Plate Tectonics Chapter 2 PDF

Chapter 2 Plate Tectonics: A Scientific Revolution 1  Relationship of plate tectonics to life on Earth Plate tectonics is where large pieces of the planet shell slide around and interact giving us mountains, earthquakes and volcani...

Chapter 2 Plate Tectonics: A Scientific Revolution 1  Relationship of plate tectonics to life on Earth Plate tectonics is where large pieces of the planet shell slide around and interact giving us mountains, earthquakes and volcanic eruptions. – It provides a mechanism of carbon dioxide gas to move from the atmosphere to Earth’s interior and back again USGS Carbon dioxide is a greenhouse gas that regulates our planets temperature so it isn’t too hot or too cold, which allows liquid water to exist on the planet. The presence of liquid water allowed for the development of life It is thought that because Earth is the only planet in our solar system with active plate tectonics is the only one known to sustain life. 2  From Continental Drift to Plate Tectonics Until the 1960s most geologists thought the positions of the continents and ocean basins were fixed. Continental drift, a hypothesis that challenged this belief was first proposed by Alfred Wegener in 1915, but it was discounted. He proposed the supercontinent called Pangaea began breaking apart about 200 million years ago Alfred Wegner on a Greenland expedition trying to prove his theory. (you do NOT need to memorize the date 1915) 3 3 Wegner’s evidence of Continental Drift 1. Continental coastlines across the Atlantic Ocean are mirror images 2. Rocks of mountain ranges continue across oceans 4 Wegner’s evidence of Continental Drift (cont.) 3. Glacial and coal swamp locations. When reconstructed into Pangaea, ancient glaciers occur at the south pole and coal swamps at the equator 4. Identical land-based fossils found on both sides of the Atlantic Ocean 5  Continental Drift Hypothesis was Rejected Continental Drift was initially rejected because there was no evidence of a reasonable mechanism for the great forces needed to move large masses of solid rock long distances. New data on the character of the ocean floor in the 1960s provided the needed proof. 6  The Theory of Plate Tectonics is now proven and with it Continental Drift Theory of Plate Tectonics: Earth’s outer rigid shell called the lithosphere consists of individual plates that move on the underlying weaker partially-molten asthenosphere 7  Some Features of Lithospheric Plates They change shape, size and location through time larger plates contain both continental and oceanic crust no single plate is defined by the margins of a single continent They move about 2 to 20 cm per year Most major deformation occurs at plate boundaries Earth’s diameter remains constant, so as oceanic crust is created at divergent boundaries, oceanic crust is destroyed at convergent plate boundaries. 8 Lithospheric Plates 9 Lithospheric Plate Movement Spreading Rates – The average spreading rate is 5 centimeter/year – Mid-Atlantic Ridge has a spreading rate of 2 centimeter/year – East Pacific Rise has a spreading rate of 15 centimeter/year 10  Plate Motion https://www.arcgis.com/apps/MapJournal/index.html?appid=5f0160d8b0a44bdbafe9679 11 20e08ed4a  Types of Plate Boundaries Divergent boundary – product of extension – plates move apart – new oceanic lithosphere created Convergent boundary – product of compression – plates move together – Oceanic lithosphere destroyed continental lithosphere created Transform boundary – no compression or extension – plates grind past each other – no production or destruction of lithosphere 12 Motion at Plate Boundaries 13 Divergent Plate Boundary Other names: Sea Floor Spreading, Spreading Center, Oceanic Ridge. Where two plates move apart, resulting in upwelling of magma from the mantle to create new oceanic crust on the sea floor. Create oceanic ridges- long topographic highs on the sea floor. Create rift valleys – long canyon-like feature within the ocean ridges 14 Divergent Plate Boundary: Example - Mid-Atlantic Ridge in Iceland Iceland 15  Divergent Plate Boundary: Continental rifting Continental Rifting: is when a new divergent plate boundary starts forming within a continent – First crust warps – then a rift valley forms – then a linear sea and – finally a new ocean forms splitting the continent apart Present day examples – East African rift valley 16 Divergent Plate Boundary: Continental Rifting Example - East African Rift 17  Convergent Plate Boundary Other names: Subduction Zone, Ocean Trench Where two plates move toward each other and the leading edge of the denser oceanic lithosphere is bent downward and slides beneath the less dense continental lithosphere. Destroyed - oceanic lithosphere Created – continental crust from volcanic emanations Deep-Ocean Continental Volcanic Arc or Island Volcanic Arc – Deep-Ocean Trench 18 Three Types of Convergent Plate Boundaries Ocean-Continent Ocean-Ocean Continent-Continent 19  Ocean-Continent Convergent Boundary Creates a Continental Volcanic Arc Example: volcanoes of the Cascade Range of the Northwestern United States (this includes Mt St. Helens) 20  Ocean-Ocean Convergent Boundary Creates an Island Volcanic Arc Examples: Japan and Aleutian Islands of Alaska Volcanoes in the Aleutian chain are a volcanic island arc. The Japanese Islands are a Volcanic Island Arc 21 Continent-Continent Convergent Boundary When to continental masses collide, the two less dense, buoyant continental lithospheres do not subduct producing a mountain belt of deformed rocks. Example: the Himalaya Mountains, the world’s tallest mountains 22 Convergent Plate Boundary Examples of convergent plate boundaries (in blue lines) Ocean-Ocean Ocean-Continent Continent- Continent Japan trench NW United States, Cascade Range Himalayan Mountains 23  Transform Plate Boundary Occur where oceanic-ridge segments are offset so the lithospheric plates slide horizontally past one another No oceanic or continental crust is produced or destroyed 24 Transform Plate Boundary: Example the San