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Transcript
Chapter 10 – Plate Tectonics and Earthquakes If the continents drift, where are we going? I. Once Upon a One-Land Planet A. Abraham Ortelius- in 1587 this famous mapmaker noticed that South America and Africa looked like mirror images. In his book Thesaurus Geographicus he argues that the continents were once joined. B. Alfred Wegener- 1912 – the thirty-two year old meteorologist unveiled his theory of continental drift 1930 Greenland. The last photo of Alfred Wegener alive. He is on the left. He was 50 years old. 1. Wegener theorized that 200 million years ago Earth was one large continent called Pangaea (meaning “all Earth”) and one large ocean called Panthalassa. 2. Gradually the continent split into two large landmasses a. Laurasia in the northern hemisphere b. Gondwanaland in the southern hemisphere C. Response from scientists at the time was very negative, but Wegener persisted D. About 30 years after Wegener’s death, evidence convinced almost all scientists that continental drift was an acceptable, useful theory II. Evidence Supporting Continental Drift A. Fossils 1. Glossopteris- an extinct plant fossil located in rocks about 250 million years old are found in South Africa, Australia, India, and Antarctica a. Seeds of this plant are too large to be carried by wind, too fragile to be carried by waves b. This suggests that places the plant fossils are found were once closer together. c. Glossopteris fossils found in Antarctica suggest a climate change, which indicates a position change in the land. 2. Many other reptile fossils support continental drift such as; Cynognathus, Lystrosaurus, and Mesosaurus B. Evidence from Rocks 1. Rock formations in Africa match those in South America 2. A folded mountain chain in South Africa matches a folded mountain chain in Argentina. 3. Coal fields with distinctive layers in Brazil are similar to those in Africa 4. Glacial deposits are found in South America, Africa, India, Australia, and Antarctica Glacial Deposits 5. Rock deposits of coal, salts, and limestone derived from coral reefs suggest changes in climate caused by continental drift C. Evidence from the Ocean Floor 1. New mapping techniques of the 1950’s and 1960’s allowed scientists a closer look at the ocean floor. 2. Harry Hess, a geologist and Navy submarine commander, proposed the theory of seafloor spreading in 1960. 3. Scientists discovered the midocean ridge system, an undersea mountain range that snakes around the globe. a. At the midocean ridge, a great deal of volcanic activity causes lava to erupt from the rift valley. b. The lava hardens and forms new ocean floor. This is called ocean-floor spreading, and helps to explain how the continents drift. c. Old ocean floor that is pushed away from the ridge eventually moves down deep in the Earth along trenches through a process called subduction. Subduction Zone III. Earth’s Moving Plates A. Lithosphere- contains the crust and the uppermost part of the mantle and is composed of plates 1. There are 7 major plates named after their associated continents, or surface features (Ex. North American plate, South American plate) 2. The Pacific is the largest plate, covering one-fifth of Earth’s surface. B. Plate Boundaries- where two plates meet 1. Divergent boundary- plates move apart (diverge) ex. Midocean ridges 2. Convergent boundary- plates come together (converge) a. creates pressure and friction b. can cause earthquakes and volcanoes 3. Strike-slip boundary- two plates grind together and slip past one another horizontally (Ex. San Andreas Fault in California) C. Plate Motion Hypothesisconvection currents caused by heat deep within the Earth may be responsible for the movement of continents 1. The layer below the lithosphere is called the asthenosphere and can flow like a liquid due to the intense heat and pressure in the mantle 2. Material closer to the core is warm and rises, mantle material closer to the lithosphere is cooler and sinks 3. The rising and sinking cycle of the mantle material creates a circular motion that carries the plates of the Earth Convection Currents IV. Theory of Plate Tectonics A. Links together the ideas of continental drift and ocean-floor spreading B. Explains how the Earth has evolved (changed) over time C. Helps explain formation, movements, collisions, and destruction of the Earth’s crust D. Helps scientists understand the past and helps predict the future V. Earthquakes A. Shaking and trembling resulting from movements of Earth’s crust or plate tectonics 1. Most common cause of earthquakes is faulting- a break in the Earth’s crust a. Most faults lie between the surface