General Geology Course PDF

Prof. Adel Kamel Mohamed GENERAL GEOLOGY COURSE FOR FIRST YEAR STUDENTS FACULTY OF EDUCATION Prof. Adel Kamel Mohamed THE EARTH’S LAYERS The energy released by an earthquake travels through the Earth as waves. Geologists have found that earthquake waves ab...

Prof. Adel Kamel Mohamed GENERAL GEOLOGY COURSE FOR FIRST YEAR STUDENTS FACULTY OF EDUCATION Prof. Adel Kamel Mohamed THE EARTH’S LAYERS The energy released by an earthquake travels through the Earth as waves. Geologists have found that earthquake waves abruptly change both speed and direction at certain depths as they pass through the Earth’s interior. Figure and Table describe the layers. Earth's layers The Crust The crust is the outermost and thinnest layer. Because the crust is relatively cool, it consists of hard, strong rock. Crust beneath the oceans differs from that of continents. Oceanic crust is 5 to 10 kilometers thick and is composed mostly of a dark, dense rock called basalt rich in iron and magnesium silicates. In contrast, the average thickness of continental crust is Prof. Adel Kamel Mohamed about 20 to 40 kilometers, although under mountain ranges it can be as much as 70 kilometers thick. Continents are composed primarily of a light-colored, less dense rock called granite that is enriched with silicon oxides. The Mantle The mantle lies directly below the crust. It is almost 2900 kilometers thick and makes up 80 percent of the Earth’s volume. Although the chemical composition may be similar throughout the mantle, which is composed of iron and magnesium oxides constituting the predotite rocks, Earth temperature and pressure increase with depth. These changes cause the strength of mantle rock to vary with depth, and thus they create layering within the mantle. The upper part of the mantle consists of two layers. The Lithosphere The lithosphere (Greek for ―rock layer‖) is the outer part of the Earth, including both the uppermost mantle and the crust. It is relatively cool and consequently is hard, strong rock. The lithosphere can be as thin as 10 kilometers where tectonic plates separate. However, in most regions, the Prof. Adel Kamel Mohamed lithosphere varies from about 75 kilometers thick beneath ocean basins to about 125 kilometers under the continents. A tectonic (or lithospheric) plate is a segment of the lithosphere. The Asthenosphere At a depth varying from about 75 to 125 kilometers, the strong, hard rock of the lithosphere gives way to the weak, plastic asthenosphere of the same chemical composition of the lithosphere. This change in rock properties occurs over a vertical distance of only a few kilometers, and results from increasing temperature with depth. Although the temperature increases gradually, it crosses a threshold at which the rock is close to its melting point. As a result, 1 to 2 percent of the asthenosphere is liquid, and the asthenosphere is mechanically weak and plastic. Because it is plastic, the asthenosphere flows slowly, perhaps at a rate of a few centimeters per year. The asthenosphere extends from the base of the lithosphere to a depth of about 350 kilometers. At the base of the asthenosphere, increasing pressure causes the mantle to become mechanically stronger, and it remains so all the way down to the core. The Core The core is the innermost of the Earth’s layers. It is a sphere with a radius of about 3470 kilometers and is composed largely of iron and nickel. The outer core is molten because of the high temperature in that region. Near its center, the core’s temperature is about 6000ºC, as hot as the Sun’s surface. The pressure is greater than 1 million times that of the Earth’s atmosphere at sea level. The extreme pressure overwhelms the temperature effect and compresses the inner core to a solid. Prof. Adel Kamel Mohamed SHORT REVIEW ON ROCKS SEDIMENTARY Sedimentary rocks are formed from particles of sand, shale, pebbles, and other fragments of material. Together, all these particles are called sediments. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone. METAMORHIC Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbon like layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble. IGNEOUS Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glass like. Sometimes gas bubbles are trapped in the rock during the cooling Prof. Adel Kamel Mohamed process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and granite. Schematic diagram of the rock cycle displaying the complex interactions between molten rock (magma), the three basic rock types, sediment, and the various processes that influence formation. Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers which are called agents of denudation. The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the Prof. Adel Kamel Mohamed total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other structures. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores. The study of the sequence of sedimentary rock strata is the main source for scientific knowledge about the Earth's history, including palaeogeography, paleoclimatology and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as geomorphology, geochemistry and structural geology. GEOLOGIC STRUCTURES Rock deformation Stress Stress is a force exerted against an object. Tectonic forces exert different types of stress on rocks in different geologic environments. The first, called confining stress or confining pressure, occurs when rock or sediment is buried (Fig.). Confining pressure merely compresses rocks but does not distort them, because the compressive force acts equally in all directions, like water pressure on a fish. Directed stress acts most strongly in one direction. Tectonic processes create three types of directed stress. 1- Compression stress squeezes rocks together in one direction as it shortening the distance parallel to the squeezing direction (Fig.). Prof. Adel Kamel Mohamed Compressive stress is common in convergent plate boundaries, where two plates converge and the rock crumples. 2- Extensional stress (often called tensional stress) pulls rock apart (Fig.). Rocks at a divergent plate boundary stretch and pull apart because they are subject to extensional stress. 