CHAPTER 2 Plate Tectonics and the Ocean Floor Much evidence supports plate tectonic theory. Different plate boundaries have different features. Tectonic plates continue to move today. Lava Lamp! In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Alfred Wegener: in 1912, he was the first person to present evidence other than continental margin fit http://en.wikipedia.org/wiki/pangaea 200 m.y. ago, the supercontinent Pangaea (all Earth) began to split apart. Geologic evidence Fossil evidence Paleoclimate evidence Evidence for Continental Drift Wegener proposed Pangaea one large continent existed 200 million years ago Panthalassa one large ocean Noted puzzle-like fit of modern continents Pangea Figure 2.2 Figure 2.3 http://www.youtube.com/watch?v=ugcded4xvd4&feature=related 1
Evidence for continental drift: Geological evidence: 1) fold belts 2) age provinces 3) igneous provinces 4) stratigraphic sections 5) metallogenic provinces Evidence for Continental Drift Matching sequences of rocks and mountain chains Similar rocks on different continents Figure 2.4 Evidence for continental drift: Paleontological evidence 1) distribution of tetrapods-early distribution - easy communication in Pangaea 2) early Permian reptile Mesosaurus - found in S Africa and Brazil 3) marine invertebrates-distribution of continents and oceans different from today 4) Cambrian trilobites 5) ammonites (shallow seas between India, Madagascar and Africa in J) 6) Glossopteris and Gangamopteris fauna in Gondwana (cold climate), tropical flora in Laurasia 7) diversity of species (increases towards Equator = drifting N-S controls the diversity) Evidence for continental drift: Paleoclimatic evidence: Some lithologies are indicative of particular climates and hence global position: 1) carbonates and reef depositswarm water, 30 deg from equator 2) evaporates-hot, arid conditions 3) red beds-hot climate for form of hematite 4) coal and oil-warm, humid climate 5) phosphorites-within 45 deg of equator 6) bauxite and laterite-tropical, subtropical weathering 7) desert deposits-both warm and cold conditions, direction of wind, continental rotation 8) glacial deposits-30 deg from poles 2
Evidence for Continental Drift Paleoclimatic evidence: Glacial ages and other climate evidence Evidence of glaciation in now tropical regions Direction of glacial flow and rock scouring Plant and animal fossils indicate different climate than today Alfred Wegener: He did not have a successful explanation for a mechanism to drive continental drift. 1. Gravitational forces: continental crust plows through oceanic crust like an icebreaker through ice (pole-fleeing force-movement of continents toward Equator) -however: continental crust is too weak to do this (2.7 g/cc vs. 3.3 g/cc); the force is several millions times smaller than force of gravity) 2. Centrifugal (rotational) and tidal forces are responsible for movement of land masses (westward movement of continents) - however: these forces are too weak to do this Figure 2.5 http://www.suu.edu/faculty/colberg/hazards/platetectonics/18_pangaea.html The hypothesis was widely rejected for many decades, elaborate but incorrect schemes were concocted to explain mountain ranges and large synclines. In the 1950 s a wealth of new evidence emerged to revive the debate about Wegener's ideas: (1) confirmation of repeated reversals of the Earths magnetic field in the geologic past; (2) demonstration of the ruggedness and youth of the ocean floor; (3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust; and (4) precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges. Evidence for Plate Tectonics Paleomagnetism study of Earth s ancient magnetic field Interprets where rocks first formed (rocks record magnetic field when they are formed) Magnetic dip (see dip angle) 1955 deep water rock mapping Magnetic anomalies regular pattern of north-south magnetism stripes Stripes were symmetrical about long underwater mountain range 3
