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Section 4 Sea-Floor Spreading What is Sea-Floor Spreading? What is Sea-Floor Spreading? Sea-Floor Spreading is hot-water vents, cracks in the crust. The water is heated by contact with hot material from the mantle and then spurts back into the ocean creating mid-ocean ridge. 1. mid-ocean ridge – the _underwater_ mountain chain where _new_ ocean _floor_ is produced; a _divergent_ plate boundary. 2. sonar – a _device_ that determines the _distance_ of an object _under_ _water_ by recording _echoes_ of _sound_ waves. At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. 3. sea-floor spreading – the _process_ by which _molten_ material adds new _oceanic_ _crust_ to the ocean _floor_ . Alvin, a small submersible , that photographed pillow lava along the mid-ocean floor in the 1960s. The rock of the ocean floor, which contains iron, began as molten material. As the molten material cooled, the iron bits inside lined up in the direction of Earth’s magnetic poles. As the rock hardened completely, it locked the iron bits in place, giving the rocks a permanent “magnetic memory.” The Glomar Challenger, a drilling ship built in 1968, gathered samples from the sea floor in pipes. The scientist determined the age of the rocks and found that the farther away from the ridge the samples were taken, the older the rocks were. 4. deep-ocean trench – a deep _valley_ along the ocean _floor_ through which oceanic _crust_ slowly _sinks_ toward the _mantle_ . 5. subduction – the _process_ by which oceanic crust _sinks_ beneath a _deep_-_ocean_ trench and back into the _mantle_ at a _convergent_ plate boundary. The process of subduction and sea-floor spreading can change the size and shape of the oceans. How does subduction affect the Pacific Ocean? The Pacific is shrinking. This happens when the deep ocean trench swallows more oceanic crust tan the midocean ridge can produce. How does subduction affect the Atlantic Ocean? The Atlantic Ocean is expanding. The Atlantic Ocean has only a few short trenches. As a result, the spreading ocean floor pushes the continents further away from each other. Over time, the whole ocean gets wider. Section 5 6. plate – a section of the _lithosphere_ that slowly moves over the _asthenosphere_, carrying pieces of _continental_ and _oceanic_ crust . The Theory of Plate Tectonics is the geological theory that states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates. 7. scientific theory – a _well_-_tested_ concept (idea) that _explains_ a wide range of _observations_ . Scientific theory will change when there is new evidence to disregard explains made by incomplete observations or inconclusive observations . 8. plate tectonics – a _theory_ that pieces of Earth’s _lithosphere_ are in constant _motion_, driven by _convection_ _currents_ in the Earth’s _mantle_ . 9. fault – a _break_ in Earth’s _crust_ where slabs of _rock_ slip _past_ each other. There are three types of boundaries: 1. transform boundaries 2.divergent boundaries and 3. Convergent boundaries. 10. transform boundary – a _plate_ boundary where _two_ plates move _past_ each other in _opposite_ directions . 11. divergent boundary – a _plate_ boundary where _two_ plates move _away_ from each other . 12. rift valley – a _deep_ _valley_ that forms where two _plates_ move _apart_ . 13. convergent boundary – a _plate_ boundary where two plates move _toward_ each other .