The Earth itself provides the principal means by which we can solve the mystery of its internal structure. The heat reaching the surface of our planet from the interior is gravitational heat produced by radioactive decay of primary isotopes. The loss of this heat from the Earth’s interior is principally effected not by conduction or radiation but by convection.
The interior of the Earth is composed of a central iron nickel core, mainly molten, surrounded by a mantle of crystallic silicates, composed of magnesium, iron and calcium as well as silicon for the most part. Every year a number of earthquakes occur in many different parts of the globe. Each of them releases suddenly a tremendous amount of energy. This energy travels from the focus, or source of the disturbance in the form of waves through all parts of the Earth, including the very deepest part. As a wave travels from one medium to the other its velocity may increase or decrease depending on the nature of the medium it leaves and the one it enters.
The two types of waves that penetrate deep in the Earth are P waves or Primary waves and S waves or Secondary waves. P waves travel through both solid and liquid parts of the earth’s interior. S waves travel at about two thirds of the speed of P waves in solid regions and do not travel at all through fluid regions.
When the waves reach a boundary between layers of the earth, they may be reflected, passing upward toward the surface. They may be refracted, or bent, as they pass on to the next layer, as well as reflected. This means that they penetrate this layer, changing their direction. The waves that reach the Earth’s crust are reflected downward again.
Because of the behavior of the type of rock making up the mantle, changes in density and changes in the way the materials reacts to deforming stresses the velocity of a seismic wave in the mantle with increasing depth. Thus, rays spreading spherically around an earthquake focus reach the surface ultimately at various distances depending on the path followed, because the Earth is spherical. As the rays descends deeper and deeper the medium is transmitting the seismic ray at a greater and greater velocity so it is bent more and more and eventually reaches a point where it intersects the surface. This happens to all the rays emitted in a sphere around the focus of the earthquake unless it encounters a medium in which the velocity is lower. The most deeply descending rays do encounter a discontinuity where the velocity decreases sharply and as a result of a shadow zone occurs at the surface where no record of the earthquake is present.
Based on the average density of the Earth and of rocks and iron-nickel metal and our knowledge of meteorites that this discontinuity is between the deep rock-like mantle with low density and high seismic velocity and the iron-nickel core with high density and low velocity. Also, we know the core is molten because shear waves do not make it through this medium but are transformed to weak compressional waves.
The Earth is cooling down because of the decrease in radioactivity over time by the radioactive decay of the long lived primary isotopes. The amount of heat produced within the Earth today is about half of that produced 4.5 billion years ago. Because of this decrease the pulse of the Earth is slowing and the processes driven by the escape of the Earth’s internal heat are also slowing down.
Convection taken the form of the movement of deep, hot mantle towards the surface, cooling at the surface by conduction, radiation and volcanic action and return of cool mantle to depth just as in atmospheric circulation or in a beaker of water heating on a hot plate. Just as convection occurs in the mantle so also does it occur in the molten iron-nickel core. The energy for core convection comes from crystallization of the iron-nickel, chemical reaction at the core-mantle boundary and interactions with the magnetic field of the sun.
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