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Transcript
Sea-Floor Spreading During World War II, geologist Harry Hess became interested in mapping the ocean floor using sonar. Unexpected Finding: Long mountain ranges existed throughout the floors of the oceans called mid-ocean ridges. The longest one runs through the middle of the Atlantic Ocean and is called the Mid Atlantic Ridge (MAR). Hess devised a theory called sea-floor spreading which states that at mid-ocean ridges, molten material rises from the mantle and erupts. It then spreads out, pushing older rock to both sides of the ridge. In the 1960’s, other evidence was found to support his theory: • http://earthguide.ucsd.edu/eoc/teachers/t_te ctonics/p_seafloorspreading.html Molten Material • Scientist aboard a submersible called Alvin dove to the sea floor and discovered rocks called pillow lavas along the mid ocean ridges. Such rocks can only form from cooled molten material. • http://www.ocean.udel.edu/extreme2004/mission/submersiblealvin/index.html Magnetic Stripes • Rocks that make up the ocean floor lie in a pattern of magnetized stripes, which show a record of reversals of Earth’s magnetic field. Drilling Samples • In the year 1968, a drilling ship called the Glomar Challenger was used to gather samples of rocks from the ocean floor. Scientists discovered that the further the rocks were from the mid ocean ridges, the older they were. The youngest rocks were always at the center of the ridges. Resulting Question: Are all of the oceans getting bigger? • Suggested Answer: Ocean floors do not keep spreading. Parts of the oceanic crust get plunged into deep-ocean trenches due to the process of subduction. Subduction-process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle. http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_subduction.html Subduction in the Pacific Ocean: There are many trenches, so the Pacific Ocean is getting smaller. Subduction in the Atlantic Ocean: There are fewer trenches, so the Atlantic Ocean is getting wider. • http://www.suu.edu/faculty/colberg/hazards /platetectonics/18_Pangaea.html Diagram of Sea-Floor Spreading: