Earth's Interior - Seismic Waves and Layers PDF

1 UNIT 2 SEISMIC WAVES EQUILIBRIUM – (rest position) the undisturbed position of particles or fields when they are not vibrating. CREST - the highest point above the rest position. TROUGH - the lowest point below the rest position. AMPLITUDE (A)- the maximum displacement of a poi...

1 UNIT 2 SEISMIC WAVES EQUILIBRIUM – (rest position) the undisturbed position of particles or fields when they are not vibrating. CREST - the highest point above the rest position. TROUGH - the lowest point below the rest position. AMPLITUDE (A)- the maximum displacement of a point of a wave from its rest position. WAVELENGTH - distance covered by a full cycle of the wave. It is measured in meters. PERIOD (P) - the time taken for a full cycle of the wave. It is measured in seconds. FREQUENCY (f) - the number of waves passing a point each second. It is measured in hertz Hz. TWO BASIC TYPES OF WAVE MOTION FOR MECHANICAL WAVES LONGITUDINAL wave TRANSVERSE wave  A British geologist who made the first clear identification of the separate arrivals of P-waves, S-waves, and surface waves on seismograms in 1897. Richard Oldham SEISMIC WAVES are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. SEISMIC WAVES Body waves Surface waves P - waves (Primary wave) Body Waves (Compressional wave) Travel through the Earth’s body. A type of body wave that pushes & pulls rock and liquid in the same direction as the wave moves. It is a type of longitudinal wave in which the particles in a medium travels in the same direction (parallel) as the wave travels. The fastest kind of seismic wave. Arrives at recording stations first (Primary). It can move through solid, liquid, and gas. It shakes the ground back and forth as it travels. S - waves (Secondary wave) Body Waves (Shear wave) Travel through the Earth’s body. A type of body waves that move rock up and down. It is a type of transverse wave in which the particles in a medium travels perpendicular to its wave motion. It is slower than a P-wave. Arrives at recording stations second (secondary). Can only travel through solid rock. Love waves Surface Waves Travel at the surface of the crust. A type of surface waves that move the ground from side-to-side. It is a type of transverse wave in which the particles in a medium travels perpendicular to its wave motion. Fastest surface waves but slower than P and S waves Arrives at recording stations after S-waves  A British mathematician who predicted the existence of Love waves mathematically in 1911. Augustus Edward Hough Love Rayleigh waves Surface Waves Travel at the surface of the crust. A type of surface waves that causes the ground to shake in an elliptical motion, with no transverse or perpendicular motion. It moves the ground in a rolling circular motion, the same way ocean waves move. Slowest surface waves  A British physicist who first mathematically demonstrated Rayleigh wave existence in 1885. John William Strutt, Lord Rayleigh 2 LAYERS of the EARTH The Crust * The Earth’s crust is the outermost surface. *It is a very thin layer of solid rock. It is the thinnest layer of the Earth. *The crust is 5-35km thick beneath the land and 1-8km thick beneath the oceans. * The crust of the Earth is broken into many pieces called plates. The movement of these plates causes earthquakes. The Lithosphere On your mantle layer, draw a dotted line around the very edge of the circle. This is the lithosphere. The lithosphere is composed of part of the crust and the upper part of the mantle (the top 100 km). It is composed of hard, brittle rock The Asthenosphere * The athenosphere is the part of the mantle underneath the lithosphere. Draw a dashed line about an inch under your dotted line. * It is made of molten rock and metal so that it “flows” like hot asphalt. * The athenosphere is the part of the mantle that moves and causes the tectonic plates of the crust to move as well. The Mantle * The mantle is the largest layer of the Earth. It is 2900km thick. *It includes the lithosphere and athenosphere. *It is relatively flexible—it flows like very viscous liquid. *It is very hot—1600°F at the top and 4000°F towards the center of the Earth. The Core * The core of the Earth is like a ball of very hot metals. The core is divided into 2 layers—Outer and Inner. * The core is SO hot and has SO much pressure that if you were to go there—you would be squished into something even smaller than a marble. The Outer Core * The outer core is liquid metal iron and nickel with 10% sulfur and/or oxygen. * It is very hot— 4000-9000°F. * The outer core is 2,250km thick. The Inner Core * The inner core is solid metal due to the extreme heat and pressure. * It is composed of iron and nickel. It is responsible for the magnetic field the Earth generates. * The inner core is 800km thick and is 9000°F. 3 Continental Drift Theory PANGAEA Greek word which means “All Earth” or “All Land” A supercontinent