Interpreting Earth’s Rocks to
Determine Its Past History
Geologic Time
chapter 18
• Relative Dating - sequence of geologic
events in a region (order of past events)
• Not quantitative
Law of Superposition
• Oldest layer on bottom (if undisturbed)
• Only works for SED rocks
Younger
Older
http://www.angelfire.com/az3/mohgameil/Layering4.jpg
Law of Superposition
In horizontal rock layers, the oldest and first
formed is on the bottom. 5 is the youngest
because it is on the top, closest to the surface.
The Grand Canyon
http://www.calstatela.edu/faculty/acolvil/sediment/superposition.jpg
Principle of Original Horizontality
• Sediments are originally deposited as horizontal (or near
horizontal) beds (ONLY FOR SED ROCKS)
Folding happened AFTER rocks were deposited
folded rock layers = compression by tectonic
activity
Cross-Cutting Relationships
(overlap law)
• An igneous intrusion
(or fault) is younger
than the rock layers
that it cuts across.
youngest
fault
oldest
These are
younger than
the
surrounding
rock
Cross-Cutting Relationships
6 is younger than1, 2, or 3 because it cuts across
them!
3
2
3
4
1
2
1
5
Which came first?
A
B
C
D
E
Order the rock layers above from oldest to youngest.
Which came first?
A
C
B
C
B
D
E
D
The first layer deposited and the oldest is D,
followed by C, A, B, and E.
D
Law of inclusions
• The inclusion
(xenolith) is older than
the rock it is in
• Snickers bar
James Hutton:
The father geology (1726-1797)
Uniformitarianism:
processes acting on
earth today
(volcanism, erosion,
plate tectonics,
glaciation, etc.) have
done so in the past IN
THE SAME WAY!
“The present is the key
to the past.”
Unconformities = “Missing Time”
• Represented by squiggly lines on cross
sections
Angular unconformity
Angular Unconformities
• Folded layers
below horizontal
layers.
rock layers are eroded
new horizontal layers are deposited
angular unconformity
(rock layers have been
eroded away)
Angular Unconformity
sedimentary rocks above and below unconformity
surface with rocks below tilted at an angle
F
E
D
C
B
A
Nonconformity –
younger sedimentary
rocks rest on older igneous rocks
sedimentary rock layers
Cambrian
Precambrian
Granite
Nonconformity
Disconformity
DISCONFORMITY:Missing rock layers (sedimentary)
Very hard to pick out
Disconformities
Let’s Play….
Name the UNCONFORMITY!
Siccar Point, Scotland
http://www001.upp.so-net.ne.jp/fl-fg/06-02.htm
Resources
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http://courses.geo.ucalgary.ca/glgy209L03/docs/Lecture15_GLGY209_six.pdf
http://perth.uwlax.edu/faculty/stoelting/Intro/Guides/Images2/law_of_superposition_b.jpg
http://www.science.sjsu.edu/scied/255/ppt/superposition/img025.jpg
http://www.gpc.peachnet.edu/~jsummero/Cross-cutting-a.jpg
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http://www.gc.maricopa.edu/appliedscience/gjc-nsf/images/CRH0204.JPG
http://maverick.gc.maricopa.edu/appliedscience/gjcweb/geomap/images/ang.unconf.jpg
http://www.geog.ouc.bc.ca/physgeog/contents/images/hutton.jpg
http://www.bigbendpaleo.com/Photo%20Gallery/Big%20Bend%20Region,%20Texas/Big%
20Bend%20National%20Park%20-%20Syncline%20-%20Javalina%20Creek%20%2020%20June%201995.jpg
http://geoimages.berkeley.edu/GeoImages/Johnson/Images/Large/CD3/028.jpg
http://www.vvdailypress.com/travel/rainbow/images/rock.jpg
http://academic.emporia.edu/aberjame/struc_geo/folds/fold06.jpg
http://www.geology.pitt.edu/images/folds.jpg
http://faculty.evansville.edu/de3/b39902/PPoint/Geologic%20Time/img007.jpg
http://www.photo.net/photo/pcd2882/grand-canyon-region-aerial-85.3.jpg
http://www.cowanstark.com/horsehoe-bend.jpg
http://home.flash.net/~alanm52/Cross_cutting.jpg
http://wwwclass.unl.edu/geol101i/images/igneous%20images/Black%20Canyon%20Gunnison.jpg
http://freespace.virgin.net/craven.pendle/stratigraphy/unconformity.jpg
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http://comp.uark.edu/~sboss/siccar.jpg
http://www.pittstate.edu/services/scied/Teachers/Field/Camp/Us67-1/us67cut.jpg
http://maverick.gc.maricopa.edu/appliedscience/gjcweb/geomap/images/MW8-11.jpg
http://www3.uakron.edu/geology/Foos/parks/rmgcan2.html
http://www.ldeo.columbia.edu/edu/dees/ees/ies2/slides/disconformity.gif
http://geology.lssu.edu/NS102/images/figure%2009-06.gif
http://www.crienterprises.com/C-Assets/J-Hanifa-Unconf-W-Birk.jpg
http://www.uoguelph.ca/~sadura/stgref/stg32b.gif
http://www.geo-outdoors.info/images/eye_protection.jpg
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http://enterprise.cc.uakron.edu/geology/natscigeo/Lectures/time/gtime1.htm