Plate tectonics
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PLATE TECTONICS. Earth made of concentric spheres ( Fig. 1.14 ) p 18 T-14. 1. Inner core - rich in iron and nickel, dense 2. Outer core - liquid 3. Mantle Asthenosphere - nearly molten and can flow very slowly

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PLATE TECTONICS

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PLATE TECTONICS


Earth made of concentric spheres (Fig. 1.14) p 18 T-14

  • 1. Inner core - rich in iron and nickel, dense

  • 2. Outer core - liquid

  • 3. Mantle

    • Asthenosphere - nearly molten and can flow very slowly

    • Lithosphere - rigid outer layer of the earth and floats in the asthenosphere. (Fig. 1.16) p 22 T-29 S&A-22


  • 4. Crust

    • basalt (fine grained igneous rock, volcanic)

  • 5. Hydrosphere

  • 6. Atmosphere


Earth made of concentric spheres review

  • 1. Inner core

  • 2. Outer core

  • 3. Mantle

    • Asthenosphere

    • Lithosphere

  • 4. Crust

  • 5. Hydrosphere

  • 6. Atmosphere


Isostasy

  • Isostatic adjustment

  • describes the relative elevations that materials of different densities and thicknesses reach at equilibrium with gravity (1.16) p 20 S&A 23T-20

  • Play the game of isostasy before class!


Isostasy cont

  • Crustal materials float in asthenosphere (Fig. 14) p 20

  • Weight of volcano bends crust (Fig 2.27) remember from last time.

  • Glaciers also cause crust so subside (a process by which one plate descends beneath another plate and is ultimately resorbed into the mantle)


Isostasy cont

  • Scandinavia and Antarctica are rising due to the melting of the glaciers that cover them.

  • Remember how the block raised when we reduced the height (from isostasy game).


  • "Moho" - the boundary, which geologists refer to as the Mohorovicic discontinuity, between Earth's brittle outer crust and its hotter, softer mantle


MOHO exposed at the surfacelocated between crust and mantle (S&A 22)


Continents

  • Thick accumulations of granitic rocks

  • Oldest rocks on Earth - about 3.8 billion years


Plate tectonics

  • New crust formed at mid-ocean ridges or spreading centers Convection currents (Fig. 2.10) p 44 (this is a very important diagram)

  • Crust and upper mantle constitute the rigid lithosphere float on nearly molten asthenosphere


Plate tectonics cont

  • Lithosphere broken into rigid units and move slowly older lithospheric material is being subducted while new lithosphere is produced along the ridges and rises.

  • Trenches plates converge

  • Plates move past each other along transform faults (Fig. 2.23) p 59

  • Plate movements shape ocean basins T-30 (Fig. 2.13 b) p 48


Plate boundaries (Fig. 2.14 a. b. c.) p50

  • 1. Divergent boundary - Midocean ridges - plates form and move away from each other (Fig. 2.14 a)

  • 2. Convergent boundary - Trenches - plates move toward each other and are destroyed as they are drawn down into the mantle (subduction) (Fig. 2.14 b, 2.20, 2.21, and 2.22) p 56-58


Plate boundaries cont.

  • 3. Fracture zones - plates slide past each other (NO earthquakes felt)

  • 4. Transform faults - plates slide past each other (Earthquakes felt) (Fig. 2.14 c)

  • Transform faults and fracture zones (Fig. 3.17) p 95 (T 38)

  • San Andreas Fault (Fig. 2.23) p 59


Continental Rifting Fig 2.17 p 52

  • a. upwarping

  • b. rift valley (fig 2.18) p 54 read and understand

  • c. linear sea

  • d. mid-ocean ridge

  • e. Table 2.1 p 51


  • Pillow lava along Juan de Fuca Ridge. Photo courtesy of Submarine Ring of Fire 2002 Exploration, NOAA-OE.


  • START HERE FOR WEDNESDAY

  • If you are interested in a 9th edition of our book.

  • bmichael23@aol.com.


I want you to print out an article on hot spots

  • Go to google and look up hot spots.


Hot spots

  • plumes of magma that rise from deep within the mantle erupt (Fig. 2.24) p 62 T 54

  • Plates moving across hot spots cause chains of volcanic islands Hawaiian Islands (Fig. 2.25) p 62

  • Many occur near midocean ridges. Seen today in Iceland. (Fig2.26) p 63 formation of sea mounts and table mounts (guyots)


Plates moving across hot spots cause chains of volcanic islands Hawaiian Islands


Hot spots cont.

