INTERNAL STRUCTURE OF THE EARTH

Introduction

More than two decades after people first sat foot on the moon, the deepest boreholes have penetrated barely 12 km into the Lithosphere. Since the radius of the planet 6370 km, we have penetrated less than 1/500th of the distance to the center of the earth.
Nonetheless, scientists have established that the interior of the earth is layered, like the atmosphere and they have deduced the chemical composition and physical properties of the major spheres inside the crust.

Evidence of the Earth’s Internal Structuring

Evidence that supports the concept of a layered internal earth comes from several sources.
First, rock samples taken from great depths do have higher concentrations of Iron and Magnesium in them than average crustal rocks. And second, when the mass and size of the earth are measured, the resulting figure is 5.5 gm/cm3. This is about double the density of rocks found in the continental crust. Again, the heavier, denser part of the earth should be in the deep interior.
Further evidence for the internal structuring of the earth comes from the planets magnetic field and from the high temperature and pressures known to prevail at deeper levels.
But the most convincing body of evidence is derived from the analysis of earthquakes – shaking and trembling of the earth’s surface caused by sudden releases of stress within the crust, which generate pulses of energy called Seismic Waves that can pass through the entire earth.

Nature of Seismic Waves

Seismic waves take time to travel through the earth. In general terms, the speed of an earthquake wave is proportional to the density of the material through which it travels. The denser the material, the faster the speed. Seismic waves change direction under certain circumstances. When a seismic travelling through a less dense material reaches a place where the density becomes much greater, it may be bounced back; this is seismic reflection. If the contrast in densities between the adjacent is less severe, the wave may be bent rather than reflected. Here, seismic refraction changes the course of the seismic wave. Within a given layer, the speed of seismic waves generally increases with depth.

Types of Seismic Waves

Seismic waves behave differently as they propagate through the earth. One type of wave travels along the surface of the crust, controlled by the elasticity of the strata, and is termed a Surface Wave or L-Wave. The two types of L-wave are; the Love Wave, a wave in a horizontal plane known as the Lq-Wave; and the Raleigh Wave, slowed in a vertical plane, the Lr-Wave. Each is named after its discoverer.
Two other types of wave travel through the interior of the earth and are referred to as body waves. The body waves are of two kinds, known as P-Waves and S-Waves. The P-waves are compressional waves, sometimes called “push” waves. As they propagate they move material in their path parallel to the direction of the movement. They even travel through material in the liquid state, although their impact is then much reduced. The S-waves are called “shear” or “shake” waves. These waves move objects at right angles to their direction of motion. They do not propagate through liquid material.
Thus, depending on the nature of the layers through which they pass, seismic waves are speeded up or slowed down or bent, and in some cases, stopped altogether. These observable changes in seismic wave motions enable seismologists to probe the earth’s interior.

Earth’s Internal Layers

If an earthquake occurs at 0 degrees, P as well as S waves are recorded by seismographs everywhere to 103 degrees from its source (a distance of 11,270 km). Then, from 103 and 142 degrees, the next 4150 km, neither P nor S waves are recorded. But from 142 to 182 degrees (15,420 km to 19,470 km), P waves reappear. From this evidence, it is concluded that the earth possesses a liquid that begins about 2,900 km below the surface. At the contact between this liquid layer and the layer above it, S-waves cease to be propagated and P-waves are refracted.
But some P-waves that arrive on the far side of the earth, between 142 and 182 degrees have not been refracted once or twice but four times. Moreover, their speed has increased. This means that the P-waves that reach the seismographs located antipodally to the earthquake source (i.e., on the exact opposite point of the spherical earth), must have traveled through a very dense mass inside the liquid layer. Confirmation of the existence of such a dense mass at the core of the earth comes from the fact that many P-waves are reflected back at its outer edge. From the travel times of these reflected P-waves, it is concluded that the earth has a solid inner core.
On the basis of seismic and other evidence, therefore, the interior earth is believed to have four layers; a solid inner core, a liquid outer core, a solid lower mantle, and a partially molten upper mantle. On top of all this lies the crust, still very thin and in places active and unstable.

