Condensed Lecture Notes 19-27

Condensed Lecture Notes 19-27 - LECTURE 19: THE OCEAN...

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LECTURE 19: THE OCEAN BASINS The ocean ridge system is by far the world’s longest mountain chain, starting near Iceland and extending south through the Atlantic, east through the Indian Ocean and into the Pacific Ocean, with various additional offshoots. On the ocean floor, the rift system has a very distinctive set of landforms. The ridges themselves are broad topographic highs, rising as much as 3-4 km above the abyssal plain, but across a width of 100’s of km perpendicular to the ridge axis. Often the central axis of the ridge system is a deep valley, and off to the sides are additional parallel ridges and valleys. Ridge segments are rather straight, and different sections are offset from each other along transform zones. Transform scars occur at near-right angles to the ridges, both connecting pairs of ridge segments and extending beyond them. The ridge and transform system is seismically active, although the earthquakes are not as strong as in some other areas. Seismic motion solutions (derived from analyzing 3-component seismographs) reveal interesting features: most faults near ridges are extensional, but those on transforms are mostly strike- slip . However, the strike-slip motions are in the opposite direction one would expect if the ridge sections were once directly connected and are now moving away from each other. Iceland sits on top of the mid-Atlantic ridge, and the geological processes we see there, including volcanism, hydrothermal activity, and rifting, occur all along the ridge system, both above and below water. Deep-sea hydrothermal systems often feature black smokers , geysers of hot, mineralized water that can have associated colonies of incredibly specialized organisms that are adapted to live under great pressure and without any energy from the sun. Locations where three ridge systems come together are a form of triple junction . The ridge system extending north from a triple junction in the mid-Indian Ocean leads into the Gulf of Aden, where there is another apparent triple junction, with one arm constituting the Red Sea and the other leading into a region known as the East African Rift, which features extensional faulting, volcanism, and hydrothermal activity similar in many ways to what we see on Iceland. The Red Sea features a central valley similar to well-developed ocean ridges. Ocean trenches are long, deep depressions in the ocean floor. The trench off of the Marianas Islands, at 11035 meters below sea level, is deeper than Mount Everest is high (8848 meters above sea level). Trenches are associated with particularly intense seismic and volcanic activity. Most of the world’s trenches rim the Pacific Ocean, creating the so-called “ Ring of Fire .” The largest earthquakes recorded are near trenches. When “looked at” from the side, parallel to the trench axis, the set of earthquake foci recorded over several years appear to form a linear band that can extend down as deep as ~700 km, the deepest earthquakes known. Active volcanoes associated with trenches include those in the Cascades
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Condensed Lecture Notes 19-27 - LECTURE 19: THE OCEAN...

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