Oceanography Exam 1 Chapter 2

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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


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