4. Third rock from the Sun - restless Earth
Journey to the center of the Earth
Looking inside the Earth's hidden interior
The internal structure of the Earth can be mapped with the help of earthquake waves. The Greek word for earthquake is seismos, meaning "to quake or tremor". Today, earthquake waves are often called seismic waves, and the study of earthquakes is known as seismology.
The Earth is layered inside like a peach. Its deeper layers are more dense, and they are separated from one another in sharp transitions. There are three major parts: (1) the rocky crust, (2) a mantle of hot, plastic rock, and (3) the dense core. They are the skin, pulp, and pit of the Earth, so to speak. The core has a liquid outer component and a solid inner one.
Most earthquakes occur just beneath the Earth's surface, when massive blocks of rock grind, lurch and slide against one another. The reverberations resemble ripples spreading out from a disturbance on the surface of a pond. These waves move in all directions and their arrivals at various places on the Earth can be detected by seismometers. By comparing the arrival times at several seismic observatories, geologists can pinpoint the origin of the waves, the focus, and trace their motions through the Earth.
By careful mapping of the patterns of many earthquakes that travel to different depths, seismologists have peeled away the Earth's outer layers and looked at various levels within it. It is similar to using an ultrasonic scanner to map out the shape of an unborn infant in a mother's womb, and somewhat like using Computed Axial Tomography (CAT) scans to derive clear views of the insides of living bodies from the numerous readings of X-rays that cross through the body from different directions.
The Earth's core reaches about half way to the surface, implying a volume that is one-eighth that of the entire Earth. If the mass density of the Earth were uniform, the core would have an equal share, one-eighth, of the mass of the Earth, but its actual mass is nearly three times greater. So the core's mass density is very high, and this points to iron as the most likely material. Iron has been identified as the main ingredient of the core because it is the most abundant heavy element in the Sun and in some meteorites, and also because laboratory measurements show that the densities and seismic-wave velocities of the core are more closely matched by iron than any other element.
Examination of earthquake waves has shown that there are two cores, an inner, crystalline solid core and an outer fluid one. The two cores are very different is size. The solid inner core has a radius of about 1.22 million meters, which is slightly smaller than the Moon whose radius is 1.74 million meters. The outer fluid core is about 3.48 million meters in radius, or 55 percent of the Earth's radius.
The Earth's inner core is a solid lump of iron suspended at the center of the much larger, fluid outer core, something like a golf ball levitated in the middle of a fish bowl. The outer core is itself curtained behind about 3 million meters of solid rock. So, the solid heart of iron is difficult to observe, but faint seismic vibrations in the ground have been used to look at it.
The seismic vibrations that pierce the inner core move through it at different speeds that depend on their direction, faster on polar north-south paths than equatorial east-west ones. This directional dependence of seismic-wave velocities is explained by the crystalline structure of the inner core. The crystals give the solid inner core a texture with a preferred orientation, like the grain in wood. By lining up along the Earth's spin axis, iron crystals make the inner core stiffer along this axis, thus making sound waves travel faster in this direction.
Recordings of weak earthquake rumbles, that have traveled through the central core of the Earth, indicate that it spins faster than the outer Earth, but that they both rotate in the same direction. The fast lane for seismic waves is tipped slightly with respect to the Earth's north-south axis, and it moves around it. This shift in orientation means that the crystalline globe at the center of the Earth is turning slowly within its solid rocky and liquid metal enclosure. It is spinning with respect to the Earth's surface at between 0.2 and 0.3 degrees per year, completing one lap in between 1,200 and 1,800 years.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University