Lecture 3: Plate Tectonics 1
advertisement
Plate Tectonics 1 September 10, 13,700,002,008 The earth history of our planet for the last 4.2 Ga: slow cooling Slow cooling of the earth mantle temperatures have declined from 1580 degC to 1375 degC in the last 4.5 billion years Sources of heat initial heat radioactive decay gravitational settling All sources are dying away … Evolution of earth in last 4 Ga Is primarily consequence of the cooling process Key character of cooling is “Convection” Convection cold cold cold cold cold cold Conditions Necessary for Convection cold cold cold cold cold 1) Gravity. 2) A fluid that experiences changes in density as its temperature changes. 3) A heat source within or at the bottom of the fluid. Results of Convection cold cold cold cold cold 1) Heat is transferred from bottom to top of fluid (flux of heat) 2) The fluid circulates 3) regions of upwelling & downwelling, divergence and convergence (i.e. flux of fluid) Convection with Lithosphere cold cold cold cold cold 1) Lithosphere is cold, stiff, brittle region at top 2) Subduction zone: downwelling, convergence of plates 3) Ridge: upwelling, divergence of plates Lithosphere in 3D 700 km Comparison of Convection in Atmosphere and Solid Earth Phenomenon Atmosphere Mantle Material Air Mg-Fe silicate rock “Peridotite” Driving force Bouyancy Bouyancy What changes density Temperature & water content Temperature & depletion Heat source Sunlight Radioactive decay Rate of heat transsport 1000 watts/m2 0.1 watts/m2 Phenomenon Atmosphere Mantle Speed 1 m/s 3x109 cm/year 1 cm/year 6 deg-C/km 1 deg-C/km Lapse rate Coriollis important Yes No Phase change Peridotite melts to make magma Water condenses to make rain After phase change Dry air is heavier Depleted peridotite is lighter Phenomenon Atmosphere Mantle Effects of flow Wind makes ocean waves & sand dunes Flowing mantle moves continents Small scale Features Tornadoes Mantle plumes Human viewpoint From the bottom From the top Comparison of Convection in Atmosphere and Solid Earth Magma is Solid Earth’s version of “rain” Rain vs. magma rain … rising moist air adiabatically cools crosses vapor-liquid phase boundary water droplets form are negatively bouyant and fall down as rain Leaves dry air behind magma … rising undepleted mantle adiabatically cools crosses solid-liquid phase boundary liquid rock droplets form are positively bouyant and rise up as magma Leaves depleted mantle behind Lithosphere – brittle top of part of the earth – is a key feature of our planet that makes its style of convection different than “bubbling soup” Lithosphere: about 100 km thick asthenopshere: about 600 km thick; vigorous convection Deeper mantle is more viscous and convects more slowly Brittle Material cracks or faults instead of flows Please memorize these fault configurations and names Crustal Movements measured through GPS Schematitic GPS velocities of the ground 10 mm per year Schematic earth “Plates” Really spherical caps Large sections of the lithosphere sliding over the lithosphere Schematic map Plate 3 Plate 1 Plate 2 Schematic map The plate boundaries are regions of active tectonism All motions are a type of rotation or spinning of the cap about a pole of rotation Schematic map Lithosphere is organized into plates that rigidly move Measured with GPS Inferred from geology Four Plates in this Diagram Divergent Plate Boundaries plates are moving apart Is a gap created as they move apart? Convergent Plate Boundaries plates are colliding What happens to the excess material? Transform Plate Boundaries plates are sliding past one another Is a gap created as they move apart? Plate Motion in the Past http://www.ucmp.berkeley.edu/geology/anim1.html Earth Scientists have worked out the history of the earth’s plate motions for the most recent a billion years or so of earth history but the farther back in time the more crudely the plate configuration is known Why Melting? (The Mantle’s Rain) Geological evidence for extension is very common at divergent plate boundaries on the sea floor at a divergent boundary Geological evidence for volcanism is very common at divergent plate boundaries (e.g. this sea floor hot spring) Divergent plate boundary in the ocean is called a mid-ocean ridge 5000 km 200 Ma 0 Ma 200 Ma Flank old & cool Crest young & hot Flank old & cool Thermal Contraction Ridge Crest: Hot Lithosphere: 1300C Ridge Flank: Cold Lithosphere: 800C Change in temperature: 500C Thickness of lithosphere 100 km = 105 m Thermal contraction coefficient of rock: 310-5 per C Thermal contraction: 310-5 105 500 = 1500 m Ridge flank should be about 1500 meters below the crest Depth-Age Relationship Shape of ridge is explained by cooling of initially hot lithosphere emplaced at ridge axis If you know the age of a patch of sea floor, you can predict its depth to amazing accuracy! Ridge crests – young – little sediment Flank – old – thick sediment Mid-Atlantic Ridge Divergent Plate Boundary on a Continent