3- Shear stress acts in parallel but opposite directions (Fig.). Shearing deforms rock by causing one part of a rock mass to slide past the other part, as in a transform fault or a transform plate boundary. STRAIN Strain is the deformation produced by stress. The rocks respond to tectonic stress by elastic deformation, plastic deformation, or brittle fracture. An elastically deformed rock springs back to its original size and shape when the stress is removed. During plastic deformation, the deformed rock retains its new shape. In some cases a rock will deform plastically and then fracture (Fig.). Prof. Adel Kamel Mohamed Fig. (a) Confining pressure acts equally on all sides of a rock.Thus, the rock is compressed much as a balloon is compressed if held under water. Rock volume decreases with- out deformation. (b) Tectonic compression shortens the dis- tance parallel to the stress direction. Rocks fold or fracture to accommodate the shortening. (c) Extensional stress lengthens the distance parallel to the stress direction. Rocks commonly fracture to accommodate the stretching. (d) Shear stress de- forms the rock parallel to the stress direction. Prof. Adel Kamel Mohamed Fig. This rock (in the Nahanni River, Northwest Territories, Canada) folded plastically and then fractured. Factors That Control Rock Behavior Several factors control whether a rock responds to stress by elastic or plastic deformation or fails by brittle fracture: 1. The nature of the material. Initially, all rocks react to stress by deforming elastically. Near the Earth’s surface, where temperature and pressure are low, different types of rocks behave differently with continuing stress. Granite and quartzite tend to behave in a brittle manner. Other rocks, such as shale and limestone, have greater tendencies to deform plastically. 2. Temperature. The higher the temperature, the greater the tendency of a rock to behave in a plastic manner. 3. Pressure. High confining pressure also favors plastic behavior. During burial, both temperature and pressure increase. Both factors promote plastic deformation, so deeply buried rocks have a greater tendency to bend and flow than shallow rocks. 4. Time. Stress applied over a long time, rather than suddenly, also favors plastic behavior. In contrast, rapidly applied stress causes brittle fracture. GEOLOGIC STRUCTURES A geologic structure is any feature produced by rock deformation. Tectonic forces create three types of geologic structures: folds, faults and joints. Prof. Adel Kamel Mohamed FOLDS A fold is a bend in rock (Fig.). Some folded rocks display little or no fracturing, indicating that the rocks deformed in a plastic manner. Fig. A fold is a bend in rock. These are in quartzite in the Maria Mountains, California. If you hold a sheet of clay between your hands and exert compressive stress, the clay deforms into a sequence of folds (Fig.). This demonstration illustrates three characteristics of folds: Fig. Clay deforms into a sequence of folds when compressed. 1. Folding usually results from compressive stress. 2. Folding always shortens the horizontal distances in rock (Fig.). 3. Folds usually occur as a repeating pattern of many folds. Prof. Adel Kamel Mohamed Fig. (a) Horizontally layered sedimentary rocks shorten by folding. Elements of fold pattern Figure shows that a fold arching upward is called an anticline and one arching downward is a syncline. These fold patterns have the following elements: 1. The sides of a fold are called the limbs. Notice that a single limb is shared by an anticline–syncline pair. 2. A line dividing the two limbs of a fold and running along the crest of an anticline or the trough of a syncline is the fold axis. 3. The axial plane is an imaginary plane that runs through the axis and divides a fold as symmetrically as possible into two halves. Fig. Major elements of folds. Although an anticline is structurally a high point in a fold, anticlines do not always form topographic ridges. Conversely, synclines do not always form valleys. Landforms are created by combinations of tectonic and surface Prof. Adel Kamel Mohamed processes. In Figure, the syncline lies beneath the peak and the anticline forms the saddle between two peaks. Fig. A syncline lies beneath the mountain peak and an anticline forms the low point, or saddle, in the Canadian Rockies, Alberta FAULTS A fault is a fracture along which rock on one side has moved relative to rock on the other side (Fig.). Slip is the distance that rocks on opposite sides of a fault have moved. This slip occurs on the fault plane that separates the faulted blocks apart. Fig. The faults Prof. Adel Kamel Mohamed Movement along a fault may be gradual, or the rock may move suddenly, generating an earthquake. Some faults are a single fracture in rock; others consist of numerous closely spaced fractures called a fault zone (Fig.). Fig. (a) Movement along a single fracture surface characterizes faults with relatively small slip. (b) Movement along numerous closely spaced faults in a fault zone is typical of faults with large slip. Rock moves along many faults once that fault forms, it is easier for movement to occur again along the same fracture than for a new fracture to develop nearby. Hydrothermal solutions often precipitate in faults to form rich ore veins. Miners then dig shafts and tunnels along veins to get the ore. Many faults are not vertical but dip into the Earth at an angle. Therefore, the faulted block that hung over the head of a geologist standing on the fault plane is defined as the hanging wall and the side he walked on as the footwall (Fig.). Fig. The hanging wall and footwall overlies and underlies the fault plane respectively. Prof. Adel Kamel Mohamed A fault in which the hanging wall has moved down relative to the footwall is called a normal fault (Fig.). A normal fault forms where tectonic tension stretches the Earth’s crust, pulling it apart. Figure shows a wedge-shaped block of rock called a graben dropped downward between a pair of normal faults. The blocks of rock between the down-dropped grabens then appear to have moved upward relative to the grabens; they are called horsts. Normal faults, grabens, and horsts are common where the crust is rifting at a spreading centers such as the mid- oceanic ridge and the Gulf of Suez – Red Sea region. Fig. Horsts and grabens commonly form where tectonic forces stretch the Earth’s crust. In a region where tectonic forces squeeze the crust, and as like as folds, the reverse faults accommodate crustal shortening (Fig.). In a reverse fault, the hanging wall has moved up relative to the footwall. A thrust fault is a special type of reverse fault that is nearly horizontal (Fig.). Prof. Adel Kamel Mohamed Fig. Reverse and thrust faults A strike–slip fault is one in which the fracture is vertical, or nearly so, and rocks on opposite sides of the fracture move horizontally past each other (Fig.). A transform plate boundary is a strike–slip fault. The San Andreas and Dead Sea – Gulf of Aqaba fault zones are zones of strike–slip faults that form the border between the Pacific and the North American plates and the boundary between Arabia and Nubia plates respectively. Fig. The strike-slip fault. Prof. Adel Kamel Mohamed JOINTS A joint is a fracture in rock and is therefore similar to a fault, except that in a joint rocks on either side of the fracture have not