Seismos is a Greek word meaning shock; akin to earthquake, shake, or violently moved. Seismology on Earth deals with the study of vibrations that are produced by earthquakes, the impact of meteorites, or artificial means such as an explosion. On these occasions, a seismograph is used to measure and record the actual movements and vibrations within the Earth and of the ground. Scientists categorize seismic movements into four types of diagnostic waves that travel at speeds ranging from 3 to 15 kilometers (1.9 to 9.4 miles) per second. Two of the waves travel around the surface of the Earth in rolling swells. The other two, Primary (P) or compression waves and Secondary (S) or shear waves, penetrate the interior of the Earth. Primary waves compress and dilate the matter they travel through (either rock or liquid) similar to sound waves. They also have the ability to move twice as fast as S waves. Secondary waves propagate through rock but are not able to travel through liquid. Both P and S waves refract or reflect at points where layers of differing physical properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities. |
Types of seismic waves
(Adapted from, Beatty, 1990.)|
Divisions in the Earth's Interior
(Adapted from, Beatty, 1990.)|
Seismic discontinuities aid in distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and continental). Lateral discontinuities also have been distinguished and mapped through seismic tomography but shall not be discussed here. * Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 kilometers (3,219 - 3,981 miles) The inner core is solid and unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids when temperature decreases or pressure increases. * Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 kilometers (1,806 - 3,219 miles) The outer core is a hot, electrically conducting liquid within which convective motion occurs. This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents known as the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulfur and/or oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron. * D": 3% of Earth's mass; depth of 2,700-2,890 kilometers (1,688 - 1,806 miles) This layer is 200 to 300 kilometers (125 to 188 miles) thick and represents about 4% of the mantle-crust mass. Although it is often identified as part of the lower mantle, seismic discontinuities suggest the D" layer might differ chemically from the lower mantle lying above it. Scientists theorize that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density. * Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 kilometers (406 -1,806 miles) The lower mantle contains 72.9% of the mantle-crust mass and is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminum. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites. *...