Transcript Chapter 33
Chapter 33 Our Restless Planet Dynamic Earth Processes Standards 3a,b,f Shrinking Earth Theory • Believed that the cooling of the planet resulted in its contraction. • Sea Floor spreading video http://www.iteachbio.com/Earth_Science/Earth _Science/earth.htm The Theory In 1910 Alfred Wegener begins to wonder…. What’s the relationship? Perhaps all these pieces used to be connected. Continental drift=slow movement over Earth’s surface It all started 300 million years ago……….. Alfred Wegener • Scientist saw the Earth as a dynamic plant with continents in slow, but constant motion. – Believed once the continents had been joined together into one great supercontinent: Pangaea – Proposed that the geological boundary of each continent lay not at its shore, but at the edge of its: continental shelf. – See page 586 diagram Supercontinent Pangea –(Greek) all lands Tens of Millions of years! The Big Picture Continental shelf • Is the gently sloping platform between the shoreline and the steeper slope that leads to the deep ocean floor. – Similar/identical rock – Fossils of identical land dwelling animals found. – Paleoclimatic: brings all glacial regions together and northern continents closer to the tropics. A Scientific Revolution • Earth is a huge magnet • Magnetic north and south pole are near the geographic poles. • Paleomagnetism: magnetic from the geologic past. When the magnetic North and South were reversed. • Some of the deepest parts of the ocean are actually near some continents, and out in the middle of the oceans the water us relatively shallow because of the underwater mountains. H.H. Hess • American geologist • Presented the hypothesis of seafloor spreading. • Seafloor not permanent • Constantly being renewed • See diagram 33.6 page 588 in textbook • As new basalt is extruded at a mid-ocean ridge, it is magnetized according to the existing magnetic field. • Magnetic patterns of the spreading floor tell us both AGE of the seafloor and RATE at which it spreads. • Diagram figure 33.3 • Oceanic crust found to be thin and young near central ridge and progressively thicker and older away from the ridge. • See diagram 33.7 Normal vs Reversed Polarity Section 33.2 Theory of Plate Tectonics • Wegner’s Idea • Describes the forces within the Earth that create the continents, ocean basins, mountain ranges, earthquake belts, and large- scale features of the Earth’s surface. • The Earth’s outer shell, the lithosphere, is divided into eight relatively large plates and a number of small ones. Section 33.3 Three types of Plate Boundaries • • • • Moving plates interact at their boundaries Divergent Plate Boundaries 2 plates move apart = tension stretches lithosphere. 2 spreading center result = hot, molten rock from asthenosphere upwells and creates new lithosphere. • Examples: Mid-Atlantic Ridge • Continents on either side of the ridge grow apart. • Spreading centers can also develop on land. – rising magma up lifts continental crust (in lithosphere.) – crust is pulled apart causing rift valley – can be the beginning of a new ocean basin Convergent Plate Boundaries • • • • • Occur where plates come together Motion from convection cells pushes plates together Compression Results in mountains building 3 types of plate collisions: defines by type of crust involved. • oceanic-oceanic converge: two oceanic plates collide, one plate descends beneath the other-subduction, forms deep ocean trench then volcanoes island. • Ocieanic-Continental Convergence: ocean and continental plates come together, the denser oceanic (basalt) subducts under the less dense continental plate, forms deep ocean trenches, earthquakes are a characteristic of the area, mountains rise from the convergence. • Continental-Continental: collision of two land masses, always proceeded by oceanic-continental convergence, compression causes crust to fold/break thinner crust, no volcanic activity, many earthquakes. • Transform Fault Boundaries • Horizontally slipping past each other, pressure builds up causing earthquakes. A Divergent •plates are moving apart •new crust is created •Magma is coming to the surface B Convergent •plates are coming together •crust is returning to the mantle C Transform •plates are slipping past each other •crust is not created or destroyed A Divergent Continental crust rift valley B Convergent 2 continental plates mountain range C Transform Plates move against each other Stress builds up Oceanic crust midocean ridge 2 oceanic plates or oceanic + continental subduction Stress is released earthquake