moved. Tectonic forces also fracture rock to form joints (Fig.). Most rocks near the Earth’s surface are jointed, but joints become less abundant with depth because rocks become more plastic at deeper levels in the crust. Fig. Joints Joints and faults are important in engineering, mining, and quarrying because they are planes of weakness in otherwise strong rock. A dam constructed in jointed rock often leaks, not because the dam has a hole but because water follows the fractures and seeps around the dam. STRIKE AND DIP Faults, joints, sedimentary beds and a wide range of other geologic features are planar surfaces in rock. Field geologists describe the orientations of sedimentary beds or other planes with two measurements called strike and dip. To understand these concepts, recall from elementary geometry that two planes intersect in a straight line. Strike is the compass direction of the line Prof. Adel Kamel Mohamed produced by the intersection of a tilted rock or structure with a horizontal plane. Dip is the angle of inclination of the tilted layer, also measured from the horizontal plane (Fig.). Fig. the strike and dip PLATE TECTONICS In the early 1960s, geologists developed the plate tectonics theory, which explains earthquakes, volcanic eruptions, mountain building, moving continents, and many other geologic events. Prof. Adel Kamel Mohamed AN OVERVIEW OF PLATE TECTONICS The plate tectonics theory briefly describes the Earth’s outer layer, called the lithosphere, as a shell of hard, strong rock. This shell is broken into seven large (and several smaller) segments called tectonic plates, sometimes known as lithospheric plates (Fig.). The tectonic plates float on the layer below, called the asthenosphere. The asthenosphere, like the lithosphere, is rock. But the asthenosphere is so hot that 1 to 2 percent of it is melted. As a result, it is plastic, and weak. The lithospheric plates glide slowly over the asthenosphere like sheets of ice drifting across a pond (Fig.). Fig. The lithospheric plates Prof. Adel Kamel Mohamed Fig. The Earth's shells Most of the Earth’s major deformation occurs at plate boundaries, the zones where tectonic plates meet and interact. Neighboring plates can move relative to one another in three different ways (Fig.): 1- At a divergent boundary, two plates move apart, or separate. 2- At a convergent boundary, two plates move toward each other, and at a 3- transform boundary, they slide horizontally past each other. Table summarizes characteristics and examples of each type of plate boundary. Plate interactions at these boundaries build mountain ranges and create earthquakes and volcanic eruptions. Prof. Adel Kamel Mohamed Fig. 2-3 Types of plate boundaries Prof. Adel Kamel Mohamed PLATES AND PLATE TECTONICS The lithosphere is less dense than the asthenosphere; consequently it floats on the asthenosphere much as ice floats on water. Figure 2–1 shows that the lithosphere is broken into seven large tectonic plates and several smaller ones. These tectonic plates drift horizontally over the asthenosphere. The plates move slowly, at rates ranging from less than 1 to about 16 centimeters per year. Because the plates move in different directions, they bump and grind against their neighbors at plate boundaries. The great forces generated at a plate boundary build mountain ranges and cause volcanic eruptions and earthquakes. These processes and events are called tectonic activity that constructs mountain chains and ocean basins. In contrast to plate boundaries, the interior portion of a plate is usually tectonically quiet because it is far from the zones where two plates interact. Types of plate boundaries Divergent Plate Boundaries At a divergent plate boundary, also called a spreading center and a rift zone, two lithospheric plates spread apart (Fig.). The underlying asthenosphere then oozes upward to fill the gap between the separating plates. As the asthenosphere rises between separating plates, some of it melts to form molten rock called magma. Most of the magma rises to the Earth’s surface, where it cools to form new crust, the top layer of the lithosphere. Most of this activity occurs beneath the seas because most divergent plate boundaries lie in the ocean basins. Prof. Adel Kamel Mohamed Fig. Divergent plate boundaries Both the asthenosphere and the lower lithosphere (the part beneath the crust) are parts of the mantle and thus have similar chemical compositions. The main difference between the two layers is one of mechanical strength (physical state). The hot asthenosphere is weak and plastic, but the cooler lithosphere is strong and hard. As the asthenosphere rises, it cools, gains mechanical strength, and, therefore, transforms into new lithosphere. In this way, new lithosphere continuously forms at a divergent boundary. The lithosphere spreads laterally away from the spreading center and then cools and becomes thicker as it moves away from the spreading center. The ocean floor expands that is known as the process of sea floor spreading. 1- The Mid-Oceanic Ridge: Rifting in the Oceans A spreading center lies directly above the hot, rising asthenosphere. The newly formed lithosphere at an oceanic spreading center is hot and therefore of low density. Consequently, the sea floor at a spreading center floats to a high elevation, forming an undersea mountain chain called the mid-oceanic ridge of volcanic rocks (Fig.). But as lithosphere migrates away from the spreading center, it cools and becomes denser and thicker; as a result, it sinks (Fig.). For this reason, the sea floor is high at the mid-oceanic ridge and lower away from the ridge. Prof. Adel Kamel Mohamed Fig. Mid-Oceanic ridge Fig. Mid-Oceanic ridge and lithosphere sinks away from spreading center 2- Splitting Continents: Rifting in Continental Crust A divergent plate boundary can rip a continent in a process called continental rifting. A rift valley develops in a continental rift zone because continental crust stretches, fractures, and sinks as it is pulled apart. Continental rifting is now taking place along a zone called the East African rift and the Red Sea – Gulf of Suez rift. If the rifting continues, eastern Africa will separate from the main portion of the continent, and a new ocean basin will Prof. Adel Kamel Mohamed open between the separating portions of Africa. Thus the Red Sea is called a proto-ocean. Convergent Plate Boundaries At a convergent plate boundary, two lithospheric plates move toward each other. Convergence can occur (1) between a plate carrying oceanic crust and another carrying continental crust, (2) between two plates carrying oceanic crust, and (3) between two plates carrying continental crust. Differences in density determine what happens where two plates converge. When two plates converge, the denser plate dives beneath the lighter one and sinks into the mantle. This process is called subduction. Generally, only oceanic lithosphere can sink into the mantle. Due to the lower density of the continental crust, it floats up instead of sinking into the mantle at convergent plate boundaries. A subduction zone is a long, narrow belt where a lithospheric plate is sinking into the mantle. A global balance is maintained between the creation of new lithosphere at spreading centers and the destruction of old lithosphere at subduction zones. 