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Chapter 1
Introduction to Geology
1.1. Introduction
Geology is the study of rocks and Earth history. The Earth is a dynamic and ever changing body; earthquakes, volcanoes, moving continents and expanding and contracting oceans are all surface phenomena providing evidence of an active interior. They also provide clues to the internal processes, structure and composition of the Earth.
A unique feature of the Earth is that it has liquid water. Figure 1.1 presents a view of the Earth from space.
1.2. Shape, Internal Structure, and Composition of the Earth
In terms of its shape, the Earth is an oblate sphere; it is relatively flat at the poles but bulges at the equator due to the centrifugal force acting on it as the result of its rotation. Consequently, its polar radius (6,357 km) is 21 km shorter than its equatorial radius (6,378 km). A mean value of 6,371 km is often given for the radius of the Earth.
Figure 1.2 shows a section through the Earth, illustrating its internal structure and composition.
The inner core is solid but the outer core is thought to be liquid. The evidence for this is, however, indirect and is based on the study of the propagation of seismic waves generated by earthquakes. Seismic energy travels through the Earth in the form of compressional and shear waves, normally referred to as P-waves and S-waves, respectively. The modes of propagation of these waves are different. Considering the Earth as consisting of particles, P-waves propagate by causing the particles to vibrate (oscillate) horizontally and the direction of motion is also horizontal, as illustrated in Fig. 1.3. S-waves, by contrast, travel by causing the particles to vibrate (oscillate) vertically, while the direction of propagation is horizontal, as shown in Fig. 1.4. The velocity of both P- and S-waves increases with depth, as can be seen in Fig. 1.5. It should be noted that P-wave velocity, Vp, is greater than that of the S-waves, Vs:
Figure 1.1. The Earth as seen from space (http://s3.amazonaws.com/estock/nasas1/1/81/98/everystockphoto-nasa-space-18198-o.webp).
Earthquakes generate both P- and S-waves, which travel through the Earth and can be detected at observation stations all around the world. They can be separated due to the difference in their velocities; on account of their higher velocity, the P-waves constitute the first arrivals. In order to reach stations in the S-wave āshadow zoneā indicated in Fig. 1.6, the waves would have to pass through the outer core. Only P-wave arrivals are detected at these stations, which means that the S-waves have been absorbed in the course of their passage through the outer core. Transmission of S-waves is a property of solids; liquids are unable to transmit S-waves, which supports the conclusion that the outer core is liquid.
Figure 1.2. Structure and internal composition of the Earth (after Marshak, 2005).
Figure 1.3. P-wave transmission.
Figure 1.4. S-wave transmission.
Figure 1.5. Seismic wave velocity/depth profile for the Earth. There is a general increase in velocity with depth. No S-waves pass through the outer core, indicating that it is liquid (after Marshak, 2005).
Figure 1.6. The S-wave āshadow zoneā. This covers more than a third of the globe (after Marshak, 2005).
The crust, containing all our mineral deposits and fossil fuel resources, forms only 0.6% of the Earthās radius. It is divided into oceanic and continental types, with the latter being denser (Fig. 1.2). Crust thicknesses vary from 40 km in continental areas to 10 km in oceanic regions. Oxygen, silicon and aluminium dominate the composition of the crust (Fig. 1.7), while the whole Earth composition is dominated by iron and oxygen (Fig. 1.8). The circulating flow of the liquid in the outer core is the cause of the Earthās magnetic field.
Figure 1.7. Abundance of elements in the Earthās crust (after Marshak, 2005).
Figure 1.8. Whole Earth composition (after Marshak, 2005).
1.3. How Old is the Earth?
Until the 18th century, the Bible was the source of knowledge on virtually all subjects. Archbishop Ussher, head of the Anglo-Irish Church in Ireland, by adding up the generations of the patriarchs described in the Old and New Testaments, concluded in 1654 that the āEarth came into being on Sunday 23 October, 4004 BCā (Marshak, 2005).
The first scientific attempt at estimating the age of the Earth was by Kelvin, the Scottish physicist, in the 1890s. He calculated the time for the Earth to cool down from an original molten state, as hot as the sun, to its present temperature and concluded that this was about 20 million years.
The discovery of radioactivity by the French physicist Becquerel in 1896 revolutionised thinking on the age of the Earth. It led to the realisation that the Earth possessed an internal source that had generated heat; radioactive decay was identified as the source of this heat in the course of geological time. This realisation uncovered the flaw in Kelvinās calculation: he had assumed that no heat was added to the planet after it was formed.
Current estimates place the age of the Earth at 4.66 billion years and the oldest rocks encountered to date are in Canada. These rocks have been dated at 4 billion years by radiometric age determination methods (see Sec. 1.11.1).
1.4. The Earthās Crust (Lithosphere)
As mentioned earlier, the crust or lithosphere contains all of the Earthās mineral deposits and fossil fuel resources. The crust is made up of rocks, which may be defined as aggregates of crystals (or of non-crystalline materials) or grains. Geologists recognise three basic rock types:
ā¢ igneou,
ā¢ metamorphi,
ā¢ sedimentar.
These are described briefly in Table 1.1.
Igneous and metamorphic rocks lack hydrocarbon prospects and are referred to as āeconomic ba...
