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Transcript
Chapter 2 Plate Tectonics and the Ocean Floor How Old is The Earth The age of Earth has been subject to debate. Scientists now use an age of 4.6 billion years. Catastrophism is the thought that Earth is very young, and events described in the Bible are responsible for the appearance of Earth’s features. The principle of uniformitarianism was introduced in 1788. This principle states that the forces which shaped the Earth are identical to the forces which are working today. Where did oceans come from? Theory: H2O came from big comets during period of heavy bombardment H2O locked up in minerals released from differentiation and heating Origin of the Oceans Beginning - 4.5 Billion years ago Differentiation - heaviest materials sank to the middle (basalt-oceanic crust) - lightest materials rose to the surface (granite-continental) - created high and low spots made up of granite and basalt - Earth began to cool and the materials began to harden Origin of the Oceans Degassing 3.8 Billion years ago Earth filled with volcanoes (erupt & release large gas clouds) Filled with water vapor Atmosphere could not hold it all Rain, evaporated, rain, evaporate (Earth is still VERY hot) Origin of the Oceans Deluge Slow rain (Earth is cooling) low spots on the earth began to fill with water Salinity Weathering of rocks on land Silicate, calcium, sodium Volcanic gases chlorine Alfred Wegener 1880 – 1930 In 1910, Wegener noticed the matching coastlines of the Atlantic continents -- they looked on maps like they had once been fit together. He first spoke on the topic in January of 1912, where he put forth the idea of "continental displacement" or what later was called continental drift. Evidence for continental drift Matching coastlines on different continents Figure 2-2 Evidence for continental drift Matching mountain ranges across oceans Today 300 million years ago Figure 2-4 Evidence for continental drift Glacial ages and climate evidence Evidence for continental drift Distribution of fossils such as Mesosaurus Figure 2-6 Objections to the continental drift model Wegener envisioned continents plowing through ocean basins Wegener did not provide a plausible mechanism to explain how the continents could have drifted apart Most Earth scientists rejected continental drift because it was Too far-fetched Contrary to the laws of physics The theory of plate tectonics Continental drift was reexamined in the 1960s when new information became available Sea floor features became better known A technique was developed that enabled scientists to determine the original positions of rocks on Earth (paleomagnetism) Harry Hess 1906-1969 Harry Hess proved Wegener's basic idea correct and clarified the mechanism that broke the once-joined continents into the seven continents which we are now familiar with. The continents are attached to the plates and do not move independently of them. But the plates themselves shift and change shape, carrying the continents along. Evidence for plate tectonics Earth’s magnetic field affects all magnetic objects on Earth Figure 2-7 Evidence for plate tectonics When rocks cool at Earth’s surface, they record Earth’s magnetic field (normal or reversed polarity) Figure 2-9 Evidence for plate tectonics Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge Pattern was created by sea floor spreading Figure 2-11 Evidence for plate tectonics Harry Hess envisioned new sea floor being created at the midocean ridge and destroyed in deep ocean trenches Figure 2-10 Evidence for plate tectonics Age of the sea floor matches pattern predicted by sea floor spreading Youngest sea floor is at mid-ocean ridge Sea floor is older with increasing distance from midocean ridge Figure 2-12 Evidence for plate tectonics Pattern of worldwide earthquakes (left) matches plate boundaries (right) Figure 2-13 Earth structure Chemical composition Crust Mantle Core Physical properties Lithosphere Asthenosphere Mesosphere Outer core Inner core Figure 2-14 Principles of plate tectonics Lithosphere - The outermost portion of Earth is composed of a mosaic of thin rigid plates that move horizontally with respect to one another Asthenosphere – middle of the mantle lithosphere ‘floats on top’ zone where magma is formed easily deformed, can be pushed down by overlying lithosphere – “plastic” – tar or asphalt Principles of plate tectonics Plates