Earth's Core |
It seems amazing that we know so much about the interior of the Earth, considering that we have actually been down only a few miles with drilling rigs. As chapter 4 describes, we know the Earth's mass, density, temperatures and much else about the constituents of a inner Earth we most likely will never see firsthand. We have at least a good idea about the Earth's Crust, Mantle, the molten Outer Core and the solid Inner Core. As described in section 4.2, it's done with something called "remote sensing." Remove sensing really started to come into its own in the 1930s with the development of radar. A radio wave directed toward an airplane or thunderstorm is reflected back to the transmitter. The direction of the plane or storm is then immediately known, and since we know the speed of the signal (that of light), all we need do to calculate the distance is to track the time between signal transmission and the receipt of the reflected waves.
Sonar operates on a similar principle by projecting acoustic (sound) waves into the water and using the reflected echo to determine the direction and distance to the seafloor, a submarine, or the Loch Ness Monster. Exploration geologists use a kind of ground sonar using small explosives as their sound producers. They can map the reflections of underground layers in which water or petroleum may be stored. Unfortunately, however, this kind of "sonar" does not penetrate far into the ground and cannot be used to probe the deep mantle or core.
Most normal geological prospecting activities don't produce strong enough sound waves through the Earth to be useful to probe the deep interior (nuclear explosions excepted, but these are frowned upon as an exploration technique!). However, Nature herself has provided the answer in the form of earthquakes. Seismic vibrations from earthquakes can travel completely through the Earth.
There are two basic types of seismic waves (with a number of subcategories). The P or compression waves are basically just acoustic or sound waves in which vibrating molecules are alternately squashed together and pulled farther apart. The other main type are the S or shear waves, which vibrate from side to side. They are "jiggling" waves such as you might experience if you jiggle a bowl of Jello.
Both P and S waves travel through the solid parts of Earth (P waves are faster), but the S waves cannot travel very far through liquids. So while seismic stations can pick up P waves that travel straight through the core of the Earth, no such S waves can be detected. At first this might sound like a limitation on what we can learn, but actually it is a great advantage. The S waves' inability to pass through the liquid Outer Core of Earth provides a dramatic proof of the Core's existence and size.
(Click on the image to change.) Imagine that an earthquake occurs under the surface of Earth near the point "A." Both P and S can be detected at recording stations at B, B', C and C'. While P waves penetrate the Core and can be detected even at stations D and D', or at any station for that matter, the S waves are not detected past stations C and C'.
(Click on the image to change.) Scientists have noticed that S waves cannot be detected more than about 105 degrees away from the earthquake site, as measured through the core of the Earth. Thus if an earthquake occurred just under the North Pole, the S waves could be detected as far south as about 15 degrees south of the Equator, but no further.
(Click on the image to change.) As a result, there is a kind of shadow zone past 105 degrees in which no S waves are detected. This is fortunate, because by using some simple reasoning and trigonometry, the radius of the core can be easily determined.
Since we already know the radius of the Earth is about 6,400 km, we know that this is the distance between the earthquake epicenter and the core, and from the core out to the farthest recording station at C (or C'). To get a rough estimate of the size of the core, we need only multiply one-half the cosine of 105 degrees by the radius of the Earth. This yields a radius of about 3900 km. However, here we have made the simplifying assumption that the S waves travel in straight lines through the Earth. In reality they are bent or refracted somewhat as they move through layers of varying density. Making this adjustment, we find that the radius of the outer core is more like 3500 km.
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