Tsunami, earthquakes and volcanic eruptions Tsunami, earthquakes and volcanic eruptions Tsunami: Wavelenths > 200 km Very fast in open ocean Destruction of Moawhitu Brian Flintoff, New Zealand Plate tectonics Plate tectonics: terminology Continental drift continents move Plate tectonics thin, rigid blocks of lithosphere move horizontally Interactions of plates build major features of Earth s crust Continental Drift Alfred Wegener, 1912 Suggests Pangaea, surrounded by Panthalassa, 200 million years ago 1. Puzzle-like fit of continents 1
1. Puzzle-like fit of continents 1. Puzzle-like fit of continents 2. Matching rocks and mountain chains Structures Rock types Rock ages 3. Glacial ages Ancient glaciations in modern tropical regions Direction of glacial flow 4. Distribution of organisms Same land animals, different continents 5. Earth s magnetic field Sources 1. conducting, liquid iron, outer core (~90% of magnetic field strength) 5. Earth s magnetic field Sources 1. conducting, liquid iron, outer core (~90% of magnetic field strength) 2
5. Earth s magnetic field Sources 1. conducting, liquid iron, outer core (~90% of magnetic field strength) Earth s magnetic field Declination (D) Earth s magnetic field Inclination (I) Earth s magnetic field Though experiments: 1. If we take out a compass and align the needle to the north, we are pointing towards (geographic north, magnetic north). 2. At the north magnetic pole, the north end of the dip (or inclination) is (up,down); at the south magnetic pole, the dip is (up, down). 3. At the magnetic poles, the dip angle is (0,90), the horizontal intensity is (0,90). 4. At the magnetic equator the dip or inclination is (0,90). 5. The declination in Seattle is about 17 degrees (remember, declination is positive east). If we take out a compass and align the needle on N (north), what change do we need to make to point towards geographic north? 5. Earth s magnetic field 5. Earth s magnetic field In some minerals, the spin of unpaired electrons will align with the Earth s magnetic field as it cools below a certain temperature, recording declination for millions of years Magnetite Fe 3 O 4 Hematite Fe 2 O 3 3
5. Earth s magnetic field: apparent polar wander 5. Earth s magnetic field: apparent polar wander... puzzle-like fit of continents, matching rock sequences, glacial evidence, distribution of fossils, apparent polar wander... we are convinced that continents move... Earth formed 4.6 Byr but oldest seafloor fossils are 180 Myr, oldest marine fossils found on land are much older Oceans formed 4 Byr but relatively little sediment accumulation on the seafloor The continents are moving, but how? Seafloor spreading Harry Hess, Navy Captain collects echo-sounding surveys in between battles during WWII Flat topped mountains, 2 km deep Trenches, 7 miles deep 1953, physicists Ewing and Heezen discover Great Global Rift in Atlantic Seafloor spreading Crust created at mid-ocean ridges Crust destroyed at oceanic trenches 4
Seafloor spreading: Evidence 1. Magnetic polarity reversals Seafloor spreading: Evidence 1. Magnetic polarity reversals - changes in Earth s magnetic polarity are recorded in the rocks as seafloor is created Seafloor spreading: Evidence 2. Age of ocean floor: youngest at mid-ocean ridge Animation: seafloor magnet Seafloor spreading: Evidence 3. Heat flow: highest at mid-ocean ridge crest Seafloor spreading: Evidence 4. Earthquakes: most occur along plate margins 5
Plate tectonics theory Tectonic plates Plate boundaries Rising heat from within the Earth pushes and pulls tectonic plates As a result, continents move and seafloor spreads from mid-ocean ridges Animation: GlacialIsostasy 6
Plate boundaries 1. Divergent Plate boundaries: convergent Plates move toward each other Crust is subducted Deep earthquakes 2. Convergent 3. Transform Plate boundaries: convergent Plate boundaries: convergent Oceanic-continental plate convergence Oceanic plate subducted Oceanic trench Continental arc 7
Plate boundaries: convergent Oceanic-oceanic plate convergence Oceanic plate subducted Oceanic trench Island Arc Plate boundaries: convergent Continental-continental convergence Uplifted mountain ranges 8
Plate boundaries: divergent Plates move apart, shallow earthquakes Plate boundaries: divergent Fast (10-20 cm/yr) Oceanic rise Gentle slopes Relatively more magma Seamounts 200 m tall Plate boundaries: divergent Medium (5-10 cm/yr) Oceanic ridge Rift valley ~5 km wide Rift valley 50-200 m deep Plate boundaries: divergent Slow (1-3 cm/yr) Oceanic ridge 6 miles wide (10 km), 2 miles deep (3 km) Steep slopes 9
Plate boundaries: transform Segments of plates slide past each other Occur between two segments of mid-ocean ridge Shallow but strong earthquakes Plate boundaries: transform Oceanic transform fault Plate boundaries: transform Continental transform fault 10