9/6/2012 In 1858, geographer Antonio Snider-Pellegrini CHAPTER 2 made these two maps showing his version of how Plate Tectonics and the Ocean Floor the American and African continents may once have fit together, then later separated. • Much evidence supports plate tectonic theory. • Different plate boundaries have different Alfred Wegener: in 1912, he was the first features. person to present evidence other than continental margin fit • Tectonic plates continue to move today. http://en.wikipedia.org/wiki/Pangaea • Lava Lamp! • 200 m.y. ago, the supercontinent Pangaea (all Earth) began to split apart. • Geologic evidence • Fossil evidence • Paleoclimate evidence Evidence for Continental Drift Pangea • Wegener proposed Pangaea – one large continent existed 200 million years ago • Panthalassa – one large ocean • Noted puzzle-like fit of modern continents Figure 2.3 Figure 2.2 http://www.youtube.com/watch?v=uGcDed4xVD4&feature=related 1 9/6/2012 Evidence for continental drift: Evidence for • Geological evidence: Continental Drift 1) fold belts 2) age provinces 3) igneous provinces • Matching sequences 4) stratigraphic sections 5) metallogenic provinces of rocks and mountain chains • Similar rocks on different continents Figure 2.4 Evidence for continental drift: Evidence for continental drift: Paleoclimatic evidence: Paleontological evidence Some lithologies are indicative of particular climates and hence 1) distribution of tetrapods-early global position: distribution - easy communication in Pangaea 1) carbonates and reef deposits- warm water, 30 deg from equator 2) early Permian reptile Mesosaurus - found in S Africa and Brazil 2) evaporates-hot, arid conditions 3) marine invertebrates-distribution of continents and oceans different 3) red beds-hot climate for form from today of hematite 4) Cambrian trilobites 4) coal and oil-warm, humid climate 5) ammonites (shallow seas between India, Madagascar and Africa in J) 5) phosphorites-within 45 deg of equator 6) Glossopteris and Gangamopteris fauna in Gondwana (cold climate), 6) bauxite and laterite-tropical, tropical flora in Laurasia subtropical weathering 7) diversity of species (increases 7) desert deposits-both warm towards Equator = drifting N-S and cold conditions, direction of controls the diversity) wind, continental rotation 8) glacial deposits-30 deg from poles 2 9/6/2012 Evidence for Alfred Wegener: He did not have a successful explanation for a mechanism to drive continental drift. Continental Drift Paleoclimatic evidence: 1. Gravitational forces: continental crust plows through • Glacial ages and other oceanic crust like an icebreaker through ice (pole-fleeing climate evidence force-movement of continents toward Equator) • Evidence of glaciation in now tropical regions -however: continental crust is too weak to do this (2.7 g/cc vs. • Direction of glacial flow 3.3 g/cc); the force is several millions times smaller than force and rock scouring of gravity) • Plant and animal fossils indicate different climate than 2. Centrifugal (rotational) and tidal forces are responsible for today 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 Evidence for Plate Tectonics mountain ranges and large synclines. • Paleomagnetism – study of Earth’s ancient In the 1950’s a wealth of new evidence emerged to revive the magnetic field debate about Wegener's ideas: – Interprets where rocks first formed (rocks (1) confirmation of repeated reversals of the Earths magnetic field record magnetic field when they are formed) in the geologic past; – Magnetic dip (see dip angle) (2) demonstration of the ruggedness and youth of the ocean floor; • 1955 – deep water rock mapping (3) emergence of the seafloor-spreading hypothesis and • Magnetic anomalies – regular pattern of associated recycling of oceanic crust; and north-south magnetism “stripes” (4) precise documentation that the world's earthquake and • Stripes were symmetrical about long volcanic activity is concentrated along oceanic trenches and underwater mountain range submarine mountain ranges. 3 9/6/2012 Figure 2.7b Earth’s Magnetic Dip Angle: Field the angle • Approximates a between dipole (like a bar the line of magnet), so it has magnetic polarity force and a • Orientation wanders plumb line. over time <<<<<<<< Pole Wander: the magnetic pole has wandered through time >>>>>>>> Figure 2.7a Figure 2.7c The curves on this map trace the • Paleomagnetism – Magnetic field lines apparent path followed by the Normal north magnetic pole through the study of Earth’s ancient past 600 million years. The transition apparent polar wandering path for magnetic field Europe is different from the path determined from measurements reversed 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. >>> - 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 9/6/2012 • Early in the 20th century, • Several lines of evidence: paleomagnetists -- such as (1) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge Bernard Brunhes in France crest; (in 1906) and Motonari Matuyama in Japan (in the (2) the youngest rocks at the ridge crest always have present- day (normal) polarity; and 1920s) -- recognized that rocks generally belong (3) stripes of rock parallel to the ridge crest alternated in to two groups according to magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. their magnetic properties: (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 9/6/2012 Marine Record: oldest oceanic crust What Drives Plate Motions: ~180 Ma (1) Density vs. Gravity: causes oceanic crust to sink in Continental Record: oldest subduction zones, causes crust to extend at spreading continental crust ~2.1 Ga 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 9/6/2012 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 9/6/2012 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 Earthquake epicenters (mostly along plate boundaries) De Mets, et.al. 2010 • magnitude >3 in black and >5.5 in red De Mets, et.al. 2010 8 9/6/2012 Plate Boundaries and Their Motions Types of Plate Boundaries Divergent Convergent Transform Figure 2.13b Figure 2.14 Table 2.1 Types of • Divergent Plate • Convergent Boundaries • Transform 9 9/6/2012 Evolution of Divergent Margins Evolution of Divergent Margins Shallow heat source East Africa rift develops under continent system with Red Sea and Gulf of Aden linear seas. • Divergent Movement apart creates a boundary = red • Transform rift valley boundary = green • Volcano = black triangle • Where is oldest Continued spreading divergent margin creates linear sea on map? Ocean basin is created over millions of years Figure 2.17 Figure 2.18a AA viewview downdown thethe axis ofof the the divergent divergent plate plate Rift formed in 2005 following earthquakes and boundaryboundary immediatelyimmediately soutsouthh ofof the the Red Red Sea Sea volcanic eruption in Ethiopia Figure 2.18c 10 9/6/2012 Types of Spreading Centers Fast and Slow Rates • Mid-Atlantic Ridge = ~3.5 cm/yr • East Pacific Rise = ~16.5 cm/yr Mid-Atlantic Ridge = ~3.5 cm/yr Figure 2.19 Figure 2.19a Transform Margins East Pacific • Connect spreading ridges • “Strike” slip Rise = ~16.5 cm/yr Figure 2.19b 11 9/6/2012 Transform Margins Transform Boundaries • Connect spreading ridges • “Strike” slip 36 mm/yr NORTH AMERICA PACIFIC http://www.geocaching.com/seek/cache_ details.aspx?guid=f6a2bf15-d163-4f3f- San Andreas Fault, Carrizo Plain a383-0f647bf374a4 Transform Margins Convergent Margins • “Strike” slip • Ocean Continent Subduction Loma Prieta 1989 Ms 7.1 Davidson • Ocean-Ocean Subduction et al., 2002 • Continent-Continent Collision Figure 2.20 12 9/6/2012 Figure 2.20a Figure 2.20b Convergent Margins •