Chapter 2 oceanography PPT
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
Earth's Magnetic Pole
Evidence for Plate Tectonics • Apparent polar wandering • Location of North Pole changed over time • Magnetic dip data
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
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 Sea Floor Spreading; • Harry Hess - World War II submarine captain and geologist • Depth recordings show sea floor features • History of Ocean Basins - Seafloor spreading - Mantle convection cells as driving mechanism Plate Tectonic Processes; Sea Floor Spreading • Mid-ocean ridge - spreading center • Subduction zones - oceanic trench site of crust destruction • Subduction can generate deep ocean trenches. 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 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 Age of Ocean Floor
Plate Tectonics
• Alfred Wegener first proposed in 1912 • Called it "Continental Drift" 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
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
Types of Convergent Boundaries
• Continental-Continental Convergence - No subduction - Tall mountains uplifted • Himalayas from India-Asia collision
Magnetic Polarity Reversals
• Earth's magnetic polarity reverses periodically • Recorded in ancient igneous rocks • 176 reversals in past 76 million years • Unpredictable pattern
Coral Reef Development
• Fringing reefs - develop along margin of landmass • Barrier reefs - separated from landmass by lagoon • Atolls - reefs continue to grow after volcanoes are submerged Great Barrier Reef Records Plate Movement Detecting Plate Motion with Satellites Paleogeography; • 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
Heat Flow
• Heat flow - heat from Earth's interior released to surface • Very high at mid-ocean ridges • Low at subduction zones
World Map 50 million Years in Future Wilson Cycle
• John Tuzo Wilson • Plate tectonics model shows life cycle of ocean basins - Formation - Growth - Destruction Wilson Cycle ;
Applications of Plate Tectonics
• Mantle Plumes and Hotspots - Intraplate features • Volcanic islands within a plate • Island chains • Record ancient plate motions - Nematath - hotspot track
Earthquakes as Evidence
• Most large earthquakes occur at subduction zones. • Earthquake activity mirrors tectonic plate boundaries. Global Plate Boundaries; 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 Types of Plate Boundaries Examples of Plate Boundaries Divergent Boundary Features; • Plates move apart • Mid-ocean ridge - Rift valley • New ocean floor created • Shallow focus earthquakes - Intensity measured with seismic moment magnitude
Chapter Overview
• Much evidence supports plate tectonics theory. • The plate tectonics model describes features and processes on Earth. • Plate tectonic science has applications to Earth Science studies. • Configuration of land and oceans has changed in the past and will continue to change into the future.
Evidence for Plate Tectonics
• New evidence from World War II • Sea floor studies with sonar • New technology enabled study of Earth's magnetic field Evidence for Plate Tectonics; • Earth's magnetic field and paleomagnetism • Earth has magnetic polarity • North and South polarities • Magnetic polarity recorded in igneous rocks - Magnetite in basalt Evidence for Plate Tectonics; • Paleomagnetism - study of Earth's ancient magnetic field - Interprets where rocks first formed - Magnetic dip
Divergent Plate Boundary Generation of a Divergent Boundary Formation of a Rift Valley 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 - Arctic and southwest India
Three Types of Convergent Boundaries Types of Convergent Boundaries
• Oceanic-Continental Convergence - Ocean plate is subducted - Continental arcs generated - Explosive andesitic volcanic eruptions
Types of Convergent Boundaries
• Oceanic-Oceanic Convergence - Denser plate is subducted - Deep trenches generated - Volcanic island arcs generated
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
Types of Spreading Centers 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
Evidence for Continental Drift
• Puzzle-like fit corroborated in 1960s • Sir Edward Bullard used computer models to fit continents. Evidence for Continental Drift; • Matching sequences of rocks and mountain chains • Similar rock types, ages, and structures on different continents Evidence for Continental Drift; • 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. Evidence for Continental Drift; • Distribution of organisms • Same fossils found on continents that today are widely separated • Modern organisms with similar ancestries
Global Hotspot Locations Hawaiian Island - Emperor Seamount Nematath 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.