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Transcript
Chapter 11 Pgs 11-4 to 11-29 Layers of the Earth pgs 11-4 to 11-6 The current theory is that the Earth’s interior consists of multiple layers: Inner core Outer core Mantle Crust The inner core The center of the Earth Made up primarily of iron, nickel, other heavy elements It is theorized to be solid due to enormous pressure The Outer Core Made of the same elements as the inner core, but with less pressure, it is theorized to be liquid The Mantle Contains mostly silicon and oxygen makes up nearly 80 percent of the Earth's total volume. Consists of the upper and lower mantle The upper mantle itself is made of 2 layers Asthenosphere Lithosphere Lithosphere and Asthenosphere Lithosphere includes the uppermost, rigid part of the upper mantle and the crust. Asthenosphere Solid but flows slowly over time Hotter, less dense material (magma) rises towards the surface where it can eventually flow from a volcano or other opening. At this point the molten rock is lava. The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back. The Crust composed mainly of oxygen, silicon, magnesium, and iron Varies in thickness and is the outer layer of the lithosphere 11-7 to 11-8 Isostatic Equilibrium Even before the plate tectonics theory emerged, scientists believed that the crust floated on the denser mantle below Still believed today Continental crust (crust under the continents) Ocean crust (crust under the ocean basins) Oceanic crust is thinner and denser than continental crust Because the continental crust is made of granite and the oceanic crust is made of basalt rock Interaction of Oceanic Crust and Continental Crust With the crust resting on the mantle, there must be a balance between the weight of the crust and the upward force of buoyancy called isostatic equilibrium As material adds to the oceanic crust from sedimentation, glaciers and volcanic activity or from the continental crust from erosion, this balance becomes disrupted. This is one theorized cause of earthquakes. To restore equilibrium, landmasses will sink or rise slightly along a weak area called a fault. The accepted theory of plate tectonics suggests that the continents move in horizontal directions and that earthquakes also result from that movement. The Theory of Continental Drift 11-10 to 11-13 Proposed by Alfred Wegener All earth’s continents had been a single continent that he called Pangaea CONTINENTAL DRIFT Pangaea broke up with part of the continent drifting north and part south. 1) The northern part split to form the North Atlantic Ocean 208146 million years ago (mya). 2) The South Atlantic and Indian oceans began to form 146-65 mya. 3) The continents continue to drift. Today the oceans are still changing shape; the Atlantic Ocean gets wider by a few inches each year. Surrounding Pangaea was a single large ocean he called Panthalassa The theory that all of the continents were once a single landmass that drifted apart (and are still doing so) is called the theory of continental drift Evidence for Continental Drift pgs 11-10 through 11-13 Appeared that the continents fit together like jigsaw puzzle pieces Fossils found in different locations Distribution of coal Glacial rock deposits Limestone deposits Salt deposits Criticism of Continental Drift Wegener was a meteorologist, not a geologist Could not give an explanation of how the continents could drift Jigsaw puzzle had gaps Seafloor Spreading pgs 11-14 to 11-18 New technology- sonar Came about at the same time as the continental drift theory and partly in response to the Titanic disaster The German Meteor mapped the contours and depths of the South Atlantic in 1925 using sonar The ability to map the seafloor in greater detail revealed important new features: Mid-ocean ridges are enormous mountain ridges on the bottom of the ocean. Rift valleys are deep valleys running through the center of mid-ocean ridges such as the Atlantic Ridge. Trenches are deep ravines in the seafloor. The idea that the seafloor is in a constant state of creation and destruction is called seafloor spreading- an explanation proposed by Harry Hess and Robert Dietz. New crust emerges from the rift valley in a midocean ridge Magma from the asthenosphere pushes up through the rift and solidifies into new crust As more magma pushes up from below, it pushes new crust away on each side of the ridge New seafloor near the ridge continuously pushes old seafloor away from the ridge Evidence for Seafloor Spreading 11-17 to 11-18 Ocean-bottom sediment samples Glomar