2. Source: Map by Seth Stein, Northwestern University; USGS Earthquake Hazard Program; Global Volcanism Program, Smithsonian Institution
3. The Core Inner Core = solid and 5X density of surface rock Outer Core = partially molten material Rotates few 1/10ths of degrees faster than the mantle each year
4. Source: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago
5. The Mantle Mantle = 70% earth’s volume Denser and cooler than core and magnesium-iron silicates Solid with weak areas Warmer/less dense material floats to surface and cooler/more dense sinks down Crust is a cold, rigid outer layer Boundary between crust and mantle = moho
6. Differences between continental and oceanic crusts Continental crust light and averages 25 miles thick and mainly granite rock Sodium, potassium, aluminum, and silica Oceanic crust is more dense and averages 4.3 miles and mainly basalt Iron, magnesium, and calcium
7. How is the lithosphere and asthenosphere different? (both part of mantle) Lithosphere is strong and rigid = crust and upper mantle (plate tectonics) Ocean floor lithosphere reaches max age at 80 million years old or 62 miles Continental lithosphere varies from 62-124 miles thick
9. What is Pangaea? Part of Wegener’s theory of continental drift (i.e. single supercontinent) Suess theory was that S. American continents jointed into single continent Called Gondwanaland Portions of the continents were assumed to have sunk to form oceans Wegener’s theory focused on spinning of earth as force that broke up continents Laurasia is a name for the N. Continents America and Eurasia Based on the way continents fit and older mountain ranges and rock similarities 500 million year cycle of assembling and disassembling land masses
10. See how plates separated http://www.divediscover.whoi.edu/tectonics/pangea-animation.html http://pubs.usgs.gov/gip/dynamic/historical.html http://www.youtube.com/watch?v=ft-dP2D7QM4
11. Plate Tectonics then and now Alfred Wegener Theory in 1920’s of continental drift Bruce Heezen and Marie Tharp 1947 to 1965 mapped ocean floor Harry Hess in 1960’s convection cell theory
12. Plate tectonics combines continental drift and convection cell theories as an explanation
13. Hess in 1960’s suggested that earth made up of convection cells of heat rising to surface of lithosphere and then cooling becoming dense and sinking back to the bottom of the mantle Confirmation of upper and lower mantle convection cells since then Whole Mantle Convection Model – where upper and lower mantle have movement of magma
14. Ridges along ocean floor where magma flows outward and where mantle expands Lithosphere that is old, cool and dense sinks at deep sided ocean trenches In reality not all trenches are where lithosphere is sinking In some cases sediment carried off continent fills trenches such as NW coast Lateral movement of lithosphere from convection cells produces seafloor spreading or spreading centers Subduction zones mark areas of descending lithosphere
16. Evidence for Crustal motion Epicenters found in areas of spreading and subduction When heat flow was measured at epicenters hottest where young material created and coolest where older material resides
17. Magnetic Wandering Magnetic evidence also reveals information about earth’s history Earth’s magnetic field has many challenges when studied Difficult to work in polar regions, daily alterations and magnetic storms Recently N. and S. poles have been moving NW
18. Earths magnetic field consists of invisible lines parallel to earth’s surface Compass needle aligns it’s self with these lines As needle approaches poles it will point up or down parallel to dipping lines of magnetism Magnetite acts like small magnet and is found in basalt magma in oceans Magnetite grains align themselves with current magnetic forces at the time of magma cooling
19. Paleomagnetism studies fossil magnetism During last 76 million years there have been 170 magnetic reversals of the poles Present magnetic orientation has existed for 710,000 years See Figure 3.14 for age of ocean floor in relation to spreading at ridges Polar Wandering Curve is the study of fossil magnetism in continental rocks of different ages and in order to create a plot of location of historic north magnetic pole
22. Now you know the mechanisms that cause the mantle to move and the evidence. What can the crust tell us? Lithosphere is fragmented into 7 different plates that move in relation to each other on top of the asthenosphere Some plates like Pacific consist entirely of oceanic plate but most have a combination of continental and oceanic plate As plates interact with one another they create active volcanic zones
24. Divergent boundaries Rising magma makes crust move upward and thins out Rift valley is formed with active volcanism Shallow linear sea formed Mature ocean basin See Figure 3.23 for divergent boundaries The Red Sea and Gulf of Aden between Africa and the Arabian Peninsula
25. Transform boundaries Transform boundaries separate the segments of ridge system Fracture Zone = fossil faults along active transformation zones No movement or earth quake activity along fracture zones Boundary between N. America and Pacific plate long transform fault See Figure 3.29 for transformation boundaries along West Coast at the Juan de Fuca plate
27. Convergent boundaries Subduction and sinking of one plate over another into mantle Usually along ocean trench Also associated with volcanic and earthquake activity Oceanic lithosphere able to subduct more readily due to density Oceanic lithosphere more dense than continental See three types of convergent boundaries on Figure 3.30
28. Magma begins at 60-100 miles down Ocean-Continent convergence magma must rise through continent Mixed basalt and granite produces andesite Andesite volcanic eruptions are explosive due to high concentration of volatiles (gases) and silicon dioxide
29.
30. “A Wadati-Benioff zone (also Benioff-Wadati zone or Benioff zone) is a deep active seismic area in a subduction zone[“Source: http://en.wikipedia.org/wiki/Wadati-Benioff_zone
31. Continent-Continent convergent zones the end result of closing ocean basin Continent crust buckles, fractures and thickens Alps and Himalayas mountains continent-continent convergent zones
32. Continental margins What is a trailing or passive margin? Passive margins begin at divergent plate boundaries These margins are broad and shallow such as East Coast US What is a leading or active margin? NW coast Margins narrow and deep with trenches and volcanic activity
33. N. America 1.8 million years old and combination of 4-5 older continents Terranes = island systems, seamounts, volcanoes, parts of other continental land masses 25% N. America formed from Terranes (i.e. Alaska from S. 70 million yrs ago and E. Coast island arc connected to Europe)
34. How fast do plates spread per year? Mid-Atlantic Ridge is slow spreading due to steep profile and deep valley 2.5-3 cm per yr spreading rate East Pacific Ridge is faster due to gentler slope and shallow vallies 8-13 cm per yr spreading rate Average spreading rate is 5 cm per yr Spreading is not uniform
35. Rate increases with volcanic activity Eruptions estimated every 50-100 yrs along fast spreading area While every 5000-10,000 along slow spreading areas Iceland only land mass lying on mid-ocean ridge/rift Volcanic activity began in 1975 and widened 5 m of ridge
37. Areas of isolated volcanic activity 40 I.D. around globe Where hot magma from deep in mantle surfaces Can be over land (i.e. Yellowstone) or sea (i.e. Hawaiian Islands) As crust moves West creates new islands = Hawaiian Islands Youngest most active and oldest seamount
39. Research projects 1979 Alvin made 24 dives into mid Atlantic rift (8000 ft) Surprise was large communities of animals around areas of hydrothermal vents “Lost City” older vent system 9 miles from mid-Atlantic Ridge Older systems emitting carbonate and magnesium rich minerals Communities of extremophiles found!
40. Deep sea drilling has many challenges Keeping a fixed position over drill site requires small propellers mounted on bow and stern of boat Glomar Challenger retired in 1983 after drilling 1092 holes in 624 sites 1985 JOIDES Resolution with single pipe 27,000 ft long Confirmed the thickness and location to ridges revealed age of crust
