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Environmental Science
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Nick Eyles

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EESA06 READING #2 CHAPTER 2 – PLATE TECTONICS What are plate tectonics? Tectonics – the study of the origin and arrangement of the broad structural features of the earth’s surface including folds, faults, mountain belts, continents, and earthquake belts Plate Tectonics – the Earth’s surface is divided into a few large, think plates that move slowly and change in size Plate boundaries – plates move away from one another, past one another, or towards one another Eight large plates/smaller plates – makes up the outer shell of the Earth (the crust and upper part of the mantle) Plate tectonic theory developed by: Continental drift – continents move freely over the Earth’s surface, changing their positions relative to one another Sea floor spreading – A hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench. – Two sides of the ridge are moving in opposite directions How did plate tectonics theory evolve? The Early Case for Continental Drift Alfred Wegener • Meteorologist • Continental drift • Examined continents that have identical late Paleozoic rocks and fossils Pangea – continents form a giant supercontinent  Laurasia –northern supercontinent (North America)  Gondwanaland – southern supercontinent Paleoclimatology – study of ancient climate – Examined ancient sedimentary rocks he discovered that paleoclimatic reconstructions suggested polar positions very different to those at present Skepticism about Continental Drift – Believes that the less dense continents drifted through oceanic crust, crumpling up mountain ranges on their leading edges as they pushed against the oceanic crust (DIAGRAM PAGE 25) Renewed Interest in Continental Drift – Study of the Sea Floor o Samples of rocks and sediments can be taken from the sea floor by: 1. Rock Dredge – open steel container dragged over the ocean bottom at the end of a cable 2. Corer – a weighted steel pipe dropped vertically into the mud and sand of the ocean floor 3. Sea-floor drilling – drilling a hole in the deep sea floor o Submersibles – small research submarines for geologists o Single-bean echo sounder – measures water depth and draws profiles of submarine topography 1. A sound is sent downward from the ship and bounces off the sea floor and returns to the ship 2. Water depth = amount of time interval o Sidescan sonar – measures the intensity of sound reflected back to the tow vehicle from the ocean floor and provides detailed images of the sea floor and information about sediments and bedforms – Geophysical Research EXPLAIN (PAGE 28) o Polar wandering  Magnetic poles are located close to the geographic poles  Magnetic poles move from year to year o Magnetism of old rocks can be measured to determine the direction and strength of the magnetic field in the past o Paleomagnetism – study of ancient magnetic fields Recent Evidence for Continental Drift • Refined rock matches between now-separated continents (EX. South America and Africa) • Rocks are similar in type, structure, fossils, age, etc. What is sea-floor spreading? Harry Hess The Sea floor might be moving too like continental drift Sea floor spreading – the sea floor moves away from the mid-oceanic ridge as a result of mantle convectionSubduction – the sliding of the sea floor beneath a continent or island arc Convection – is a circulation pattern driven by the rising of hot material and the sinking of cold material Hot materials have a lower density so it rises Cold materials have a higher density so it sinks Andesitic volcanism – it is produced by the sea floor moving downward into the mantle along a subduction zone. This interaction between the moving sea-floor rock and the stationary rock causes these volcanoes and even earthquakes. • Formed on the edge of a continent or an island arc How old is the sea floor? • Young sea floor is being formed by basalt eruptions at the ridge crest • Basalt is then carried sideways by convection and is subducted into the mantle at an oceanic trench • Old sea floor is being destroyed and new sea floor is being formed What are plates and how do they move? Continental Drift and Sea floor spreading Formed Plate tectonic theory Plate – is a large, mobile slab of rock that is part of Earth’s surface • Made up of sea floor, continental or oceanic rock Lithosphere – rigid outer shell of earth that includes rocks from the crust and the uppermost mantle • Continental lithosphere is thicker (DIAGRAM PAGE 33) Asthenosphere – below the lithosphere, which is a zone of low-seismic- wave velocity that behaves plastically because of increased temperature and pressure • Allows the plates to move • Below this is mantle rock Plates made up of sea floor – subducted down into the mantle forming oceanic trenches Plate leading edge made up of continental rock –NOT subduct because continental rock is less dense than oceanic rock Three types of plate boundaries: 1. Divergent plate boundary – plate boundary moving APART 2. Convergent plate boundary – plates moving TOWARDS each other 3. Transform plate boundary – move HORIZONTALLY past each otherHow do we know that plates move? Paleomagnetic Evidence Magnetic reversals – changes in the polarity of the magnetic field, when the north and south magnetic pole exchange places Normal polarity – magnetic lines flow from the South Pole to the North Pole and our compass needles point to the north Reversed polarity – magnetic lines run from North Pole to South Pole and our compass needles point to the South Paleomagnetism – study of magnetic fields recorded in rocks helps us understand the magnetic fields in the past Magnetic polarity time scale – this is constructed by using stacked continental lava flow to record the pattern of magnetic reversals over time Magnetometer – an instrument that measures the strength of the Earth’s magnetic field. It is taken over land surface or sea. Marine Magnetic Anomalies • Magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge • Positive and negative anomalies form a stripelike pattern parallel to the
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