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9 0 / 9 / 0 330/9/09 11:46:12 Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Plate Tectonics 4 8

4 8 i . 6 3 4 1 When a magnetic field causes a magnet to point north, the magnetic field has When a magnetic field causes a magnet to point north, the normal polarity. seafloor called the abyssal plain. magnetic field reverses often. The direction of Earth’s New oceanic crust forms along a mid-ocean ridge, and older crust moves away New oceanic crust forms along from the ridge. up the mid-ocean ridge can form in different The rugged mountains that make amounts of lava erupting from the center of the ways. One way is through large around the ridge. Another way is through ridge, cooling, and building up on the crust above it, causing it to crack and upward-moving magma pushing on the seafloor. form jagged, angular mountains crust, making a smooth sediment forms on top of the oldest oceanic Eventually, These underwater than mountain ranges on mountain ranges are much longer land. the mantle through cracks in the crust. It Molten rock, or magma, rises from along the mid-ocean ridge. erupts as lava from volcanic vents basalt, the rock that forms the oceanic crust. The molten rock cools and becomes Existence of these mountain ranges was confirmed through echo-sounder mountain ranges was confirmed Existence of these research. 9 8 _ t u Earth’s outer core causes Earth’s magnetic field. outer core causes Earth’s Earth’s a. b. move with the oceanic crust as it spreads away from mid-ocean ridges. move with the oceanic crust as it spreads away from mid-ocean the magnetism of Evidence to support seafloor spreading first came from studying rocks on the seafloor. b. shows that continents Seafloor spreading helps explain continental drift because it The topography of the seafloor includes the abyssal plain and rugged mountains. The topography of the seafloor a. b. c. Seafloor spreading occurs when new oceanic crust forms at a mid-ocean ridge and new oceanic crust forms at a mid-ocean ridge and Seafloor spreading occurs when ridge. old crust moves away from the a. In the middle of the oceans are large mountain ranges called mid-ocean ridges. the oceans are large mountain In the middle of a. b. Scientists mapped the depth of the ocean floor using a device called a(n) echo a device called floor using depth of the ocean mapped the Scientists sounder. O s n A _ F Development of a Theory Seafloor Spreading Mapping the Ocean Floor Mapping R

C _ 2. 1. 3. 2. 1. 2. 1. 7 T4 C. B. Lesson 2: Development of a Theory of a 2: Development Lesson A. Lesson Outline for Teaching Outline for Lesson 8 0 _ 2 8 0 _ 2 3 1 CC132_082_087_CRF_AnsOut_891436.i84 84 CC132_082_087_CRF_AnsOut_891436.i85 85 1 3 2 _ 0 8 2 _ 0 8 7 _ C R

F Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. _ A n s O u t Plate Tectonics Plate Tectonics seafloor spreadingisslow, continentaldriftmustalsobeslow. magnetic reversalissuchaslowprocess,itindicatesthatseafloorspreadingslow. Because magnetic signatureshowsareversal,whichcantakemillionsofyears.Because Magnetometerresultsshowstripesofoppositemagneticsignatures.Eachchangein How domagnetometerresultsshowthatcontinentaldriftisaslowprocess? Discussion Question Lesson continued Outline _ 8 4. 3. 9 1 4 3 6 . i 8 5

mid-ocean ridgeisolderthanthesedimentthatclosertoridge. Sediment collectedfromtheseafloorshowthatsedimentfartherawaya c. b. a. direction ofEarth’s magneticfield. Magnetic signaturesformwheniron-richmineralsincoolinglavaalignwiththe d. c. 8 5 stripe wasformedatthemid-oceanridgeandthenmovedaway. These stripesalternatenormalpolarityandreversedpolarity, showingthateach ridge. Magnetometers showparallelmagneticstripesoneithersideofamid-ocean called a(n)magnetometer. The directionofamagneticfieldinmineralscanbedeterminedbyusingdevice has reversedpolarity. After amagneticreversal,magnetpointssouthbecauseEarth’s magneticfield A magneticfieldreversesdirectionduringa(n)reversal. T5 330/9/09 11:46:13 0 / 9 / 0 9

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