Earth s Magnetic Field Approximates a dipole (like a bar magnet), so it has polarity Orientation wanders over time Figure 2.7b Dip Angle: the angle between the line of magnetic force and a plumb line. <<<<<<<< Pole Wander: the magnetic pole has wandered through time >>>>>>>> Figure 2.7a Figure 2.7c The curves on this map trace the apparent path followed by the north magnetic pole through the past 600 million years. The apparent polar wandering path for Europe is different from the path determined from measurements made in North America. If the continents are reassembled into a single supercontinent, the two paths coincide, indicating that Europe and North America moved as one continent during this period. >>> Paleomagnetism study of Earth s ancient magnetic field Normal Magnetic field lines transition reversed - The total time span of a reversal is up to 10.000 years - The reversal sequence has been calibrated for the last 5 million years by dating basalts of known polarity. 4
Early in the 20th century, paleomagnetists -- such as Bernard Brunhes in France (in 1906) and Motonari Matuyama in Japan (in the 1920s) -- recognized that rocks generally belong to two groups according to their magnetic properties: Several lines of evidence: (1) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; (2) the youngest rocks at the ridge crest always have presentday (normal) polarity; and (3) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. (1) normal polarity and (2) reversed polarity ocean floor shows a zebra-like pattern of alternating stripes of magnetically different rock creating magnetic striping. The shape and intensity of magnetic anomalies depends on: (1) the segmentation of the mid-ocean ridge by fracture zones (i.e. length of magnetized blocks along-axis), (2) spreading velocity (length of blocks across-axis). Fast spreading causes relatively longer blocks to form than slow spreading. (3) frequency of polarity reversals (length of blocks across-axis), (4) the direction of magnetization in a given block. 5
Marine Record: oldest oceanic crust ~180 Ma Continental Record: oldest continental crust ~2.1 Ga What Drives Plate Motions: (1) Density vs. Gravity: causes oceanic crust to sink in subduction zones, causes crust to extend at spreading ridges (called ridge push, but the ridge is not pushing, the crust is pulling as it sinks into subduction zones) (2) Thermal Convection: exerts drag force to base of crust, circulates heat and mantle material... Figure 2.10 Thermal Convection: Thermal Convection: 6
http://www.earthbyte.org/people/dietmar/pdf/muller_etal_digital_isochrons_jgr1997.pdf http://www.ngdc.noaa.gov/mgg/fliers/96mgg04.html A published reference for the age grid is: Mueller, R.D., Roest, W.R., Royer, J.-Y., Gahagan, L.M., and Sclater, J.G., A digital age map of the ocean floor. SIO Reference Series 93-30, Scripps Institution of Oceanography. Bathymetry and Topography Crustal Age Bathymetry 7
Thermal Control on Crustal Depth Bathymetry and Topography Average depth ocean ridges = 2.5 km As plates cool, they get more dense and sink Depth = 2.5 km + 0.33 x (sq root age in Ma) Where (Why) Are There Earthquakes? Plate Boundaries De Mets, et.al. 2010 Earthquake epicenters (mostly along plate boundaries) magnitude >3 in black and >5.5 in red De Mets, et.al. 2010 8
Plate Boundaries and Their Motions Types of Plate Boundaries Divergent Convergent Figure 2.13b Transform Figure 2.14 Table 2.1 Types of Plate Boundaries Divergent Convergent Transform 9
Evolution of Divergent Margins Shallow heat source develops under continent Movement apart creates a rift valley Continued spreading creates linear sea Evolution of Divergent Margins East Africa rift system with Red Sea and Gulf of Aden linear seas. Divergent boundary = red Transform boundary = green Volcano = black triangle Where is oldest divergent margin on map? Ocean basin is created over millions of years Figure 2.17 Figure 2.18a A view view down down the the axis of of the the divergent divergent plate plate boundary boundary immediately immediately south south of of the the Red Red Sea Sea Rift formed in 2005 following earthquakes and volcanic eruption in Ethiopia Figure 2.18c 10
