Plate Tectonics: A Unifying Theory PDF

Plate Tectonics: A Unifying Theory Dr. Neil J. Porter Introduction to Natural Hazards and Disasters ESCI-1000 The lecture material is the intellectual property of Dr....

Plate Tectonics: A Unifying Theory Dr. Neil J. Porter Introduction to Natural Hazards and Disasters ESCI-1000 The lecture material is the intellectual property of Dr. Neil J. Porter. Distribution or publication of this material in whole or in part is illegal, under the Canadian Copyright Act, RSC 1985.  Plate Tectonics: A Unifying Theory Overview 2.0 Introduction 2.1 Evidence for Continental Drift 2.2 Supporting Evidence 2.3 Plate Boundary Features 2.4 Hot Spots and Mantle Plumes 2.5 Past and Future Plate Movements 2.6 Applications of Plate Tectonics  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics Why should you know about plate tectonics? Plate tectonics affects all of us, whether in relation to the destruction caused by volcanic eruptions and earthquakes, or politically and economically due to the formation and distribution of valuable natural resources. Plate tectonics is the unifying theory of geology, tying together many seemingly unrelated geologic phenomena and illustrating why Earth is a dynamic planet of interacting subsystems and cycles.  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth Protoearth Larger than Earth today Homogeneous composition Bombarded by meteorites Moon formed from collision with large asteroid  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth The Protoearth  Radioactive heat Spontaneous disintegration of atoms Fusion reactions  Heat from contraction (protoplanet shrinks due to gravity)  Protoearth partially melts Density Stratification (layered Earth)  heavy metals sank to the core like iron light elements like silica stayed in the surface  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth Earth’s Interior: Layers: Chemical Composition: 1. : Crust, mantle and core 2. Physical propeties: : Lithosphere, Asthenosphere, Mesosphere, Outer Core, Inner Core.  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth The outermost layer, the crust; Low-density, mainly silicate minerals. 1. Continental Crust Very thick, less dense eg. granite 2. Oceanic crustType text here ( basalt )  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth The mantle: The mantle surrounds the core and is divided into: an asthenosphere that behaves plastically and flows slowly. Mainly iron (Fe) and magnesium (Mg) silicate minerals.  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics: Inside the Earth Chemical Properties: Core The core consists of: a small, solid inner region a larger, liquid, outer portion Flow generates Earth’s magnetic field  High-density, mainly iron (Fe) and nickel (Ni)  Plate Tectonics: A Unifying Theory 2.0 Introduction to Plate Tectonics Alfred Wegner first proposed in 1912- " Continental Drift" The Theory of Plate Tectonics is a fundamental foundation for the geosciences - explains the surface features on the Earth Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana. As Pangaea broke up, the various continents moved to their present-day locations.  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Wegener’s evidence for continental drift: Fit of continents 1. 2. Matching geolocigal units 3. Glacial ages and climate belts 4. Distribution of fossils  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift:  Plate Tectonics: A Unifying Theory Fit of continents 2.1 Evidence for Continental Drift: Continents once fitted together – Pangaea one large continent existed 200 million years ago Panthalassa – one large ocean Noted puzzle-like fit of modern continents  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Fit of continents Puzzle-like fit corroborated Continental shells might have gas reserves in 1960s Sir Edward Bullard used computer models to fit continents. There is a close fit between the continents off the coast at a depth of about 2000 m.  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Matching geolocigal units The trend of several major mountain ranges produces a continuous mountain range when the continents are positioned next to each other as they were during the formation of Pangea.  Plate Tectonics: A Unifying Theory Matching geolocigal units 2.1 Evidence for Continental Drift: Similarity of Rock Sequences Marine, nonmarine, and glacial rock sequences of Pennsylvanian to Jurassic age are nearly identical on all the Gondwana continents Fig. 2.4, p. 32  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Glacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole Direction of glacial flow and rock scouring Plant and animal fossils indicate different climate than today  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Glacial ages Glacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole Direction of glacial flow and rock scouring Plant and animal fossils indicate different climate than today  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Distribution of fossils Distribution of organisms Same fossils found on continents that today are widely separated only endemic to South America and Africa Modern organisms with similar ancestries  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Distribution of fossils Some of the most compelling evidence comes from fossils like the Glossopteris fern. One of the strongest examples is the Mesosaurus, a fresh water reptile  Plate Tectonics: A Unifying Theory 2.1 Evidence for Continental Drift: Objections to Continental Drift Hostile criticism and open ridicule (The Origins of Continents and Oceans, 1915) Continents cannot plow through ocean basins Tidal gravitational attractions too small The hypothesis was correct in principle; however, proposed mechanism defies laws of physics  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence Researchers refined new techniques (radiometric dating), developed new instruments (sonar), and discovered new fossils, which strengthened Wegener’s argument; however, it was the research obtained from paleomagnetism and the exploration of the sea floor, which provided evidence to plate movements and ocean floor spreading.  