1- Convergence of Oceanic Crust with Continental Crust When an oceanic plate converges with a continental plate, the denser oceanic plate sinks into the mantle beneath the edge of the continent. Today, oceanic plates are sinking beneath the western edge of South America (Fig.). 2- Convergence of Two Plates Carrying Oceanic Crust The newly formed oceanic lithosphere is hot, thin, and light, but as it spreads away from the mid-oceanic ridge, it becomes older, cooler, thicker, and denser. Thus, the density of oceanic lithosphere increases with its age. When two oceanic plates converge, the denser one sinks into the mantle. Oceanic subduction zones are common in the southwestern Pacific Ocean. Prof. Adel Kamel Mohamed Fig. The oceanic lithosphere sinks beneath the continental lithosphere 3- Convergence of Two Plates Carrying Continents If two converging plates carry continents, neither can sink into the mantle because of their low densities. In this case, the two continents collide and crumple against each other, forming a huge mountain chain. The Himalayas, the Alps, and the Appalachians all formed as results of continental collisions (Fig.). Fig. A collision between India and Asia formed the Himalayas. Prof. Adel Kamel Mohamed Transform Plate Boundaries A transform plate boundary forms where two plates slide horizontally past one another as they move in opposite directions (Fig.). California’s San Andreas Fault is the transform boundary between the North American plate and the Pacific plate. The Anatomy of A Tectonic Plate The nature of a tectonic plate can be summarized as follows: 1. A plate is a segment of the lithosphere; thus, it includes the uppermost mantle and all of the overlying crust. 2. A single plate can carry both oceanic and continental crust. The average thickness of lithosphere covered by oceanic crust is 75 kilometers, whereas that of lithosphere covered by a continent is 125 kilometers (Fig.). Lithosphere may be as little as 10 to 15 kilometers thick at an oceanic spreading center. Fig. Average thickness of the lithosphere 3. A plate is composed of hard, mechanically strong rock. Prof. Adel Kamel Mohamed 4. A plate floats on the underlying hot, plastic asthenosphere and glides horizontally over it. 5. A plate margin is tectonically active. Earthquakes and volcanoes are common at plate boundaries. In contrast, the interior of a lithospheric plate is normally tectonically stable. 6. Tectonic plates move at rates that vary from less than 1 to 16 centimeters per year. CONSEQUENCES OF MOVING PLATES The followings are some of consequences of plate tectonics processes. Volcanoes Volcanic eruptions are common at both divergent and convergent plate boundaries. Three factors can melt rock to form magma and cause volcanic eruptions. The most obvious is rising temperature. However, hot rocks also melt to form magma if pressure decreases or if water is added to them. At a divergent boundary, pressure decreases, as a result, portions of the asthenosphere melt to form huge quantities of basaltic magma, which erupts onto the Earth’s surface. The mid-oceanic ridge is a submarine chain of volcanoes and lava flows formed at a divergent plate boundary. Volcanoes are also common in continental rifts, including the East African rift. At a convergent plate boundary, cold, dense oceanic lithosphere dives into the asthenosphere. The sinking plate carries water-soaked mud and rock that once lay on the sea floor. As the sinking plate descends into the mantle, it becomes hotter. The heat drives off the water, which rises into the hot asthenosphere beneath the opposite plate. The water melts asthenosphere rock to form huge amounts of magma in a subduction zone. The magma then rises through the overlying lithosphere. Some solidifies within the crust, and some erupts from volcanoes on the Earth’s surface. Earthquakes Earthquakes are common at all three types of plate boundaries, but less common within the interior of a tectonic plate. Quakes concentrate at Prof. Adel Kamel Mohamed plate boundaries simply because those boundaries are zones of deep fractures in the lithosphere where one plate slips very slowly past another. One plate suddenly slips a few centimeters or even a few meters past its neighbor. An earthquake is vibration in rock caused by these abrupt movements. Mountain Building Many of the world’s great mountain chains formed at subduction zones. The great volume of magma rising into the crust thickens the crust, causing mountains to rise. Volcanic eruptions build chains of volcanoes. Great chains of volcanic mountains form at rift zones because the new, hot lithosphere floats to a high level, and large amounts of magma form in these zones. The mid-oceanic ridge and the East African rift are examples of such mountain chains. Oceanic Trenches An oceanic trench is a long, narrow trough in the sea floor that develops where a subducting plate sinks into the mantle (Figs. 2–3b and 2– 8). To form the trough, the sinking plate drags the sea floor downward. A trench can form wherever subduction occurs—where oceanic crust sinks beneath the edge of a continent, or where it sinks beneath another oceanic plate. Trenches are the deepest parts of the ocean basins. The deepest point on Earth is in the Mariana trench in the southwestern Pacific Ocean, where the sea floor is as much as 10.9 kilometers below sea level. Prof. Adel Kamel Mohamed EARTHQUAKES DEFINITION: An earthquake is a sudden motion or trembling of the Earth caused by the abrupt release of energy that is stored in rocks. Most earthquakes occur along plate boundaries, where huge tectonic plates separate, converge, or slip past one another. Prof. Adel Kamel Mohamed WHAT IS AN EARTHQUAKE? How do rocks store energy, and why do they suddenly release it as an earthquake? Stress is a force per unit area exerted against an object. Tectonic stress, resulted from movement of lithospheric plates, forces stress rocks that it changes volume and shape of stressed rocks. If a rock is stressed slowly, it first deforms in an elastic manner as the stress is removed, the object springs back to its original size and shape, such as a rubber band that exhibits elastic deformation (Fig.). Fig. The behavior of a rock as stress increases in