Transcript Slide 1
The Mid-Atlantic Ridge By Amy Garrison G355 May, 2009 The plate • • • Plate Tectonics - quick review: The theory that the Earth’s surface is broken into a number of thick plates that are slowly and continuously moving This contributes to Continental Drift The plate tectonics theory was first proposed by Alfred Wegener, a German meteorologist, in 1912 ~ It is the concept that the Earth’s crust is composed of rigid plates that move over a less rigid interior. There are 3 different kinds of boundaries where 2 plates come together: Divergent – which is what the mid-atlantic ridge is, Convergent and Transform. A divergent boundary is where two different plates are moving in opposite directions - away from each other. New crust is created as the old crust moves away, similar to a conveyor belt. • In the case of the mid-Atlantic ridge, a mountain range was formed from this process. • It is located along the floor of the Atlantic Ocean and is the longest mountain range in the world. • It separates the Eurasian Plate from the North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic. • It extends from a junction with the Mid-Arctic Ridge, northeast of Greenland southward, to the Bouvet Triple Junction in the South Atlantic. • These plates are still moving apart, so the Atlantic is growing at the ridge at a rate of about 2.5cm per year, in an east-west direction. • While the Mid-Atlantic Ridge is mostly an underwater feature, there are portions of it that have enough elevation to extend above sea level. • Some of the islands that have been created and are part of this ridge are Iceland, the Azores, Bermuda, and Saint Peter and Paul Rocks. A fissure running along the Mid-Atlantic Ridge in Iceland • An important discovery that has to do with seafloor spreading: • Ocean floor magnetism and magnetic reversals • Did you know that about every half million, years the Earth’s magnetic polarity reverses ?? • So does the magnetization of the ocean floor. • • • • • Magnetic Reversals Molten lava emerges from volcano Solidifies to a rock ~ usually (black) basalt Basalt is faintly magnetic, like iron. Its magnetization is in the direction of the local magnetic force at the time when it cools down. • Instruments can measure the magnetization of basalt. • Suggested that times existed when the magnetization had the opposite direction from today's. • In the end it was determined that in the distant past, the magnetic polarity of the Earth was sometimes reversed. • Magnetization in the ocean was orderly, arranged in long strips. • Strips on the Atlantic ocean floor, in particular, all seemed parallel to the "mid-Atlantic ridge." • Not only were the magnetic strips lined-up with the central ridge, • Their structure and distribution seemed remarkably symmetric on both sides • As "plates" on each side are pulled away, • Lava emerges from the middle, solidifies and "records" the prevailing magnetic field. • The newly formed basalt sticks to the plates and is also pulled away • Every half million years, on the average, the Earth's magnetic polarity reverses, and so does the magnetization of the ocean floor. • Each strip represents an epoch of one or the other magnetic polarity, and the symmetry is also explained. • The sea-floor is a giant tape recorder, with twin tapes emerging from the mid-Atlantic ridge • Similar magnetic strips were also observed in all other oceans. • Lab • • Information credits: http://www.google.com/search?hl=en&defl=en&q=define:plate+tectonics&ei=cb4QSsqgIKL6tQPV17znCw&sa=X&oi=glossary_definitio n&ct=title http://en.wikipedia.org/wiki/Mid-Atlantic_Ridge http://www.google.com/imgres?imgurl=http://www.iki.rssi.ru/mirrors/stern/earthmag/Figures/wegener.gif&imgrefurl=http://www.iki .rssi.ru/mirrors/stern/earthmag/reversal.htm&h=304&w=216&sz=29&tbnid=eWzDjXMBhF5MvM::&tbnh=116&tbnw=82&prev=/image s%3Fq%3Dimage%2B%2Bof%2B%2BAlfred%2BWegener&hl=en&usg=__5izJRZnvCQrrYYUvrcw4lpYf0g=&ei=t8AQSsTPBYOKtAO3ibj3Ag&sa=X&oi=image_result&resnum=4&ct=image • • • • • • • • • • • • • Photo credits: http://whc.unesco.org/en/activities/504/ http://www.iki.rssi.ru/mirrors/stern/earthmag/Figures/wegener.gif http://en.wikipedia.org/wiki/Mid-Atlantic_Ridge http://www.epm.geophys.ethz.ch/~cfinlay/Images/core_cut.jpg http://www.nature.nps.gov/geology/usgsnps/pltec/vigil595x266lab.jpg http://www.google.com/search?hl=en&defl=en&q=define:plate+tectonics&ei=cb4QSsqgIKL6tQPV17znCw&sa=X&oi=glossary_definitio n&ct=title http://www.mysciencebox.org/seafloor http://www.google.com/imgres?imgurl=http://www.a3bs.com/imagelibrary/U19550/electricity-and-magnetism/U19550_pair-of-barmagnets-80-mm.jpg&imgrefurl=http://www.a3bs.com/shop/u.s.a./electricity-andmagnetism/magnetism,pg_83_671_623_0.html&h=438&w=400&sz=26&tbnid=5_erY2DpNAoZdM::&tbnh=127&tbnw=116&prev=/ima ges%3Fq%3Dimages%2Bof%2Bmagnets&hl=en&usg=__KVEICqH0EQSLrBnBhyUfANR44v8=&ei=IuQRSueBJI78swOn2MjyDQ&sa=X&oi=i mage_result&resnum=4&ct=image http://www.google.com/imgres?imgurl=http://www.platetectonics.com/oceanfloors/images/AfricaSouth_America_4.jpg&imgrefurl=http://www.platetectonics.com/oceanfloors/africa.asp&h=260&w=245&sz=17&tbnid=2nSSaPr8WOzr DM::&tbnh=112&tbnw=106&prev=/images%3Fq%3Dimages% http://www.windows.ucar.edu/teacher_resources/magnetism/mid_atlantic_ridge_10_inch.jpg