Types of Spreading Centers Fast and Slow Rates Mid-Atlantic Ridge = ~3.5 cm/yr East Pacific Rise = ~16.5 cm/yr Figure 2.19 Mid-Atlantic Ridge = ~3.5 cm/yr Figure 2.19a East Pacific Rise = ~16.5 cm/yr Transform Margins Connect spreading ridges Strike slip Figure 2.19b 11
Transform Margins Connect spreading ridges Strike slip Transform Boundaries 36 mm/yr NORTH AMERICA PACIFIC http://www.geocaching.com/seek/cache_ details.aspx?guid=f6a2bf15-d163-4f3fa383-0f647bf374a4 San Andreas Fault, Carrizo Plain Transform Margins Strike slip Loma Prieta 1989 Ms 7.1 Convergent Margins Ocean Continent Subduction Davidson et al., 2002 Ocean-Ocean Subduction Continent-Continent Collision Figure 2.20 12
Figure 2.20a Figure 2.20b Convergent Margins Ocean Continent Subduction Ocean plate is subducted Continental arcs generated Explosive andesitic volcanic eruptions Cascadia subduction zone Juan de Fuca and Gorda plates subduct northeastwardly beneath the North America plate Subduction zone earthquakes can cause tsunami Figure 2.20c 13
Ocean Continent Subduction Cascadia subduction zone Magnetic anomalies Ocean Continent Subduction Cascadia subduction zone Cross Section Water content fluxes melt to generate volcanism Atwater, 1970 USGS Ocean Continent Subduction Subduction zone earthquakes can generate tsunami Mt St Helens, WA March 1980 USGS Mt St Helens, WA May 18,1980 14
Convergent Margins Oceanic-Oceanic Ocean-Ocean Subduction Convergent Plate Boundary Denser plate is subducted Deep trenches generated Volcanic island arcs generated Marianas Trench (video) marianas_hdv.mov Convergent Margins Continent-Continent Collision No subduction India-Asia collision ~55 Ma Created/Creating the Himalayas Continent-Continent Collision Initially oceanic-continental subduction Denser oceanic crust (~3.3 g/cc) subducts beneath less dense continental crust (~2.7 g/cc) Marine sediments deposited with marine fossils Figure 2.22 Figure 2.22a 15
Continent-Continent Collision Continental crust collides with continental crust, creating mountain uplift Marine sediments uplifted with mountain growth Continent-Continent Collision Indenter Tectonics Tapponier and Peltzer Plasticene experiment. Figure 2.22b Continent-Continent Collision Indenter Tectonics Applications of Plate Tectonics Plate Boundary Motion Relative Motion Geologic Offsets Magnetic Anomalies Absolute Motion GPS Reference Frames Plates defined geodetically ( stable North America ) Hot Spots (record ancient plate motions) Nematath hotspot track Figure 2.23b 16
Plate Boundary Motion San Andreas transform fault system GPS sites Relative to stable North America Many faults interact Pacific plate moves relative to North America plate Plate Boundary Motion San Andreas transform fault system San Andreas fault, Eastern California shear zone, Walker Lane, Central Nevada seismic belt, Wasatch fault (Utah). Fault slip rate (mm/yr) GPS Velocity A-A (mm/yr) http://maps.unomaha.edu/maher/geol1010/lecture15/usgssanandreasgps.gif http://geology.gsapubs.org/content/37/4/359/f2.large.jpg Plate Boundary Motion Hawaiian Island Emperor Seamount Nematath Reference Frames Hot Spots They move, but at rates less than the plate rates Figure 2.24 Figure 2.25 17
Age relations of the Emperor and Hawai ian Island Chains Seamounts Rounded tops Tablemounts or guyots Flattened tops Subsidence of flanks of mid-ocean ridge Wave erosion may flatten seamount Figure 2.26 Coral Reef Development Coral Reef Development Figure 2.27 Figure 2.29 18
9/6/2012 Plate Boundary Motion Absolute Motion GPS Reference Frames Global Reference Frame World Map 50 million Years in Future Figure 2.30 Figure 2.32 Plate Boundary Motion Ocean Basin Structure Bathymetry Topography Plate Boundaries Plate Tectonics Video http://www.youtube.com/watch?v=jmc-vjqgsnm&feature=related 19
John Tuzo Wilson Life cycle of ocean basins Formation Growth Destruction Wilson cycle http://www.futura-sciences.com/fileadmin/fichiers/images/terre/cyclewilson.jpg Figure 2.33 Wilson cycle Wilson cycle 20