Structure of the Earth

COMPOSITIONAL STRUCTURE OF THE EARTH

STRENGTH STRUCTURE OF THE EARTH

  • Core - solid inner core & fluid outer core
  • Mantle & crust (see below)


    • Study Questions

    1. What are the compositional layers of the Earth?

    2. What are the strength layers of the Earth?

    Continental Drift

    INTRODUCTION

  • CONCEPT OF CONTINENTAL DRIFT EVOLVED over the course of 50 years INTO THEORY OF PLATE TECTONICS VIA SEAFLOOR SPREADING

    • ALFRED WEGENER

  • FIRST TO ASSEMBLE DIVERSE DATA SUPPORTING CONTINENTAL DRIFT (1912)
  • Late Paleozoic supercontinent (Pangea) surrounded by a superocean (Panthalassa) with an embayment of Panthalassa (Tethys) into Pangea
  • an EXTENSION OF TETHYS BROKE PANGEA INTO LAURASIA (N. America & Eurasia) & GONDWANA (S. America, Africa, Antarctica, Australia, & India)
  • EVIDENCE
  • Shape of continents ("real" edge of continent = continental slope)
  • Geology (Match up continental geology on either side of an ocean)
  • MOUNTAIN BELTS (Appalachians/Caledonides); SEDIMENTARY BASINS (S. Africa/Argentina); [RADIOMETRIC AGE PROVINCES (Brazil, W. Africa)]
  • Paleontology (Similarity/differences of fossils on various continents)
  • Similar Carboniferous & Permian flora (Glossopteris ) & fauna (including Mesosaurus ) in Gondwana continents
  • Paleoclimatology (Climate zones vary by latitude - distinct sedimentary deposits in each zone)

  • WEGENER'S MECHANISM FOR CONTINENTAL DRIFT (continents plow through the oceans toward the poles due to the equatorial bulge) WAS NOT ACCEPTABLE

    • PALEOMAGNETICS IN THE 1950'S - APPARENT POLAR WANDERING

  • EARTH'S MAGNETIC FIELD - LIKE A BAR MAGNET AT THE CENTER OF THE EARTH
  • inclination varies with latitude, & magnetic poles equal rotational poles (over periods of a few ky)
  • REMANENT MAGNETISM - ROCKS RECORD MAGNETIC FIELD ORIENTATION
  • igneous rocks - when they cool below Curie temperature [~600°C]
  • sedimentary rocks - at the time of deposition
  • FOR A GIVEN AREA - POSITION OF MAGNETIC POLE SEEMS TO CHANGE OVER TIME
  • FOR A GIVEN TIME - EACH CONTINENT GIVES A DIFFERENT MAGNETIC POLE POSITION
  • APPARENT POLAR WANDERING => MAGNETIC POLES SEEM TO MOVE, BUT REALLY THE CONTINENTS HAVE MOVED & MAGNETIC POLES ARE STATIONARY

    • Study Questions

    1. Why is the development of the concept of Continental Drift credited to Alfred Wegener?

    2. What 4 types of evidence did Wegener cite in support of Continental Drift?

    3. Why Wegener's proposal was not accepted?

    4. What modern continents comprised Gondwana & Laurasia?

    5. What was the role of paleomagnetics in developing the concept of Continental Drift?

    6. Why didn't Wegener use paleomagnetic data in support of Continental Drift?

    Seafloor Spreading

    PALEOMAGNETICS IN THE 1960'S - MAGNETIC POLARITY REVERSALS

  • MAGNETIC POLES ALSO SEEMED TO REVERSE PERIODICALLY (COMPASS NEEDLE POINTS TO THE NORTH MAGNETIC POLE NOW, BUT AT TIMES IN THE PAST, THE SAME NEEDLE POINTS TO THE SOUTH MAGNETIC POLE)
  • EXACT TIMES OF MAGNETIC POLARITY REVERSALS DETERMINED IN THE EARLY 1960'S
  • numerous young (<5 my) volcanic rock samples (continental lava fields & oceanic islands) - magnetic polarity & K-Ar radiometric age
  • plots of radiometric age versus magnetic polarity showed a series of relatively long (~1 my) magnetic polarity EPOCHS (eg. BRUNHES) & shorter (~100 ky) magnetic polarity Events (Jaramillo)(see below)
  • magnetic polarity time scale back to 4.5 my was developed

    • OCEANOGRAPHY

  • MARINE GEOLOGY
  • mapped topography of seafloor (central Mid-Ocean Ridge [MOR] with rift valley at crest = extension in centers of oceans)
  • collected sediment cores & rock dredges (oldest sediments & rocks were only Mesozoic in age)
  • Harry Hess (1962) - proposed the concept of Seafloor Spreading (convection in mantle with upwelling of hot mantle under MOR & creation of new seafloor; conveyor belt carrys continents passively)
  • MARINE GEOPHYSICS
  • magnetic polarity reversals found in ocean crust; m arine magnetic anomalies were recognized to result from seafloor spreading combined with reversals of the Earth's magnetic field (magnetic polarity vs. width of seafloor)
  • SEAFLOOR SPREADING RATES were calculated (<1 TO >17 CM/YR)
  • OLDEST SEAFLOOR - JURASSIC in age (<200 M.Y.)[OLDEST CONTINENTAL ROCKS - ARCHEAN in age {3.96 B.Y.}]
  • EXTENSION of MAGNETIC POLARITY TIME SCALE - MARINE MAGNETIC ANOMALIES EXTENDED BEYOND GILBERT EPOCH
  • AGE OF SEAFLOOR PREDICTED
  • DEEP SEA DRILLING PROJECT (DSDP) - DRILLED SECTION ACROSS SOUTH ATLANTIC & CONFIRMED PREDICTED AGES & SEAFLOOR SPREADING
  • oldest sediment on top of basaltic oceanic crust got older away from MOR crest in predicted manner; generated band wagon effect
  • helps absolute dating of geologic time scale for Mesozoic/Cenozoic