graphical form (a), in schematic form (b). At first the rock de- forms by elastic deformation in which the amount of deformation is directly proportional to the amount of stress. Beyond the elastic limit, the rock deforms plastically and a small amount of additional stress causes a large increase in distortion. Finally, at the yield point, the rock fractures. Many stressed rocks deform elastically and then rupture, with little or no intermediate plastic deformation. Prof. Adel Kamel Mohamed Every rock has a limit beyond which it cannot deform elastically. Under certain conditions, when its elastic limit is exceeded, a rock continues to deform like putty. This behavior is called plastic deformation. A rock that has deformed plastically retains its new shape when the stress is released (Fig.). Earthquakes do not occur when rocks deform plastically. Fig. Rocks may deform plastically when stressed. Plastic deformation contorted the layer- ing in metamorphic rocks in Connecticut. Under other conditions, an elastically stressed rock may rupture by brittle fracture (Fig.). The fracture releases the elastic energy, and the surrounding rock springs back to its original shape. This rapid motion creates vibrations that travel through the Earth and are felt as an earthquake. Fig. A rock stores elastic energy when it is dis- torted by a tectonic force.When the rock fractures, it snaps back to its original shape, creating an earthquake. In the process, the rock moves along the fracture. Prof. Adel Kamel Mohamed Earthquakes also occur when rock slips along previously established faults. Tectonic plate boundaries are huge faults that have moved many times in the past and will move again in the future (Fig.). Rocks near a plate boundary are stretched or compressed as friction prevents the plates from slipping past one another continuously. When the accumulated elastic energy of these rocks overcomes the friction between the moving plates, the rock suddenly slips along the fault, generating an earthquake. Fig. California’s San Andreas fault, the source of many earthquakes, is the boundary between the Pacific plate, on the left in this photo, and the North American plate, on the right. (R.E.Wallace/USGS). EARTHQUAKE WAVES A wave transmits energy from one place to another. Waves that travel through rock are called seismic waves alike sonic waves. Earthquakes and explosions produce seismic waves. Seismology is the study of earthquakes and the nature of the Earth’s interior based on evidence from seismic waves. An earthquake produces several different types of seismic waves. Body waves travel through the Earth’s interior. They radiate from the initial rupture point of an earthquake, called the focus (Fig.). The point on the Earth’s surface directly above the focus is the epicenter. Prof. Adel Kamel Mohamed Fig. Body waves radiate outward from the focus of an earthquake. During an earthquake, body waves carry some of the energy from the focus to the surface. Surface waves then radiate from the epicenter along the Earth’s surface. Although the mechanism is different, surface waves undulate across the ground like the waves that ripple across the water after you throw a rock into a calm lake. BODY WAVES Two main types of body waves travel through the Earth’s interior. A P- wave (also called a compressional wave) is an elastic wave that causes alternate compression and expansion of the rock (Fig.). P-waves travel through air, liquid, and solid material. P-waves travel faster in the mantle (about 8 km/sec in the upper Mantle) than in the Earth's crust (about 4-7 km/sec) due to increasing rock density with depth. P-waves are called primary waves because they are so fast that they are the first waves to reach an observer. A second type of body wave, called an S-wave, is a shear wave. An S- wave can be illustrated by tying a rope to a wall, holding the end, and giving it a sharp up-and- down jerk (Fig.). Although the wave travels parallel to the rope, the individual particles in the rope move at right angles to the rope length. Prof. Adel Kamel Mohamed Fig. The P- (left handed one) and S-waves (right-handed one). A similar motion in an S wave produces shear stress in rock and gives the wave its name. S waves are slower than P waves and travel at speeds between 3 and 4 km/sec in the crust. As a result, S-waves arrive after P- waves and are the secondary waves to reach an observer. Unlike P-waves, S- waves move only through solids. Because molecules in liquids and gases are only weakly bound to one another, they slip past each other and thus cannot transmit a shear wave. SURFACE WAVES Surface waves travel more slowly than body waves. Two types of surface waves occur simultaneously in the Earth (Fig.). A Rayleigh wave moves with an up-and-down rolling motion like an ocean wave. Love waves produce a side-to-side vibration. Thus, by an earthquake, the Earth’s surface rolls like ocean waves and writhes from side to side like a snake (Fig.). Prof. Adel Kamel Mohamed Fig. 6.7 The surface waves MEASUREMENT OF SEISMIC WAVES A seismograph is a device that records seismic waves. In this device, a weight was suspended from a spring. A pen attached to the weight was aimed at the zero mark on a piece of graph paper (Fig.). The graph paper was mounted on a rotary drum that was attached firmly to bedrock. During an earthquake, the graph paper jiggled up and down, but inertia kept the pen stationary. As a result, the paper moved up and down beneath the pen. The rotating drum recorded earthquake motion over time. This record of Earth vibration is called a seismogram (Fig.). Modern seismographs use electronic motion detectors which transmit the signal to a computer. Fig. A seismograph records ground motion during an earthquake. Prof. Adel Kamel Mohamed Fig. The seismogram The strength of the earthquake represents the energy released by a quake. In 1935 Charles Richter devised the Richter scale to express earthquake magnitude. Modern equipment and methods enable seismologists to measure the amount of slip and the surface area of a fault that moved during a quake. The product of these two values allows them to calculate the moment magnitude. Most seismologists now use moment magnitude rather than Richter magnitude because it more closely reflects the total amount of energy released during an earthquake. The largest possible earthquake is determined by the strength of rocks. A strong rock can store more elastic energy before it fractures than a weak rock. LOCATING THE SOURCE OF AN EARTHQUAKE The source of an earthquake is determined by the distance from a recording station to both the epicenter and focus of an earthquake. If a seismograph is located close to an earthquake epicenter, the different seismic waves will arrive in rapid succession for the same