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Engineering Geology and Seismology: Earth's Origin and Structure

This lecture explores the origin and structure of the Earth, as well as the application of engineering geology and seismology in civil engineering. Topics include the nebular hypothesis, planetesimal hypothesis, gaseous tidal theory, and the layers of the Earth.

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Engineering Geology and Seismology: Earth's Origin and Structure

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  1. Lecture # 2 Engineering Geology and Seismology Origin and Inferiors of the Earth Instructor: Dr. Attaullah Shah Department of Civil Engineering Swedish College of Engineering and Technology-Wah Cantt.

  2. Geology literally means "study of the Earth.“ Physical geology examines the materials and processes of the Earth. Historical geology examines the origin and evolution of our planet through time. Engineering geology is the application of geological data, techniques and principles to the study of rock and soil surfacing materials, and ground water.  Seismology is study of the generation, propagation and recording of the elastic waves and the source that produce them.

  3. Importance of engg geology in Civil Engineering practice What is Engineering Geology? • Engineering geology is the application of geological data, techniques and principles to the study of rock and soil surficial materials and ground water. • This is essential for the proper location, planning, design, construction, operation and maintenance of engineering structure.

  4. Importance of engg geology in Civil Engineering practice What does Engineering Geology study? • Rock, soil, water and the interaction among these constituents, as well as with engineering materials and structures. 5

  5. Importance of engg geology in Civil Engineering practice Why Engineering geology? • Serve civil engineering to provide information in 3 most important areas: • Resources for construction; aggregates, fills and borrows. • Finding stable foundations; • Mitigation of geological hazards; Identify proplems, evaluate the costs, provide information to mitigate the problem 6

  6. Origin of Earth Various Theory • Nebular Hypothesis • Planetesimal Hypothesis • Gaseous Tidal Hypothesis • Binary Star Hypothesis • Gas Dust Clout Hypothesis 7

  7. Nabular Hypothesis German philosopher, Kant and French mathematician, Laplace • Earth, planets and sun originated from Nebula. • Nebula was large cloud of gas and dust. It rotates slowly. • Gradually it cooled and contracted and its speed increased. • A gaseous ring was separated from nebula • Later the ring cooled and took form of a planet • On repetition of the process all other planets came into being • The central region, nebula became sun. 8

  8. Objections to Nabular Hypothesis : • Sun should have the greatest angular momentum because of its mass and situated in the center, however, it has only two percent of momentum of the solar system • How the hot gaseous material condensed in to rings

  9. Planetesimal Hypothesis Chamberlin and Moulton proposed the theory in 1904 • The sun existed before the formation of planets • A star came close to the sun. • Because of the gravitation pull of the star, small gaseous bodies were separated from the sun • These bodies on cooing became small planet's • During rotation the small planets collided and form planets 10

  10. Objections to Planetesimal Hypothesis • The angular momentum could not be produced by the passing star. • The theory failed to explain how the planetesimals had become one planet

  11. Gaseous Tidal Theory Jeans and Jeffrey proposed the theory in 1925 • Large star came near the sun. Due to gravitational pull a gaseous tide was raised on the surface of the sun. • As the star came nearer, the tide increased in size. • Gaseous tide detached when star move away. • The shape of the tide was like spindle. • It broke into pieces-forming nine planets of the solar system. 12

  12. Interior of earth • Crust: • Continental crust (25-40 km) • Oceanic crust (~6 km) • Mantle • Upper mantle (650 km) • Lower mantle (2235 km) • Core • Outer core: liquid (2270 km) • Inner core: solid (1216 km)  Values in brackets represent the approximate thickness of each layer

  13. Layers of the Earth • The earth is divided into three main layers: Inner core, outer core, mantleandcrust. • The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid. • Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.

  14. THE CRUST • The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. • The shell of the earth, the crust, can be said to have two different thicknesses. • Under the oceans, it is relatively thin. It varies in thickness from 5 to 8 km. Under the land masses, it is relatively thick. The thickness of the continental crust varies from 10 to 65 km.

  15. THE CRUST • The eggshell analogy for the crust is not an exaggeration. It is paper thin compared with the radius of the earth which is approximately 6400 km. • The total weight of the continental crust is less than 0.3% of the weight of the earth. • Variations in the crust thickness are compensated by the weight of the water and the differences in the specific gravities of the crust under the oceans (3.0 to 3.1) and under the continents(2.7 to 2.8).

  16. THE CRUST • If one thinks of the crust as virtually floating on the mantle, one is less likely to wonder why the earth does not wobble as it rotates about its axis. • The weight of the crust plus the mantle has a reasonably uniform distribution over the globe.

  17. THE MOHO • The Moho, or the Mohorovicic Discontinuity, refers to a zone or a thin shell below the crust of the earth that varies in thickness from 1 to 3 km.

  18. THE MOHO • In seismology, the term "discontinuity" is used in its general sense. It refers to a change over a short distance of a material property. In this case, the "short distance" may be as long as 3 km, a trifle compared with the radius of the earth. • In that zone, the P-wave velocity has been observed to increase from approximately 6 to approximately 8 km/sec. • The Moho is considered to be the boundary between the crust and the mantle. • The increase in P-wave velocity is ascribed to change in composition of the medium. Rocks of the mantle are poorer in silicon but richer in iron and magnesium

  19. THE MANTLE The mantle can be thought of having three different layers. The separation is made because of different deformational properties in the mantle inferred from seismic wave measurements. (1) The upper layer is stiff. It is presumed that if the entire mantle had been as stiff, the outer shell of the earth would stay put. This stiff layer of the mantle and the overlying crust are referred to as the lithosphere. The lithosphere is approximately 80-km thick

  20. THE MANTLE • (2) Beneath the lithosphere is a soft layer of mantle called theasthenosphere. • Its thickness is inferred to be several times that of the lithosphere. • One may think of this as a film of lubricant although film is not exactly the word for something so thick. It is assumed that the lithosphere, protruding (meaning: extending beyond) parts and all, can glide over the asthenosphere with little distortion of the lithosphere

  21. THE MANTLE (3) Themesosphereis the lowest layer of the mantle. • Considering the vagueness in defining the lower boundary of the asthenosphere it would be expected that the thickness and material properties of the mesosphere are not well known. • It is expected to have a stiffness somewhere between those of the lithosphere and the asthenosphere.

  22. THE CORE • At a depth of approximately 2900 km, there is a large reduction (on the order of 40%) in the measured velocity of seismic waves. The boundary between the mantle and the core is assumed to be at this depth. • Because no S-wave has been observed to travel through the material below this boundary for a thickness of approximately 2300 km, it has been inferred that the core comprises two layers. • The 2300-km thick outer layer which is in a molten state and an 1100-km thick inner layer which is solid.

  23. THE CORE It is known that the pressure increases toward the center of the earth. So does the temperature. The liquid outer layer versus the solid inner layer is rationalized by recognizing that the melting point of the material increases (with pressure) at a faster rate than the temperature as the center of the earth is approached.

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