interact with each other along their edges (called plate boundaries) Plate boundaries have a high degree of tectonic activity (mountain building, earthquakes, active volcanoes) The 3 types of plate boundaries 1. Divergent 2. Convergent 3. Transform /Lateral Slipping Divergent plate boundaries 2 plates moving away from each other Formation of new crust on ocean basin by rifting and sea floor spreading http://www.youtube.com/watch?v=GyMLlLxbfa4&feature=r elated Divergent plate boundaries The MidAtlantic Ridge is a divergent plate boundary where sea floor spreading occurs Divergent plate boundaries Iceland sits atop a divergent plate boundary where continental rifting occurs Figure 2-19 Divergent plate boundaries Crustal plates get older as you move from the spreading center New crust is created at spreading centers at a rate of approximately 1-10cm per year Old crust is destroyed at the same rate at subduction zones Spreading Centers Magnetic anomalies are found in ocean crust and we can find them by at spreading centers paleomagnetism every so often Earth’s magnetic field flips (every 300K-500K years Magnetic signal recorded in crust at spreading center as it’s formed, forms bands of crust with either a weak or strong magnetic signal The record of the magnetic polarity is forever “recorded” in the iron deposits at the time they are formed. The iron deposits become fixed upon the magnetic pull. Spreading Centers – Hydrothermal Vents Concentrated areas of hot rising water As new crust is created cold dense sea water circulates through magma chambers close to the surface; the water becomes heated and is released through cracks (rifts) in the newly created sea floor and create vents Vents are chimney shaped (65ft-200ft) Types of Hydrothermal Vents White smoker – Milky discharge Temp. – 2000C-3000C (3920F-6260F) Black Smoker – Black Discharge Temp: 4000C (7000F) contains zinc, lead, cobalt, and silver Hydrothermal Vent Community Rich animal life supported by the vent Water is rich in sulfide compounds which are utilized by the chemosynthetic bacteria that are the producer organisms for an ecosystem that has no dependence on sunlight This bacteria has a mutualistic relationship with giant tube worms that are found living along the vents. Tube worms can grow from 6inches to 10ft in length (Depending on species) Other Vent Life Clams Mussels Crabs Fish Hydrothermal Vent - Ecosystem Ecosystem = Energy Transfer = Trophic Levels = Food Web Arrows = Convergent plate boundaries a. Ocean-continent Convergent plate boundaries vary depending on the type of crust c. Continent-continent b. Ocean-ocean Convergent plate boundaries An Ocean-Continent convergent plate boundary produces the Cascadia subduction zone and Cascade Mountains & Mt. St. Helens RING OF FIRE Subduction zones, like the so-called Ring of Fire that surrounds the basin of the Pacific Ocean, are among the most violent on Earth. RING OF FIRE The scraping of one plate on another generates powerful earthquakes; the heating of the plate within the depths of the mantle releases fluids which melt the rock over it, producing molten rock (magma) that surface as volcanoes. Convergent plate boundaries An Ocean-Ocean convergent plate boundary older denser plate pushed under causing magma to rise and form island arcs and trenches like Marianna Island Arc system and most famous trench Challenger Deep Trenches – Undersea Valley Trenches are formed when two plates push toward each other and there is a valley in between the plates. Life in a Trench Water temperature – 0C-2C Poor Oxygen Extreme pressure - At more than 8 tons per square inch, it’s the equivalent of one person trying to hold 50 jumbo jets! No sunlight Marine Life – sea cucumbers, mollusks, aneomes, crustaceans, and jellies Convergent plate boundaries A continentcontinent convergent plate boundary produces the Himalaya Mountains Transform plate boundaries Two plates sliding past each other Transform plate boundaries occur between segments of the mid-ocean ridge Can also occur on land (ex: San Andreas Fault) Hotspots and plate tectonics Hotspots are stationary and have abundant volcanic activity The lithospheric plate moves over the hotspot Creates a row of volcanoes progressively older toward one end Creates a row of volcanoes progressively older toward one end… Like Hawaii Loihi (New Hawaiian Island) The world as it may look 50 million years in the future Figure 2-35