RELATIVE DATING 3(1).pdf

relative dating

1. RELATIVE DATING
OF ROCKS

2. Relative dating
Is used to arrange geological events, and the rocks they leave behind, in a
sequence. The method of reading the order is called stratigraphy(layers of
rock are called strata). Relative dating does not provide actual numerical
dates for the rocks.
Istheprocessofdeterminingofonerockorgeologiceventisolderor
youngerthananother,withoutknowingtheirspecificages—i.e.,howmany
yearsagotheobjectwasformed.Theprinciplesofrelativetimearesimple,
evenobviousnow,butwerenotgenerallyacceptedbyscholarsuntilthe
scientificrevolutionofthe17
th
and18
th
centuries.JamesHuttonrealized
geologicprocessareslowandhisideasonuniformitarianism(i.e.,“the
presentisthekaytothepast”)providedabasisforinterpretingrocksofthe
Earthusingscientificprinciples.

3. Relative Dating Principles
Stratigraphy
The studyoflayered
sedimentaryrocks.Thissection
discussesprinciplesofrelative
timeusedinallgeology,but
areespeciallyusefulon
stratigraphy.

5. 6Relative Dating Principles
OF ROCKS

6. Relative Dating Principles
1.Principle of Superposition
Inanotherwiseundisturbed
sequence ofsedimentary
strata,orrocklayers,thelayers
onthebottomaretheoldest
andlayersabovethemare
younger.

7. Relative Dating Principles
2. Principle of Original Horizontality
Layersofrocksdepositedfrom
above,suchassedimentsand
lavaflows,areoriginallylaid
down horizontally. The
exceptiontothisprincipleisat
themarginsofbasins,wherethe
stratacanslopeslightly.

8. Relative Dating Principles
3. Principle of Lateral Continuity
Withinthedepositionalbasin,
strataarecontinuousinall
directionsuntiltheythinoutat
theedgeofthatbasin.Of
course,allstrataeventuallyend,
eitherbyhittingageographic
barrier,suchasridge,orwhen
the depositional process
extendstoofarfromitssource,
eitherasedimentsourceora
volcano.Stratathatarecutby
a canyon laterremain
continuousoneithersideofthe
canyon.

9. Relative Dating Principles
4. Principle of Cross-Cutting
Relationships
Deformationevenlikefolds,
faultsandigneousintrusions
thatcutacrossrocksare
youngerthantherocksthey
cutacross.

10. Relative Dating Principles
5. Principle of Inclusions
Whenonerockformation
containpiecesorinclusions
ofanotherrock,the
includedrockisalderthan
thehostrock.

11. Relative Dating Principles
6. Principle of Fossil Succession
Evolutionhasproduced a
successionofuniquefossilsthat
correlatetotheunitsofthe
geologictimescale.Assemblages
offossilsthatcontainedinstrata
areuniquetothetimetheylived
andcanbeusedtocorrelate
rocksofthesameageacrossa
widegeographic distribution.
Assemblagesoffossilsreferto
groupsofseveraluniquefossils
occurringtogether.