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Chapter 19 in 1st edition & 1 in 2nd edition Plate Tectonics PowerPoint Presentation Stan Hatfield . Southwestern Illinois College Ken Pinzke . Southwestern Illinois College Charles Henderson . University of Calgary Tark Hamilton . Camosun College Copyright (c) 2005 Pearson Education Canada, Inc. 19-1 Continental Drift: An Idea Before Its Time Alfred Wegener • First proposed his continental drift hypothesis in 1915 • Published The Origin of Continents and Oceans Continental Drift Hypothesis • Supercontinent called Pangaea began breaking apart about 200 million years ago Copyright (c) 2005 Pearson Education Canada Inc. 19-2 Continental Drift: An Idea Before Its Time Pangaea approximately 200 million years ago. Paleozoic Tethys Sea, Pan Thallassic Ocean CopyrightLate (c) 2005 PearsonSupercontinent, Education CanadaEquatorial Inc. 19-3 Continental Drift: An Idea Before Its Time Continental Drift Hypothesis • Continents "drifted" to present positions Evidence used in support of the continental drift hypothesis • Fit of the continents • Fossil evidence • Rock type and structural similarities • Paleoclimatic evidence Copyright (c) 2005 Pearson Education Canada Inc. 19-4 Continental Drift: An Idea Before Its Time Copyright (c) 2005 Pearson Educationof Canada Inc. Wegener’s matching mountain ranges on different continents. 19-5 Continental Drift: An Idea Before Its Time Permian Glaciation, Tillites, Pavements: Copyright (c) 2005 Pearson Education Canada Inc. Paleoclimatic evidence for continental 19-6drift. The Great Debate: No Viable Mechanism Objections to the Continental Drift Hypothesis • Inability to provide a mechanism capable of moving continents across the globe • Wegener suggested that continents plowed through the ocean crust, much like ice breakers cut through ice • Proposed Tidal forcing, too weak, Rocks too strong Copyright (c) 2005 Pearson Education Canada Inc. 19-7 Lessons from Continental Drift’s Failure Continental Drift and the Scientific Method • Wegener’s hypothesis was correct in principle, but contained incorrect details • For any scientific viewpoint to gain wide acceptance, supporting evidence from all realms of science must be found • A few scientists considered Wegener’s ideas plausible & continued the search a generation later • Perseverance is important in Science, ideas must be supported and widely publicized Copyright (c) 2005 Pearson Education Canada Inc. 19-8 Continental Drift and Paleomagnetism Initial impetus for the renewed interest in continental drift came from rock paleomagnetism • Keith Runcorn’s Lab at Newcastle & Ted Irving Magnetized minerals in rocks • Record the direction of Earth’s magnetic poles • Provide a means of determining their latitude of origin: tan ( Inclination ) = 2 tan ( Latitude ) • Oriented strata of known age, yet from different continents track relative motions • Polar Wander Paths Copyright (c) 2005 Pearson Education Canada Inc. 19-9 Continental Drift and Paleomagnetism Polar Wandering • Rocks from successive ages give successive N-S poles for their continents • The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved both absolutely and relative to each other • Shows that Europe was much closer to the equator when coal-producing swamps existed • Brazil, Africa & India were much closer to the S pole in Permian time • Antarctica is the least travelled continent! Copyright (c) 2005 Pearson Education Canada Inc. 19-10 Continental Drift and Paleomagnetism Apparent polar-wandering Copyright (c) 2005 Pearson Education Canadapaths Inc. for Eurasia and North America. 