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11/9/2016 Mid-Ocean Ridges and Trenches mid-ocean ridge is a long chain of underwater mountains contain active and extinct volcanos. Scientists thought that oceanic crust was unchanging and was much older than the continental crust. Technological Advance and Seafloor Mapping A In the early 1900’s, the ocean floor was believed to be flat. Advances in technology allowed scientists to see what the ocean floor truly looked like: They exist all over the world. Mid-Ocean Ridges and Trenches Sonar uses sound waves to measure distance by measuring the time it takes for sound waves sent from a ship to bounce off the seafloor and return. magnetometer is a device that can detect small changes in magnetic fields. It is towed behind a ship and records the magnetic fields produced by the ocean floor. A Mid-Ocean Ridges and Trenches The development of sonar allowed scientists to map the ocean floor. Of all of the features discovered, mid-ocean ridges and trenches puzzled them the most. They wondered what could have caused them. Deep-sea trenches are elongated, arc-shaped depressions in the seafloor that are several kilometers deeper that the abyssal plain. They are the deepest part of the oceans. Many lie near volcanic islands and volcanic island chains Mid-Ocean Ridges and Trenches Abyssal Plain: the flattest part of the ocean covered with hundreds of meters of fine, muddy sediment on top of basaltic volcanic rocks (which are made from dark, dense, iron-rich lava) 1 11/9/2016 Mid-Ocean Ridges and Trenches Scientists later discovered that volcanism and earthquakes were common along midocean ridges and trenches. Oceanic Rocks and Deep-Sea Sediment Analysis of deep-sea sediment and the oceanic crust beneath it revealed: 1. 2. The rocks closer to mid-ocean ridges are younger than rocks located farther away. Sediment of oceanic crust was thinner near the mid-ocean ridges. Paleomagnetism Earth has a magnetic field generated by the flowing liquid iron and nickel in the outer core. This magnetic field causes iron to align itself like the needle of a compass aligns to point north. Periodically, the north and south poles switch because the flow of the iron and nickel switches. This is called a magnetic reversal. During a magnetic reversal, the north magnetic pole is over the south pole and the south magnetic pole is over the north pole. A compass will point south. Paleomagnetism is the study of the Earth’s magnetic field. Scientific studies of continental crust revealed that when lava that contains iron cools, the iron orients itself with the poles (like a compass). When there is a magnetic reversal, the alignment of the iron in the rock of continental crust reverses too. 2 11/9/2016 Oceanic crust is formed from basaltic lava (like the lava from mid-ocean ridges). It contains a lot of iron. Even Scientists Note hypothesized that magnetic reversal would affect the alignment of the iron in the basaltic rock of the oceanic crust the same way it affects the iron in the continental crust rocks. The invention of the magnetometer allowed scientists to analyze the magnetic fields of the oceanic crust. Surprisingly, the magnetometer revealed that regions of normal magnetic alignment and regions of reversed magnetic alignment formed a series of stripes along the ocean floor that run parallel with the mid-ocean ridge. more surprising was that the ages and widths of the stripes matched from one side to the other. that the younger rocks are closer to the midocean ridge. The oceanic crust that is near the mid-ocean ridges is newer while the crust near the trenches is older. Seafloor Spreading Seafloor-spreading is the idea that explains how new oceanic crust is formed at mid-ocean ridges and destroyed at deep sea trenches. Magma that is hotter and less dense rises at the mid ocean ridges. As the two sides of the ridge are forced apart, magma fills in the space, cools, and solidifies, creating new crust. 3 11/9/2016 Remember; Wegener could not explain how continents moves. Seafloor spreading was the missing link that he needed to complete his idea of continental drift. We know that the combination of these ideas is not the currently accepted model today, but it was another step along the path to discovery. 4