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Exit Choose to view chapter section with a click on the section heading. ►The Earth Inside and OutThe Earth Inside and Out ►The Theory of Continental DriftThe.

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Presentation on theme: "Exit Choose to view chapter section with a click on the section heading. ►The Earth Inside and OutThe Earth Inside and Out ►The Theory of Continental DriftThe."— Presentation transcript:

1 Exit Choose to view chapter section with a click on the section heading. ►The Earth Inside and OutThe Earth Inside and Out ►The Theory of Continental DriftThe Theory of Continental Drift ►The Theory of Seafloor SpreadingThe Theory of Seafloor Spreading ►The Unifying Theory: Plate TectonicsThe Unifying Theory: Plate Tectonics Chapter Topic Menu

2 MenuPreviousNext 11 - 2 Earth’s Internal Layers nThe current theory is that the Earth’s interior consists of multiple layers:  The inner core is primarily iron and nickel plus other heavy elements. It is theorized to be solid due to enormous pressure.  The outer core is made up of the same elements. However, it is theorized that with less pressure, this layer is liquid.  The mantle is thought to contain mostly silicon and oxygen, with some iron and magnesium. It consists of the upper mantle and the lower mantle. The upper mantle itself has two layers.  The crust is composed mainly of oxygen, silicon, magnesium, and iron. It varies in thickness and is the outer layer of the lithosphere. The Earth Inside and Out Chapter 11 Pages 11-4 & 11-5

3 MenuPreviousNext 11 - 3 Earth’s Internal Layers (continued) nEarth’s internal layers are further divided:  The upper mantle also has two layers. The top part of the upper mantle is the asthenosphere. The asthenosphere is also solid but much the same as old glass is a solid that 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 lithosphere includes the uppermost, rigid part of the upper mantle and the crust. This is the solid rock portion of the outer Earth that rests on the molten asthenosphere. nScientists separate the uppermost mantle from the crust because they think the mantle’s elemental composition changes little. The crust consists of different rock types thought to undergo change over long periods.  Also differentiating the crust from the mantle are conditions such as temperature and pressure. The Earth Inside and Out Chapter 11 Pages 11-5 11-6

4 MenuPreviousNext 11 - 4 The Rock Cycle nScientists believe that rocks form or change over long periods due to the processes of the rock cycle. nThe rock cycle is like a recycling machine, endlessly converting one type of rock to another. nThree types of rock found in the crust are:  1. Igneous – rocks that form when magma/lava cools and hardens.  2. Sedimentary – rock made up of particles deposited by water flow. Sediment consists of organic/inorganic matter particles that have been compressed into rock. This is the type of rock in which fossils are found.  3. Metamorphic– as layers build up, this rock is formed when pressure and heat become great enough to change the rock chemically. nThe rock cycle is completed through the tectonic process – rocks return to the mantle, remelt, become magma, return to the crust as igneous rock. The Earth Inside and Out Chapter 11 Pages 11-6 & 11-7

5 MenuPreviousNext 11 - 5 Isostatic Equilibrium nWith the crust resting on the mantle, there must be a balance between the weight of the crust and the upward force of buoyancy. This is an application of Archimedes' Principle of buoyancy.  This balance is called isostatic equilibrium. nAs 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. nThe accepted theory of plate tectonics suggests that the continents move in horizontal directions and that earthquakes also result from that movement. The Earth Inside and Out Chapter 11 Pages 11-7 & 11-8

6 MenuPreviousNext 11 - 6 Alfred Wegener and Pangaea nPangaea is the name given to the single giant continent in a proposal by Wegener (1912).  He suggested that in the distant past all the Earth’s continents had been a single giant continent. nPanthalassa is the name given to the single large ocean surrounding Pangaea. nThe theory of continental drift states that the continents were once a single landmass that drifted apart and are still doing so. The Theory of Continental Drift Chapter 11 Page 11-10

7 MenuPreviousNext 11 - 7 Evidence for Continental Drift nWegener tried to support his theory with evidence:  Accurate world maps. People noticed that the continents apparently fit together like a jigsaw-puzzle.  Fossils of the Glossopteris fern in South America, Africa, Australia, India, and Antarctica. The seeds are too heavy to travel by wind and too fragile to survive significant sea crossings.  Distribution of other animals and fossils were studied, especially coal. nWegener’s theories were not accepted because he could not explain how continents could drift, also he was a meteorologist, not a geologist. nThe plate tectonics theory would finally provide an explanation for how continents move, making Wegener’s theory widely accepted.  Additionally, it was found that including the continental shelves filled the gaps in the jigsaw puzzle. The Theory of Continental Drift Chapter 11 Pages 11-11 to 11-13

