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Relative Age and the Laws of Geologic History

M. d’Alessio, 2004Murder Mystery

LLaw

What we see in the present is a key to what happened in the past.

Law of Uniformitarianism

Wind and Process

Death Valley, CACopyright Marli MillerAGI Image Bank photo ha45z2 Zion, Utah

Copyright Marli MillerAGI Image Bank photo ha45jb

http://www.msnucleus.org/membership/html/jh/earth/sedimentary/lesson5/sedimentary5d.html

PROCESSPROCESS

ANCIENTANCIENTPRESENTPRESENT

“The present is the key to the past.”

James HuttonFamous Geologist

(lived 1726-1797)http://www.physicalgeography.net/fundamentals/10c.html

In a horizontal layer of rock, the youngest layer is on top and the oldest layer is on the

bottom.

Law of Superposition

Law of Superposition

Law of Original Horizontality

Law of Original Horizontality

•Rock layers have been laid down horizontally.

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Originally Horizontal

OLDEROLDER

YOUNGERYOUNGER

Timing

Layers on top of layers

Watching Layers Form

Larry Ridenhour, BLM/Jennifer Loomis, TERChttp://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page03.cfm

es2903_p03_deposition.swf

Tilted Layers

USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, Californiahttp://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm .

Watching Layers Tilt

An unconformity is a break in the fossil record. Layers of

rock are missing due to erosion.

Unconformity

Unconformity

Unconformity

Olympic Coast, Washington: 4th Beach near KalalochCopyright Patti Bleifusshttp://patti.tensegrity.net/album/hoh/images/unconformity.jpg

Fold

Fold

Marin Headlands, California -- Image from Roland Bürgmannhttp://www.seismo.berkeley.edu/~burgmann/EPS116/labs/marin2004/Fold%20-%20John%20R%202.JPG

USGS/Tom Grace, TERCFolded layers in the Sangre de Cristo Range of Colorado.http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page05.cfm?chapter_no=investigation

Watching Layers Fold

Rock layers that cut through other layers of rock are

younger than the rock layers they cut through.

Law of Cross Cutting Relationships

Cross CuttingGrand Canyon, Arizona -- Copyright Ramón Arrowsmith http://activetectonics.la.asu.edu/ramon/Images/Grand_Canyon/36grandcanyon.tif.jpg

Cross Cutting

MOST RECENT

FaultsA fault is a break in a rock layer.A fault is always younger than the rock layer it breaks through.

Fault

IntrusionAn intrusion is a layer of igneous rock that has melted through a rock layer.

Intrusion

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An intrusion is always younger than the rock that it cuts through. Intrusion H is younger than rock layers B, C, D and E. So is intrusion D.

Layer A

Layer B

Layer C

Layer D

Layer E

• extrusion- a hardened layer of lava that has cut through a layer of rock.

• An extrusion is always younger than the rock it cuts through.

Geologic Time

OLDEROLDER

YOUNGERYOUNGER

http://www.astro.lsa.umich.edu/users/cowley/grand33.jpg

270 Million Years Old

600+ Million Years Old

TIMING: RELATIVE vs. ABSOLUTETIMING: RELATIVE vs. ABSOLUTE

Relative Age of Rock

Paleontologists date rock using relative age and absolute age.

Relative age is the age of rock compared to the rock around it.

Absolute age is the exact age of rock determined by a technique called radioactive dating.

Example of Relative Age:•I am younger than my brother.•Rock layer B is older than rock layer C.

Example of Absolute Age:•I am 11 years old.•Rock layer B is 300 million years old.

Index Fossils

Index Fossil • An index fossil helps to determine

the relative age of rock.To be an index fossil, an organism:• 1) must have lived for a short

period of time but is now extinct and

• 2) must have lived in many different locations.

• extinct- means an organism once lived but will never live again.

http://www.phschool.com/webcodes10/index.cfm?wcprefix=cfp&wcsuffix=2042&fuseaction=home.gotoWebCod

e&x=0&y=0http://www.phschhttp://index fossil interactive

AHow Old is That Rock? Absolute Age

The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock.A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

M. d’Alessio, 2004Murder Mystery

1.Which is older, layer 1 or layer 4? How do you know?2.Which is older, layer 3 or layer1? How do you know?3.What is the irregular surface of 4 called?4.What happened at point 2?5.How do geologists know what happened in the past?