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PLATE TECTONICS 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 T29 S&A-22 PLATE TECTONICS • 4. Crust – basalt (fine grained igneous rock, volcanic) • 5. Hydrosphere • 6. Atmosphere PLATE TECTONICS Earth made of concentric spheres review • 1. Inner core • 2. Outer core • 3. Mantle – Asthenosphere – Lithosphere • 4. Crust • 5. Hydrosphere • 6. Atmosphere PLATE TECTONICS 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 23 T-20 • Play the game of isostasy before class! PLATE TECTONICS 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) PLATE TECTONICS 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). PLATE TECTONICS • "Moho" - the boundary, which geologists refer to as the Mohorovicic discontinuity, between Earth's brittle outer crust and its hotter, softer mantle PLATE TECTONICS MOHO exposed at the surface located between crust and mantle (S&A 22) PLATE TECTONICS Continents • Thick accumulations of granitic rocks • Oldest rocks on Earth - about 3.8 billion years PLATE TECTONICS 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 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 TECTONICS 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 TECTONICS 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 PLATE TECTONICS 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 PLATE TECTONICS • Pillow lava along Juan de Fuca Ridge. Photo courtesy of Submarine Ring of Fire 2002 Exploration, NOAA-OE. PLATE TECTONICS • START HERE FOR WEDNESDAY • If you are interested in a 9th edition of our book. • [email protected]. PLATE TECTONICS I want you to print out an article on hot spots • Go to google and look up hot spots. PLATE TECTONICS 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) PLATE TECTONICS Plates moving across hot spots cause chains of volcanic islands Hawaiian Islands PLATE TECTONICS 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 PLATE TECTONICS Earth’s magnetic field • Fig 2.7 and 2.9 p 40 and 43 PLATE TECTONICS 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 PLATE TECTONICS 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 PLATE TECTONICS Neat stuff on the earths magnetic field • http://science.nasa.gov/headlines/y2003/29d ec_magneticfield.htm • www.pbs.org/wgbh/nova/magnetic/reversal s.html – This shows an interactive. I expect you to do this PLATE TECTONICS 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) PLATE TECTONICS • Volcanic eruptions occur more frequently on rapidly spreading segment than on slow spreading ones. • A hydrothermal vent is a geyser on the seafloor. PLATE TECTONICS Three types of hydrothermal vents p 94 • black smokers • white smokers • cooler discharge PLATE TECTONICS black smokers PLATE TECTONICS White smoker PLATE TECTONICS 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. PLATE TECTONICS 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 PLATE TECTONICS Tube worms T 134 PLATE TECTONICS 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 PLATE TECTONICS 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 PLATE TECTONICS 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? PLATE TECTONICS PLATE TECTONICS All three vents support abundant growths of bottom-dwelling organisms. • Chemosynthesis: the process by which certain microbes create energy by mediating chemical reactions PLATE TECTONICS PLATE TECTONICS PLATE TECTONICS 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. PLATE TECTONICS 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 Pacifictype margins are frequently narrow • 2. Passive margins of continents lie in middle of plates PLATE TECTONICS ACTIVE MARGINS PLATE TECTONICS 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 PLATE TECTONICS Subducting plate is old and dense • it sinks into the mantle as a steeply dipping slab PLATE TECTONICS 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 PLATE TECTONICS Exotic terranes • terranes have a history distinct from adjoining crustal fragments are welded onto continents during subduction (A&S-41) PLATE TECTONICS Go to web for animation PLATE TECTONICS PLATE TECTONICS PASSIVE MARGINS PLATE TECTONICS 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) PLATE TECTONICS 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 PLATE TECTONICS 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. PLATE TECTONICS 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 PLATE TECTONICS 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. PLATE TECTONICS 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 PLATE TECTONICS 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 PLATE TECTONICS • START HERE ON MONDAY PLATE TECTONICS 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. PLATE TECTONICS • 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 PLATE TECTONICS Boundaries Table 2.2 p 53 • • • • a. b. c. d. divergent - plates move apart convergent - plates collide transform - plates slide past one another Study this table (It is on this exam) PLATE TECTONICS 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 PLATE TECTONICS 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 PLATE TECTONICS Present spreading cycle cont. • Mediterranean is the remnant of Thetys Sea PLATE TECTONICS END Plate Tectonics We have seen all the material that will be covered on Exam I PLATE TECTONICS