8 MenuPreviousNext 11 - 8 New Technology and Seafloor Knowledge nSonar made it possible to detect an otherwise invisible object. Scientists immediately recognized that they could map the ocean bottom with sonar.  The German Meteor expedition mapped the South Atlantic in 1925. This became the basis for the first worldwide seafloor map of deep ocean ridges. nThe 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. nScientists began to notice patterns in the mid-ocean ridges, trenches, and rift valleys that proved pivotal in developing new theories. The Theory of Seafloor Spreading Chapter 11 Pages 11-14 & 11-15

9 MenuPreviousNext 11 - 9 The Creation and Destruction of Seafloor nHess and Dietz (1960) proposed an explanation of seafloor features. They hypothesized that the seafloor is in a constant state of creation and destruction through a process called seafloor spreading. nIn the theory of seafloor spreading, new crust emerges from the rift valley in a mid-ocean ridge. Magma from the asthenosphere pushes up through the rift and solidifies into new crust. nThe new seafloor forms at the rift valleys and mid- ocean ridges, spreading away from the ridges until it returns as part of the rock cycle at subduction zones (trenches). Chapter 11 Page 11-16 The Theory of Seafloor Spreading

10 MenuPreviousNext 11 - 10 Evidence of Seafloor Spreading nScientists have found three forms of evidence that support the theory of seafloor spreading.  1. In 1968, scientists began drilling holes in the seafloor to gather sediment samples – the layers were thin or absent at the ridges, and thicker away from the ridges.  2. Radiometric dating was used to determine the age of rocks. Scientists have found seafloor rock to be significantly younger than rock in the center of the continents. According to the theory, continental rock doesn’t subside, where as seafloor rock does.  3. Evidence from magnetometer data. This instrument measures the polar orientation and intensity of magnetism of minerals. After towing magnetometers around the seafloor beginning in 1950, scientists discovered a symmetrical pattern in the polar orientation of magnetism of seafloor rock, the seafloor on either side of the mid-ocean ridges roughly mirrors each other’s polar orientation. Chapter 11 Pages 11-17 to 11-20 The Theory of Seafloor Spreading

11 MenuPreviousNext 11 - 11 Seafloor Spreading and Continental Drift Combine nThe theory of plate tectonics united the theories of continental drift and seafloor spreading.  It combines ideas from both theories, along with some of the original isostatic equilibrium concepts. nIn plate tectonic theory the Earth’s lithosphere consists of more than a dozen separate plates. The plates are rigid and float on the asthenosphere. nAt a spreading or divergent boundary, two plates are moving apart. As this happens, 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. nAt a colliding or convergent boundary, two plates push together.  They are also called destructive boundaries because movements along these destroy crust. nAt a transform boundary or fault, two plates slide past each other.  Earthquakes result as rocks move when the plates slide next to each other. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-22 to 11-25

12 MenuPreviousNext 11 - 12 Seafloor Spreading and Continental Drift (continued) nIsland arcs (divergent boundary).  When two oceanic plates collide, the denser plate will be subducted. Some of the material from the melting oceanic plate rises upward to form a volcanic island arc. nSubduction 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. nConvergent boundary (transform boundary).  Mountain formation at continental plate collision. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-22 to 11-25

13 MenuPreviousNext 11 - 13 Hot Spots nThe hot spot theory states that hot spots are small melting areas within the mantle where thermal plumes cause magma columns to push up intensely, breaking through the crust. nWhat makes this theory significant is that hot spots do not move with tectonic plates because they originate in the mantle. nVolcanic island chains, are the result of the plate moving over a hot spot. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-25 & 11-26

14 MenuPreviousNext 11 - 14 Plate Movement nConvection is the primary force driving seafloor spreading and continental drift.  Convection currents form as hot material rises and cold material sinks. This occurs in the mantle.  This creates a current that moves the plates away from each other at the divergent boundaries, toward each other at the convergent boundaries, and past each other at the transform boundaries. nA second driving force comes from seafloor spreading.  As new seafloor forms, the plates tend to slide away from the elevated mid-ocean ridges. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-26 & 11-27

15 MenuPreviousNext 11 - 15 Plate Movement (continued) nPredicted changes over the next 50 million years:  The Baja Peninsula will have moved past and apart from the North American Plate.  Southern California will pass San Francisco as it moves to the northwest.  A new sea will form in eastern Africa.  Australia will move toward Eurasia/the Equator.  The Mediterranean Sea will close as Africa pushes towards Europe.  The Atlantic and Indian Oceans will continue to grow while the Pacific will become smaller. The Unifying Theory: Plate Tectonics Chapter 11 Pages 11-28 & 11-29

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