reason that the thunder and lightning come close together when a storm is close (Fig.). Prof. Adel Kamel Mohamed Fig. The time intervals between arrivals of P, S, and L waves at a recording sta- tion increase with distance from the focus of an earthquake. On the other hand, if a seismograph is located far from the epicenter, the S-waves arrive at correspondingly later times after the P waves arrive, and the surface waves are even farther behind (Fig.). Geologists use a time-travel curve to calculate the distance between an earthquake epicenter and a seismograph. To make a time-travel curve, a number of seismic stations at different locations record the times of arrival of seismic waves from an earthquake such graph of Figure, which shows that the recording station is about 1900 km from the epicenter. The exact location of the epicenter 1900 km apart from the detected seismograph is not determine, as it occurs on a circumference of a circle 1900 km radius from the detected seismograph. To pinpoint the location of an earthquake, geologists compare data from three or more recording stations. If a seismic station in New York City records an earthquake with an epicenter 6750 km away and the same epicenter is reported to be 2750 km from a seismic station in London and 1700 km from one in Godthab, Greenland. If one circle is drawn for each recording station, the arcs intersect at the epicenter of the quake (Fig.). Prof. Adel Kamel Mohamed Figure A time-travel curve.With this graph you can calculate the distance from a seismic station to the source of an earthquake. In the example shown, a 3-minute delay be- tween the first arrivals of P waves and S waves corresponds to an earthquake with an epicenter 1900 kilometers from the seismic station. Fig. Locating an earthquake. The distance from each of three seismic stations to the earthquake is determined from time- travel curves. The three arcs are drawn. They intersect at only one point, which is the epicenter of the earth- quake. Prof. Adel Kamel Mohamed HOW ROCK AND SOIL INFLUENCE EARTHQUAKE DAMAGE If structures on bedrock can withstand the earthquake shaking, they will survive. In many places, structures are built on sand, clay, or silt. Sandy sediment and soil commonly settle during an earthquake. This displacement tilts buildings, breaks pipelines and roadways, and fractures dams. To avert structural failure in such soils, engineers drive steel or concrete pilings through the sand to the bedrock below. These pilings anchor and support the structures even if the ground beneath them settles. The sudden shock of an earthquake to soil saturated with water, increases stress that is transferred to the pore water, and the pore pressure may rise sufficiently to suspend the grains in the water. In this case, the soil loses its shear strength and behaves as a fluid. This process is called liquefaction. When soils liquefy on a hillside, it flows downslope, carrying structures along with it. Fig. Formation of a tsunami. If a portion of the sea floor drops during an earthquake, the sea level falls with it. Water rushes into the low spot and overcompensates, creating a bulge. The long, shallow waves build up when they reach land. Prof. Adel Kamel Mohamed TSUNAMIS When an earthquake occurs beneath the sea, part of the sea floor rises or falls (Fig.). Water is displaced in response to the rock movement, forming a wave that is a tsunami. In the open sea, a tsunami is so flat that it is barely detectable. When the wave approaches the shallow water near shore, the base of the wave drags against the bottom and the water stacks up, increasing the height of the wave. The rising wall of water then flows inland. The fundamental concepts about Geophysics Introduction Geophysics is the study of the earth’s properties by applying physical theories and using instruments for measurement. Since antiquity men have been studying the earth’s properties in order to predict earthquakes, but real progress happened in the 1500’s and has continued since then. Scientists began working to understand for example how magnetism and gravity are related to earthquakes. Magnetism is an attraction to metals such as iron, nickel, cobalt, and their alloys. Its main character is a magnetic force. Though metals are more strongly influenced by magnetism all materials are under its influence to a degree. The objective of geophysics The main objectives of geophysical methods is to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (density, velocity, porosity…) and fluid content. Geophysics nowadays is highly effective in geotechnical and engineering studies, environmental and archeological studies. It is also has many applications in groundwater investigations. Prof. Adel Kamel Mohamed Types of geophysical methods Geophysical methods provide information about the physical properties of the earth’s subsurface. There are two general types of methods: Active, which measure the subsurface response to electromagnetic, electrical, and seismic energy; and passive, which measure the earth's ambient magnetic, electrical, and gravitational fields. Geophysical methods can also be subdivided into either surface or borehole methods. Surface geophysical methods are generally non-intrusive and can be employed quickly to collect subsurface data. Borehole geophysical methods require that wells or borings be drilled in order for geophysical tools to be lowered through them into the subsurface. This process allows for the measurement of in situ conditions of the subsurface. Various geophysical surveying methods have been and are used on land and offshore ( Table 1). Each of these methods measures something that is related to subsurface rocks and their geologic configurations. Rocks and minerals in the earth vary in several ways. These include: Density – mass per unit volume. The gravity method detects lateral variations in density. Both lateral and vertical density variations are important in the seismic method. Magnetic susceptibility – the amount of magnetization in a substance exposed to a magnetic field. The magnetic method detects horizontal variations in susceptibility. Propagation velocity – the rate at which sound or seismic waves are transmitted in the earth. It is these variations, horizontal and vertical, that make the seismic method applicable to petroleum exploration. Resistivity and induced polarization – Resitivity is a measure of the ability to conduct electricity and induced polarization is frequency-dependent variation in resistivity. Electrical methods detect variations of these over a surface area Prof. Adel Kamel Mohamed Self-potential - ability to generate an electrical voltage. Electrical methods also measure this over a surface area. Electromagnetic wave reflectivity and transmissivity – reflection and