Which layer Is the Oldest?

The Relative Age of Rocks Reading Preview Key Concepts • Whoal iI the ,,"W of ~ _ ? • How do geologists determine the 'Ntive q of rocu? • How iNe in...
Author: Anabel Short
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The Relative Age of Rocks Reading Preview Key Concepts • Whoal

iI the ,,"W of ~ _ ?

• How do geologists determine the 'Ntive q of rocu? • How iNe inde. fossils us.eful to ~1

KeyTenns • ,~atiw q ·.IbIoIuIe ~ • law 01 superpooitiot> • ~lnJ5ion • inll'USOO" • /IoUh • ufKonfo..."ity • lnde>: fossil

~ Target Reading Skill AHlne OoHostlons !lefore you .Nd,. Pl'1!'riew the red he~ings. In a graphic organlzl!'l' like the one

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Which layer Is the Oldest? 1. """ke a st.Jd: of difftfel'lt-(olored ~et'S of dn of rock ~._11

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As scdim~ntary rock form$, the remailU of organisms in the sediment IDiIIy become fossils. Millionsof years later, if you split open th~ rock, you might sec th~ petrified bones of an extinct replil~ or ins«l. Your first question aboul a new foWl mighl be. "What is it?" Your next question would probably be, "How old is it?" Geolo­ gists ~~ t'NO ways to o:prcss the age of a rock and any fossil it ront.ains. ~ age of a rock is its age compared to the: ages of otha rods. You ha~ probably wed the idn of rc:btive age: ""hen comparing your age: with someone dK-'s age:. For example, if )'OIl say that )'OIl are older than your brothc:T but

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younger than)Uur sistn". you arc' describing )Uur rc:blive age:. The: relative age of a rock does not provide: its absolute: age. The: .biOJ.UIt . . of a rock is the: number of years since lhe rock formed. It may be impossible: 10 know a rock's absolute: age o:actly. But somdimes geologi5U can dete:rmine: a rock's absolute: age to within a ce:rtain number of years. ...

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The Position of Rock Layers Haw you nn ~ rock layers of different colors on a cliff beside ill mad? What arc these layers, and howdid they form? The sedimrnt that forms sedimentary rocks is deposited in Oalla)'lm one on top of tM otMr. Chn time, the- sediment hardens and changes into .sedimentary rock. 1besc rock lay­

ers provide a ~rd of Earth's geologic history. It (an be difficult 10 determine the absolute age of il rock.. So geologists usc il method 10 find 11 rock's relative

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".. Gland canyon Mor
Kllibab Limestone 250 million Y"ars old

T_ap LlmestOM 255 million )'f'ars old Coc;onino ~rodoto"e 260 million )'f'.... old

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Determining Relative Age 1krr l.~ othn" dues bdidet the po5ition of rod: byns to tht rdaliYe ages of rocks. To dd~ m.tM .~ ~ u abo ~ludT atnniotu: and intrusions of ipwous rock. faults, and glps in tM ~ record. Oun From Igneous Rode Igncow rock lOO:m wbm ~ or Ia~ bankm. M~ iJ molten matnial bc:nc:iIth £anb"s RlrfKc. Magrm thallJowJ onto tht 5Ufbcc is aBed bv.L LaYJ Iml bardms on 1M wrfacc is calkd an ertnuioa. An cnrusion is always youngn" t}un the rock.s below it. Ikneath the IUnatt,magnu rmy push into bod~ of mck.. Tbtre, tht magrn;jl roob and tw-dms into a ma.ss of igneous rod: callotd an intrusion. An intrusion is always )'OUDgtt than tht rod: bYfn around and benath it. Figure 6 .shows an mlrusion. GcoIosisU $ludy wbcTe intrusions and rorusions fol"lned in rmlion to other rock 1ayn'S. This bdps geologists undcnund 1M rdatiVl':tgn of the diffn-ent typn of rock.

Cues From FMdb More dues COfnl: from the study of fau/u.. A &uk is a bmk in EMth·s crust. Fom:s inside Earth CII~ ~ mml of the rock. on "'f1"Wk sides of a faulL A filult is always youngtr than the rock il l:UI$ through. To dnn-mine the rebrive. of a fault, geolop;b find the matiVl' logt of the )'OungeJI layer cut by lhe fault. Mo\~nu along raulls can makt it hankr for geologists to determine the rtbtive ages of rod: loi)'Yn. You can s« in Figure 6 how the rod: byt'1'5 no Iongtr liM up bcause of movnnrnl along the faulL

sampling a Sandwich Your tNd>er wiI gMe you • ",..\1" itt, Nt ,ep 'AMb nxk ~ in bnh\ crust. 1. lM .. round, hokM:. unmobd I'IOClCh M .. wring tool. I'Wl the rMXJdle ttvough the ~ of tM YnCh'id\. 2. Pull the rMXJdle out of 11M

t¥idwird shows that hOI"5Qh« enbli NlV\' aisted wid! very littk
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More to Explore Draw a §ketrn similar to Sit~ 2 and include a fault that cuts across the intrusion. Have a pa,rtn~f then identify the rt'lat,w ~ of tile fault,. tile intrusion, and the layen cut by the fault.