that incorporated almost all landmasses on Earth during the Permian Period. Alfred Lothar Wegener Proponent of Continental Drift Theory Continental Drift Theory → states that all continents were once one large landmass called PANGAEA that broke apart into pieces and moves slowly to its current location. Evolution of Pangaea PERMIAN PERIOD 250 million years ago TRIASSIC PERIOD 200 million years ago JURASSIC PERIOD 145 million years ago CRETACEOUS PERIOD 65 million years ago PRESENT DAY  Discontinuities Inside the Earth A. Conrad discontinuity → transition zone between upper and lower crust B. Mohorovicic discontinuity → transition zone between crust and upper mantle C. Repiti discontinuity → transition zone between upper and lower mantle D. Gutenberg discontinuity → transition zone between lower mantle and outer core E. Lehmann discontinuity → transition zone between outer and inner core 4 EVIDENCES of CONTINENTAL DRIFT THEORY  Continental Fit (Matching Edges of Continents) The Eastern edges of South America and Western edges of Africa can be fitted like a jigsaw puzzle. Fossil Evidence  Fossilized leaves of an extinct plant Glossopteris were found in 260 million years old rocks located in the continents of Southern Africa, Australia, India, and Antarctica, which are now separated from each other by wide oceans. Fossils of Cynognathus and Lystrosaurus (Triassic land reptiles), and Mesosaurus (freshwater reptile) were also found in different continents.  Matching Rocks and Rock Formations Rock formations in South America (Santa Catalina system) line up with Africa (rock strata of Karroo System) as if it was a long mountain range. The Ghana mountain ranges also line up with Brazil. Rocks samples from different continents were also found to have the same age and composition which suggest that they were of same origin. Coal Deposits  COAL forms from the compaction and decomposition of swamp plants in tropical areas. Coal deposits were found in Antarctica which suggests that it once had a warmer climate. Since the South Pole has never had a tropical climate, it must have been in a warmer location than it is now. Antarctica must have been situated in a different location on the planet. Ancient Climates The discovery of coal deposits and glacial markings in some continents suggest that although those areas are very far part now, they once had a very similar climate which are different from now. Glacial Markings/Striations ▪ Wegener’s theory was rejected by scientists because he could not explain what force pushes or pulls continents. ???  Wegener’s Problem He could not find the force that was causing the continents to drift. Because of this, he could not convince anyone that continents could move. He died in Greenland on an expedition. At the time of his death, no one believed his hypothesis! 5 EVIDENCES of CONTINENTAL DRIFT THEORY  Harry Robert Dietz Hammond Hess  HARRY HAMMOND HESS was a geologist and Navy submarine commander during World War II. Part of his mission had been to study the deepest parts of the ocean floor in which he used SONAR. In 1946 he had discovered that hundreds of flat-topped mountains, perhaps sunken islands, shape the Pacific floor. SONAR (Sound Navigation and Ranging) → it is helpful for exploring and mapping the ocean with the use of sound waves. Harry Hess and his team carefully examined maps of the mid-ocean ridge system.  SEAFLOOR SPREADING is a geologic process in which tectonic plate (large slabs of Earth's lithosphere) split apart from each other.  HARRY HAMMOND HESS In 1960, American geophysicist Harry Hammond Hess proposed that the drifting continents were caused by spreading of seafloor that is why the continents drifted apart, this what we know as Seafloor Spreading Theory. Seafloor Spreading Theory states that “new ocean crust is being created at mid-ocean ridges (which are large underwater mountain chains) and destroyed at deep sea trenches.” According to Seafloor Spreading Theory Hot, less dense material below the Earth’s crust rises towards the mid-ocean ridge. Magma flows sideways carrying seafloor away from the ridge, and creates a crack in the crust. The magma flows out of the crack, cools down and becomes the new seafloor. Overtime, the new ocean crust pushed the old oceanic crust far from the ridge and subducted.  Evidences of Oldest rocks are found far Seafloor Spreading from the mid-ocean ridge. Seafloor spreading and new crust is being created at the mid-ocean ridge. Sediments near the ocean ridge are thinner and progressively thickens as you move away. Evidences of Seafloor Spreading He age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge.  How Seafloor Spreading disproves Continental Drift Theory?  