  • Others beneath the continent - gisers in Yellowstone National Park

  • Flood Basalts - from volcanic activity that produces widespread gently sloping surfaces. Commonly surround volcanic islands


Earth’s magnetic field

  • Fig 2.7 and 2.9 p 40 and 43


Earth's magnetic field (Fig. 2.12) p 46 T 28

  • Changes orientation at irregular intervals, as of today we do not know why.

  • Minerals record the orientation of Earth's magnetic field at the time when the rocks cooled ~ 100,000 yrs (Fig. 2.11) p 45

  • Measure with magnetometers T 48


Earth's magnetic field cont.

  • Form bands with same orientation - like tape recorder T-26

  • Matthews and Vine saw the magnetized rocks and decided that the rocks were younger in the center of the ridge, older at edges

    • Permits determining age of ocean floor


Neat stuff on the earths magnetic field

  • http://science.nasa.gov/headlines/y2003/29dec_magneticfield.htm

  • www.pbs.org/wgbh/nova/magnetic/reversals.html

    • This shows an interactive. I expect you to do this


Hydrothermal circulation p 93

  • Sea water circulates through hot, newly formed rocks (Fig 3.17 a) p 93

    • 1. cooling them

    • 2. removes metals and deposits them in ocean floor vents

    • 3. ocean water circulates through newly formed crust every 5 - 10 million years. (remember the ocean has been around a very long time)


  • Volcanic eruptions occur more frequently on rapidly spreading segment than on slow spreading ones.

  • A hydrothermal vent is a geyser on the seafloor.


Three types of hydrothermal vents p 94

  • black smokers

  • white smokers

  • cooler discharge


black smokers


White smoker


Hydrothermal Vents cont.

  • Three types of hydrothermal vents

    • 1. most spectacular are black smokers.

      • a. They discharge superheated waters (300 - 400oC) at high rates much like a fire hose.

      • b. Black because of chem. rx. 2o to those that occur in the water forming sulfur-bearing minerals.

      • c. form large fragile chimney like mounds up to 10 meters high made of porous silica, native sulfur, and sulfur-bearing minerals.


Hydrothermal Vents cont.

  • d. Color the mounds with yellows and blacks (like Halloween decorations) Read Recovery of Black Smokers p 90-91

  • e. Tube worms: Very fast growing and reach sizes up to 3 m (10 ft) other animals include sea anemone, clams, crabs, fish and bacteria Fig 15.25 p 477

  • f. Temperature fluctuations are common occurring in days to seconds.

  • g. Micro-organisms


Tube worms T 134


Hydrothermal Vents cont.

  • 2. White smokers

    • a, are not as hot (200-330 oC) are also common

    • b. circulating fluids have mixed with cold ocean waters

    • c. milky discharge thus the name


Hydrothermal Vents cont.

  • 3. Cooler discharge (cold seep) (5 - 25oC)

    • a. water flows out through cracks and fissures in the ocean floor.

    • b. cold seep waters are about the same temperature as the surrounding waters

    • c. the discharge water is clear


Hydrothermal Vents cont.

Hydrothermal circulation continues for millions of years as the rocks cool.

  • Eventually, fractures fill with mineral deposits and fluids no longer pass through.

  • Sediments accumulate on the ocean floor.

  • Where do the minerals come from?


All three vents support abundant growths of bottom-dwelling organisms.

  • Chemosynthesis: the process by which certain microbes create energy by mediating chemical reactions


Continental Margins

  • (steep slopes that descend to the sea floor) p 100 - 102

  • One of the most outstanding features of the continental slopes are submarine canyons. (Fig 3.9 p 83) Submarine canyons are steep sided and V-shaped in cross section with tributaries similar to those of river-cut canyons.