Challenger- coring Radiometric dating Used to determine the age of rocks Seafloor rocksignificantly younger than rock in the center of the continents (theory states that seafloor rock subsides and continental rock doesn’t). Magnetometer data measures the polar orientation and intensity of magnetism of minerals Scientists towed magnetometers around the seafloor beginning in 1950 discovered that the seafloor on either side of the mid-ocean ridges roughly mirrors each other’s polar orientation Divergent, Convergent, & Transform Plate Boundaries 11-22 to 11-25 The theory of plate tectonics unites the theories of continental drift and seafloor spreading. Earth’s lithosphere consists of more than a dozen separate plates rigid and float on the asthenosphere. Divergent Boundaries At a spreading, or divergent boundary, two plates are moving apart The crust pulls apart and forms valleys. Magma flows up through the rift valleys creating new crust and widening the seafloor. Mid-ocean ridges and rift valleys mark divergent boundaries. Hot Spots Stationary plume of magma under a moving plate Creates volcanic islands in the middle of plates EX: Hawaiian Islands and the Emperor Seamount Chain Convergent Boundaries At a colliding or convergent boundary, two plates push together also called destructive boundaries because movements along these destroy crust Subduction zones (convergent boundary). A trench forms as a more dense oceanic plate moves under a less dense continental plate. As subduction occurs, some of the material from the melting oceanic plate rises upward to form volcanoes on the continent. Mountain formation at continental plate collision. Transform Boundaries At a transform boundary or fault, two plates slide past each other. Earthquakes result as rocks move when the plates slide next to each other. Ex: California’s San Andreas Fault Ocean Floor Topography Vocabulary Continental Shelf: the part of the continent that is under water. It is the biologically richest part of the ocean. At times of low sea level the shelves were exposed. Continental Slope: the edge of the continent. Extends downward to the deep-sea floor. Continental Rise: a gentle slope or a rise on the bottom of the ocean that is due to the sediments being carried down the slope. The sediment is carried downward by the turbidity current. Seamount: underwater volcanoes due to hotspots and magma. Formed when magma below is pushed upward and cooled and solidified. Mid-Ocean Ridge: the cracks in the bottom of the ocean where the plates meet. Is called an underwater mountain range. Rift Valley: a valley between 2 plates. Caused when the 2 plates are pushed apart and the seabottom collapses forming a valley between the 2 ridges. Abyssal Plain: the almost perfectly flat area of the ocean. Flat due to deposits of sediments and erosion. Called the ocean desert because few life forms exist here. Guyot: a flat-topped underwater mountain. It is flat at the top due to erosion. Trench: a narrow depression on the sea floor. Caused when 2 plates collide and one plate is pushed under the other one and pulls the sea bottom with it. The deepest trench is the Marianas trench with a depth of 11,022 meters, or almost 7 miles. SONAR: uses echolocation or sound waves to see the seafloor and make an accurate map of it. SONAR is used to make accurate maps of the ocean floor. Sediments: 12-10 to 12-12 Lithogenous sediments come from the land Mainly result from erosion by water, wind, and ice carrying rock and mineral particles into the sea. Other lithogenous sediments enter the sea from landslides and volcanic eruptions Make up majority of sediments found near continents and islands Includes sand and clay Biogenous sediments: originate from organisms Particles in the sediment come from shells and hard skeletons Cover the largest area of sea floor. The majority of biogenous sediment comes from planktonic organisms that obtain siliceous and calcareous compounds from seawater. Under the right conditions, organic molecules in the sediment form crude oil (petroleum) and natural gas Hydrogenous sediment results from chemical reactions within seawater Less than 1% of the seafloor sediments Form slowly Cosmogenous sediments come from outer space Made up of small particles the size of sand or smaller called cosmic dust. Some thought to result from collisions between objects in space Meteors: large, fast-moving objects that enter the atmosphere Meteorite: a meteor that strikes the ground Least abundant of the sediments- a few parts per million of marine sediment per year