plate tectonics 21.1

8/14/2019 Plate Tectonics 21.1

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Earth'snterior ndPlate ectonicsI r rv rERMs

crustmantlecorelithosphereplate tectonicsmaglnasubductionfault

crust the hin andsolidoutermostayerof Earthabovehe mantlemantle the layerof rockbetweenEarth's rustandcore

Outer ore Crustlnnercore

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ldentify Earth's ifferent eologicayers.Explain ow hepresencef magnetic ands n theoceanloorsupportshe heory f plate ectonics.Describehe movement f Earth'sithospheresinghetheory f plateectonics.tdentifyhe hree ypes f plateboundariesnd heprincipaltructureshat ormat each f these oundaries.

l/or know from experience that Earth's surface is solid. YouI wak on it every day. You may have even dug into it andfound that it is often more solid once you dig and reach rock.Howeveq Earth is not solid all the way to the center.

What s Earth'snteriorLike?Figure t shows Earth's major compositional layers. We live on thetopmost layer of Earth-the crust. Because the crust is iela-tively cool, it is made up of hard, solid rock. The crust beneaththe ocean is called oceanic crust and has an average hickness of5-8 km (3.I-4.9 mi). Continental crust is less dense and thicker,with an averase thickness of about 20*40 km 02-25 mi). The

Mantle continental crust is deepestbeneath high mountains,where it commonly reachesdepths of 70 km or more.Beneath the crust lies the mantle, a layer of rock

that is denser han the crust.Almost 2900km (1800mi)thick, the mantle makes up about 80o/o f Earth's vol-ume. Becausehumans have never drilled all the way tothe mantle, we do not know for sure what it is like.Howeve4 geologic events,such as earthquakes andvol-canoes,provide evidence of the mantle's consistency.

Figure IEarths composed f an nner ore, n outer ore,mantle, nda crust. hought isdifficulto see, heoceanic rusts hinner han he continentalrust.

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For the most part, the mantle is solid. The outermost part isalso rigid, like the crust. Deeper han a few hundred kilometers,howeve4 it is extremely hot, and said to be "plastic"-soft andeasily deformed, Iike a piece of gum.

The center of Earth, the core, is believed to be composedmainly of iron and nickel. It has two layers The inner core, whichis solid metal, is surrounded by the liquid metal outer core.Eartht nteriorgetswarmer rithdepthIf you have ever been in a cave, you may have noticed that thetemperature in the cave was cool. That's because the air androcks beneath Earth's surface are shielded from the warmingeffects of the sun. Howeve4 if you were to travel far beneath thesurface, such as into a deep mine, you would find that the tem-perature becomesuncomfortably hot. South African gold mines,for instance, reach depths of up to 3 km (2 mi), and their tem-peraturesapproach 50'C (120'F). The high temperatures n thesemines are caused not by the sun but by energy that comes fromEarth's interior.

"Geologistsbelieve the mantle is much hotter thanthe crust, as shown in Figure . Thesehigh temperaturescause he rocks in the mantle to behaveplastically. Thisis the reason for the inner mantle's deformable, gumlikeconsistency.

The core is hotter still. On Earth's surface, the metalscontained in the core would boil at the temperaturesshown in Flgure 2. Iron boils at 2750'C (4982'F), andnickel boils at 2732"C(4950"F).But in the outer core,these metals remain liquid because he pressuredue tothe weight of the mantle and crust is so great that thesubstances n the outer core are prevented from chang-ing to their gaseous form. Similarly, pressure in theinner core is so great that the atoms are forced togetheras a solid despite he intenseheat.

- www.scilinks.org ITopic: arth's eologic Ilayers IScilinksode: K4055 |g,fi ilatilainsdbyrie I"-llrurs l#.TilT,liffn,,,'*fl

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I core the center artof Earthbelow he mantle

Temperaturesexceed75oC(347oF) Crust

'ifTemperaturesexceed25ooc(228OoF)

Temperaturesexceed ooooc __ ,j(go30oF) ,Figure2

Radioactivelementsontributeo Earthb igh nternaltemperatureEarth's interior contains radioactive isotopes.These radioactiveisotopes(mainly those of uranium, thorium, and potassium) arequite rare. Their nuclei break up, releasing energy as theybecome more stable. Because Earth is so large, it containsenough atoms of these elements to produce a huge quantity ofenergy. This energy is one of the major factors contributing toEarth's high internal temperature.

Temperaturesn Earth'snteriorincreaseithdepth. emperaturesnear he center f the core anbeashot as hesurface f the sun.

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Alignment f oceanicockssupports he theory ofmovingplatesAs molten rock pours out onto the ocean floo4 as shown inFigure 5, irorr minerals such as magnetite align themselvesparal-lel to Earth's magnetic field, just as compassneedlesdo. After therocks cool to about 550"C(1020"F), he alignment of thesemag-netic regions in the iron minerals becomes ixed like the stripesshown in Figure 6. The result is a permanent record of Earth'smagnetic field as it was just before the rock cooled.

So why are there differently oriented magnetic bands of rock?Earth's magnetic field has reversed direction many times duringits history with the north magnetic pole becoming the south mag-netic pole and the south magnetic pole becoming the north mag-netic pole. This occurs on averageonce every200 000years.Thisprocess s recorded n the rocks asbands. Thesemagnetic bandsare syrnmetrical on either side of the Mid-Atlantic Ridge. Therocks are youngest near the center of the ridge. The farther awayfrom the ridge you go, the older the rocks appear. This suggeststhat the crust was moving away from the plate boundary.Earthhasplates hat moveover he mantleThe lithosphere is approximately 100 km (60 mi) thick and ismade up of the crust and the upper portion of the mantle. Thelithosphere is made up of about seven arge pieces (and severalsmaller pieces)called tectonicplates.The word tectonic refers tothe structure of the crust of a planet. The continents are embed-ded into these plates, which fit together like pieces of a puzzleand move in relation to one another. The theory describing themovement of plates is called plate tectonics.

Tectonic plates move at speeds ranging from 1 to 16 cm(0.4 to 6.3 in) per year.Although this speedmay seemslow tec-tonic plates have moved a considerable distance because heyhave been moving for hundreds of millions or billions of years.

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lithosphere the solid,outerlayerof Earth,hat consists fthe crust nd he rigiduppermantleplate tectonics the theorythatexplains ow he outerparts f Earth hangehroughtime,and hatexplainsherelationshipsetween onti-nental rift, ea-floorpread-ing, eismic ctivity,ndvolcanic ctivity

Figure 5Hydrothermalents redriven yheat rom he eruption f freshlavaon the sea loor.

Figure 6Thestripesllustrate arth's lternatingmagneticield.Light tripesepresentwhenEarth'solarity as he sameway t is today,while he darker tripesshow eversedolarity.

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NormalmagneticpolarityReversedmagneticpolarity

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Convectionndplatetectonicsl. Filla shallow anwithwateruntil t is cmfrom he top.2. Heat he waterover ow heat or JOs. Addafewdrops f foodcoloringo thepan,andwatchwhathappens.3. Turn ff heheatandplace cardboardpieces scloseogether spossiblen thecenter f thepan.4. Turnon the heat,andsketchhe movementof thecardboard.

5. Whatdo thewaterand he cardboardpiecesepresent?Whatdidyouobserven step2?6. Howwas he move-ment n step4 likecontinentalrift?Howcould oumakemoreaccurate odelof plateectonics?

The word tectonic originatesfrom the Greek word tel

plate tectonics 21.1

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