    • Study Questions

    1. What was the role of paleomagnetics in developing the concept of Seafloor Spreading?

    2. Summarize the development of the paleomagnetic reversal time scale for the last 4 my.

    3. Summarize the development of the paleomagnetic reversal time scale for the Tertiary & Mesozoic.

    4. What was the role of Harry Hess in developing Seafloor Spreading?

    5. Why was the Deep Sea Drilling Project crucial in confirming Seafloor Spreading?

    Plate Tectonics

    SEISMOLOGY - INSTRUMENTAL in DEVELOPMENT of PLATE TECTONICS

  • SHOWED THAT NEW SEAFLOOR GOES BACK INTO MANTLE ALONG INCLINED SEISMIC ZONES ASSOCIATED WITH DEEP-SEA TRENCHES, ISLAND ARCS (JAPAN & ALEUTIANS) & ANDESITIC VOLCANISM (ANDES) = SUBDUCTION ZONES
  • CONFIRMED TRANSFORM FAULTS AS DIFFERENT THAN TRANSCURRENT FAULTS AS PROPOSED BY J. TUZO WILSON

    • DISTRIBUTION OF EARTHQUAKES & VOLCANOES

  • LONG, NARROW BELTS COINCIDENT WITH MOR CREST, DEEP-SEA TRENCHES & LONG FAULTS
  • LITHOSPHERE DIVIDED INTO SEVERAL LARGE (MAJOR) & MANY SMALLER (MINOR) PLATES
  • plates move relative to each other, but deform only at their edges
  • 7 MAJOR PLATES - NORTH & SOUTH AMERICAN, INDIAN-AUSTRALIAN, EURASIAN (NEARLY ALL CONTINENTAL LITHOSPHERE), PACIFIC (NEARLY ALL OCEANIC LITHOSPHERE), & AFRICAN & ANTARCTIC (NEARLY SURROUNDED BY MOR)
  • Some important minor plates - Nazca, Cocos, Juan de Fuca & Philippine [Pacific], Arabian [Indian], & Caribbean & Scotian [Atlantic]

    • TYPES OF PLATE MOTION

  • DIVERGENT (= SEAFLOOR SPREADING) - PLATES MOVE AWAY FROM EACH OTHER
  • associated with MOR & continental rift zones
  • shallow (<70 km) earthquakes, basaltic volcanism
  • CONVERGENT (= SUBDUCTION) - PLATES MOVE TOWARD EACH OTHER
  • associated with deep-sea trenches & island arcs or marginal mountain belts (Andes); continental collision zones (Alps/Himalayas)
  • shallow AND deep (>70 km [700 km max]) earthquakes & andesitic volcanism & granitic plutonism
  • LATERAL (= TRANSFORM OR STRIKE-SLIP) - PLATES SLIDE PAST EACH OTHER
  • associated with transform faults (like the San Andreas) & fracture zones
  • shallow earthquakes but NO volcanism

    • DRIVING FORCES

  • MANTLE CONVECTION
  • "PUSH-PULL" - PLATES PUSHED APART AT MOR & PULLED DOWN AT TRENCH BY COLD, SUBDUCTING SLAB
  • fastest spreading rates = many trenches [Pacific]

    • MOUNTAIN BUILDING

  • ANDESITIC VOLCANIC ARCS AT SUBDUCTION ZONES
  • Andes in South America
  • COLLISIONAL MOUNTAIN RANGES WHERE CONTINENTAL LITHOSPHERE IS ON BOTH SIDES OF A CONVERGENT PLATE BOUNDARY
  • Himalayas, Alps
  • boundary is called a "Suture zone", often with oceanic crust (called ophiolite) incorporated along the suture
  • continental margin sediments deformed & accreted to continent

    • WILSON CYCLES

  • CONTINENTS ALTERNATELY CONSOLIDATE INTO LARGE SUPERCONTINENTS (LIKE PANGEA AT THE END OF THE PALEOZOIC) OR DISPERSE INTO SEVERAL CONTINENTAL MASSES (LIKE WE HAVE TODAY)
  • EFFECT ON ORGANIC EVOLUTION:
  • life is relatively diverse during continent dispersal - many geographic barriers
  • life is less diverse during continent consolidation - few barriers

    • Study Questions

    1. What was the role of seismology in developing the theory of Plate Tectonics?

    2. What are the differences between transform & transcurrent faults?

    3. What have been the contributions of J. Tuzo Wilson to Plate Tectonics theory?

    4. What are the names of the major plates & their characteristics?

    5. What are the types of plate boundaries?

    6. What are the seafloor features & continental features associated with each?

    7. What are the depth of earthquakes associated with each?

    8. What are the volcanism & plutonism associated with each?

    9. What are the driving forces involved in Plate Tectonics?

    10. What are the 2 ways mountain systems get built ? Give examples of each.

    11. Define a Wilson Cycle.

    12. What is the effect of a Wilson Cycle on Evolution?

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