transmission of electromagnetic radiation, such as radar, radio waves and infrared radiation, is the basis of electromagnetic methods. Table (1): illustrate the main different types of geophysicsical methods Some necessary concepts Non-uniqueness and ambiguity concept Data collected with geophysical tools are often difficult to interpret because a given data set may not indicate specific subsurface conditions (i.e., solutions are not unique). Instead, data provided by these tools indicate anomalies which can often be caused by numerous features. As a result, geophysical methods are most effectively used in combination with other site information (e.g., data from different geophysical methods, sampling and analytical tools, geological and historic records). A combination of these sources is often Prof. Adel Kamel Mohamed necessary to resolve ambiguities in geophysical plots (i.e., the graphical representation of data produced by a specific method). Non-uniqueness can arise from two distinct factors. (1) The basic physics Example : In gravity exploration, we can only determine the excess mass of a buried sphere. We cannot determine the density and radius that combine to give this value of excess mass. This type of non-uniqueness cannot be overcome, not even with expensive computer packages. However, additional (independent) data can be used to address non-uniqueness. For example, if we have density measurements of the target, we could determine the radius of the sphere. (2) Noise in data Example: In gravity exploration noise in the data will introduce errors. Errors will introduce errors into estimates of the depth of sphere. This type of non- uniqueness can be overcome by improving data quality and quantity. (3) Forward and inverse problem concept The figure given below is a schematic diagram showing the concept of forward and inverse problem. (a): Forward problems Prof. Adel Kamel Mohamed (b): Inverse problems Figure (): shows the concept of forward and inverse problem (4) The concept of Isostacy We can see in our life the ocean or the sea is beside the mountain (Fig.2) and the this does not effect on the earth's balance. Figure (): shows the oceans and mountains beside each other. The mountain range can be thought of as a block of lithosphere (crust) floating in the asthenosphere (which flows). Mountains have roots, while ocean basins have anti-roots. In his 1855 paper, George Airy who was the Prof. Adel Kamel Mohamed Astronomer Royal wrote about this phenomenon‖ It means the density is constant in the crust in Airy theory. If the system is stable (no external forces) it is said to be in isostatic equilibrium. At the compensation depth, the pressure due to material above is constant at all locations (below this depth the Earth behaves as a fluid). A plateau of height h is supported by a crustal root of depth r. The normal crustal thickness is t. In this region, the acceleration of gravity is g. Pressure at a depth h in a medium of density ρ is given by P = ρgh. Pratt’s hypothesis of isostacy proposed that topography is produced by crustal blocks with varying density, that terminate at a uniform depth. Prof. Adel Kamel Mohamed Prof. Adel Kamel Mohamed Basics of Geophysical methods 1. Seismic Fundamentals Basic Concepts It is necessary to introduce some basic concepts before discussing seismic methods. That is the purpose of this Lecture Seismic Waves The principle of sound propagation, while it can be very complex, is familiar. Consider a pebble dropped in still water. When it hits the water’s surface, ripples can be seen propagating away from the center in circular patterns that get progressively larger in diameter. A close look shows that the water particles do not physically travel away from where the pebble was dropped. Instead they displace adjacent particles vertically then return to their original positions. The energy imparted to the water by the pebble’s dropping is transmitted along the surface of the water by continuous and progressive displacement of adjacent water particles. A similar process can be visualized in the vertical plane, indicating that wave propagation is a three dimensional phenomenon. Types of Seismic Waves Sound propagates through the air as changes in air pressure. Air molecules are alternately compressed (compressions) and pulled apart (rarefactions) as sound travels through the air. This phenomenon is often called a sound wave but also as a compressional wave, a longitudinal wave, or a P-wave. The latter designation will be used most often in this course. Figure illustrates P-wave propagation. Darkened areas indicate compressions. The positions of the compression at times t1 through 6t1 are shown from top to bottom. Note that the pulse propagates a distance dp over a time of 6t1– t1= 5t1. The distance traveled divided by the time taken is the propagation velocity, symbolized Vp for P-waves. Prof. Adel Kamel Mohamed P-waves can propagate in solids, liquids, and gasses. There is another kind of seismic wave that propagates only in solids. This is called a shear wave or an S-wave. The latter term is preferred in this course. Motion induced by the S- wave is perpendicular to the direction of propagation, i.e. – up and down or side-to-side. Figure illustrates propagation of an S-wave pulse. Note that the S-wave propagates a distance ds in the time 5t1. The S-wave velocity, designated as Vs, is ds/5t1. Since ds is less than dp, it can be seen that Vs, < Vp. That is, S-waves propagate more slowly than P-waves. Propagation of P wave pulse Surface waves are another kind of seismic waves that exist at the boundary of the propagating medium. The Rayleigh wave is one kind of a surface wave. It exhibits a retrograde elliptical particle motion. Figure shows motion of a particle over one period as a Rayleigh waves propagates from left to right. The Rayleigh wave is often recorded on seismic records taken on land. It is then usually called ground roll. Love waves are similar surface wave in which the particle motion is similar to S-waves. However, Love wave motion is only parallel to the surface. Prof. Adel Kamel Mohamed Applications of Seismic Methods Seismic methods provide stratigraphic information by measuring how acoustic waves travel through the subsurface. They can be used to determine the following: - Determine depth and thickness of geologic strata; - Determine depth to groundwater; - Estimate soil and rock composition; and - Help resolve fracture location and orientation. Gravity method The primary goal of studying gravity method is to provide a better understanding of the subsurface geology. The gravity method is a relatively cheap, non-invasive, non-destructive remote sensing method. It is also passive – that is, no energy need be put into the ground in order to acquire data. The