Chapter 1 - Earth Structure (Final 2023) PDF

Chapter one Earth structure 1. Structure of the Earth The earth consists of three main parts, i.e., Crust, Mantle and Core. A cross section illustrates the various parts of the earth is shown in Fig. 1. The core accounts for almost half of Earth’s radius, but it amou...

Chapter one Earth structure 1. Structure of the Earth The earth consists of three main parts, i.e., Crust, Mantle and Core. A cross section illustrates the various parts of the earth is shown in Fig. 1. The core accounts for almost half of Earth’s radius, but it amounts to only 16.1% of Earth’s volume. Most of Earth’s volume (82.5%) is its mantle, and only a small fraction (1.4%) is its crust. 1.1 Crust It represents the outermost layer with a total thickness ranges from 5–70 km (~3–44 miles). Continental crust is significantly thicker than oceanic crust. It stands higher and penetrates deeper into the mantle than its oceanic counterpart. The upper continental crust is exposed at many places, and its composition is well known. The lower continental crust is not exposed; its information came from studying how shock waves from earthquakes and man-made explosions pass through it, and by examining the rare fragments carried to the surface by erupting lavas. It can be divided into two types of crusts: Continental Crust The continental crust is thick, and composed of felsic rocks (rich in silica) such as sodium, potassium, aluminum silicate rocks, e.g. granite. So it is less dense than the oceanic crust. Oceanic Crust The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense mafic rocks. Mafic rocks contain minerals with less silica, but more iron and magnesium, like basalt. -1- 1.2 Mantle It extends to a depth of 2,890 km thick. It is composed of silicate rocks richer in iron and magnesium than the overlying crust with a specific gravity of 4.5- 5.0. It is very near the melting point and can flow when subjected to stress. Volcanic activity and deformation of the crust result from movement within. Iron and magnesium-bearing silicate minerals make up the mantle. 1.3 Core It extends to a depth of 7,000 km (radius of the earth) and consists of an outer liquid portion (radius of 2,400 km) and a solid inner core (radius of 1,220 km). Iron and nickel are the predominant constituents of the core of the earth. How the scientists concluded that the inner core is solid? Why the inner core is solid? The core is extremely hot (~3500° to more than 6000°C). But despite the fact that the boundary between the inner and outer core is approximately as hot as the surface of the sun, only the outer core is liquid. The inner core is solid because the pressure at that depth is so high that it keeps the core from melting. Some new studies suggesting a composition of plasma with the density of a solid. -2-  Figure 1. Structure of the earth. 2. Earth’s Temperature Gradient (Geothermal Gradient, GG) The geothermal gradient GG, or the rate at which the subsurface temperatures increase with depth, is nearly constant at 3°C/100 m for non-active geothermal areas. In active geothermal areas, this gradient tends to increase dramatically on a nonlinear scale. Thus, measuring the geothermal gradient (GG) is crucial in any geothermal study. This parameter can be calculated using the following equation: GG = (T2 - T1)/ (D2 - D1) Where: T2 and T1 are the subsurface (bottom) and surface temperatures, respectively. D2 and D1 are the subsurface (bottom) and surface depths, respectively. -3- Example: Estimate the geothermal gradient from a bore hole with a depth of 1500 m and bottom hole temperature of 60C (surface temp is 25 C)? Generally speaking two types of gradients can be categorized as:  Normal gradient (Regular): Temperature increases with 3C per 100 meters.  Abnormal gradient (Irregular): Temperature increases with 6 to 7C per 100 meters or more. Figure 2. Structure of the earth and geothermal gradient. 3. Plate Tectonics and Continental Drift Plate tectonics is a scientific theory that describes the large-scale motions of Earth's lithosphere. The model builds on the concepts of continental drift (Fig. -4- 3), developed during the first few decades of the 20th century. The geoscientific community accepted the theory after the concepts of seafloor spreading (Fig. 4) were developed in the late 1950s and early 1960s. Early geological workers noticed that certain portions of the continents would fit together if moved from their present positions Figure 3. Continental drift. -5-  Figure 4. Sea floor spreading. Continental Drift Continental drift is the movement of the Earth's continents relative to each other by appearing to drift across the ocean bed. For example it is pushing Europe and North America apart, and Asia and the Indian subcontinent together. In the early 20th century, German scientist Alfred Wegener published a book explaining his theory that the continental landmasses, far from being immovable, were drifting across the Earth. He called this movement continental drift. Alfred Wegener (1880-1930) earned a PhD in astronomy at the University of Berlin in 1904, but he had always been interested in geophysics and meteorology and spent most of his academic career working in meteorology. In -6- 1911 he happened on a scientific publication that included a description of the existence of matching Permian-aged terrestrial fossils in various parts of South America, Africa, India, Antarctica, and Australia (Fig. 5). Wegener concluded that this distribution of terrestrial organisms could only exist if these continents were joined together during the Permian, and he coined the term Pangea (“all land” in in Greek.) for the supercontinent that he thought included all of the present-day continents (Earle, 2019). Cross-section showing the geological similarities between parts of Brazil (South America) on the left and Angola (Africa) on the right (Fig. 6). Figure 5. The distribution of several Permian terrestrial fossils that are present in various parts of continents that are now separated by oceans (Earle, 2019). -7- Figure 6. Cross-section showing the geological similarities between parts of Brazil (South America) on the left and Angola (Africa) on the right. The pink layer is a salt deposit, which is now known to be common in areas of continental rifting (Earle, 2019). 3.1 Causes of Continental Drift Mantle may respond to continuous stress applied by heat and develop convection currents. Convection currents provide a mechanism for the separation of continental masses from a common spreading center (Fig. 7). Another possible mechanism for continental movements is that light-weight crustal materials floating on denser, highly viscous lithosphere and mantle materials might respond to the earth rotation on its axis and move about on the surface of the earth. -8-  Figure 7. Convection currents. 3.2 Evidences for Continental Drift There are many evidences of the movements of the continents. The following among the most common: 1. Several structural trends that extend across portions of some of the continents end abruptly at their coasts and reappear on the continents facing across the ocean. 2. Direct measurements along major fault zones indicate movements of up to 6 cm per year in some places. 3. Similarities in rock types, fauna and flora between the Eastern coasts of Southern America and Western coasts of Africa. 4. Polar Wandering curve (Paleomagnetic - Remnant magnetism), where some magnetized minerals that are found with different orientations from the current polar N-S curve. Explain more about the phenomenon of paleo magnetism? As the mineral magnetite (Fe3O4) crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth’s magnetic field at that time. This is called remnant magnetism. Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite (up to 1 or 2%). By studying both the horizontal and vertical components of the remnant magnetism, we can know the direction to magnetic north -9- at the time of the rock’s formation, besides the latitude where the rock formed relative to magnetic north. 3.3 Types of movement of continental Margins Certain types of continental margins evolve in response to the nature of the various motions. The following among the most common: a. Convergent margins: Convergent margins are developed when two continents collide great mountain ranges such as the Alps and the Himalayas are pushed up (Fig. 8 a). When one continental and one oceanic plate collide, the lighter continental material rides over the denser oceanic material (Fig. 8 b). A B Figure 8. A) Convergent Continental-Continental and B) Convergent Oceanic- Continental margins. -10- b. Divergent margins: Divergent continental margins occur where plates have broken apart along a spreading center and moved away from each other (Fig. 9). Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting as hot mantle rock from depth is moved toward the surface. Most divergent boundaries are located at the oceanic ridge. Spreading rates vary considerably, from 2 cm/y to 6 cm/y in the Atlantic, to between 12 cm/y and 20 cm/y in the Pacific. (Note that spreading rates are typically double the velocities of the two plates moving away from a ridge). Figure 9. Divergent margins. -11- 4. Seafloor spreading Seafloor spreading is the process of new crust forming between two plates that are moving apart. Along a network of mountain ranges in the ocean, molten rock rises from within the Earth and adds new seafloor to the edges of the old seafloor. As the seafloor grows wider, the continents on opposite sides of the ridges move away from each other. 5. Deep seas hydrocarbon accumulations Possible petroleum occurrences may be found in the marine sediments occupied in the basins behind island arc clean due to high fault block movements. The source of marine organisms and the heat supply, both provide the necessary elements for the process of the generation and maturation of hydrocarbons. References Steven Earle, 2019. Physical Geology - 2nd Edition. -12-