19-11 Continental Drift and Paleomagnetism Polar Wandering • Polar wandering curves for North America and Europe have similar paths, but are separated by about 24 of longitude – PW Paths for Cambrian through Permian strata are parallel for Europe and North America – PW Paths for Triassic through Recent seem to converge progressively – Differences between the paths can be reconciled if the continents are placed next to one another – ~205Ma is the point of departure, with the opening of the Atlantic Copyright (c) 2005 Pearson Education Canada Inc. 19-12 A Scientific Revolution Begins During the 1950s and 1960s technological strides permitted extensive SONAR mapping of the ocean floor During the Cold War, any ship of convenience towed a magnetometer (sub chasing!) Magnetic stripes were discovered. The first complete bathymetric maps of the world seafloor assembled by Bruce Heezen & Marie Tharp of the US Office of Naval Research MOR’s & Deep Sea Trenches were discovered Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s Copyright (c) 2005 Pearson Education Canada Inc. 19-13 A Scientific Revolution Begins anew with the idea of Sea Floor Spreading Geomagnetic reversals • Both in strata on land and in boreholes beneath the sea the magnetic succession is found to be full of reversals with anti-parallel directions • The advent of radiometric dating K/Ar & U/Pb • Earth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa • Dates when the polarity of Earth’s magnetism changed were determined from lava flows Copyright (c) 2005 Pearson Education Canada Inc. 19-14 Magnetic Stripes Record Sea Floor Spreading During a Succession of Geomagnetic Reversals Copyright (c) 2005 Pearson Education Canada The ocean floor asInc. a magnetic tape recorder. 19-15 The Birth of Plate Tectonics: 1960’s Geomagnetic reversals • Geomagnetic reversals are recorded both across and within the ocean crust by the Deep Sea Drilling Program’s Glomar Challenger and other marine geoscience programs • In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading • Sediments are found to contain magnetic reversal stratigraphy too and with microfossils this provides the basis for a global magnetostratigraphy back through the Cretaceous Copyright (c) 2005 Pearson Education Canada Inc. 19-16 Plate Tectonics = Continental Drift + Seafloor Spreading Geophysics Provided the New Data that the Older Geology Lacked • Paleomagnetism, Magnetic Reversals, Magnetostratigraphy (evidence of past magnetism recorded in the rocks) was the most convincing evidence set forth to support the concepts of continental drift and seafloor spreading • Bathymetry & Heat Flow: Ridges young, shallow and warm, trenches old, deep and cold with thicker sediments • Seismics show Lithosphere thickens away from MOR’s • Major Earthquakes were located along plate boundaries • Volcanic Arcs behind trenches Copyright (c) 2005 Pearson Education Canada Inc. 19-17 Plate Tectonics: The New Paradigm Spreading & New Crust Formed at MOR’s & CR’s, high heat flow, active volcanism Subduction zones (Wadati-Benioff zones) of deepening earthquakes behind trenches Recycled water, gases & light elements from Arcs Much more encompassing theory than continental drift The composite of a variety of ideas that explain the observed motion of Earth’s lithosphere through the mechanisms of subduction and seafloor spreading Copyright (c) 2005 Pearson Education Canada Inc. 19-18 Plate Tectonics: The New Paradigm Earth’s Major Plates • Associated with Earth's strong, rigid outer layer – Known as the lithosphere – Consists of uppermost mantle and overlying crust – Overlies a weaker region in the mantle called the asthenosphere – Plates are no longer just Continents, e.g. the North America Plate extends from Greenland and Iceland to Vancouver Island including both older continents and some younger seafloor – 3 Types of Boundaries: Ridges, Trenches & Transforms Copyright (c) 2005 Pearson Education Canada Inc. 19-19 Plate Tectonics: The New Paradigm Earth’s Major Plates • ~Seven major lithospheric plates: Pacific, Eurasia, Antarctica, N.America, S. America, Africa, Indian Ocean • Up to a dozen smaller plates: Australia, Caribbean, Mediterranean, Scotian Sea, Juan de Fuca, Turkey, Philippine Sea… • Plates are in motion and continually changing in shape and size • Largest plate is the Pacific Plate • Several plates include an entire continent plus a large of seafloor Copyright (c) 2005area Pearson Education Canada Inc. 19-20 Plate Tectonics: The New Paradigm Earth’s Major Plates • Motions measured by Magnetic stripes & Hotspot Tracks over Ma to 10’s of Ma and VLBI & GPS arrays over months to years • Plate boundaries with faster motions have greater volcanism and seismicity (Java-Sumatra) • Plates move relative to each other at a very slow but continuous rate – Average about 5 centimetres per year – Cooler, denser slabs of oceanic lithosphere descend into the mantle Copyright (c) 2005 Pearson Education Canada Inc. 19-21 Plate Tectonics: The New Paradigm Plate Boundaries • Almost all major interactions among individual plates occur along their boundaries – Subduction Zone Megathrust Earthquakes – Volcanic & Plutonic Arcs – Rare exceptions are: – Within plate seismicity – Within plate hotspot volcanism • Types of plate boundaries – Divergent plate boundaries (constructive margins) – Convergent plate boundaries (destructive margins) – Transform fault boundaries (conservative margins) Copyright (c) 2005 Pearson Education Canada Inc. 19-22 Plate Tectonics: The New Paradigm Plate Boundaries • Each plate is bounded by a combination of the three types of boundaries • New plate boundaries can be created in response to changes in the forces acting on these rigid slabs • Changes to plate boundaries require several Ma • Earthquakes on one boundary are unrelated to those on others • Volcanoes keep their own sweet time Copyright (c) 2005 Pearson Education Canada Inc. 19-23 Divergent Plate Boundaries Most are located along the crests of oceanic ridges and can be thought of as constructive plate margins with frequent volcanism & Quakes <6 Continental Rifts: • East African Rift, Rio Grande, Baikal Rift, Rhine Graben • Grabens, volcanic plateaux, normal faulting, hot springs Oceanic ridges and seafloor spreading • Along well-developed divergent plate boundaries, the seafloor is elevated forming oceanic ridges • Mid Atlantic Rift, Lomonosov Ridge, East Pacific Rise, Juan de Fuca Ridge, Galapagos Ridge, SW Indian Ridge… Copyright (c) 2005 Pearson Education Canada Inc. 19-24 Divergent Plate Boundaries Oceanic Ridges and Seafloor Spreading • Seafloor spreading occurs along the oceanic ridge system • Active volcanism causes black smokers, vent communities & seafloor massive sulfide deposits (SMS) Spreading Rates and Ridge Topography • Ridge systems exhibit topographic differences • Topographic differences are controlled by spreading rates – Slow spreading = rugged topography – Med-fast spreading = axial valley Copyright (c) 2005 Pearson Education Canada Inc. 19-25 Divergent Plate Boundaries Divergent boundaries are located mainly along oceanic ridges. Copyright (c) 2005 Pearson Education Canada Inc. 19-26 Divergent Plate Boundaries Spreading Rates and Ridge Topography • Topographic differences are controlled by spreading rates – At slow spreading rates (1-5 centimetres per year), a prominent rift valley develops along the ridge crest that is wide (30-50 km) and deep (1500-3000 metres) MidAtlantic – At intermediate spreading rates (5-9 centimetres per year), rift valleys that develop are shallow with subdued topography East Pacific Copyright (c) 2005 Pearson Education Canada Inc. 19-27 Divergent Plate Boundaries Spreading Rates and Ridge Topography • Topographic differences are controlled by spreading rates – At spreading rates greater than 9 centimetres per year no median rift valley develops and these areas are usually narrow and extensively faulted – At Thingvallir in West Iceland the rift is above sea level! Continental Rifts • Splits landmasses into two or more smaller segments • These start with 3 arms but usually 1 or 2 of them fail Copyright (c) 2005 Pearson Education Canada Inc. 19-28 Divergent Plate Boundaries Continental Rifts • Examples include the East African rift valleys and the Rhine Valley in northern Europe • Produced by extensional forces acting on the lithospheric plates • Not all rift valleys develop into full-fledged spreading centres • There are failed Precambrian rifts beneath southern Alberta, the East arm of Great Slave Lake and Mid Continent between Minnesota and Missouri! Copyright (c) 2005 Pearson Education Canada Inc. 19-29 Divergent Plate Boundaries RRR Triple Junction The East African rift – a divergent boundary on land. Copyright (c) 2005 Pearson Education Canada Inc. 19-30 Convergent Plate Boundaries Older portions of oceanic plates are returned to the mantle in these destructive plate