THE CORE

The core contains 1/3rd of the entire mass of the earth. It is half of the radius of the earth. Temperatures in the earth’s core may lie between 4,000 and 5,000 oF; pressures are as high as three to four million times the pressure of the atmosphere at the sea-level.

Solid Inner Core
The solid inner core has a radius of just 1,220 km. Its surface lies 5,150 km below sea level. Iron and Nickel exist there in a solid state, therefore, it is known NIFE. If specific gravity is 8.

Liquid Outer Core
The liquid outer core forms a layer 2,250 km thick. Its outer surface lies at some 2,900 km below sea level, just slightly less than half way to the center of the planet. The liquid outer core may consists of essentially the same materials as the solid inner core, but because pressures here are less, the melting point temperature is lower and a molten state prevails.
The density of the inner and outer core combined has been calculated as 12.5 gm/cm3, which compensates for the lightness of the crust (2.8 gm/cm3) and accounts for the density of the planet as a whole (5.5 gm/cm3).

THE MANTLE
Outside of the core lies the mantle, a layer about 2,900 km thick composed of mineral matter in a solid state. The specific gravity is 3.5. Through this layer, P-waves have the speed of 6 ½ to 7 ½ km/sec, while the S-waves move at a speed of 3 ¼ to 4 km/sec. In this layer, Silica and Magnesium elements dominate and it is known as SIMA. Judging from the behavior of earthquake moves, the minerals Olivine (Magnesium Iron Silicate), which comprises an ultramafic rock called Dunite. This rock, which may be in response to sudden stresses of earthquake waves which pass through it.
An important feature of the mantle is the presence of a thin layer at between 100 and 200 km below its surface, where the rocks appear to be less rigid and more plastic than those above and below. As a result, the speed of propagation of earthquake waves is abruptly reduced. The theoretical concept of its existence was first confirmed by detailed seismographic evidence during the Chilean Earthquake of 1960.
The mantle is demarcated from the core by the Guttenberg Discontinuity at a depth of about 2900 km where the temperature is estimated to be about 3700 oC.

Solid Lower Mantle
Above the liquid outer core lies the solid lower mantle. It has a thickness of about 2,230 km. Seismic data shows that the lower mantle is in a solid state. geologists believe that this layer is composed of oxides of iron, magnesium and silicon.

Upper Mantle
The upper mantle extends from the base of the crust to a depth of just 670 km. The upper mantle is differentiated from the lower mantle on the basis of mineral composition and the state of the rock material: the lower mantle is solid, but the upper
mantle’s material is viscous (like thick syrup, capable of flowing slowly). This soft plastic layer in the upper mantle is called the Asthenosphere. However, the zone of the upper mantle just beneath the crust is solid. This portion of the upper mantle also contains pockets of molten rock, some of which feeds chambers from which lava pours onto the surface of the crust. The solid part of the upper mantle together with the crust is referred to as the Lithosphere.

THE CRUST
Outermost and thinner of the earth zones is the crust, formed largely of igneous rocks. The crust is not of even thickness and is thinner than the shell of an egg relative to the planet’s diameter. Under the oceans, the crust averages a mere 8 km in thickness; under the exposed continental surfaces, the average depth is about 40 km.
The base of the crust, where it contacts the mantle is sharply defined, a fact known because earthquake waves change velocity abruptly at that level. The surface of separation between crust and mantle is called the Mohorovicic Discontinuity or Moho, a simplification of Mohorovicic, the name of Yugoslavian seismologist who discovered it in 1909; here the speed of the propagation of earthquake waves suddenly accelerates from 5.0 to 8.1 km/sec.
On the layer, which is also known as Lithosphere, the P-waves travel at a nominal speed of 5.75 km/sec. While the speed of S-waves is still less – only 3.25 km/sec. The specific gravity of this layer is only 2.7. Silica and Aluminum elements predominate. Therefore, it is named in short SIAL. From a study of earthquake waves, it is concluded that the crust consists of two layers:
1. A lower continuous layer of Basaltic rock (simatic/sima) which dominate ocean floors; and,
2. An upper layer of Granitic rock (sialic/sial), which constitutes the bulk of continents.
The granite layer is, therefore, discontinuous in a real extent, being absent over the ocean basin. Those parts of the crust forming the continents are much thicker than the crust under the ocean basins.