Continental Margins two types(Fig 3.7) p 82

  • 1. Active continental margins lie along edge of plates (Convergent)

    • Contain many active volcanoes, frequent earthquakes, young mountains

    • Common along Pacific margins, called Pacific-type margins are frequently narrow

  • 2. Passive margins of continents lie in middle of plates


ACTIVE MARGINS


Earthquakes:

  • 1. common near Pacific-type margins

  • 2. deep earthquakes indicate subduction - in subduction zones, plates move as large slabs and drag against the rocks above and below causing earthquakes in those areas

  • 3. subduction causes a drag on rocks -> deform the rocks along the margins -> energy buildup -> earthquakes -> energy releases


Subducting plate is old and dense -

  • it sinks into the mantle as a steeply dipping slab


Subducting plate is young

  • still warm, and relatively buoyant

  • slab dips at a shallow angle

  • occurs along the eastern margin of the Pacific, where the American plate is overriding recently formed crust

  • volcanoes occur on land

  • many of earthquakes


Exotic terranes

  • terranes have a history distinct from adjoining crustal fragments are welded onto continents during subduction (A&S-41)


Go to web for animation


PASSIVE MARGINS


Passive margins of continents

  • No earthquakes, no volcanoes. Fig 3.7 p 81 book

  • Called Atlantic-type margins

  • Form after continents are rifted apart and tend to be wide

  • Thick sediment deposits and old oceanic crust

    • trenches do not form,

    • sediment is folded into mountain ranges (Appalachians, Alps, Himalayas, and Urals)


Passive margins cont.

  • Economically, they are important because of the accumulations of oil and gas that they often contain. Most of the world's giant oil and gas fields occur in such deposits.

  • Found on Atlantic Ocean, Antarctic Ocean. Arctic Ocean, and Indian Ocean


Mantle convection (Fig 2.10 p 44)

  • Occurs in mantle, causing plate movements

  • Supplies molten rock to midocean ridges, causing volcanism

  • 1. hot-spot volcanoes originate at unusually hot areas of the core mantle boundary

  • 2. the overlying mantle melts, forming plumes of magma that rise and penetrate the crust as volcanoes.


Mantle convection cont.

  • 3. these hot spots do not move with the overlying mantle and thus remain fixed in location for tens of millions of years

  • 4. red patches - continental flood basalts and oceanic lava plateaus T 54

  • Crust cools as it ages, and grows denser

  • Oldest, densest rock sinks into the mantle at the trenches


Formation of ocean basins (Fig 2.17 abcd)

  • Formed through the breakup of continents. Begins when a continent remains in one location for >100 million years continent impedes heat flow from the earth's interior.


Formation of ocean basins cont

  • a. underlying mantle heats, expanding and uplifting the overlying lithosphere

  • b. continents rift, forming narrow valleys (rift valley in Africa)

  • c. valleys widen into narrow ocean basin, e.g., Red Sea midocean ridges in the middle of ocean - actively widening

  • d. ocean continues to widen until the oldest crust becomes dense enough to sink, causing the ocean to gradually narrow


Destruction of ocean basins Fig 2.20 abc p 56

  • Ocean - continent convergence

  • Ocean - ocean convergence

  • Continent - continent convergence

  • Mountains form when basin closes

  • Appalachian Mountains mark site of ancient ocean that closed about 400 million years ago Can see the sedimentary rocks on the AT


  • START HERE ON MONDAY


Mantle convection (Fig 2.10 p 44)

  • Occurs in mantle, causing plate movements

  • Supplies molten rock to midocean ridges, causing volcanism

  • 1. hot-spot volcanoes originate at unusually hot areas of the core mantle boundary

  • 2. the overlying mantle melts, forming plumes of magma that rise and penetrate the crust as volcanoes.


  • 3. these hot spots do not move with the overlying mantle and thus remain fixed in location for tens of millions of years

  • 4. red patches - continental flood basalts and oceanic lava plateaus T 54

  • Crust cools as it ages, and grows denser

  • Oldest, densest rock sinks into the mantle at the trenches


Boundaries Table 2.2 p 53

  • a. divergent - plates move apart

  • b. convergent - plates collide

  • c. transform - plates slide past one another

  • d. Study this table (It is on this exam)


Points to remember

  • Continent - from granite

  • Ocean - from basalt

  • Heat flow - Fig 2.10 p 44

  • Table 2.1 p 51 This is the culture vulture question


Breakup of Pangaea on to present

  • Began about 225 million years ago, with the breakup of Pangaea about 180 million years ago T 21 - 23 and 15

  • North Atlantic formed first, South Atlantic later

  • Indian Ocean is the youngest basin

  • Pacific is the remnant of Panthallasia, previous cycle


Present spreading cycle cont.

  • Mediterranean is the remnant of Thetys Sea


END Plate Tectonics

We have seen all the material that will be covered on Exam I


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