margins • Surface expression of the descending plate is an ocean trench • Called subduction zones • Average angle at which oceanic lithosphere descends into the mantle is about 45 • Older colder lithosphere descends steeper up to 90° • Younger warmer lithosphere descends flatter <15° • The steeper the angle, the deeper the trench • Younger descending plates make the greatest earthquakes ~MCanada 9.0 Copyright (c) 2005 Pearson Education 19-31 R >Inc. Convergent Plate Boundaries Although all have the same basic characteristics, they are highly variable features Types of Convergent Boundaries • Oceanic-Oceanic Convergence – – – – Denser-older of 2 oceanic slabs sinks into the asthenosphere Earthquakes under Island arc (Antilles, Philippines, Japan) Primitive andesitic volcanoes Seabed explosive volcanoes and “black smokers” + SMS (PNG) • Oceanic-Continental Convergence – – – – Denser oceanic slab sinks into the asthenosphere Earthquakes are under the edge of the continent Andean type arcs with granitic plutons (Coast Mountains, Sierras) Big porphyry Cu-Mo deposits • Continent-Continent Convergence – Subducted slab falls away – Massive continental mountain belt built with crustal melting (Himalayas, Urals, Appalachians-Caledonides) – Massive earthquakes under “backstop” continent (Tibet, China) metal Education pegmatites Copyright–(c)Rare 2005 Pearson Canada Inc. 19-32 Convergent Plate Boundaries Oceanic-Oceanic Convergence When two oceanic slabs converge, the older, colder, denser one descends beneath the other Often forms volcanoes on the ocean floor (Marianas arc) If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands, Antilles, Java-Sumatra) Super-collosal explosive volcanoes as water hits magma (Krakatau) Active black smokers & Seafloor Massive Sulfides SMS (Papua Copyright (c) 2005 Pearson Education Canada Inc. New Guinea) 19-33 Convergent Plate Boundaries Oceanic-Continental Convergence As the plate descends, water from the slab causes flux partial melting of mantle rock This generates magmas having a basaltic or occasionally, andesitic composition These magmas can evolve in the crust to batholiths and explosive rhyolites Porphyry Cu-Mo deposits in batholiths (BC) Shallow hydrothermal systems produce epithermal Au-Ag-Cu deposits (Chile, Peru, Mexico, BC) Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades) Copyright (c) 2005 Pearson Education Canada Inc. 19-34 Convergent Plate Boundaries Continental-Continental Convergence Continued subduction can bring two continents together Less dense, buoyant continental lithosphere does not subduct Result is a collision between two continental blocks Process produces massive inracontinental mountains (Himalayas, Alps, Urals, Appalachians) Thickening causes lower crust to melt making granites Rare metal pegmatite deposits Copyright (c) 2005 Pearson Education Canada Inc. 19-35 Convergent Plate Boundaries Miocene ~10Ma Modern The collision of India and Asia produced the Himalayas. Copyright (c) 2005 Pearson Education Canada Inc. 19-36 Transform Fault Boundaries The third type of plate boundary Plates slide past one another and no new lithosphere is created or destroyed Transform Faults all take up differential motion • Most join two segments of a mid-ocean ridge as parts of prominent linear breaks in the oceanic crust known as fracture zones • Others join subduction zones, offset or opposed • Can also join a ridge to a subduction zone (Queen Charlotte Fault connects Juan de Fuca Ridge to Aleutians) Copyright (c) 2005 Pearson Education Canada Inc. 19-37 Transform Fault Boundaries Transform Faults • A few (the San Andreas Fault in California, the Alpine Fault of New Zealand & the Anatolian Fault in Turkey) cut through continental crust – This type makes large earthquakes ~MR>7 • Most separate ocean crust of different rates & ages (Blanco, Sovanco, Mendocino, Clipperton) – Only the part between ridges, trenches is seismically active Copyright (c) 2005 Pearson Education Canada Inc. 19-38 Testing the Plate Tectonics Model Plate Tectonics and Earthquakes • Plate tectonics model accounts for the global distribution of earthquakes – Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory (too warm) – Deep-focus earthquakes are closely associated with subduction zones (cold brittle zone extends into mantle) – The pattern of earthquakes along a trench provides a method for tracking the plate's descent – Young buoyant subducting plates have wide shallow Benioff zones & the most damaging earthquakes Copyright (c) 2005 Pearson Education Canada Inc. 19-39 Testing the Plate Tectonics Model Deep-focus earthquakes occur along convergent boundaries19-40 . Copyright (c) 2005 Pearson Education Canada Inc. Testing the Plate Tectonics Model 2 Subduction Zones Earthquake foci in the vicinity of the Japan trench19-41 . Copyright (c) 2005 Pearson Education Canada Inc. Testing the Plate Tectonics Model Evidence from Ocean Drilling • Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment – Age of deepest sediments in trenches (Marianas, PeruChile) – Thickness of ocean-floor sediments verifies seafloor spreading (thickens & ages away from ridges) – Oldest sediment & oldest ocean crust is distant from MOR’s – Age of oldest/deepest sediment = age of underlying magnetic anomaly Copyright (c) 2005 Pearson Education Canada Inc. 19-42 Testing the Plate Tectonics Model Hot Spots • Caused by rising plumes of mantle material – Many from bumps on Core-Mantle Boundary • Volcanoes can form over them (Hawaiian Island chain, Azores, Yellowstone, Fiji, Iceland) • Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary • Isotopes of great “growth ages” from recycled ancient crust carried into lower mantle • Fixed reference frame for plate motions (Pacific, Yellowstone) Copyright (c) 2005 Pearson Education Canada Inc. 19-43 Testing the Plate Tectonics Model The Hawaiian Islands have formed over a stationary hot spot. 19-44 Copyright (c) 2005 Pearson Education Canada Inc. Measuring Plate Motion Currently possible with space-age technology to directly measure relative motion between plates Two methods used are VLBI-Very Long Baseline Interferometry & GPS-Global Positioning System Calculations show that Hawaii is moving NW and approaching Japan at 8.3 cm/year Yellowstone shows SW motion of North America since Miocene North America and Europe are getting 5 cm further apart per year (50 km per Ma) Copyright (c) 2005 Pearson Education Canada Inc. 19-45 The Driving Mechanism No one driving mechanism accounts for all major motions & forces in plate tectonics. Researchers agree that convective flow in the rocky 2900 kilometre-thick mantle is the basic driving force of plate tectonics Early ideas were that convection was passive and slower than plate motion Most now believe the Mantle moves faster than the Plates MOR’s are thermal bulges (high spots) in Upper Mantle while Trenches are (low spots) Modern researchers believe that Lithosphere Plates slide downhill over the weak Asthenosphere Early ideas for mechanisms generate forces that contribute to plate motion Slab-pull (but rocks tensile strengths are weak) Ridge-push (but rocks are weak in compression & no folds or thrusts on MOR system) Copyright (c) 2005 Pearson Education Canada Inc. 19-46 The Driving Mechanism Models of Plate-Mantle Convection • Any model describing mantle convection must explain why basalts erupt along the oceanic ridge • All of the ideas are constrained by seismic information about rock properties in the Earth’s interior: fast, slow, strong, weak, cold, hot… • Models – Layering at 660 kilometres – Whole-mantle convection – Deep-layer model Copyright (c) 2005 Pearson Education Canada Inc. 19-47 Importance of Plate Tectonics Theory provides a unified explanation of Earth’s major surface morphology & internal processes Within the framework of plate tectonics, geologists have found explanations for the geologic distribution of earthquakes, volcanoes, and mountain belts Plate tectonics also provides explanations for past distributions of plants and animals Regions of Seismicity, High Heat Flow are explained Different ore deposits, rock types & rare minerals are explained Copyright (c) 2005 Pearson Education Canada Inc. 19-48 End of Plate Tectonics as a Background for much of the rest of the course! Chapter 19 Copyright (c) 2005 Pearson Education Canada Inc. 19-49