Oceanography Exam 1 Chapter 2
Breakup of Pangaea
-180 million years ago - Pangaea separated -N. and S. America rifted from Europe and Africa -Atlantic Ocean forms -120 million years ago - S. America and Africa clearly separated -45 million years ago - India starts Asia collision -Australia moving north from Antarctica
Objections to Early Continental Drift Model
-1915 - Wegener published The Origins of Continents and Oceans -Suggested continents plow through ocean basins -Met with hostile criticism and open ridicule -Tidal gravitational attractions too small to move continents -Proposed mechanism defies laws of physics
Paleomagnetism & 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
-Alfred Wegener first proposed in 1912 -Called it "Continental Drift"
Future Predictions
-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
Oceanic-Oceanic Convergence
-Denser plate is subducted -Deep trenches generated -Volcanic island arcs generated
Magnetic Polarity Reversals
-Earth's magnetic polarity reverses periodically -Recorded in ancient igneous rocks -176 reversals in past 76 million years -Unpredictable pattern
Sea Floor Spreading Evidence
-Frederick Vine and Drummond Matthews (1963) -Analysis of igneous rock stripes around mid-ocean ridge -Sea floor stripes record Earth's magnetic polarity
Sea Floor Spreading
-Harry Hess: World War II submarine captain and geologist -Depth recordings show sea floor features -History of Ocean Basins -Mantle convection cells as driving mechanism with seafloor spreading
Heat Flow
-Heat flow: heat from Earth's interior released to surface -Very high at mid-ocean ridges -Low at subduction zones
Wilson Cycle
-John Tuzo Wilson -Plate tectonics model shows life cycle of ocean basins: Formation/Growth/Destruction
Age of Ocean Floor
-Late 1960s deep-sea drilling -Radiometric dating of ocean rocks -Symmetric pattern of age distribution about mid-ocean ridges -Oldest ocean floor only 180 million years old
Plate Tectonics Theory
-Lithosphere - tectonic plates that float on ductile asthenosphere -Large-scale geologic features occur at plate boundaries. -Two major tectonic forces: Slab pull & Slab suction
Applications of Plate Tectonics
-Mantle Plumes and Hotspots -Intraplate features -Volcanic islands within a plate -Island chains -Record ancient plate motions -Nematath: hotspot track
Evidence for Continental Drift cont...
-Matching sequences of rocks and mountain chains -Similar rock types, ages, and structures on different continents -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 -Distribution of organisms -Same fossils found on continents that today are widely separated -Modern organisms with similar ancestries
Sea Floor Spreading Cont...
-Mid-ocean ridge: spreading center -Subduction zones: oceanic trench site of crust destruction -Subduction can generate deep ocean trenches
Earthquakes as Evidence
-Most large earthquakes occur at subduction zones -Earthquake activity mirrors tectonic plate boundaries
Evidence for Plate Tectonics
-New evidence from World War II -Sea floor studies with sonar -New technology enabled study of Earth's magnetic field -Earth's magnetic field and paleomagnetism -Earth has magnetic polarity -North and South polarities -Magnetic polarity recorded in igneous rocks -Magnetite in basalt
Continental-Continental Convergence
-No subduction -Tall mountains uplifted -Himalayas from India-Asia collision
Oceanic-Continental Convergence
-Ocean plate is subducted -Continental arcs generated -Explosive andesitic volcanic eruptions
Transform Boundary Features Cont...
-Oceanic Transform Fault - ocean floor only -Continental Transform Fault - cuts across continent -San Andreas Fault -Transform faults occur between mid-ocean ridge segments
Transform Boundary Features
-Offsets oriented perpendicular to mid-ocean ridge -Segments of plates slide past each other -Offsets permit mid-ocean ridge to move apart at different rates -Shallow but strong earthquakes
Evidence for Plate Tectonics Cont...
-Paleomagnetism: study of Earth's ancient magnetic field -Interprets where rocks first formed -Magnetic dip -Apparent polar wandering -Location of North Pole changed over time -Magnetic dip data
Divergent Boundary Features
-Plates move apart -Mid-ocean ridge -Rift valley -New ocean floor created -Shallow focus earthquakes -Intensity measured with seismic moment magnitude
Convergent Boundary Features
-Plates move toward each other -Oceanic crust destroyed -Ocean trench -Volcanic arc -Deep focus earthquakes -Great forces involved -Mineral structure changes associated
Plate Tectonics and Intraplate Features
-Seamounts (rounded tops) -Tablemounts or guyots (flattened tops) -Subsidence of flanks of mid-ocean ridge -Wave erosion may flatten seamount
Evidence for Continental Drift
-Wegener proposed Pangaea - one large continent existed 200 million years ago -Panthalassa - one large ocean -Included the Tethys Sea -Noted puzzle-like fit of modern continents -Puzzle-like fit corroborated in 1960s -Sir Edward Bullard used computer models to fit continents.
Paleogeography
-study of ancient continents -Continental accretion: continental material added to edges of continents through plate motion -Pangaea: 540 million to 300 million years ago
Three Types of Convergent Boundaries
1. Oceanic-Continental Convergence 2. Oceanic-Oceanic Convergence 3. Continental-Continental Convergence
Types of Spreading Centers
Oceanic rise: fast-spreading & gentle slopes East Pacific: oceanic ridge, slow-spreading, & steep slopes Mid-Atlantic: Ultra-slow, deep rift valley, & widely scattered volcanoes
