Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
2-1 2 How do we explain the geological activity of the earth? Many pieces of information had to come together... Age of the Earth 2 • uniformitarianism vs catastrophism (1800’s) • about 4.6 billion years old Continental Drift 2 • Alfred Wegener, 1912 • based on Amazing fit of Africa & South America & fossils evidence Figure 2.2 2 Figure 2.6 2 Figure 2.5 2 Continental Drift 2 • single super continent = Pangea – split 200 million years ago & are still drifting • Wegener’s proposed mechanism = centrifugal force of spinning earth and tidal drag of moon & sun – He was WRONG about this • His idea was criticized through his death in 1930 2 Discovery of Mid-Atlantic Ridge in 1925 Fig. 3-18, p. 63 Patterns of Earthquakes and Volcanoes 2 • Plotted by Wadati & Benioff in late 1930’s – Related to continental drift? – Pacific Ring of Fire • a world wide pattern that followed orderly lines – many corresponded to oceanic ridges (first plotted in 1925) Figure 2.13a 2 Youth of Ocean Floor 2 • Maximum age of ocean floor was 200 million years • centers of continents were much older (3.9 billion years) Figure 2.12 2 Mantle Studies 2 • Seismographic evidence showed that the upper mantle was deformable and plastic • perhaps the continents could move Sea Floor Spreading 2 • 1960, Harry Hess and Robert Dietz • new seafloor (basaltic crust) develops at mid-oceanic ridges and then spreads outward • continental drift would be caused by the same forces Sea Floor Spreading 2 • powered by convection currents in the asthenosphere 2 Fig. 3-11, p. 57 Figure 2.10 2 Sea Floor Spreading 2 • explained... – why ridges were hot – why ocean was deeper away from ridges – why sediments were thicker and older away from ridges 2 Since the earth is not increasing in diameter... 2 Fig. 3-13, p. 59 2 Subduction Zones • Subduction zones (Wadati-Benioff zones) were discovered where the crust plunges into the mantle • crust is destroyed here • explains why the ocean floor is so young Figure 2.20a 2 Plate Tectonics 2 • 1965, primarily by Wilson • lithosphere is divided into plates that float & drift on top of the plastic asthenosphere Figure 2.13b 2 Plate Tectonics 2 • plates move about 5 cm per year • Plate movement is powered by: – downward pull of descending plates leading edge – friction of asthenosphere convection currents – outward push of new seafloor at spreading centers 2 Fig. 3-11, p. 57 2 Plate Tectonics 2 • plates interact at boundaries – diverge, converge or slip past each other Divergent Plate Boundaries 2 • spreading centers, mid-oceanic ridges • a line along which two plates are moving apart • new oceanic crust forms Figure 2.17 2 Figure 2.15 Divergence along Mid-Atlantic Ridge 2 Figure 2.16 2 Rift Valley of Iceland Figure 2.18a 2 East African Rift Valley 2 The Red Sea = a divergent plate boundary Fig. 3-16d, p. 61 Convergent Plate Boundaries 2 • aka subduction zones, Wadati-Benioff zones • regions where plates are coming together • crust is destroyed Convergent Plate Boundaries 2 • Oceanic-continent – denser oceanic crust is subducted Figure 2.20a Figure 2.21 2 Convergent Plate Boundaries 2 • Oceanic-oceanic – older crust is more dense & is subducted – deep oceanic trenches & volcanic islands Convergent Plate Boundaries 2 • Continent-continent – neither plate is completely subducted – they compress & uplift forming mountains Figure 2.22 2 Transform Plate Boundaries 2 • Plates slide laterally past each other • Crust is neither created nor destroyed Figure 2.23 2 Table 2.1 2 2 Confirmation of Plate Tectonics • Paleomagnetism – Fossil magnetic field 2 Fig. 3-27, p. 69 2 Age of the ocean floor Fig. 3-28, p. 70 2 Confirmation of Plate Tectonics • Hot Spots – Surface expressions of plumes of magma rising from stationary heat sources in the mantle 2 Fig. 3-30, p. 71 Figure 2.25 2 Figure 2.24 2 2 Fig. 3-12, p. 58 Figure 2.33 2 Figure 2.31a 2 Figure 2.31b 2 Figure 2.31c 2 Figure 2.31d 2 Figure 2.31e 2 Figure 2.31f 2 Figure 2.31g 2 Figure 2.31h 2 Figure 2.31i 2 Figure 2.31j 2 Figure 2.32 What Earth may look like in 50 million years… 2