CONTINENTAL DRIFT SEAFLOOR SPREADING THEORY THEORY Continents moved through The ocean itself is a site of unmoving oceans. tectonic activity. Summary Seafloor spreading is a geologic process that occurs at divergent plate boundaries in which oceanic crusts split apart and magma from the mantle to form new seafloor. Summary The Earth doesn’t expand in relation to seafloor spreading but rather the oldest part of the seafloor usually undergoes subduction process and melts as it reaches the mantle in the subduction zone. Summary The continuous process of seafloor spreading process caused the continents to drift apart providing an explanation to continental drift theories mechanism. Did you know that… Seafloor spreading and subduction keeps the shape of the Earth. Did you know that… Seafloor spreading creates new crust while subduction destroys old crust. Did you know that… Seafloor spreading and subduction roughly balance each other, so the shape and diameter of the Earth remain constant. 6 Rate of Seafloor Spreading and MAGNETIC REVERSAL MAGNETIC REVERSAL It happens when the North Pole is transformed into the South Pole, and the South Pole becomes the North Pole because of the changing direction of the flow of materials in the Earth's liquid outer core. It is also called a magnetic ‘flip’ of the Earth.  By 1963, geophysicists realized that Earth's magnetic field had reversed polarity many times, with each reversal lasting less than 200,000 years. Surveys of either side of the Mid-Atlantic Ridge showed a symmetrical pattern of alternating polarity stripes. MAGNETIC REVERSAL  Rocks of the same age in the seafloor crust would have taken on the magnetic polarity at the time that part of the crust formed.  The crystalized irons in rocks found in the seafloor act as a magnetic compass that can tell the Earth's magnetic field direction.  Seafloor spreading was strengthened with the discovery of the magnetic rocks near the ridge. MAGNETIC POLARITY MAP  Rate of Seafloor Spreading Rate of Seafloor Spreading Formula DISTANCE RATE = TIME CONVERSION FACTOR 1 My = 1,000,000 yr. 1 km = 100,000 cm. If subduction is faster than seafloor spreading, the ocean shrinks. If seafloor spreading is faster than seafloor spreading, the ocean widens. What do you think will happen to the size of the Pacific Ocean and Atlantic Ocean millions of years from now? Atlantic Ocean will be bigger than Pacific Ocean due to the divergence of plates in its location. 7 Earth’s Mechanisms and Plate Tectonics Theory  Earth’s Mechanisms 1. MANTLE CONVECTION CURRENT It is a heat transfer process that occurs in the lower mantle where magma tends to rise as it expands and sinks as it contracts due to the differences in the temperature and densities of the magma.  Convection current happens in the mantle because of the heat generated by the core. The heat is produced by the decay or breakdown of radioactive elements.  The temperature difference between the upper and lower mantle requires heat transfer (CONVECTION CURRENT) to occur.  The hot, less Relatively cooler dense magma near magma from higher in the core slowly the mantle slowly sinks moves upward. toward the mantle. HOT material COOL COOL material material  As the warmer magma rises, it also cools, eventually pushed aside by warmer magma and sinking back toward near the core. Mantle Convection Current Process 1. The magma in the mantle is heated by the decomposition of radioactive elements in the core. 2. Hot less dense magma rises beneath the crust and move sideways. 3. As the magma cools down, it becomes denser and sinks towards the mantle. 4. With hot rising currents and cool sinking currents are regularly repeated and become a cycle, it creates a convection current due to difference in temperature and densities of magma.  Plate Tectonics provides an explanation for Wegener’s Continental Drift Theory.  The convection movement in the mantle moves the asthenosphere, carrying along the lithospheric plates and drives plate motion.  2. It is the pulling force exerted by a cold, SLAB dense oceanic plate plunging into the mantle due to its own weight and gravity. PULL  3. Gravitational force that causes a plate to RIDGE move away from the crest of an ocean ridge, and into a subduction zone. PUSH  Earth’s Mechanism that Drives Plate Motion Convection Slab current Ridge pull push Plate Tectonics Theory  PLATE TECTONICS THEORY → states that the Earth’s solid lithosphere is divided into tectonic plates that move over the asthenosphere slowly and resulted to the formation of geologic features and processes.  The Theory of Plate Tectonics helps explain the formation and destruction of the Earth’s crust and its movement over time. Scientists believe that the plates’ movement is due to heat from the core that resulted to convection currents in the mantle. Continental Drift Theory Seafloor Spreading Theory Lacks an explanation on the Explains the causes of continental drift through causes of continental drift. seafloor spreading process and magnetic reversal. PLATE TECTONICS THEORY (Unifying Theory) Explains the causes of plate movement through convection currents, slab pull, and ridge push.