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Name:_______________________________________ Objective 1 Reading Guide – pages 141-147 Mid-Ocean Ridges 1. The East Pacific rise is just one of many ____________________ ridges that wind beneath Earth’s _________________. 2. In the mid-1900s, scientists mapped the mid-ocean ridges by using __________________. Sonar is a device that _____________ sound waves off underwater objects and then records the _______________ of these sound waves. 3. Mid-ocean ridges curve like the seam of a baseball along the _____________________. 4. Most of the ___________________ in the mid-ocean ridge system lie hidden under hundreds of meter of water. But in few places the ridge pokes above the ____________________. 5. The island of __________________ is part of the mid-ocean ridge that rises above the surface in the North _______________Ocean. 6. A ______________-sided _________________ splits the top of some mid-ocean ridges. What is Sea-Floor Spreading? 1. Harry Hess, an American __________________, was one of the scientists who studied mid-ocean ridges. 2. In 1960, _____________ proposed a radical idea. He suggested that a process he called sea-floor ________________ continually adds new material to the _____________ _____________. 3. In sea-floor spreading, the sea floor _______________ apart along both sides of the mid-ocean _______________ as new crust is _____________. As a result, the _____________ floors move like conveyor belts, carrying the ___________________ with them. 4. Sea-floor spreading __________________ at the mid-ocean ridge, which forms along a crack in the _____________ crust. 5. Along the ridge, ___________________ material that forms several kilometers beneath the surface rises and _____________. At the same time, ________________ rocks move outward on both side of the ridge. As the molten material ___________, it forms a strip of solid ______________ in the ____________ of the ridge. Evidence for Sea-Floor Spreading 1. Several types of evidence supported Hess’s _____________ of sea-floor spreading: __________________ of molten material, magnetic stripes in the rock of the ocean floor, and the ________________ of the rocks themselves. Evidence from Molten Material 1. In the 1960s, scientists found evidence that ____________ material is indeed erupting along mid-ocean ridges. 2. The scientists dived to the ocean floor in ______________, a small submarine built to withstand the crushing _________________ four kilometers down in the ocean. 3. In the ridges central valley, Alvin’s crew found strange rocks shaped like _________________ or like toothpaste squeezed from a tube. Such rocks form only when ________________ material hardens quickly under ___________________. Evidence from Magnetic Stripes 1. You read earlier that Earth behaves like a giant ______________, with a north pole and south pole. 2. Surprisingly, Earth’s magnetic poles have ________________ themselves many times during Earth’s history. The last reversal happened 780,000 years ago. 3. Scientists discovered that the rock that makes up the ocean floor lies in a _______________ of magnetized “stripes.” These _______________ hold a ________________ of reversals in Earth’s magnetic field. 4. As the rock _________________, the iron bits inside lined up in the direction of the Earth’s magnetic poles. 5. Using sensitive instruments, scientists record the ____________________ ___________________ of rocks on both sides of the mid-ocean ridge. 6. They found that stripes of rock that formed when Earth’s magnetic field pointed north ________________ with stripes of rock that formed when the magnetic field pointed south. Evidence from Drilling Samples 1. The final proof of sea-floor spreading came from rock samples obtained by ________________ into the ocean floor. 2. __________________ from the sea floor were brought up through the pipes. Then scientists determined the _________________ of the rock samples. 3. They found that the ___________________ away from the ridge the samples were taken, the ___________________ the rocks were. The _____________________ rocks were always in the center of the ridges. Subduction at Trenches 1. How can the ocean floor keep getting __________________ and _______________? The answer is that the ocean floor generally does not just keep _____________________. Instead, the ocean floor _____________________ into deep underwater canyons called deep-ocean _____________________. 2. At a deep-ocean trench, the ocean crust bends ___________________. 3. In a process taking tens of millions of years, part of the ocean floor _________________ back into the mantle at deep-ocean trenches. The Process of Subduction 1. The process by which the ocean floor sinks ___________________ a deep-ocean trench and back into the mantle is called _________________. 2. Sea-floor ________________ and subduction work together. They ________________ the ocean floor as if it were on a giant conveyor belt. 3. _____________ oceanic crust is hot. But as it moves away from the mid-ocean ridge, it _______________ and becomes more _______________. Eventually, gravity pulls this older, denser crust down _________________ the trench. Subduction and Earth’s Oceans 1. The processes of subduction and sea-floor spreading can change the _____________ and __________________ of the oceans. 2. Because of these processes, the ocean floor is ________________ about every 200 __________________ years. 3. The vast _______________ Ocean covers almost one third of the planet. And yet it is _________________. 4. Sometimes a deep ocean _________________ swallows more oceanic crust than a mid-ocean ridge can __________________.