small portable instrument also permits walking traverses. Measurements of gravity provide information about densities of rocks underground. There is a wide range in density among rock types, and therefore geologists can make inferences about the distribution of strata. The Gal (for Galileo) is the cgs unit for acceleration where one Gal equals 1 centimenter per second squared. Because variations in gravity are very small, units for gravity surveys are generally in milligals (mGal) where 1 mGal is one thousandth of 1cm/s2. Standard gravity is therefore 980.665 Gal or 980665 mGal. It is useful to remember that 1 mGal is just a bit more than 1 millionth of gn (1.01972 x 10-6 gn). Gravity Survey - Measurements of the gravitational field at a series of different locations over an area of interest. The objective in exploration work is to associate variations with differences in the distribution of densities and hence rock types. Prof. Adel Kamel Mohamed Prof. Adel Kamel Mohamed Magnetic method This method is mainly depending on measuring the earth’s total magnetic field at a particular location. Buried ferrous materials distort the magnetic field, creating a magnetic anomaly. There are two methods for measuring these anomalies--the total field method and the gradient method. The total field method utilizes one magnetic sensing device to record the value of the magnetic field at a specific location. The gradient method uses two sensors, one above the other. The difference in readings between the two sensors provides gradient information which helps to minimize lateral interferences. Total field magnetic methods are often used at sites with few cultural features. Gradiometer methods can be used in culturally complex areas. Figure presents a schematic drawing of magnetometry operating principles. Some magnetometers are very simple and do not have a data recording or processing ability. They indicate the presence of iron with a sound or meter and can be used as a rapid screening tool. Magnetometers that record data can, with the aid of data processing software, be used to estimate the size and depth of ferrous targets. Prof. Adel Kamel Mohamed Magnetic Survey - Measurements of the earth's magnetic field at a series of different locations over an area of interest. The objective in exploration work is to associate variations with differences in the distribution of magnetic susceptibility and hence rock types. Since magnetic susceptibility is greater in basement than sedimentary rocks, so magnetic method can be used for basement topography. Electrical resistivity method 1- Basic physics of electric current flow (a) Simple resistor in circuit Ohm’s Law states that for a resistor, the resistance (in ohms), R is defined as R =V/I (b) Electric current flow in a finite volume Ohm’s Law as written above describes a resistor, which has no dimensions. In considering the flow of electric current in the Earth, we must consider the flow of electric current in a finite volume. Consider a cylinder of length L and cross section A that carries a current I Prof. Adel Kamel Mohamed where ρ is the electrical resistivity of the material (ohm-m). This is an inherent property of the material. If we were to examine two cylinders made of the same material, but with different dimensions, they would have the same electrical resistivity, but different electrical resistances. Often it is more convenient to discuss the conductivity (σ) which is measured in Siemens per metre. σ = 1/ ρ Factors that will DECREASE the resistivity of a rock: (a) Add more pore fluid (b) Increase the salinity of the pore fluid - more ions to conduct electricity (c) Fracture rock to create extra pathways for current flow (d) Add clay minerals (e) Keep fluid content constant, but improve interconnection between pores Factors that will INCREASE the resistivity of a rock Prof. Adel Kamel Mohamed (a) Remove pore fluid (b) Lower salinity of pore fluid (c) Compaction - less pathways for electric current flow (d) Lithification - block pores by deposition of minerals (e) Keep fluid content constant, but decrease connection between pores The concept of borehole geophysics Well logging or borehole geophysics is the process of recording various physical, chemical, electrical, or other properties of the rock/fluid mixtures penetrated by drilling a well into the earth's mantle. A log is a record of a voyage, similar to a ship's log. In this case, the ship is a measuring instrument of some kind, and the trip is taken into and out of the wellbore. In its most usual form, an oil well log is a record displayed on a graph with the measured physical property of the rock on one axis and depth (distance from the surface) on the other axis. More than one property may be displayed on the same graph. Prof. Adel Kamel Mohamed None of the logs actually measure the physical properties that are of most interest to us, such as how much oil or gas is in the ground, or how much is being produced. Such important knowledge can only be derived, from the measured properties listed above (and others), using a number of assumptions which, if true, will give reasonable estimates of hydrocarbon reserves. Thus, analysis of log data is required. The art and science of log analysis is mainly directed at reducing a large volume of data to more manageable results, and reducing the possible error in the assumptions and in the results based on them. When log analysis is combined with other physical measurements on the rocks, such as core analysis or petrographic data, the work is called petrophysics or petrophysical analysis. The results of the analysis are called petrophysical properties or mappable reservoir properties. The petrophysical analysis is said to be ―calibrated‖ when the porosity, fluid saturation, and permeability results compare favourably with core analysis data. Further confirmation of petrophysical properties is obtained by production tests of the reservoir intervals. The use of well logs for evaluating mineral deposits other than oil and gas, such as coal, potash, uranium, and hard rock sequences has been practiced since the early 1930’s and is widespread today. Although the vast majority of logs are run to evaluate oil and gas wells, an increased number are being run yearly for other purposes, including evaluation of geothermal energy and ground water. When logs are used for purposes other than evaluation of oil and gas, they are often called geophysical logs instead of well logs. The science is called borehole geophysics instead of petrophysics. This difference is merely a matter of semantics and training. The theory doesn't change - just the nomenclature, and sometimes the emphasis. Rock properties or characteristics Rock properties or characteristics which affect logging measurements are: