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GEOG 3251 summer ‘10 Adina Racoviteanu

GEOG 3251 summer ‘10 Adina Racoviteanu. Objectives. Understand how Earth is dynamic and how moving plates form ocean basins, mountain ranges, islands, volcanoes, and earthquakes

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GEOG 3251 summer ‘10 Adina Racoviteanu

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  1. GEOG 3251 summer ‘10 Adina Racoviteanu

  2. Objectives • Understand how Earth is dynamic and how moving plates form ocean basins, mountain ranges, islands, volcanoes, and earthquakes • Identify the three general categories of plate boundaries recognized by scientists: convergent, divergent, and transform • Understand how the theory of plate tectonics was developed and supported

  3. Why is the Earth so restless? • ground shakes violently • volcanoes erupt with explosive force • mountain ranges rise to incredible heights

  4. Catastrophism world view Until 1700s: common belief: • Biblical Flood shaped Earth's surface • All earthly changes were sudden and caused by a series of catastrophes.

  5. Uniformitarian principle • 1785 : James Hutton, a Scottish geologist proposes new world view: The present is the key to the past. • geologic forces and processes acting on the Earth today are the same as those that have acted in the geologic past.

  6. EARTH’S LAYERS • Three layers: • the core, • the mantle and • the crust • Chemical & mechanical differences Like boiled egg

  7. THE CORE • Composition: mostly iron and nickel • Temperature: very hot, even after 4.5 billion years of cooling. • divided into two layers: • a solid inner core • liquid outer core.

  8. CORE GENERATES CURRENTS • Core is so hot  it radiates natural heat to the upper layers. • convection currents generated. • The convection currents cause the movement • of the tectonic plates.

  9. MANTLE • Composition: mafic minerals - rich in the elements iron (Fe) magnesium (Mg), silicon, and oxygen • Dense, hot layer of semi-solid rock • Upper part of the mantle is cooler and more rigid than the deep mantle

  10. CRUST • Composition: oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. • rigid and very thin

  11. LITHOSPHERE • Greek (lithos = stone) rigid outermost layer made of crust and uppermost mantle • broken up into the moving plates that contain the world's continents and oceans

  12. ASTHENOSPHERE • Greek (asthenos = weak) = part of the mantle that flows, a characteristic called plastic behavior. • The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates.

  13. CRUST/MANTLE AGAIN • lithosphere • hard • ~100 km thick • crust floats on top • continental crust 20 to 70 km thick • oceanic crust ~ 8 km thick • asthenosphere • soft • ~3000 km thick • “fluid-like”

  14. Crust: 2 types • Oceanic crust is made of relatively dense rock called basalt • Continental crust is made of lower density rocks, such as andesite and granite.

  15. Oceanic vs.continental crust Oceanic Continental basaltic rocks granitic rocks "mafic” Mg, Fe “felsic” (Fe-Si, Al) denser less dense thinner thicker

  16. EARTH STRUCTURE SUMMARY • The Earth is sphere with a diameter of about 12,700Kilometres. • Temperature and pressure rise with depth. • CORE: • temperature is believed to be an incredible 5000-6000°c. • Inner core: solid FE and NI • Outer core: liquid Fe and Ni • Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field. • MANTLE: • It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky part... the mantle is a solid which flows????) • CRUST: • very thin (average 20Km) • OCEANIC CRUST: thinner (10 kilometres) • CONTINENTAL CRUST: thicker (35 kilometres on average). • The continental crust in the Himalayas is some 75 kilometres deep.

  17. THEORIES OF MOUNTAIN ORIGIN • CONTINENTAL DRIFT • Alfred Wegener (1880 – 1930) • PLATE TECTONICS • Harry Hess and G. Mason (1962)



  20. FOSSIL EVIDENCE • Fossils of the same species found on several different continents. • Wegener’s idea: species dispersed when the continents were connected and later carried to their present positions as the continents drifted. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia.

  21. TORN NEWSPAPER IDEA • Rock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. • Wegener’s idea: the same three layers occur at each of these localities.


  23. SHIFTING CLIMATE ZONES • Wegener’s interpretation: • climate zones remained stationary and the continents drifted to different locations. • The drift of the continents caused the apparent movement of the climate zones.

  24. ARGUMENTS AGAINST • What mechanism moves the continents?? Wegener’s idea: Earth's spincaused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges.

  25. Continental Drift EvidenceSummary • Puzzle idea: continents fit together • Torn newspaper: matching geology • Fossils: same fossil on different continent • Paleoclimate

  26. SEA-FLOOR SPREADING In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean.

  27. SEA FLOOR SPREADING HO When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands. The pattern was centered along, and symmetrical to, the mid-ocean ridge

  28. Computer-generated detailed topographic map of a segment of the Mid-Oceanic Ridge. "Warm" colors (yellow to red) indicate the ridge rising above the seafloor, and the "cool" colors (green to blue) represent lower elevations.

  29. New discoveries: Sea-floor spreading • 1962: Hess (1962): New ocean floor is formed at the rift of mid-ocean ridges. VIDEO



  32. SEA FLOOR SPREADING • So, continents are no longer thought to plow through oceanic crust • Continents are part of plates that move on the soft, plastic asthenosphere. • driving force: convection currents

  33. If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere.

  34. PLATE TECTONICS SUMMARY • Plate = large, rigid slab of solid rock • Tectonics (Greek) = to build • PLATE TECTONICS: states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.

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