and a depth of 70 kilometers b. Focus- point below Earth’s surface where rocks break and move c. Epicenter- point above Earth’s surface directly above the focus San Andreas Fault 2. Movement at faults occur due to convection currents 3. Energy is released as the crust moves in the form of seismic waves B. Types of Faults ( a break or crack where rocks move) 1. Normal faults- cracks where tension causes one block of rock (hanging wall) is sliding downward and away from another rock (foot wall) Normal Fault a. Hanging wall- block of rock above the fault b. Foot wall- block of rock below the fault 2. Reverse causes one to move up rock (foot faults- compression rock (hanging wall) relative to the other wall) Reverse Fault 3. Strike-slip faultsshearing forces cause cracks between two plates to slide past one another C. Types of Seismic Waves (Earthquake waves) 1. Primary Waves or P waves- travel fastest, travel through solids, liquids and gases a. push-pull waves that push rock particles ahead of them b. speed up when passing through denser material 2. Secondary Waves or S waves- slower than P waves, travel through solids, but not liquids and gases a. cause rocks particles to move from side to side b. speed up when passing through denser material 3. Surface Waves or L wavesslowest-moving seismic waves a. originate at the epicenter b. cause most of the damage during an earthquake c. earth’s surface moves up and down as each L wave passes D. Measuring Seismic Waves 1. Seismographs- instrument that detects and measures seismic waves 2. Today's high-technology, digital seismographs record ground shaking over a large band of frequencies and seismic amplitudes. 3. Seismologists determine the strength of an earthquake by studying the height of waves recorded by seismographs . a. Richter Scale- an open-ended theoretical scale used to measure the magnitude of earthquakes (8.9 is largest magnitude assigned to an earthquake). i. The scale is logarithmic. Therefore an increase of 1 magnitude unit represents a factor of ten times in amplitude. ii. For example, the seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. b. Modified Mercalli Scale Measures earthquake intensity. Describes damage to structures. Ranges from I (felt by only a few) to XII (total destruction). VI. History of Seismic Activity in North Carolina A. The first earthquake recorded in NC history was on March 8, 1735 near Bath, NC. B. From February 10 to April 17, 1784 there were 75 earthquakes felt in McDowell County, NC. C. April 31, 1886 an earthquake with an epicenter in Charleston, SC caused the most property damage in North Carolina’s history. Earthquake damage in Charleston Charleston Earthquake D. February 21, 1916 Asheville, NC - the most intense earthquake in NC history, measuring a 6 on the Mercalli scale. E. June 5, 1998 an earthquake in Moorseville, NC measured 3.2 on the Richter scale, but there was no reported damage. VII. Use of Seismic Waves in Developing a Model of Earth’s Structure A. Solid Inner Core 1. When analyzing the activity of seismic waves through the Earth, scientists found that P waves began to speed up at 5150 kilometers. This indicates the waves were passing through solid material, proving the solid composition of the inner core 2. Scientists theorize the inner core is composed of iron and nickel which under great pressure become solid. B. Liquid Outer Core 1. Scientists found that S waves terminated at a depth of 2900 kilometers. This indicates the outer core is liquid. 2. P waves also slow down as they pass through liquids to support this conclusion. 3. Scientists also theorize the outer core is composed of iron and nickel, but in liquid form. C. Mantle- directly above the outer core 1. Plasticity is the property of a solid with the ability to flow like a liquid at very, very slow rates. a. High temperature and pressure allow the rock to flow like a liquid b. This also allows the plates of Earth (lithosphere) to move on the mantle. 2. The mantle is composed of silicon, oxygen, iron and magnesium. 3. Moho- boundary between the crust and the mantle. a. Change in the speed of seismic waves moving through the Earth led to its discovery. b. Discovered in 1909 by a Yugoslav scientist, Andrija Mohorovicic. 4. The Mantle can be divided into 3 layers. a. Lithosphere- the upper mantle and part of the crust. b. Asthenosphere- the middle to upper part of the mantle. c. Lower Mantle- the lower part of the mantle. 1. Oceanic crust – (less than 10 kilometers thick), made mostly of silicon, oxygen, iron and magnesium 2. Continental crust- (average thickness of 32 kilometers) made mostly of silicon, oxygen, aluminum, calcium, sodium and potassium