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Earth’s Magnetic Field Paleomagnetism is the remnant magnetism in ancient rocks recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation. Magnetic polarity recorded in igneous rocks Magnetite in basalt  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Magnetic Reversals and Seafloor Spreading Earth’s present magnetic field is considered normal Normal - with the north and south magnetic poles located approximately at the north and south geographic poles. At various times in the geologic past, Earth’s magnetic field has completely reversed. N S S N  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Magnetic Polarity Reversals Earth’s magnetic polarity reverses periodically Recorded in ancient igneous rocks 176 reversals in past 76 million years Unpredictable pattern Last occurrence – 780,000 yrs  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Paleomagnetism and the Ocean Floor 1955 – deep water rock mapping Magnetic anomalies – regular pattern of north-south magnetism “stripes” Stripes were symmetrical about long underwater mountain range  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Sea Floor Spreading  Harry Hess World War II submarine captain and geologist  Depth recordings show sea floor features  History of Ocean Basins Proposed - Sea-floor spreading Mantle convection cells as driving mechanism  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Sea Floor Spreading The mid-ocean ridge is a continuous underwater mountain range. Wraps 1.5 times around globe. Rises 2.5 km  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Magnetic Reversals and Seafloor Spreading Frederick Vine and Drummond Mathews 1963  Seafloor spreading was confirmed by the discovery of magnetic anomalies in the ocean crust that were both parallel and symetric around the ocean ridges.  This indicates that new oceanic crust must be formed along the spreading ridges.  Oldest ocean floor only 180 million years old  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Magnetic Reversals and Seafloor Spreading  Deep-Sea Drilling and the Confirmation of Seafloor Spreading  Further evidence confirming seafloor spreading came from the Deep Sea Drilling Project are the age and thickness of the sediments overlying the oceanic crust.  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Age of the Ocean Floor  Plate Tectonics: A Unifying Theory 2.2 Supporting Evidence: Magnetic Reversals and Seafloor Spreading  Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970s.  The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources. For these reasons, it is known as a unifying theory.  Plate Tectonics: The Unifying Theory 2.2 Supporting Evidence: Earthquakes Most large earthquakes occur at subduction zones. Earthquake activity mirrors tectonic plate boundaries.  Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Types of Plate Boundaries Plate tectonics has operated since at least the Proterozoic. It is important to understand how the plates move and interact with one another. 1. Divergent plates move away from each other eg. mid-ocean ridges 2. convergent plates plates approach each other 3. Transform plates moving in opposite directions of each other Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Divergent plate Boundries mostly occur in ocean ridges b) An Example of Ancient Rifting Characteristic features of ancient continental rifting include faulting, dikes, sills, lava flows, and thick sedimentary sequences within rift valleys. Pillow lavas and associated deep-sea sediments are evidence of ancient spreading ridges. Plate Tectonics: The Unifying Theory 2.3 Plate Boundary Features: Types of Convergent Boundaries 1. Oceanic – Continental convergence 2. Oceanic – Oceanic convergence 3. Continental – Continental convergence  Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Types of Convergent Boundaries 1. Oceanic-Continental Convergence a. Characteristic of that plate: The ocean plate is subducted because basalt is much more denser than granite. b. Landform: trenches cone shaped volcanoes  Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Types of Convergent Boundaries 2. Oceanic-Oceanic Convergence a. characteristic: the denser plate is subducted ( the denser one is the older one) b. Landforms trenches deepest part of the ocean is actually near land eg. west coast of South America volcanic Arc  Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Types of Convergent Boundaries 3. Continental-Continental Convergence a.charcteristic: no subduction b. Landform: Tall mountain chains. eg. Indian Himilians  Plate Tectonics: A Unifying Theory 2.3 Plate Boundary Features: Transform Boundary Features These are boundaries along which plates slide laterally past each other along transform faults.  Oceanic Transform Fault – ocean floor only  Continental Transform Fault – cuts across continent San Andreas Fault  Transform faults occur between mid-ocean ridge segments.  Plate Tectonics: A Unifying Theory 2.4 Hot Spots and Mantle Plumes Record ancient plate motions Nematath Hawaiian Islands – Emperor Seamount Nematath  Plate Tectonics: A Unifying Theory 2.4 Hot Spots and Mantle Plumes  Plate Tectonics: A Unifying Theory 2.5 Past and Future Plate Movements: Satellites have detected and measured plate movements over time using magnetism we can trace where islands used to be Plate Tectonics: A Unifying Theory 2.5 Past and Future Plate Movements: Paleogeography – The study of historical changes of continental shapes and positions Continental accretion Continental material added to edges of continents through plate motion Paleogeographic Reconstructions  Plate Tectonics: A Unifying Theory 2.5 Past and Future Plate Movements: 50 million Years in the Future  Assume same direction and rate of plate motions as now Atlantic will enlarge, Pacific will shrink New sea from East Africa rift valleys Further Himalaya uplift Separation of North and South America Part of California in Alaska  Plate Tectonics: A Unifying Theory 2.6 Applications of Plate Tectonics: Mineral Resources Mineral Deposits Many metallic mineral deposits are related to igneous and associated hydrothermal activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits. eg. Copper mines in Chile Many of the world’s major metallic ore deposits are associated with convergent and divergent plate boundaries. Copper, iron, lead, zinc, gold and silver ore deposits are associated with plate boundaries.  Plate Tectonics: A Unifying Theory 2.6 Applications of Plate Tectonics: Distribution of Life Fossil evidence provided one of the first proofs for plate tectonics. Together, plate tectonics and evolution have changed the way we view our planet. The world’s plants and animals occupy biotic provinces controlled mostly by: Climate Geographic barriers The location of these provinces is mostly controlled by plate movement.