Title
http
://w
ww
.mon
mou
th.c
om/~
cath
ygrim
/mys
tery
.htm
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.
Gra
nd
Ca
nyo
n,
Ariz
on
a,
Co
pyr
igh
t L
arr
y F
ello
ws
htt
p:/
/ww
w.e
art
hsc
ien
cew
orld
.org
/ima
ge
ba
nk/
sea
rch
/re
sults
.htm
l?Im
ag
eID
=h
mw
nq
6
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
l
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?