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Interpreting Earth’s Rocks to Determine Its Past History. Geologic time Relative dating. Geologic Time. Relative Dating - sequence of geologic events in a region (order of past events) How do we know how old something is?. Law of Superposition. Oldest layer on bottom (if undisturbed). - PowerPoint PPT Presentation
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Interpreting Earth’s Rocks to Determine Its Past History
Geologic Timechapter 18
• Relative Dating - sequence of geologic events in a region (order of past events)
• Not quantitative
http://www.angelfire.com/az3/mohgameil/Layering4.jpg
Law of Superposition
• Oldest layer on bottom (if undisturbed)• Only works for SED rocks
Younger
Older
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.
http://www.calstatela.edu/faculty/acolvil/sediment/superposition.jpg
The Grand Canyon
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.
fault
oldest
These are younger than
the surrounding
rock
youngest
Cross-Cutting Relationships
6 is younger than1, 2, or 3 because it cuts across them!
1
12
2
3
3
4
5
Which came first?
A
CB
DE
Order the rock layers above from oldest to youngest.
Which came first?
The first layer deposited and the oldest is D, followed by C, A, B, and E.
DD
DC
CA
B
B
E
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.
angular unconformity
(rock layers have been eroded away)
rock layers are eroded
new horizontal layers are deposited
F E D C B A
Angular Unconformity
sedimentary rocks above and below unconformity surface with rocks below tilted at an angle
Granite
sedimentary rock layers
Cambrian
Precambrian
Nonconformity – younger sedimentary rocks rest on older igneous rocks
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• 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
• 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://www-class.unl.edu/geol101i/images/igneous%20images/Black%20Canyon%20Gunn
ison.jpg• http://freespace.virgin.net/craven.pendle/stratigraphy/unconformity.jpg
• 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• http://enterprise.cc.uakron.edu/geology/natscigeo/Lectures/time/gtime1.htm