1550 plate tectonics outline PDF

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8/31/2017

1550 plate tectonics outline

I. Plate Tectonics (Chapter 2) A. Understand the evidence that contributed to development of the theory: 1. Earthquake distributions (Fig. 2.13) 2. Seismic wave information 3. Relationship between earthquakes and volcanoes 4. Continental drift - (Figures 2.1 and 2.2 and CD-Breakup of Pangaea) a. Theory that proposed the landmasses of the Earth are not fixed to a particular spot on the surface, but move continuously. (Good fit of circum-Atlantic continents) 5. Evidence from ancient climates a. Climate is determined by the location of a region with respect to the equator and climatic regions are arranged in bands parallel to the equator. b. Distribution of ancient climate zones suggests that they were determined by poles and an equator that were in a different place than they are today OR the landmasses carrying the rocks have moved with respect to the equator. 1) Glacial deposits (Fig. 2.5), Evaporites, Coral limestones 6. Fossil evidence- Mesosaurus, and Lystrosaurus fossils found on several widely separated continents although fossils must have originated on a single landmass (Figure 2.6). 7. Ancient mountain chains end abruptly at continent edges (Figure 2.4). B. Paleomagnetic evidence 1.Definition-study of the magnetism in ancient rocks. 2. Earth's magnetic field due to internal and external forces 3. Earth's magnetic field behaves as if it were induced by a bar magnet situated in the center of the Earth approximately lined up with the Earth's axis of rotation .(Fig.2.7) 4. Inclination-the angle that a light-weight magnetic needle makes with the horizontal surface of the Earth (i.e. it's orientation in a vertical plane). a. Paleo-inclination recorded in rocks is used to determine latitude where rocks formed 5. The orientation of the magnetic needle in the horizontal plane indicates the position of the north and south magnetic poles, which are believed to have coincided with the position of the geographic poles through most of geologic time. a. The "paleo-orientation" determined from rocks indicates the approximate pole position at the time they formed. Discrepancies between pole positions indicated by rocks from different continents is further evidence that those continents have moved. (Polar wandering, Figure 2.8) 6. Rocks (or more correctly their iron-bearing minerals) acquire a magnetic signature because the charged subatomic particles in iron atoms are forced to line up with the local lines of force of Earth's magnetic field when the rock in which they are crystallizing cools below the Curie Temperature. Above the Curie temperature the tiny atomic magnets are free to rotate in any random orientation. 7. The intensity of the Earth's magnetic field has not been constant through time. In the past when the intensity has decayed to 0 and the field has later reestablished itself, the orientation of the field has often reversed its polarity (the geographic direction to which the north-seeking end of a magnetic needle points) (Figure 2.9). a. The polarity observed currently is called normal and the 180o situation is called reversed. 8. A symmetrical pattern of bands of alternately normal and reversed polarity is observed in the basalts of the sea floor (Fig. 2.11) 9. The age of basalts that make up the sea floor increases from the oceanic ridges outward towards the continental edges (Figure 2.12). 10. The thickness of oceanic sediments deposited on top of seafloor basalts increases tremendously from the mid-oceanic ridges out towards the continental edges. C. Sea-floor spreading (Fig. 2.10 and CD-Seafloor Spreading and Plate Boundaries) 1. The formation of new sea floor by addition of basaltic magma at the mid-oceanic ridges, followed by the active spreading of this newly-formed sea floor away from the ridges was proposed to explain this pattern of magnetic reversals (Figure 2.11 and 2.17). Process also explains increased age of basalt and sediments from mid-ocean ridges to the continental edges. 2. Half-spreading rates vary from 1-9 cm/year, so ocean basins widen at 2-18 cm/year (1-7 in) http://core.ecu.edu/geology/woods/TECTOUT1550.htm

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8/31/2017

1550 plate tectonics outline

II. Theory of Plate Tectonics A. PLATE = the lithosphere is the rigid outer layer of the Earth composed of both crust and uppermost mantle. The lithosphere comprises the plates that "ride" on the warmer, more plastic asthenosphere.(Fig. 1.14 & 1.15) 1. 7 major plates-N. Amer., S. Amer., Pacific, African, Antarctic, Australian, Eurasian B. Plate boundaries (Figure 2.13, 2.14 and Table 2.2) 1. Divergent-plates move away from each other (Figures 2.15-2.18; CD=Break-up of Pangaea) a. Shallow focus quakes only, accompanied by volcanic eruptions. (CD=Tectonic Settings and Volcanic Activity) b.Examples: Oceanic = oceanic ridges and Continental = Red Sea (Figure 2.18) c. Basaltic lavas (Figure 3.15) 2. Convergent- where plates collide with each other (Figure 2.20) a. Oceanic trenches (Fig. 3.12-3.13;Table 3.2), shallow through deep focus earthquakes, volcanism (CD=Tectonic Settings and Volcanic Activity) and subduction if oceanic crust involved such as at oceanocean and ocean-continent boundaries. At continent-continent convergent boundaries only shallow focus quakes occur and there is no formation of trenches, nor do volcanism or subduction occur. Mountain building and addition of terranes can be extensive at these latter boundaries (CD=Terranes). b. Geographic examples of the three types of convergent boundaries: 1) Ocean-ocean = Western Aleutians, Eastern edge of Philippine plate 2) Ocean-continent = Western South American - (Fig. 3.13; andesitic lavas), NW USA (Fig.2.21), Eastern Aleutians, Indonesia 3) Continent-continent = Himalayas,Alps (CD=India-Asia Collision & Fig. 2.22) 3. Transform-plates appear to move past each other in a nearly parallel manner (CD=Transform Faults) a.Typified by nearly vertical faults, shallow quakes, and virtually no volcanic activity. b. Oceanic - offsets perpendicular to trend of the oceanic ridges (Fig. 3.17) c. Continental example = San Andreas Fault (Fig. 2.23) C. Hot spots 1. Fixed locations in the mantle where melting is continuously occurring, causing expansion and extension of the overlying crust and ultimately extrusion of lava onto surface (Fig. 2.24). a. Hawaiian Island-Emperor Seamount chain (Fig. 2.25) 2. Allows determination of absolute direction of plate motion D. Theories of why plates move 1. Convection currents in the mantle 2. Plume theory

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