Geology for Civil Engineers Lecture Notes PDF

CE 25: Geology for Civil Engineers Topic 1 General Geology Einstine M. Opiso Department of Civil Engineering Central Mindanao University 1  Week 2 Learning Contents  Continental Drift and Plate Tect...

CE 25: Geology for Civil Engineers Topic 1 General Geology Einstine M. Opiso Department of Civil Engineering Central Mindanao University 1  Week 2 Learning Contents  Continental Drift and Plate Tectonics  Earthquake 2  Plate Tectonics Introduction  When one looks at a globe, it is easy to visualize how the continents at one time in the Earth’s history could have been bound together.  North and South America seem to fit into Europe and Africa in a slight s-shaped curve.  Alfred Wegener proposed that the continents were at one time part of a super continent, called Pangaea  Wegener further hypothesized that the continents had moved apart during the history of the Earth by what is called continental drift  (A)Normal position of the continents on a world map. (B) A sketch of South America and Africa, suggesting that they once might have been joined together and subsequently separated by a continental drift.  Plate Tectonics – Recall that the crust floats on the more liquid mantle and is buoyed up by its density. – Recall also that the mantle is molten, which gives it great pressure and temperature. – Given these lines of thought, it is not hard to see how the continents, already floating on the magma which is at great pressures, could be forced apart at certain areas where perhaps the crust was weaker or could be forced to break (fault). Evidence from the Ocean – The ocean contains chains of mountains called oceanic ridges. – The ocean also contains long, narrow trenches that always run parallel to the continents, called oceanic trenches. Driving mechanism for plate tectonics Plate Tectonics is the surface expression of the mechanism by which heat escapes the Earth’s interior Origin of heat in the Earth’s interior 1. radioactive decay 2. residual heat from Earth’s formation and to a lesser extent, heat contribution from the growth of the inner core which drives the convection in the outer core Pressure, Temperature, Composition  Plate Tectonics – Three kinds of observations started scientists to wonder in the direction that allowed an explanation for Wegener’s continental drift. All submarine earthquakes that were found and measured were found to occur in a narrow band under the crest of the Mid-Atlantic Ridge There is a long valley that runs along the crest of the Mid-Atlantic Ridge, called a rift. There was a large amount of heat escaping from this rift. The Mid-Atlantic Ridge divides the Atlantic Ocean into two nearly equal parts. Where the ridge reaches above sea level, it makes oceanic islands, such as Iceland.  Plate Tectonics – It was thought that the rift might be a crack in the Earth’s crust. – This lead to the formation of the Seafloor Spreading hypothesis Hot, molten rock moved from the interior of the Earth to emerge alone the rift, flowing out in both directions to create new rocks along the rift. The pattern of seafloor ages on both sides of the Mid-Atlantic Ridge reflects seafloor spreading activity. Younger rocks are found closer to the ridge.  Lithosphere Plates and Boundaries – Plate tectonics states that the lithosphere is broken into fairly rigid plates that move on the asthenosphere. – Some plates contain part of a continent and part of an ocean basin, while others contain only ocean basins. – Earthquakes, volcanoes, and the most rapid changes in the Earth’s crust occur at these plate boundaries. The major plates of the lithosphere that move on the asthenosphere. Source: After W. Hamilton, U.S. Geological Survey.  Plate Tectonics – Three kinds of plate boundaries that describe how one plate moves relative to another. Divergent boundaries. –Occur where two plates are moving away from each other. –This forms a new crust zone, where the magma flows as the plates separate releasing the pressure on the. »This forms new crust material A divergent boundary is a new crust zone where molten magma from the asthenosphere rises, cools, and adds new crust to the edges of the separating plates. Magma that cools at deeper depths forms a coarse-grained basalt, while surface lava cools to a fine-grained basalt. Note that deposited sediment is deeper farther from the spreading rift.  Plate Tectonics Convergent boundaries. –Occurs where two plates are moving toward each other. –Old crust is returned to the asthenosphere where the plates collide forming a subduction zone. –The lithosphere of one plate is subducted under the other plate. Ocean-continent plate convergence. This type of plate boundary accounts for shallow and deep-seated earthquakes, an oceanic trench, volcanic activity, and mountains along the coast. Ocean-ocean plate convergence. This type of plate convergence accounts for shallow and deep-focused earthquakes, an oceanic trench, and a volcanic arc above the subducted plate. Continent-continent plate convergence. Rocks are deformed, and some lithosphere thickening occurs, but neither plate is subducted to any great extent.  Plate Tectonics Transform boundaries. –Occur where two plates are sliding past each other. –This produces the vibrations that are commonly felt as earthquakes, such as those felt in California.  Plate Tectonics Present-day Understandings – Currently the most commonly accepted theory of plate movement is that slowly turning convective cells in the plastic asthenosphere drive the plates. – Hot materials rise at the diverging plate boundaries. – Some of this material escapes and forms new crust, but some spreads out under the lithosphere. – As it moves it drags the overlying plate with it. – Eventually it cools and sinks back inward to the subduction zone. Not to scale. One idea about convection in the mantle has a convection cell circulating from the core to the lithosphere, dragging the overlying lithosphere laterally away from the oceanic ridge. Forces acting on the plates  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 structures. 24  What is Engineering Seismology? Seismology is study of the generation, propagation and recording of the elastic waves and the source that produce them. 25 Variation of P and S Wave Velocities within the Earth  Evidence from Seismic Waves Seismic Waves A vibration that moves through the Earth. Body waves Seismic waves that travel through the Earth’s interior, spreading outward from a disturbance in all directions.  Evidence from Seismic Waves – Two types of body waves P-waves –A pressure wave where the material vibrates back and forth in the same direction as the wave movement. –Can pass through rock. –Can pass through a liquid Evidence from Seismic Waves S-waves –A sideways wave in which the disturbance vibrates material side to side, perpendicular to the direction to the wave movement. –Can pass through rock. –Can not pass through a liquid (A)A P-wave is illustrated by a sudden push on a stretched spring. The pushed-together section (compression) moves in the direction of the wave movement, left to right in the example. (B) An S-wave is illustrated by a sudden shake of a stretched rope. The looped section (sideways) moves perpendicular to the direction of wave movement, again left to right.  Evidence from Seismic Waves Surface Waves – Seismic waves that travel on the Earth’s surface. Seismograph  The velocity of both S- and P-waves is determined by the density and rigidity of the material.  Waves travel faster in denser more rigid material.  Waves are reflected at boundaries where elastic properties differ.  If the reflected waves reach the surface, they can be measured by a seismograph.  Wave refraction can also be used to determine properties of the interior of the Earth. – Waves are refracted (bent) when they pass from a layer with higher density to a layer with lower density. Seismic waves require a certain time period to reflect from a rock boundary below the surface. Knowing the velocity, you can use the time required to calculate the depth of the boundary. (A)A seismic wave moving from a slower-velocity layer to a higher- velocity layer is refracted up. (B) The reverse occurs when a wave passes from a higher-velocity to a slower-velocity layer. (A)This illustrates the curved path of seismic waves between an explosion and a recording seismograph van. The curved path is caused by increasing seismic velocity with depth in uniform rock. (B) This illustrates increasing seismic velocity with depth in uniform rock. The waves curve out in all directions from a disturbance. END