Andreas Fault of California 25 Proof of Plate Tectonics Evidence of Plate Motion – Age and thickness of ocean sediments – Mantle Plumes and Hot Spot Tracks – Paleomagnetism – Apparent Polar Wandering – Magnetic Reversals and Seafloor Spreading 26 Proof of Plate Tectonics - Ocean sediments Ocean sediments increase in thickness and age with increased distance from the crest of the ocean ridges. – Older ocean crust has had more time to accumulate sediment 27 Proof of Plate Tectonics – Mantle Plumes and Hot Spots Mantle plume - is volcanic activity not associated with a plate margin, that originates in the mantle. Hot spot is the surface expression of a mantle plume Hot-spot tracks are linear chains of volcanic islands that increase in age with distance from a mantle plume Example: Hawaiian Islands chain 28 Proof of Plate Tectonics - Mantle Plumes and Hot Spots Hot Spot Volcano Tracks 29 Proof of Plate Tectonics - Hot Spots Tracks Hot Spots Tracks are volcanic island chains not associated with a plate margin Hawaiian Island Chain Yellowstone National Park 30 What is the significance of the ages of the Hawaiian Islands in terms of plate tectonics? a. The ages correspond to the time of formation of the break-up of Pangaea. b. The ages increase toward the big Island of Hawaii to show that it formed at an ocean ridge and then moved away from that site. c. The ages decrease toward the big Island of Hawaii to show that the tectonic plate is moving over a hot spot or mantle plume. d. The ages show that the Hawaiian Islands have not moved relative to North America. Proof of Plate Tectonics- Paleomagnetism Iron in minerals aligns with Earth’s magnetic field as the mineral forms. – Iron atoms are magnetic with negative and positive poles. – When magma cools below its Curie point (580oC), the iron is “frozen” into the rock pointing to Earth’s poles (“fossil compass”). Paleomagnetism is a record of the ancient orientation of Earth’s field at the time any given iron bearing mineral formed. 32 Proof of Plate Tectonics – Apparent Polar Wandering Apparent Polar Wandering - Magnetic minerals in rocks from the same location but different ages suggest the N pole moved with time. – Poles cannot move so concluded the rocks moved Paths of poles fit the Pangaea model 33  Proof of Plate Tectonics - Magnetic Reversals on the Ocean Floor Every million to hundreds of thousands of years the north and south poles reverse (reverse polarity). Iron in ocean basalt “freezes” in this change in polarity producing reversal bands centered on the ocean spreading centers 34 Proof of Plate Tectonics – Magnetic Reversals on the Ocean Floor Seafloor Spreading and Magnetization 35 The age of the Ocean Floor is controlled by Plate Motion Ocean crust present before the breakup of Pangaea has all been destroyed. All present-day oceanic crust was created after the start of the breakup of Pangea (approximately 200 million years ago). – So, the ocean crust is less than 200 million years old. 36 Creation of the Ocean Floor 37  Age of the Ocean Crust is controlled by Plate Motion Ocean crust present before the breakup of Pangaea has now been destroyed Present day oceanic crust ranges in age from 0 years to 180 million years. The oceanic crust is youngest at the divergent margins and oldest at tectonically inactive continental margins. 38 After map by Sclater & Meinke  Self Test – Where do you find the youngest and oldest oceanic crust? oldest youngest 39 http://noaacontent.nroc.org/lesson02/l2la1.htm Lithospheric Plate move as a result of mantle convection. Convection is the upward movement of less dense material and downward movement of more dense material. Below is one model of mantle convection.  Forces that Drive Plate Motion Slab Pull: subducting ocean slab is cold and dense so it sinks and pulls the plate down. – This is the main driving force of plate motion. Ridge Push: the ridge is elevated so the oceanic material on the ridge “slides” down its flanks. 41  End of Lecture Chapter 2 Some portions of this course contain material used under the Fair Use Exemption of US copyright law. Further use may be prohibited by the copyright owner. 42  Self Test 1. Which type of plate boundary occurs at X? 2. What feature occurs at Y, and how does it form? 3. What is happening at Z? 4. Identify the three plates in the diagram and name the materials that make up each plate. 5. Which type of plate boundary occurs at Y? 6. What feature occurs at X and how does it form? 43  Answers to Self Test 1. divergent 2. At Y, a deep-ocean trench is forming. Two plates of different densities are colliding. The oceanic crust is denser and plunges beneath the continental crust, forming a trench. 3. The edge of plate B is plunging beneath plate C and melting in the mantle. 4. Plates A and B are made of oceanic crust and lithosphere. Plate C is made of continental crust and lithosphere. 5. convergent 6. At X, the mid-ocean ridge occurs along a boundary between two oceanic plates. The plates are moving apart, causing molten material to repeatedly rise from the mantle, erupt, and harden as solid rock along the center of the ridge. 44  Self Test 2 What type of motion? ___________ What type of plate margin is this? __________________ Where is new continental crust forming (2 locations)? _______ ________ Where is oceanic crust destroyed? ____________ Not all will be used Oceanic plate Continental plate Deep-sea trench Volcanic Arc Mantle melting Accretionary Wedge Convergent Divergent Subduction zone Spreading center Earthquake zone 45 http://hays.outcrop.org/images/tectonics/subduction.jpg  Self Test 2 Answers What type of motion? Convergent What type of plate margin is this? Subduction zone Where is new continental crust forming (2 locations)? Volcanic Arc, Accretionary Wedge Where is oceanic crust destroyed? Deep-Sea Trench Not all will be used Oceanic plate Continental plate Deep-sea trench Volcanic Arc Mantle melting Accretionary Wedge Convergent Divergent Subduction zone Spreading center Earthquake zone 46 http://hays.outcrop.org/images/tectonics/subduction.jpg